Abstract

Water processing now has to address contaminant classes that are not reliably removed by conventional clarification and biological treatment. These include refractory organics, pharmaceuticals, pesticides, dyes, solvents, pathogens, algal cells, viruses, and complex solids contained in waste activated sludge. Hydrodynamic cavitation (HC) is relevant because it can impose mechanical, thermal, and chemical effects in the same flow through device, commonly at near ambient bulk temperature and without requiring an optical path through the liquid [1-8].

13. Hydrodynamic Cavitation for Wastewater Processing

13.1 Introduction

Water processing now has to address contaminant classes that are not reliably removed by conventional clarification and biological treatment. These include refractory organics, pharmaceuticals, pesticides, dyes, solvents, pathogens, algal cells, viruses, and complex solids contained in waste activated sludge. Hydrodynamic cavitation (HC) is relevant because it can impose mechanical, thermal, and chemical effects in the same flow through device, commonly at near ambient bulk temperature and without requiring an optical path through the liquid [1-8].

Cavitation is the formation, growth, oscillation, and collapse of vapor or gas vapor cavities when the local liquid pressure approaches or falls below the vapor pressure. In HC equipment, the pressure trajectory is created by the flow itself. Static devices use constrictions, Venturi passages, orifices, nozzles, vortex chambers, and bluff bodies. Dynamic devices use rotating surfaces, teeth, cavities, holes, or rotor stator gaps. The useful treatment event is not cavity formation alone. It is the controlled history of inception, growth, transport, and collapse, together with the spatial distribution of these events through the treated volume [3-10].

Technology can serve several distinct process functions. It can be a pretreatment that makes refractory material more biodegradable, a disintegration step that exposes intracellular and extracellular substrates, an oxidation reactor that creates or activates radicals, a contactor that improves gas or oxidant transfer, or a disinfection step that damages microorganisms mechanically and chemically. These functions should not be conflated. A configuration optimized for cell disruption may not be optimal for oxidation of a hydrophilic pharmaceutical, and a configuration optimized for rapid parent compound disappearance may not achieve adequate total organic carbon removal or toxicity reduction [4,5,8].

The supplied materials span foundational reviews, pilot experiments, industrial wastewater studies, sludge processing, jet generated cavitation, ballast water systems, and patents. Reported values are study specific. Comparisons require attention to reactor volume, number of passes, pressure definition, pump efficiency, water chemistry, temperature, contaminant concentration, and the analytical endpoint.

Figure 13.1. Mechanistic scope of hydrodynamic cavitation in wastewater processing

13.2 Water Phase Reaction Chemistry and Process Response

The common pressure, cavitation number, bubble dynamics, device, and energy relations are consolidated in Section 2.7, Chapter 3, and Chapter 5. The following discussion therefore focuses on aqueous radical chemistry, water matrix scavenging, biological response, and treatment train performance.

13.2.1 Reaction zones and radical chemistry

Three conceptual reaction zones help interpret contaminant behavior. Volatile compounds can enter the expanding cavity and undergo pyrolytic or radical reactions in the gas vapor core. Hydrophobic and nonvolatile compounds tend to accumulate at the gas liquid interface, where radical concentration, temperature gradients, and shear are high. Hydrophilic nonvolatile compounds remain in the liquid bulk and depend on radical escape, micromixing, and oxidant activation. The dominant route therefore changes with volatility, Henry coefficient, hydrophobicity, concentration, and matrix scavenging [2,4,17,20,44].

$${\mathrm{H}}_2\mathrm{O}\to \cdot \mathrm{OH}+\cdot \mathrm{H}$$

(13.1)

$$2\cdot \mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(13.2)

$${\mathrm{H}}_2{\mathrm{O}}_2\to 2\cdot \mathrm{OH}$$

(13.3)

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}^{3+}+\cdot \mathrm{OH}+\mathrm{O}{\mathrm{H}}^{-}$$

(13.4)

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}^{2+}+\mathrm{H}{\mathrm{O}}_2\cdot +{\mathrm{H}}^{+}$$

(13.5)

$${\mathrm{H}}_2{\mathrm{O}}_2+\cdot \mathrm{OH}\to \mathrm{H}{\mathrm{O}}_2\cdot +{\mathrm{H}}_2\mathrm{O}$$

(13.6)

Equations 13.3 to 11.6 show why added hydrogen peroxide can intensify treatment at an optimum dose but reduce performance when overdosed. Peroxide supplies additional hydroxyl radicals, yet it also scavenges them. Similar optima occur with ozone, persulfate, peroxymonosulfate, Fenton reagents, catalysts, and injected gas. Hybrid chemistry should therefore be optimized on the basis of net mineralization, toxicity, chemical consumption, and energy, not on the largest oxidant dose [20-24,42,44-50].

Figure 13.2. Reaction locations and transport pathways around a cavitating structure.

13.2.2 Hydraulic state, cavity population, and reactor form

For water processing, the controlling variable is the population of cavities that experiences a productive pressure history. Minimum local pressure is necessary for inception, but it does not by itself determine chemical or biological performance. Nuclei concentration, dissolved gas, surface tension, viscosity, pressure recovery rate, cavity residence time, cloud interaction, and the location of collapse can cause two devices with similar nominal cavitation numbers to produce different outcomes [3,8,11-14].

Figure 13.3. Cavity lifecycle and engineering meaning of controlled collapse.

Static flow through devices create cavitation by converting pressure into velocity through an orifice, Venturi passage, nozzle, vortex element, bluff body, or jet. They are mechanically simple and can be installed in a recycle loop or a once through line. Their principal engineering limitations are pump duty, possible blockage by fibrous or coarse solids, nonuniform distribution among parallel openings, and the tendency for developed cavities to merge into long vapor structures when backpressure is too low.

Dynamic devices create repeated low pressure regions with a rotor, stator, dimpled disk, toothed surface, or narrow rotating gap. Local velocity and pressure oscillation can therefore be increased without forcing the entire plant flow through one small constriction. This can be advantageous for sludge, concentrated industrial streams, and high throughput disinfection. The tradeoff is the introduction of bearings, seals, rotating clearances, vibration, torque demand, and additional wear surfaces [6-8,30,61].

Figure 13.4. Principal reactor families used in wastewater processing.

Reactor selection should begin with process function. Oxidation requires radical production and transfer to the relevant reaction domain. Sludge pretreatment requires floc breakup, cell damage, and controlled solubilization. Disinfection may depend on pressure cycling, microjets, shear, radicals, or bulk heating. Gas transfer and precipitation applications require interfacial renewal and mixing rather than maximum collapse intensity. A device that is effective for one function should not be assumed to be optimal for another.

13.2.3 Cavitating jets, nozzles, and controlled collapse

Cavitating jets are useful when the collapse region must be placed in a defined chamber. A shear layer forms as liquid exits the nozzle, microscopic nuclei grow in the low pressure jet, and the cloud collapses as it enters a higher pressure zone or strikes a stagnation surface. Arrays of jets can distribute active regions through a vessel, while a downstream target or collision ring can intensify collapse at a selected location [14,55,58].

Figure 13.5. Jet forming nozzle with first and second shear zones., US Patent 6,200,486 B1 [68].

Figure 13.6. Recirculating cavitation treatment loop with reaction chamber, pump, heat exchanger, and pH control. [68].

Figure 13.7. Stroboscopic views of resonant and swirling cavitating jets. [16].

Kalumuck and Chahine used recirculating jet loops and multistage orifice plates with downstream stagnation surfaces to oxidize dilute p nitrophenol. Under the reported experimental conditions, submerged cavitating jets provided approximately two orders of magnitude greater energy efficiency than the ultrasonic comparison. The result was attributed to broader cavitation distribution and the relatively high efficiency of hydraulic pumps, but it should be interpreted as a study specific comparison rather than a universal ratio for all reactors and contaminants [55].

Backpressure is an active design variable. If downstream pressure recovery is insufficient, a stable or supercavitating vapor region can persist with weak collapse. If recovery is excessively abrupt or occurs on a vulnerable wall, erosion, noise, and vibration can increase. Productive operation therefore requires control of both inception and collapse, including the axial position of cloud shedding and the distance from sensitive equipment surfaces [9,10,14,55].

Figure 13.8. Pressure can accelerate contaminant removal while reducing hydraulic energy efficiency [16].

Figure 13.9. Effect of recycle loop volume on the energy required for one order of magnitude methyl orange reduction. [16].

13.3 Hydrodynamic Cavitation for Wastewater Treatment

13.3.1 Position in the treatment train

Hydrodynamic cavitation is most defensible as a targeted unit operation rather than a universal replacement for primary, biological, and separation processes. In a municipal plant, it may be placed before biological treatment to improve biodegradability, after biological treatment to reduce micropollutants and residual pathogens, or in a sludge sidestream. In industrial plants, it may pretreat concentrated or toxic streams, intensify oxidant contact, or serve as a polishing step before reuse. The location should follow the dominant limitation: mass transfer, biodegradability, particle accessibility, toxicity, or microbial risk [1-8].

Literature consistently warns that results obtained with a single compound in clean water can overstate performance in real effluent. Carbonate, bicarbonate, chloride, natural organic matter, suspended solids, oils, surfactants, and multiple contaminants compete for radicals and alter bubble dynamics. Conversely, solids and gas bubbles can also supply nuclei and increase the number of cavitation events. These opposing effects explain why matrix specific trials remain necessary [1,2,4,8].

Figure 13.10. Targeted positions for hydrodynamic cavitation in municipal and industrial wastewater treatment trains.

13.3.2 Model compounds and optimization of hydraulic severity

The p nitrophenol study by Capocelli and coworkers provides a useful example of a nonmonotonic pressure response. In a 1.5 L recycle apparatus with a transparent Venturi, the tested inlet pressure range was 0.2 to 0.7 MPa. Removal rate and energy efficiency were optimized near 0.4 to 0.45 MPa, corresponding to a reported cavitation number close to 0.25. At 0.45 MPa, the observed first order constant was approximately 1.13 × 10⁻² min⁻¹ and EEO was 66.7 kWh m⁻³ per order [15]. Increasing pressure beyond the optimum increased hydraulic demand and changed cavity behavior without a proportional chemical benefit.

The implication is broader than this one compound. Very weak cavitation provides too few intense collapses. Developed cavitation can provide a large population of productive events. At excessive severity, vapor shielding, coalescence, long attached cavities, choked flow, or energy loss can reduce the fraction of input energy converted to useful collapse. The optimum should be located experimentally using a chemical or biological response and an energy normalized metric.

13.3.3 Real industrial wastewater, landfill leachate, and nutrient removal

Real wastewater studies reveal both the potential and the limitations of standalone HC. Korniluk and Ozonek treated mature landfill leachate in a recycle loop with a four slot orifice reactor. The system operated at approximately 7 bar, 46.5 L min⁻¹, and a total liquid volume of about 40 L. After 30 minutes, COD decreased by 6.7% and TOC by 5.1%, while temperature increased by approximately 11 °C [18]. These modest changes are consistent with the high concentration and refractory nature of old leachate. In such cases, HC is more likely to be useful as a pretreatment or oxidant activation step than as the sole mineralization process.

Badve and coworkers studied actual wood finishing wastewater with an initial COD of approximately 38,000 mg L⁻¹ and pH 6.18. The rotating cavitator used a rotor with 204 surface indentations. COD reduction depended on rotor speed, residence time, and peroxide dose. The study reported an optimum near 2200 rpm and an optimum H₂O₂ loading near 5 g L⁻¹. Under the tested conditions, HC alone produced substantial COD reduction, while the optimized HC peroxide process increased cavitational yield by approximately 46% [19]. Higher peroxide loading reduced the benefit, which is consistent with radical scavenging.

Industrial advanced Fenton case. The industrial wastewater manuscript describes a 25 L recirculating hydrocavitator coupled to zero valent iron and hydrogen peroxide. The feed was a complex substituted phenolic effluent with reported pH 1.7, COD about 42,000 mg L⁻¹, and TOC about 14,000 mg L⁻¹. At otherwise comparable conditions, TOC removal increased from about 50% at 500 psi to 52% at 1000 psi and 60% at 1500 psi. A short 15 minute active treatment followed by storage also produced continued mineralization, showing that cavitation can initiate chemistry that persists after hydraulic exposure [69].

Figure 13.11. Effect of hydrocavitator inlet pressure on total organic carbon mineralization in industrial wastewater treated with heterogeneous advanced Fenton chemistry. [69].

Figure 13.12. Continuous treatment and latent remediation after a short active cavitation period [69].

Figure 13.13. Response of mature landfill leachate during 30 minutes of recirculating hydrodynamic cavitation. [18].

A separate phosphate removal study used 20% calcium hydroxide slurry before HC to intensify precipitation and dispersion. About 70% phosphate reduction was reported, with a jet nozzle giving the best result among the tested devices [25]. This is a physicochemical precipitation application rather than radical oxidation. It illustrates an important principle: HC can increase dissolution, dispersion, nucleation, and contact efficiency even when the primary removal mechanism is not oxidative.

Micropollutant polishing. In the pharmaceutical study, one liter of water containing six model compounds was repeatedly transferred through a narrow Venturi at initial pressures of 4, 5, and 6 bar with hydrogen peroxide. The 6 bar condition produced the highest reported removals, but compound response varied widely, from approximately 45% for clofibric acid to 86% for naproxen [43]. A more recent study applied ozone assisted hydrodynamic cavitation to antibiotic contaminated aquaculture seawater, illustrating the shift toward gas liquid transfer, oxidant activation, and treatment train integration rather than cavitation alone [48].

Figure 13.14. Phosphate reduction reported for mixing, orifice plate, and jet nozzle configurations using calcium hydroxide slurry. [25].

Figure 13.15. Removal of six pharmaceuticals after hydrodynamic cavitation with hydrogen peroxide at 6 bar. [43

Figure 13.16. Conceptual ozone assisted hydrodynamic cavitation treatment train for antibiotic contaminated water. [7,8,48].

Table 13.1. Representative HC wastewater treatment studies from the supplied and recent literature.

Matrix or target	Reactor and treatment	Representative result	Engineering interpretation
p Nitrophenol	Recycle Venturi, 0.2-0.7 MPa	Optimum near 0.4-0.45 MPa; EEO 66.7 kWh m⁻³ per order	Maximum pressure was not the optimum [15]
Landfill leachate	Four slot orifice, 7 bar, 30 min	COD 6.7%; TOC 5.1%	Standalone HC gave limited mineralization in a refractory matrix [18]
Wood finishing wastewater	Rotor stator with indentations, HC and H₂O₂	COD reduction and yield improved at an optimum dose; yield +46%	Rotor speed, residence time, and oxidant dose must be jointly optimized [19]
2,4-D herbicide	HC plus zero valent iron and H₂O₂	Residual TOC about 30% after 20 min in the reported continuous treatment	Advanced Fenton chemistry supplied radicals beyond those generated by water alone [21]
2-Chlorophenol	Venturi enhanced cavitation oxidation process	Higher H₂O₂ dose increased degradation, but efficiency favored mild cavitation and controlled injection	Fast treatment and minimum energy can require different operating points [20]
Phosphate containing wastewater	Ca(OH)₂ slurry plus orifice or jet HC	About 70% phosphate reduction	HC intensified precipitation and solids contact [25]
Wastewater pharmaceuticals	HC with H₂O₂, ozone, Fenton, persulfate, plasma, or catalysts	Performance depends strongly on compound and water matrix	Hybrid selection should be driven by reaction domain and scavenging [44-46]

The landfill leachate experiment also demonstrates why temperature must be recorded as a response variable. During 30 minutes of recirculation, the liquid temperature increased by approximately 11 °C while COD and TOC decreased by only 6.7% and 5.1%. The absorbed hydraulic energy was therefore expressed mainly as bulk heating rather than deep mineralization. A comparison based only on removal percentage would miss this energy partitioning [18].

For concentrated and highly scavenging wastewater, modest COD or TOC change does not necessarily mean that pretreatment has no value. Molecular weight distribution, biodegradability, toxicity, color, volatile compounds, and downstream biological oxygen demand may change before extensive mineralization occurs. These secondary endpoints should be measured when the intended role of HC is pretreatment rather than final oxidation.

13.3.4 Design implications for real effluents

The treatment objective should be defined before selecting operating conditions. Parent compound removal alone is appropriate when the transformation products are known and acceptable. COD and TOC are required when bulk oxidation or mineralization is the goal. Biochemical oxygen demand to COD ratio, respirometry, or biological methane potential can quantify improved biodegradability. Toxicity assays are needed when reactive intermediates may form. Color removal and UV absorbance are useful but can be misleading if chromophores are destroyed without corresponding carbon removal.

A practical staged approach is often preferable. Coarse solids or free oil can be removed first to avoid unnecessary hydraulic duty. HC can then be applied at a concentration and pH window where the target is accessible. Biological treatment can remove biodegradable products. A final polishing step can address residual micropollutants or microbial risk. This sequence uses the high intensity process only where it adds value and avoids attempting to mineralize the entire organic load by cavitation alone [22-24].

13.3.5 Equipment selection and hydraulic integration

The reactor must be integrated with wastewater rather than selected independently of it. Low solids pharmaceutical polishing can use narrow Venturi or orifice passages and controlled oxidant injection. Fibrous, oily, or particulate streams require larger flow paths, maceration, flushing provisions, or dynamic devices with clearances that tolerate solids. Sludge service requires positive displacement or progressive cavity pumping, controlled temperature rise, and protection from grit. Highly corrosive industrial effluent may require lined piping, compatible elastomers, and replaceable cavitation elements.

The controlled cavitation equipment described by Nicholas and Vella used pressure equalization chambers and opposed truncated nozzles that formed a collision region inside a central chamber. A transparent demonstration chamber made the cavitation cloud visible. The arrangement illustrates how equalized feed pressure and opposed flow can localize collapse without relying on a single downstream wall. The reported sludge benefits came from a developer affiliated pilot study, so the equipment photographs are valuable design evidence but the performance claims require independent verification at the proposed duty [58].

A minimum instrumentation package should measure upstream and downstream pressure, flow, liquid temperature, motor or pump power, and treatment time. For hybrid processes it should also measure pH, oxidant dose, residual oxidant, dissolved oxygen or ozone transfer, and off gas. Sampling must distinguish instantaneous pass performance from the cumulative response of a recycle tank. The loop volume, dead volume, bypass flow, cooling duty, and number of nominal turnovers should be reported.

Table 13.2. Wastewater matrix and equipment selection considerations.

Matrix or objective	Preferred reactor features	Main risks	Essential controls
Low solids micropollutant polishing	Venturi, orifice, vortex, or jet with controlled oxidant contact	Radical scavenging, overdosage, low single pass conversion	Pressure, flow, pH, oxidant residual, EEO, transformation products
Concentrated industrial wastewater	Recycle loop, staged oxidation, heat rejection, sampling between stages	Bulk heating, incomplete mineralization, toxicity of intermediates	COD, TOC, biodegradability, toxicity, temperature, specific energy
Waste activated sludge	Large passages or rotational device, macerator, positive displacement pumping	Grit wear, plugging, excessive solubilization, foaming changes	SCOD, particle size, viscosity, BMP, torque, temperature
Oily or emulsified wastewater	Jet, vortex, or rotor with downstream flotation or separation	Stable emulsification when separation is intended	Droplet size, oil removal, gas transfer, flotation performance
Precipitation and nutrient removal	Rapid dispersion and nucleation followed by clarification or filtration	Fine solids carryover and scaling inside the reactor	pH, reagent stoichiometry, particle size, settling, pressure loss
Disinfection and reuse	Repeated pressure cycles with validated biological endpoint	Sublethal injury, regrowth, shielding by solids	Culture and infectivity assays, turbidity, temperature, LRV, residual control

13.4 Sludge Pretreatment and Anaerobic Digestion

13.4.1 Hydrolysis limitation and disintegration mechanisms

Waste activated sludge contains flocs, extracellular polymeric substances, microbial cells, cell debris, and slowly degradable particulate organic matter. Hydrolysis is commonly the rate limiting step in anaerobic digestion. HC pretreatment can decrease floc and particle size, damage cell walls, release soluble COD, expose intracellular material, and increase the surface area available to hydrolytic and fermentative microorganisms [5,26-31]. Primary sludge is often more readily biodegradable, so its relative response may be smaller than that of secondary sludge.

Figure 13.17. Hydrodynamic cavitation pretreatment before anaerobic digestion.

Degree of sludge disintegration DDCOD can be determined from the equation 13.7:

$$\mathrm{DDCOD}=100\frac{{\mathrm{SCOD}}_t-{\mathrm{SCOD}}_0}{{\mathrm{SCOD}}_{\max }-{\mathrm{SCOD}}_0}$$

(13.7)

DDCOD is the degree of disintegration based on soluble chemical oxygen demand, SCODt is soluble COD after treatment, SCOD₀ is the initial soluble COD, and SCODmax is a reference maximum obtained by a defined chemical or thermal method. Because the reference method changes the denominator, studies should state it explicitly.

Figure 13.18. Schematic shift in sludge particle size distribution after controlled cavitation. The representative centers and treated size range reflect the controlled flow cavitation observations [26,27].

13.4.2 Methane yield, severity optimum, and excessive treatment

Methane enhancement can be determined from the equation 13.8:

$$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{e}\mathrm{n}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}(\%)=100\frac{Y_{{\mathrm{CH}}_4,t}-Y_{{\mathrm{CH}}_4,c}}{Y_{{\mathrm{CH}}_4,c}}$$

(13.8)

Early controlled flow cavitation work summarized by Michel and Kozyuk reported narrower particle distributions and typical methane yield improvements in the range of 20% to 30%, together with lower residual digested solids in the cited applications [5,26,27]. Grübel and Suschka combined mild alkalization near pH 9 with HC. Their hybrid treatment increased to soluble organic release and reported 22% to 27% higher biogas production than ordinary digestion under the study conditions [28]. The alkaline step weakened cell walls and flocs, while HC supplied shear, shock, mixing, and further disintegration.

More recent results reinforce the existence of a severity optimum. Abdelrahman and coworkers treated waste activated sludge at upstream pressures of 0.83 and 1.72 MPa. Methane yields were 128.4 and 119.1 mL CH₄ g⁻¹ VS, respectively, compared with 98.3 mL CH₄ g⁻¹ VS for the control. The lower pressure condition produced the largest increase, 30.6% [29]. A Son, at el. study using an advanced rotational HC reactor also found that excessive treatment could reduce biogas yield, while optimized treatment improved overall energy efficiency [30].

Table 13.3. Selected sludge pretreatment evidence and scale up implications.

Study or system	Treatment	Reported outcome	Implication
Controlled flow cavitation [26,27]	Repeated flow through exposure	Secondary sludge distribution centered near 150 μm shifted below about 22 μm	Particle accessibility and hydrolysis can be improved without complete solubilization
Hybrid alkali HC [28]	Mild NaOH conditioning and developed Venturi cavitation	Higher SCOD release; biogas increased about 22-27%	Chemical weakening and mechanical disintegration can be synergistic
Flow pattern study [29]	Orifice HC at 0.83 and 1.72 MPa	Methane 128.4 and 119.1 versus 98.3 mL CH₄ g⁻¹ VS control	Lower pressure produced the best biological result
Advanced rotational HC [30]	Rotor based sludge disintegration	Optimized treatment improved digestibility and energy efficiency; excessive treatment was detrimental	Severity must be optimized against biological response, not only SCOD
Semi industrial rotating device [31]	Scale up to 500 L sludge pig slurry treatment	Specific solubilization energy decreased with scale	Rotational HC can benefit from scale when hydraulic duty is managed

13.4.3 Quantitative indicators of sludge disintegration

Machnicka, Grübel, and Suschka treated 25 L portions of activated sludge in a recycle system using a 1.2 mm nozzle and a pump pressure of approximately 12 bar. Soluble COD increased from about 42 to 326 mg L⁻¹ during 90 minutes of treatment, while the reported degree of disintegration rose from approximately 14% at 15 minutes to 54% at 90 minutes. The most rapid increase occurred during the first 30 minutes, showing that exposure time should be optimized against diminishing incremental solubilization [56].

Figure 13.19. Soluble chemical oxygen demand release and degree of disintegration during hydrodynamic treatment of activated sludge. [56].

Soluble COD is a convenient process indicator, but it is not a complete measure of digestion benefit. Solubilized material can include readily fermentable substrate, slowly degradable macromolecules, inert soluble compounds, or inhibitory products. The value of pretreatment must therefore be confirmed by methane yield, volatile solids destruction, dewaterability, ammonia release, viscosity, and the stability of the anaerobic microbial community.

In the same study, adding disintegrated sludge at 10%, 20%, and 30% of fermenter volume increased reported biogas production by about 22%, 95%, and 131% relative to the control. These large batch responses demonstrate that released substrate was biologically accessible under the test conditions. They should not be transferred directly to full scale continuous digesters because treated fraction, organic loading, solids retention time, inoculum adaptation, and the energy required for 90 minutes of recycle treatment differ substantially [56].

Figure 13.20. Reported biogas response after adding hydrodynamically disintegrated sludge to batch fermenters. [56].

Optical microscopy from the APV INature process shows disruption of large agglomerates and filamentous structures after low pressure homogenization. Such images provide direct evidence of structural change, but microscopy should be paired with quantitative particle size and soluble organic measurements because visual fields may not represent the entire sludge population [57].

Figure 13.21. Activated sludge structure before and after cavitation. [26,27].

13.4.4 Commercial and full scale sludge conditioning systems

Commercial sludge systems commonly combine several operations rather than using a cavitation element alone. The APV INature configuration used an upstream macerator, a high pressure piston pump, and a low pressure homogenizing valve. The supplied workshop paper identified approximately 150 bar and 5.5 kWh m⁻³ as the preferred operating point for the described installation. Reported particle size values decreased from 30 to 15 µm at X50 and from 80 to 32 µm at X90, with a claimed increase in available surface area greater than 100% [57].

Figure 13.22. Low pressure homogenizing valve used in the APV INature sludge treatment process. [57].

Figure 13.23. Reported anaerobic digestion benefits and particle size effects associated with controlled flow cavitation. [59,65,70].

The VRTX study reported improvements in soluble COD, dewatering, suspended solids measures, sludge volume index, and particle size during pilot treatment of waste activated sludge [58]. The Siemens Crown process combined maceration, high speed mixing, pressurization, a disintegration nozzle, recirculation, and discharge to the digester. Its technical literature reported biogas increases up to 30% and post digestion solids reduction up to 20% [59,65]. These figures are useful for defining commercial performance hypotheses, but they are vendor associated and should be separated from independently controlled research data in design reviews.

Figure 13.24. Crown sludge disintegration equipment and a full scale installation at Wiesbaden. [59,64,65].

Full scale value is created through the combined balance of additional methane, reduced biosolids mass, improved cake solids, reduced foaming, and avoided external carbon addition. The same system can lose value through excessive electrical demand, maintenance, grit erosion, increased soluble refractory COD, or destabilization of the digester. Economic assessment should therefore use measured plant mass balances rather than a single percentage improvement.

Table 13.4. Evidence hierarchy for sludge pretreatment design.

Evidence class	Representative source	Strength	Limitation	Recommended design use
Controlled laboratory digestion	Machnicka et al. [56]; Abdelrahman et al. [29]	Direct comparison with control and defined treatment conditions	Small volume, batch digestion, matrix specific	Identify mechanisms, exposure optimum, and biological response
Pilot sludge treatment	Nicholas and Vella [58]	Real waste activated sludge and equipment operation	Developer affiliated study and limited long term data	Define solids handling, dewatering, and instrumentation needs
Workshop or commercial case study	Fulton [57]; Schmitt [64]	Plant scale equipment, operability, and economic context	Incomplete independent controls and variable reporting	Establish retrofit configuration and candidate performance range
Vendor technical literature	Siemens [59,65]	Equipment layout and claimed operating benefits	Marketing context and selective reporting	Develop hypotheses only; require guaranteed and independently verified acceptance tests
Recent peer reviewed rotational HC	Son et al. [30]	Modern reactor, BMP response, scale analysis, excessive treatment warning	Still requires site specific validation	Support advanced rotor selection and severity optimization

13.5 Ballast Water Cavitation for Treatment

13.5.1 Regulatory and process context

Ballast water can transport bacteria, viruses, phytoplankton, zooplankton, larvae, cysts, and other organisms between ecosystems. The International Maritime Organization Ballast Water Management Convention establishes the D-2 performance standard for discharged ballast water. The phase in schedule for existing ships reached its final compliance date on 8 September 2024. The mandatory Code for Approval of Ballast Water Management Systems was adopted in 2018 and took effect in October 2019, replacing the earlier G8 approval guidelines. Systems using active substances remain subject to the additional environmental evaluation procedure associated with G9 [32-35].

As of December 2025, the Convention had 101 Parties representing 93.73% of world merchant shipping tonnage [54]. The regulatory framework continues to evolve through the IMO experience building and review process. Therefore, any commercial design, retrofit, or statement of approval status should be checked against current IMO, flag state, classification society, and port state requirements rather than inferred from historical literature.

Figure 13.25. IMO Regulation D-2 ballast water discharge performance standard. [33-35,54].

Table 13.5. IMO D-2 ballast water discharge limits.

Organism or indicator	Maximum concentration in discharged ballast water
Viable organisms, minimum dimension ≥50 μm	<10 organisms m⁻³
Viable organisms, 10 to <50 μm	<10 organisms mL⁻¹
Toxigenic Vibrio cholerae O1 and O139	<1 CFU 100 mL⁻¹ or <1 CFU g⁻¹ wet zooplankton
Escherichia coli	<250 CFU 100 mL⁻¹
Intestinal enterococci	<100 CFU 100 mL⁻¹

13.5.2 Treatment architecture and biological mechanisms

Ballast systems normally combine unit operations because no single mechanism is equally effective for all size classes and water qualities. Filtration or hydrocyclonic separation removes larger organisms and protects downstream equipment. HC can then damage cells, increase oxidant contact, and expose organisms to repeated pressure and shear. An active substance, ozone, deoxygenation, or another disinfectant may provide residual control and suppress regrowth. Neutralization may be required before discharge when an active oxidant remains [32,36,37].

Hydrodynamic cavitation inactivation mechanisms include membrane deformation, shear, microjet impact, pressure oscillation, shock waves, reactive species, and improved transport of chemical disinfectant. Large plankton and zooplankton are especially susceptible to mechanical disruption, while bacteria and viruses may require repeated exposure or chemical assistance. The salinity, temperature, turbidity, dissolved organic carbon, organism type, and holding time determine the final response.

Figure 13.26. Typical hybrid ballast water treatment architecture incorporating filtration, hydrodynamic cavitation, oxidant generation, residual monitoring, and neutralization. [32,36,37].

13.5.3 Historical systems and the HyCator case study

The review by Cvetković and coworkers identified four type approved or commercial systems that incorporated HC as one stage: JFE Ballast Ace, OceanSaver, FineBallast OZ, and Venturi Oxygen Stripping. Each combined cavitation with filtration, chemical treatment, ozonation, nitrogen supersaturation, or deoxygenation [36]. These examples are historically important because they show that commercial ballast HC was implemented as part of a treatment train. They should not be interpreted as a statement of current product availability or approval status.

Sangave, Mukherjee, and Pandit described a HyCator ballast water system that combined automatic back flushing filtration, controlled HC, and sodium hypochlorite generated by electrolysis of seawater. The reported ballasting sequence used approximately 3 mg L⁻¹ total residual oxidant, with residual disinfectant maintained during the holding period and neutralized by sodium thiosulfate during deballasting to less than 0.1 mg L⁻¹. The reported tests met the selected D-2 biological endpoints in a single pass treatment train [37]. Because the paper was authored by personnel affiliated with the technology developer, the results are best treated as vendor associated performance evidence, supplemented by independent type approval data where available.

Mechanochemical ballast treatment. Yoshimura, et al study combined cavitating water jets with sodium hypochlorite. At a 3.1 mm nozzle diameter, a flow of 30 L min⁻¹, and 5 mg L⁻¹ sodium hypochlorite, the authors reported complete extinction of the test plankton population. Ejector nozzles supported higher flow than the tested Venturi arrangement, emphasizing that chemical suction, nozzle pressure, and organism exposure must be designed as one system [71].

Figure 13.27. Plankton extinction reported for mechanochemical cavitation with sodium hypochlorite. [71].

Figure 13.28. Water treatment cavitation device with chemical injection near the constriction. US Patent 7,247,244 B2 [51].

13.5.4 Current approval, monitoring, and verification requirements

The HyCator process diagram shows the practical complexity of a physicochemical ballast system. The ballasting line included filtration, a booster pump, a cavitation reactor, a side stream electrochlorination unit, total residual oxidant monitoring, hydrogen separation, and a ballast tank. During deballasting, sodium thiosulfate was metered into the discharge line and residual oxidant was checked before overboard discharge [37]. The cavitation reactor was therefore only one controlled element in a complete safety and compliance system.

The final D2 implementation date for existing ships was 8 September 2024. By December 2025 the Convention had 101 Parties representing 93.73% of world merchant shipping by tonnage [34,54]. In 2026 the IMO adopted revised G4 guidance through resolution MEPC.409(84). The earlier G8 approval guidelines have been superseded by the mandatory BWMS Code, while systems that use active substances remain subject to the G9 procedure [35].

A present day design must document the certified system design limitations, approved flow range, salinity and temperature envelope, turbidity limits, active substance dose, neutralization capacity, monitoring accuracy, and contingency procedures. The biological result must be demonstrated by the prescribed size classes and indicator microbes, not by pump pressure, cavitation number, or oxidant residual alone. Commissioning tests and operational sampling should also verify that bypasses, valves, sensor drift, and changing water quality do not compromise treatment.

Table 13.6. Ballast water management system verification items.

Verification area	Required evidence	HC specific concern
Biological performance	D 2 organism size classes and indicator microbe limits under approved test conditions	Mechanical damage may not equal loss of viability; regrowth and assay method matter
System design limitations	Certified range for flow, salinity, temperature, UV transmittance or turbidity, and holding time	Cavitation pattern changes with vapor pressure, gas content, and backpressure
Active substances	IMO approval, dose control, byproduct evaluation, and neutralization	HC can intensify oxidant transfer and may alter residual demand
Mechanical integration	Pump, filtration, bypass protection, pressure relief, corrosion, and hydrogen safety where electrolysis is used	Pressure oscillation, erosion, noise, and gas release require engineered safeguards
Operational monitoring	Calibrated sensors, alarms, records, commissioning, and contingency actions	Pressure and power are surrogate indicators and cannot replace biological verification

13.6 Remediation and Disinfection

13.6.1 Mechanical and chemical inactivation

Microbial inactivation by HC can proceed through mechanical and chemical routes. Mechanical damage includes stretching and rupture from pressure oscillations, turbulent eddies, shear, shock waves, and microjets. Chemical damage includes oxidation of membranes, proteins, and nucleic acids by radicals or added oxidants. The relative contribution depends on bubble size, collapse frequency, organism size and structure, and water chemistry [16,38-41].

Figure 13.29. Mechanical, chemical, and thermal pathways that contribute to microbial inactivation by hydrodynamic cavitation. Original synthesis based on [4,5,16,38-41,60-63,67].

A useful biological metric is logarithmic reduction can be determined from the equation 13.9:

$$\mathrm{LRV}={\log }_{10}\left(\frac{N_0}{N_t}\right)$$

(13.9)

N₀ and Nt are the initial and treated viable concentrations. For disinfectant assisted processes, a Chick Watson form is sometimes used as an empirical exposure model:

$$\ln \left(\frac{N_t}{N_0}\right)=-k{C}_W^{n}t$$

(13.10)

CW is disinfectant concentration, n is an empirical exponent, and k is an inactivation constant. HC can change the apparent k by improving micromixing, damaging protective structures, and renewing the cell liquid interface. The relation should not be extrapolated beyond the tested range because oxidant demand and organism susceptibility change during treatment.

13.6.2 Jet generated cavitation and E. coli

Kalumuck and coworkers used submerged cavitating jets to oxidize contaminants and inactivate microorganisms. Their designs used passive acoustic excitation, bluff bodies, or swirl to intensify cavitation at moderate pump pressure. The study reported multiple orders of magnitude reduction in E. coli and substantially higher energy efficiency than the compared ultrasonic configuration [16]. The authors also observed a pressure tradeoff: higher jet pressure accelerated treatment, while a lower pressure could remove more contaminant per unit hydraulic energy.

Figure 13.30. Representative E. coli reduction history for jet generated cavitation. Original reconstruction from the study specific results reported in the CAV2003 paper [16].

13.6.3 Bacteria, viruses, algae, and zooplankton

Jyoti and Pandit found HC to be an attractive energy physical disinfection method compared with several nonchemical alternatives and showed that combining cavitation with hydrogen peroxide, ozone, or other treatments could increase the rate of inactivation [39,40,63]. Arrojo, Benito, and Martínez Tarifa compared chamber geometries and concluded that hydrodynamic disinfection under their conditions was strongly associated with mechanical disruption, especially for configurations that promoted large bubbles and extended pressure oscillations [41]. Multi barrier disinfection evidence. The Van der Walt, et al. study evaluated hydrodynamic cavitation, ultraviolet radiation, and ultrasound individually and in combination. In filtered water, the combined treatment reported average reductions above 90% for standard plate counts, total coliforms, fecal coliforms, and bacteriophages, while the average Clostridium reduction was about 79%. The results are study specific, but they demonstrate the value of matching complementary mechanisms and verifying each biological endpoint separately [72].

Figure 13.31. Average microbial reductions reported for a combined hydrodynamic cavitation, ultraviolet, and ultrasound treatment system. [72].

Sawant and coworkers studied zooplankton in seawater and developed a framework relating turbulent shear and cavity dynamics to organism damage [38]. The larger size and more fragile structures of many zooplankton make them responsive to mechanical events, but complete ballast treatment still requires control of smaller microorganisms and regrowth. Dular and coworkers later showed that different cavitation regimes should be selected for different biological targets. In their work, supercavitation was effective for Legionella inactivation, while toxic cyanobacteria, green microalgae, and rotavirus showed different sensitivities [4].

A decrease in culture count, RNA signal, membrane integrity, or metabolic activity does not necessarily mean the same thing. A robust protocol should distinguish immediate injury, loss of culturability, infectivity, delayed recovery, and regrowth. Controls should account for pump shear, temperature, sampling, and holding time. For reuse and ballast applications, treated water should be observed through the relevant storage period.

Table 13.7. Representative biological targets and dominant HC considerations.

Target	Likely dominant response	Recommended verification
Vegetative bacteria	Mechanical membrane damage plus oxidative stress	Culture, membrane integrity, delayed regrowth, residual disinfectant
Spores or resistant bacteria	Higher mechanical and chemical resistance	Validated multi barrier treatment and long holding study
Viruses	Capsid and genome damage, often requiring repeated exposure or oxidant	Infectivity assay, not nucleic acid signal alone
Cyanobacteria and microalgae	Cell rupture, buoyancy change, pigment release, toxin release risk	Cell count, viability, dissolved toxin, downstream separation
Zooplankton and larvae	Shear, pressure oscillation, microjet and shock damage	Viable organism count by size class
Mixed natural water	Matrix dependent combination of effects	Standardized challenge organisms, real water trials, regrowth and ecotoxicity

13.6.4 Rotational cavitation and the contribution of bulk heating

Sun and coworkers investigated an advanced rotational HC reactor containing a 260 mm rotor with cone shaped dimples and stationary front and rear covers with different dimple counts. A 15 kW motor operated the rotor at 2700 to 3600 rpm, while a separate pump controlled flow and inlet pressure. The repeated interaction of moving and stationary cavities produced strong local cavitation and substantial conversion of shaft power into heat [61].

The maximum reported heat generation rate was 48.15 MJ h⁻¹ and the maximum thermal efficiency was 82.18%. In the disinfection experiments, the authors concluded that bulk thermal exposure was the dominant factor. At approximately equal final temperatures near 65 °C after 14 minutes, the rotational HC treatment achieved complete plate count inactivation and an 8.3 log reduction, while conventional heating produced 24.14% inactivation and a 0.12 log reduction. The comparison indicates that thermal history and cavitational damage acted together in that reactor [61].

This result should not be generalized to every HC device. In a low pressure Venturi or jet reactor with active cooling, the bulk temperature may remain too low to contribute materially. Conversely, an uncooled high power rotor can behave as both a cavitation reactor and a rapid heater. Disinfection studies should therefore include a matched thermal control, a complete temperature time history, power measurement, and direct assessment of the nonthermal contribution.

13.6.5 Viability, sublethal injury, and regrowth

Mezule and coworkers used a small rotor device to treat E. coli in a 2 L system. Three minutes of exposure at a reported energy density of 490 W L⁻¹ stopped division in approximately 75% of the cells. However, most cells retained measurable respiratory activity. The authors interpreted this as an active but nonculturable state and recommended investigation of possible resuscitation [60]. This distinction is critical for reuse and drinking water applications.

A reliable disinfection study should combine culture or plaque assays with at least one independent measure of integrity or metabolic state. Options include membrane staining, respiration assays, ATP, flow cytometry, quantitative polymerase chain reaction with viability treatment, and host infectivity for viruses. Molecular signal reduction alone does not prove infectivity loss, while culture loss alone may miss injured organisms that later recover.

Independent virus work provides stronger evidence than genomic reduction alone. Kosel and coworkers achieved more than 4 log reduction of MS2 bacteriophage infectivity in 3 mL and 1 L Venturi reactors, demonstrating that HC can inactivate a virus surrogate when the endpoint is a plaque assay [67]. The result also shows that reactor scale, initial titer, pass number, and infectivity assay must be reported together.

13.6.6 Cyanobacteria, microalgae, and cyanotoxin control

Cyanobacterial treatment has two simultaneous objectives: control the cells and control the toxin. Mechanical rupture may reduce viable biomass but release intracellular microcystin. A suitable process must therefore either avoid uncontrolled lysis or provide sufficient oxidation after lysis. Chlorophyll, turbidity, cell count, photosynthetic activity, extracellular toxin, intracellular toxin, and post treatment regrowth should be evaluated together [4,62].

The ERDC study compared four nozzle configurations in large treatment tanks. Three conventional jet nozzles produced limited radical signatures and inconsistent biological effects. A submerged nano micro bubble nozzle supplied with compressed air produced the clearest hydroxyl and superoxide signatures and the strongest combined decrease in turbidity, chlorophyll a, and microcystin. The nozzle operated at approximately 22.3 L min⁻¹, with water pressure near 25.3 psi and air pressure near 1.2 psi [62].

Figure 13.32. Conventional and submerged gas assisted nozzles evaluated in the U.S. Army ERDC cyanobacteria study. [62].

The reported two hour treatment reduced chlorophyll a by approximately 95% and microcystin by approximately 47.5% with the nano micro bubble nozzle. The difference between these endpoints confirms that cell or pigment removal cannot be used as a surrogate for complete toxin destruction. The treated water also became visibly clearer in a separate 15 L demonstration, but visual clarity is not a substitute for toxin analysis [62].

Figure 13.33. Reported chlorophyll a and microcystin response for the ERDC nozzle configurations. Original redraw from [62].

Figure 13.34. Visual change during treatment of a Microcystis aeruginosa suspension. The supplied government report photograph was cleaned to a pale neutral background and increased in contrast [62].

Results across cyanobacteria studies remain strongly species and reactor dependent. Gas vesicles, cell wall composition, colony structure, and extracellular polymeric material affect susceptibility. HC can inhibit cyanobacterial growth without visible lysis under some conditions, while more aggressive cavitation can rupture cells. For field deployment in lakes or reservoirs, ecological selectivity, resuspension, oxygen demand, toxin release, and treatment of only a fraction of the water body must be considered.

13.7 Oxidation of Organic Components

13.7.1 Reaction domain selection

The phrase organic oxidation covers several distinct objectives: transformation of the parent compound, ring opening, dehalogenation, reduction of COD, reduction of TOC, detoxification, and complete mineralization to carbon dioxide, water, and inorganic ions. HC can accomplish some or all of these functions, but the required exposure and chemistry differ. Volatile organic compounds can enter the cavity and undergo pyrolysis or radical attack. Hydrophobic nonvolatile compounds are favored at the interface. Hydrophilic nonvolatile compounds remain in the bulk, where radical delivery is limited by short radical lifetime and scavenging [2,4,17,20].

Figure 13.35. Dominant organic contaminant reaction domain as a function of volatility, hydrophobicity, and partitioning.

The 13.11 equation represents a hydrogen abstraction reaction by a hydroxyl radical (•OH), which is a fundamental elementary step in free-radical chemistry, advanced oxidation processes, and cavitation-induced degradation.

$$\mathrm{RH}+\cdot \mathrm{OH}\to \mathrm{R}\cdot +{\mathrm{H}}_2\mathrm{O}$$

(13.11)

$$\mathrm{R}\cdot +{\mathrm{O}}_2\to \mathrm{ROO}\cdot \to \mathrm{intermediates}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}$$

(13.12)

Equation 13.12 represents a reaction sequence rather than a single elementary step. The intermediate pathway depends on the molecule and water matrix. Chlorinated compounds can form chloride and low molecular weight acids; nitrogen containing pharmaceuticals can form nitrogenous intermediates; aromatic compounds can produce hydroxylated and ring opened products. Analytical screening is therefore necessary when the treated water will be reused or discharged to a sensitive receiving environment.

13.7.2 Phenols, dyes, pesticides, and industrial organics

Phenols and dyes are frequently used as model compounds because they are analytically convenient and span different reaction domains. p Nitrophenol demonstrates the optimum pressure discussed earlier [15]. Methyl Orange and 2,4-D experiments with cavitating jets showed that rapid removal and maximum energy efficiency can occur at different jet pressures [16]. The advanced Fenton study on 2,4-D showed that HC alone generated insufficient radicals for deep mineralization, whereas the combination with hydrogen peroxide and zero valent iron reduced residual TOC to about 30% after 20 minutes in the reported system [21].

For nonvolatile compounds, Sayyaadi introduced an enhanced cavitation oxidation process in which H₂O₂ was injected upstream of a Venturi. The peroxide dose had a larger effect on 2-chlorophenol degradation than cavitation number across the tested range. Energy efficient operation favored mild cavitation, a sufficient peroxide dose, and short injection duration, while maximum decomposition rate favored more intense cavitation [20]. This distinction is valuable for design specifications: the operating point for minimum treatment time is not necessarily the operating point for minimum cost or minimum carbon footprint.

13.7.3 Pharmaceuticals and personal care products

Pharmaceuticals are challenging because they occur at low concentrations, can be biologically active, and are accompanied by natural organic matter and inorganic radical scavengers. In the supplied pharmaceutical study, six compounds were treated with HC and H₂O₂. In deionized water, naproxen, carbamazepine, and diclofenac showed high removal under optimized conditions, while clofibric acid, ibuprofen, and ketoprofen were less consistent. In real effluent, the same treatment produced lower removal unless exposure and oxidant dose were increased [4,43].

A Liu, et al. review concluded that pharmaceutical HC research now spans standalone HC and combinations with H₂O₂, persulfate, peroxymonosulfate, percarbonate, Fenton reagents, catalysts, ozone, and photocatalysis [44]. Recent scale up work combined HC with nonthermal electrical discharge plasma. A pilot reactor operating at 600 L h⁻¹ and a semi industrial reactor at 3200 L h⁻¹ achieved near quantitative degradation of ibuprofen and diclofenac solutions after 13 passes, with comparable energy efficiency and no detectable final byproducts under the reported conditions [45]. A 2026 ofloxacin study reported 74.93% degradation by optimized standalone HC at pH 3 and 4 bar, increasing to approximately 95% with advanced Fenton coupling [46]. Recent studies have also extended HC and hybrid HC treatment to cefixime, antibiotics in aquaculture seawater, triclosan, and rhodamine B, reinforcing the need to optimize aeration, ozone or peroxide dose, and hydraulic severity for each compound and matrix [47-50]. These results demonstrate progress, but real wastewater, chronic toxicity, maintenance, and life cycle comparisons remain essential before general adoption.

Table 13.8. Selected organic contaminant studies and design lessons.

Target or matrix	Treatment	Representative outcome	Lesson
p Nitrophenol [15]	Venturi HC	Optimum near 0.4-0.45 MPa	Hydraulic severity has a nonmonotonic optimum
2,4-D [16,21]	Cavitating jet; HC with advanced Fenton	Order of magnitude jet reduction; deep TOC reduction with hybrid process	Mechanical and chemical routes can be combined
2-Chlorophenol [20]	Venturi HC plus injected H₂O₂	Dose dominated response; excess oxidant reduced yield	Optimize oxidant and cavitation jointly
Wood finishing wastewater [19]	Rotor stator HC plus H₂O₂	High COD reduction at optimum speed and dose	Real wastewater can respond when volatile and interfacial compounds are present
Six pharmaceuticals [4,43]	HC plus H₂O₂	Compound- and matrix specific removal	Clean water results cannot be transferred directly to effluent
Ibuprofen and diclofenac [45]	HC plus nonthermal plasma	Pilot to 3200 L h⁻¹, 13 passes, near complete degradation	Preserving residence time and energy efficiency can enable scale up
Ofloxacin [46]	HC, H₂O₂, hypochlorite, Fenton, advanced Fenton	74.93% HC alone; up to 95.18% hybrid	Hybrid chemistry can substantially increase antibiotic removal

13.7.4 Hybrid process selection

Hybrid treatment is justified when the target is located mainly in the bulk liquid, the real matrix consumes radicals, standalone HC produces only partial transformation, or residual disinfection is required. Hydrogen peroxide is simple to dose but has a narrow optimum and must be measured. Ozone benefits from HC enhanced gas liquid transfer and decomposition but requires off gas and bromate control. Fenton chemistry can be highly effective at acidic pH but introduces iron management and sludge. Persulfate and peroxymonosulfate can generate sulfate radicals but require activation and byproduct evaluation. Plasma and photocatalysis can add reactive species without bulk chemical inventory, but electrical and reactor complexity increase [20,21,24,44-46].

Figure 13.36. Qualitative selection matrix for hydrodynamic cavitation hybrid processes.

13.7.5 Industrial, mining, and high solids effluent treatment

Industrial and mining effluents often contain several contaminant classes that require different mechanisms. Suspended mineral particles and oil can be separated physically, dissolved metals can be oxidized or precipitated, ammonia and cyanide require specific chemistry, and refractory organics may require radical oxidation. HC is most useful when it intensifies one of these steps rather than when it is treated as a universal separator.

The supplied National Research Council presentation proposed integrated process trains for drill water, mine water, flotation, oil dispersion, electrocoagulation, and ozone assisted treatment [66]. These examples are process concepts rather than a uniform performance dataset. They nevertheless illustrate a sound design principle: place HC after gross solids removal and before the reaction or separation step that benefits from rapid micromixing, gas transfer, surface renewal, or nucleation.

For metal bearing water, HC does not remove dissolved ions by itself. It can accelerate oxidation of ferrous iron, improve reagent dispersion, increase precipitation nuclei, or support electrocoagulation, after which solids must be separated. For petroleum wastewater, aggressive mixing can either assist flotation by gas dispersion or create a stable emulsion that is harder to separate. The treatment objective and the downstream separator must therefore be defined before selecting cavitation severity.

High suspended solids and abrasive grit change both hydraulics and asset life. Nuclei may increase the number of cavitation events, but particles can block small openings and erode pumps, valves, and rotor surfaces. Pilot work should use the actual particle size distribution, mineral hardness, oil content, temperature, and chemical composition. Clean water hydraulic tests are necessary for commissioning but are not sufficient for process prediction.

Table 13.9. Industrial wastewater mechanisms and downstream requirements.

Dominant contaminant	Potential role of HC	Required downstream step	Critical evidence
Refractory dissolved organics	Radical formation, oxidant activation, improved mass transfer	Biological treatment, adsorption, membrane, or final oxidation	TOC, transformation products, toxicity, EEO
Dissolved metals	Oxidation state control, reagent dispersion, precipitation nucleation	Clarification, flotation, filtration, or sludge handling	Dissolved and total metals, pH, solids settleability
il and grease	Gas dispersion, droplet breakup, interfacial renewal	Flotation, coalescer, skimming, or membrane	Droplet size, residual oil, emulsion stability
Mineral suspended solids	Agglomerate breakup or enhanced flotation contact	Hydrocyclone, clarifier, filter press	Wear rate, pressure loss, particle size, cake properties
Ammonia, cyanide, or sulfide	Targeted oxidation with selected reagent	Gas control, biological polishing, residual removal	Stoichiometry, off gas, byproducts, acute toxicity
Mixed high COD wastewater	Pretreatment to improve biodegradability	Aerobic or anaerobic biological process	BOD to COD ratio, respirometry, methane potential, inhibition

13.7.6 Recent developments

Recent research has shifted from demonstrating cavitation effects toward defining reactor function, excessive treatment limits, and scale. In waste activated sludge, Abdelrahman et al. found that 0.83 MPa produced a larger methane benefit than 1.72 MPa, while Son et al. reported biogas improvement ranging from 14.4% to 96.5% across advanced rotational conditions and warned that excessive treatment could reduce yield [29,30]. These results reinforce the need to optimize against the downstream biological response rather than maximum SCOD alone.

Pharmaceutical research now includes standalone HC and combinations with peroxide, Fenton chemistry, persulfate, peroxymonosulfate, ozone, catalysts, photocatalysis, and plasma [44]. Verdini et al. extended a cavitation plasma process from approximately 600 L h⁻¹ to 3200 L h⁻¹ and reported near complete degradation of ibuprofen and diclofenac solutions after repeated passes [45]. Gandhi and Gogate reported 74.93% ofloxacin degradation by optimized HC alone and approximately 95% with advanced Fenton coupling [46].

The trend is encouraging, but the evidence hierarchy remains important. Many high removals are obtained in prepared solutions at acidic pH and low contaminant concentration. Real effluent contains carbonate, chloride, dissolved organic matter, surfactants, solids, and mixed pollutants that consume radicals and modify cavitation. Scale claims should therefore include actual matrix results, energy per order, chemical demand, transformation product screening, toxicity, and continuous operating data.

Figure 13.37. Selected developments in hydrodynamic cavitation for water processing through 2026. Original timeline based on [7,8,29,30,44-50,61].

13.8 Process Design, Scale Up, and Energy

13.8.1 The operating window

The most common design error is to assume that stronger pressure drop, higher rotor speed, or lower cavitation number must always increase treatment. The useful response rises after inception, reaches a region in which cavity population and collapse are productive, and can then decline when vapor clouds shield collapse, flow chokes, or energy is dissipated without sufficient chemical or biological effect. Temperature can have the same nonmonotonic influence because increasing vapor pressure promotes cavity formation but increases vapor cushioning during collapse [1-5,15,20,29,30].

Figure 13.38. Conceptual operating window for hydrodynamic cavitation.

13.8.2 Similarity and scale up

Geometric similarity and a matched cavitation number do not guarantee identical treatment. Scale changes Reynolds number, turbulence structure, nuclei population, residence time distribution, pump efficiency, gas content, heat rejection, cavity cloud interactions, and the ratio of wall area to volume. The design objective is to preserve the process function: the distribution of productive collapse events and the exposure of the target population or molecules.

$$\mathrm{Re}=\frac{\rho vD}{\mu }$$

(13.13)

$$\mathrm{We}=\frac{\rho v^{2}D}{\gamma }$$

(13.14)

$$\mathrm{Da}=k_{\mathrm{reaction}}{\tau }_{\mathrm{hydraulic}}$$

(13.15)

Reynolds number, Re, characterizes inertial relative to viscous forces. Weber number, We, characterizes inertial relative to surface tension forces. A Damköhler type number, Da, compares reaction time with hydraulic exposure time. These indices supplement the cavitation number. For rotating equipment, tip speed, gap ratio, event frequency, torque, and specific power must also be considered. For jets, nozzle diameter, chamber pressure, standoff, entrainment, and jet interaction are important.

Figure 13.39. Scale up by preserving process function and verifying each development stage.

13.8.3 Energy and economic accounting

Energy accounting should include pumps, motors, variable frequency drives, cooling, gas compression, ozone generation, electrolysis, ultraviolet sources, plasma power, mixing, and downstream separation. Chemical production and neutralization should be included in the operating cost and, when possible, in life cycle assessment. The correct comparison is between complete treatment trains meeting the same endpoint, not between a cavitation pump and a single competing device [52,53].

For sludge, recovered methane and reduced biosolids disposal can offset pretreatment energy. For industrial effluent, increased biological treatability can reduce aeration and residence time. For ballast water, compactness and reduced chemical inventory can have operational value. For micropollutants, treatment cost should be normalized to water volume and required removal, while recognizing that EEO does not represent toxicity or mineralization.

Table 13.10. Bench to full scale HC design checklist.

Design stage	Required evidence before proceeding
Bench characterization	Real water matrix, target endpoint, pH and temperature range, nuclei and solids behavior, oxidant demand, byproduct screen
Hydraulic development	Pressure map, flow curve, cavitation regime, heat balance, erosion location, pump efficiency, bypass and control strategy
Pilot verification	Continuous operation, residence time distribution, energy per treated volume, maintenance intervals, analytical reproducibility, toxicity and regrowth
Full scale design	Redundancy, isolation, clean in place or inspection access, replaceable wear parts, hydraulic turndown, and maintenance access
Performance verification	Performance at minimum and maximum flow, worst case water quality, defined holding time, endpoint and uncertainty, residuals, noise, and vibration
Lifecycle operation	Confirm the pressure power relation, inspect wear, update chemistry, retain treatment records, and reassess when feed composition changes

13.8.4 Quantitative performance and energy normalization

Removal must first be stated using a clearly defined analytical endpoint. For a concentration based response, the percentage removal is

$${\eta }_R=100\frac{C_0-C_t}{C_0}$$

(13.16)

where ηR is removal efficiency, C₀ is the initial concentration, and Ct is the concentration after treatment time t. When a pseudo first order model is justified over the measured interval,

$$\ln \left(\frac{C_t}{C_0}\right)=-k_{\mathrm{obs}}t$$

(13.17)

where kobs is observed constant. A high fitted rate constant is meaningful only if the analytical method, temperature, mixing, and initial concentration are comparable.

The hydraulic power transferred across a static cavitation device is approximated by

$$P_h=\mathrm{\Delta }pQ$$

(13.18)

The associated electrical demand is

$$P_{\mathrm{el}}=\frac{P_h}{{\eta }_p{\eta }_m}$$

(13.19)

where Δp is the measured pressure difference, Q is volumetric flow rate, ηp is pump efficiency, and ηm is motor and drive efficiency. For a rotating device, measured electrical or shaft power should be used directly and separated from the auxiliary pump demand.

Specific energy input to a treated volume is

$$E_s=\frac{P_{\mathrm{el}}t}{V}$$

(13.20)

For contaminant degradation, electrical energy per order is

$$\mathrm{EEO}=\frac{P_{\mathrm{el}}t}{V{\log }_{10}\left(C_0/C_t\right)}$$

(13.21)

EEO is useful only when the same power boundary and treatment volume are used. It becomes unstable for small concentration changes and does not capture chemical consumption, cooling, or downstream value.

A concentration based cavitational yield can be expressed as

$$Y_{\mathrm{cav}}=\frac{V(C_0-C_t)}{P_{\mathrm{el}}t}$$

(13.22)

The nominal number of recycle turnovers and the mean hydraulic residence time are

$$N_{\mathrm{pass}}=\frac{Qt}{V}$$

(13.23)

$${\tau }_{\mathrm{hydraulic}}=\frac{V}{Q}$$

(13.24)

A nominal turnover is not necessarily one equal exposure for every fluid element because short circuiting, dead zones, and bypass flow create a residence time distribution. Tracer testing or computational fluid dynamics should be used when exposure uniformity is important.

Bulk temperature rise follows the overall energy balance

$$m{c}_p\frac{dT}{dt}=P_{\mathrm{el}}-P_{\mathrm{loss}}$$

(13.25)

where m is liquid mass, cp is heat capacity, and Ploss represents heat rejected to the surroundings and cooling system. This equation allows the analyst to separate useful thermal duty from uncontrolled heat accumulation.

For a hybrid process, a simple kinetic synergy index is

$$S=\frac{k_{\mathrm{H}\mathrm{C}+\mathrm{X}}}{k_{\mathrm{HC}}+k_X}$$

(13.26)

where kHC+X is the observed rate constant for the combined process and kHC and kX are the corresponding standalone values. S greater than one suggests synergy, but the conclusion should also be checked using mineralization, toxicity, and total energy or chemical cost.

A plant level net energy balance can be written as

$$E_{\mathrm{net}}=E_{\mathrm{biogas}}+E_{\mathrm{avoided}}-E_{\mathrm{HC}}-E_{\mathrm{chem}}-E_{\mathrm{cooling}}$$

(13.27)

where recovered biogas energy and avoided aeration or disposal energy are credited against cavitation, chemical, and cooling demands. Monetary value should be calculated separately because electricity, heat, disposal, and chemical prices are not equivalent energy terms.

Hydraulic similarity is commonly described using the cavitation number and, for rotating equipment, tip speed and power number:

$$\sigma =\frac{p_2-p_v}{0.5\rho v_c^2}$$

(13.28)

$$u_{\mathrm{tip}}=\pi D_{r}N$$

(13.29)

$$\mathrm{Po}=\frac{P_{\mathrm{shaft}}}{\rho N^3{D}_r^5}$$

(13.30)

Here p₂ is a downstream reference pressure, pv is vapor pressure, ρ is density, vc is characteristic velocity, Dr is rotor diameter, N is rotational speed, and Pshaft is shaft power. These quantities support comparison, but none uniquely determines bubble cloud structure or treatment performance. Scale up must retain the process function and verify the actual response.

Figure 13.40. Energy and performance accounting boundary for hydrodynamic cavitation.

Table 13.11. Minimum normalized reporting set for HC studies and pilot trials.

Category	Minimum reported variables	Reason
Hydraulics	Flow, upstream and downstream pressure, temperature, device geometry, backpressure, loop volume, pass count	Defines the actual pressure and exposure history
Power and energy	Electrical power by component, shaft or hydraulic power, cooling duty, specific energy, EEO or yield	Prevents incomplete energy comparisons
Water matrix	pH, conductivity, alkalinity, chloride, DOC or TOC, solids, dissolved gas, initial concentration	Explains radical scavenging and cavitation changes
Treatment outcome	Parent compound, COD, TOC, biodegradability, toxicity, biological response, transformation products	Separates disappearance from useful treatment
Asset condition	Noise, vibration, erosion, fouling, pressure stability, maintenance interval	Links treatment to equipment life and operability
Uncertainty	Replicates, analytical precision, blank and thermal controls, mass balance closure	Establishes whether differences are significant

13.8.5 Materials, erosion, solids handling, and control

Cavitation erosion is produced when collapse occurs repeatedly on or very near a solid surface. Water treatment reactors should therefore locate the most intense collapse in the fluid whenever possible and use replaceable sacrificial components where impact is unavoidable. Candidate materials can include stainless steels, duplex alloys, hard coatings, ceramics, or engineered polymers, but selection must account for corrosion, chloride, temperature, abrasive solids, cyclic stress, weld quality, and cleaning chemicals. Material ranking from clean water erosion tests may change in actual wastewater.

Fibers, rags, grit, precipitates, and scale are common causes of lost performance. Upstream screening or maceration, drain and flush connections, reversible flow, accessible nozzles, and differential pressure alarms should be designed from the beginning. For rotating equipment, seal flushing, bearing monitoring, vibration limits, and torque protection are essential. For oxidant systems, compatible seals and controlled injection location prevent concentrated chemical attack.

A robust control system maintains the desired operating window rather than a fixed pump pressure alone. The manipulated variables may include motor speed, control valve position, downstream backpressure, recycle ratio, gas injection, oxidant dose, and cooling flow. Protective interlocks should respond to low flow, excessive temperature, pressure instability, vibration, seal failure, high residual oxidant, and loss of neutralization capacity.

Table 13.12. Common failure modes and engineering safeguards.

Failure mode	Likely cause	Process consequence	Safeguard or diagnostic
Loss of cavitation response	Nozzle wear, blockage, gas change, backpressure drift	Lower oxidation, lysis, or inactivation	Pressure profile, acoustic or vibration signature, power and flow trend
Excessive vapor structure	Backpressure too low or temperature too high	Vapor shielding and weak collapse	Control downstream pressure and temperature
Rapid erosion	Collapse attached to wall, abrasive solids, unsuitable material	Metal loss, contamination, unplanned outage	Move collapse zone, replaceable insert, erosion inspection coupon
Plugging or ragging	Fibers, grit, precipitate, inadequate screening	Flow imbalance and pump overload	Macerator, screen, flush ports, differential pressure alarm
Biological deterioration after sludge treatment	Excessive exposure or inhibitory soluble products	Lower methane yield or unstable digestion	BMP testing, staged treated fraction, SCOD and ammonia monitoring
Oxidant inefficiency	Overdose, scavenging, poor injection, incorrect pH	Higher chemical cost and byproducts	Dose response testing, residual measurement, TOC and toxicity
Uncontrolled heating	High recirculation power and insufficient cooling	Confounded kinetics, vapor pressure change, safety risk	Heat balance, cooling loop, temperature interlock

13.8.6 Pilot protocol and acceptance criteria

Pilot development should progress through defined evidence gates. The first stage maps hydraulics in clean water and identifies stable pressure, flow, temperature, power, vibration, and backpressure ranges. The second stage uses a safe tracer or model target to confirm residence time distribution and analytical repeatability. The third stage treats actual wastewater over the expected seasonal matrix range. The final stage operates continuously long enough to reveal fouling, erosion, sensor drift, maintenance demand, and downstream biological response.

Acceptance criteria must be linked to the plant objective. A polishing system may be accepted on micropollutant removal, toxicity, and EEO. A sludge system requires net methane, solids reduction, dewatering, and digester stability. A ballast system requires certified biological performance and residual control. A precipitation system requires dissolved nutrient or metal removal and acceptable solids separation. In every case, the guaranteed boundary should include feed quality, flow, temperature, and required auxiliary chemicals.

Figure 13.41. Recommended staged pilot and acceptance program.

Table 13.13. Recommended staged pilot program.

Stage	Feed and duration	Primary measurements	Decision gate
1. Hydraulic commissioning	Clean water over full pressure and flow map	Pressure profile, flow, power, temperature, vibration, cavitation stability	Safe and repeatable operating envelope
2. Controlled response	Model solute, tracer, or standardized sludge test	Residence time distribution, removal kinetics, heat balance, replicate uncertainty	Evidence that response tracks controllable variables
3. Actual wastewater campaign	Representative low, average, and high load matrices	Target removal, TOC or COD, toxicity, SCOD or BMP, chemical demand	Site specific optimum and treatment train position
4. Continuous reliability run	Weeks to months at design duty	Wear, fouling, cleaning, sensor drift, availability, downstream process impact	Maintainable design and realistic operating cost
5. Performance acceptance	Contract feed window and independent sampling	Guaranteed endpoints, energy, chemicals, residuals, asset condition	Documented compliance and handover criteria

13.9 Conclusions

Hydrodynamic cavitation is a versatile process intensification tool for water processing because it combines mechanical disruption, interfacial renewal, localized thermal effects, and radical chemistry in a flow through device. Its central engineering advantage over localized acoustic systems is that the entire process stream can be forced through a designed cavitation region using conventional hydraulic equipment.

The strongest evidence supports targeted applications. In wastewater treatment, HC can transform model compounds, pretreat refractory industrial effluent, activate oxidants, and improve contact. In sludge processing, it can reduce particle size and release soluble substrates, increasing anaerobic digestion performance when severity is optimized. In ballast water and disinfection, it can damage larger organisms directly and can reduce the chemical dose required in hybrid systems. In organic oxidation, its effectiveness depends on the compound reaction domain and the water matrix.

Technology is not governed by a simple rule that more pressure, lower cavitation number, or higher rotor speed is better. Productive treatment normally occupies an operating window between weak inception and vapor dominated or overtreated conditions. The optimum must be located with the actual endpoint and an energy normalized metric.

Standalone HC often provides partial transformation rather than complete mineralization in concentrated or scavenging matrices. Hybrid processes with hydrogen peroxide, ozone, Fenton reagents, persulfate, catalysts, or plasma can supply additional reactive species, but they introduce chemical, energy, byproduct, and operating obligations. The correct comparison is therefore between complete treatment trains that meet the same water quality objective.

Scale up should preserve the treatment function, not only geometry or cavitation number. Continuous pilot verification should establish hydraulic distribution, energy, heat rejection, wear, chemistry, biological response, and maintenance. With this discipline, HC can be integrated as a practical pretreatment, sidestream, disinfection, or polishing operation in municipal, industrial, marine, and water reuse systems.

Nomenclature

The following symbols are used in this chapter. Unless stated otherwise, quantities are expressed in SI units. Symbols used only in a quoted source may use that source's convention; the equation text and surrounding definitions govern.

Table 13.14. Roman symbols.

Symbol	Definition	SI unit or status
Ac	Flow area at the constriction or designated cavitating section	m²
C	Contaminant concentration	mol m⁻³, mg L⁻¹, or specified
C₀	nitial contaminant concentration	same as C
Ct	Contaminant concentration at time t	same as C
CW	Disinfectant concentration in the Chick Watson model	mass volume⁻¹
D	Characteristic hydraulic length or diameter	m
Da	Damköhler type reaction to hydraulic time ratio	dimensionless
DDCOD	Degree of sludge disintegration based on soluble COD	%
EEO	Electrical energy per order of contaminant reduction	kWh m⁻³ order⁻¹
Ein	Energy input to the treatment system	J or kWh
Es	Specific electrical energy input	J m⁻³ or kWh m⁻³
g	Gravitational acceleration	m s⁻²
hL	Specific irreversible hydraulic loss	J kg⁻¹
k	Empirical inactivation or reaction constant	specified by model
kobs	Observed pseudo first order rate constant	s⁻¹ or min⁻¹
kreaction	Characteristic reaction rate constant used in Da	s⁻¹
LRV	Logarithmic reduction value	dimensionless
N₀	Initial viable organism concentration	CFU or organisms per volume
Nt	Viable organism concentration after treatment time t	CFU or organisms per volume
Npass	Nominal number of treatment loop turnovers	dimensionless
p	Static pressure	Pa
p₂	Recovered downstream reference pressure	Pa
pB	Pressure inside a cavity	Pa
p∞	Far field liquid pressure	Pa
Pel	Estimated electrical pump power	W
Ph	Hydraulic power transferred to the liquid	W
pv	Liquid vapor pressure	Pa
Q	Volumetric flow rate	m³ s⁻¹
R	Instantaneous bubble radius	m
Rmax	Maximum bubble radius before collapse	m
Re	Reynolds number	dimensionless
SCOD₀	Initial soluble chemical oxygen demand	mg L⁻¹
SCODt	Soluble chemical oxygen demand after treatment	mg L⁻¹
SCODmax	Reference maximum soluble COD	mg L⁻¹
t	Treatment time	s or min
tc	Idealized Rayleigh collapse time	s
V	Treated or recirculating liquid volume	m³
v	Mean fluid velocity	m s⁻¹
vc	Velocity in the constriction or designated cavitating region	m s⁻¹
We	Weber number	dimensionless
Ycav	Cavitational yield	mass energy⁻¹
YCH₄,c	Control methane yield	mL CH₄ g⁻¹ VS or specified
YCH₄,t	Methane yield after pretreatment	mL CH₄ g⁻¹ VS or specified
z	Elevation	m
cp	Specific heat capacity	J kg⁻¹ K⁻¹
Dr	Rotor diameter	m
Eavoided	Avoided process or disposal energy credited in the net balance	J or kWh
Ebiogas	Recoverable energy associated with additional biogas	J or kWh
Ecooling	Cooling energy demand	J or kWh
Echem	Energy or equivalent energy assigned to chemical consumption	J or kWh
EHC	Electrical energy used by the HC process	J or kWh
Enet	Net energy benefit	J or kWh
m	Mass of treated liquid	kg
N	Rotational speed	s⁻¹ or rpm
Ploss	Heat loss or rejected thermal power	W
Po	Power number	dimensionless
Pshaft	Measured shaft power	W
S	Kinetic synergy index	dimensionless
u tip	Rotor tip speed	m s⁻¹

Greek Letters

Table 13.15. Greek letters.

Symbol	Definition	SI unit or status
γ	Surface tension	N m⁻¹
Δp	Pressure difference across the treatment device or specified section	Pa
ηp	Pump efficiency	dimensionless
μ	Dynamic viscosity	Pa s
ρ	Liquid density	kg m⁻³
σ	Cavitation number	dimensionless
τhydraulic	Characteristic hydraulic exposure or residence time	s
ηm	Motor and drive efficiency	dimensionless
ηR	Removal efficiency	%
π	Circle constant	dimensionless
τ	Residence or exposure time	s

Abbreviations

Table 13.16. Abbreviations.

Abbreviation	Meaning
AOP	Advanced oxidation process
BMP	Biochemical methane potential
BOD	Biochemical oxygen demand
BWM	Ballast Water Management
BWMS	Ballast water management system
CFU	Colony forming unit
COD	Chemical oxygen demand
DBP	Disinfection byproduct
DCF	Diclofenac
DO	Dissolved oxygen
HC	Hydrodynamic cavitation
H₂O₂	Hydrogen peroxide
IBP	Ibuprofen
IMO	International Maritime Organization
NaOCl	Sodium hypochlorite
PMS	Peroxymonosulfate
ROS	Reactive oxygen species
SCOD	Soluble chemical oxygen demand
STS	Sodium thiosulfate
TOC	Total organic carbon
TRO	Total residual oxidant
VFD	Variable frequency drive
VOC	Volatile organic compound
VS	Volatile solids
WAS	Waste activated sludge
WWTP	Wastewater treatment plant
ABNC	Active but nonculturable
ARHCR	Advanced rotational hydrodynamic cavitation reactor
ATP	Adenosine triphosphate
DOC	Dissolved organic carbon
DVC	Direct viable count
EPR	Electron paramagnetic resonance
HAB	Harmful algal bloom
HCR	Hydrodynamic cavitation reactor
MS 2	Bacteriophage MS 2, used as a virus surrogate
PLC	Programmable logic controller
qPCR	Quantitative polymerase chain reaction
VBNC	Viable but nonculturable

Bibliography

[1] Gogate PR. Cavitation: an auxiliary technique in wastewater treatment schemes. Advances in Environmental Research. 2002;6:335-358. doi:10.1016/S1093-0191(01)00067-3.

[2] Gogate PR, Pandit AB. A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions. Advances in Environmental Research. 2004;8:501-551. doi:10.1016/S1093-0191(03)00032-7.

[3] Gogate PR, Pandit AB. A review and assessment of hydrodynamic cavitation as a technology for the future. Ultrasonics Sonochemistry. 2005;12:21-27.

[4] Dular M, Griessler-Bulc T, Gutierrez I, Heath E, Kosjek T, Krivograd Klemenčič A, et al. Use of hydrodynamic cavitation in (waste)water treatment. Ultrasonics Sonochemistry. 2016;29:577-588. doi:10.1016/j.ultsonch.2015.10.010.

[5] Ciriminna R, Albanese L, Meneguzzo F, Pagliaro M. Wastewater remediation via controlled hydrocavitation. Environmental Reviews. 2017;25:175-183. doi:10.1139/er-2016-0064.

[6] Pandya H, Parikh S, Banka S. Panoramic study of hydrodynamic cavitation and its applications in wastewater treatment. Environmental Technology Reviews. 2025;14:371-399. doi:10.1080/21622515.2025.2479712.

[7] Albanese L. Sustainable intensification of advanced oxidation processes by hydrodynamic cavitation: a critical review. Sustainable Chemistry. 2026;7(2):26. doi:10.3390/suschem7020026.

[8] Albanese L. Hydrodynamic cavitation in water and wastewater treatment: a critical review of applications, reactor design, and process function. Pollutants. 2026;6(3):37. doi:10.3390/pollutants6030037.

[9] Brennen CE. Cavitation and Bubble Dynamics. New York: Oxford University Press; 1995.

[10] Franc JP, Michel JM. Fundamentals of Cavitation. Dordrecht: Kluwer Academic Publishers; 2004.

[11] Rayleigh L. On the pressure developed in a liquid during the collapse of a spherical cavity. Philosophical Magazine. 1917;34:94-98.

[12] Plesset MS. The dynamics of cavitation bubbles. Journal of Applied Mechanics. 1949;16:277-282.

[13] Šarc A, Stepišnik-Perdih T, Petkovšek M, Dular M. The issue of cavitation number value in studies of water treatment by hydrodynamic cavitation. Ultrasonics Sonochemistry. 2017;34:51-59.

[14] Soyama H, Hoshino J. Experimental control of cavitating-jet intensity by downstream pressure. AIP Advances. 2016;6:045113.

[15] Capocelli M, Prisciandaro M, Lancia A, Musmarra D. Hydrodynamic cavitation of p-nitrophenol: a theoretical and experimental insight. Chemical Engineering Journal. 2014;254:1-8. doi:10.1016/j.cej.2014.05.102.

[16] Kalumuck KM, Chahine GL, Hsiao CT, Choi JK. Remediation and disinfection of water using jet generated cavitation. Proceedings of the Fifth International Symposium on Cavitation, CAV2003; Osaka, Japan; 2003.

[17] Loraine G, Chahine GL. Oxidation of pharmaceuticals and personal-care products in water using hydrodynamic cavitation. 245th American Chemical Society National Meeting; New Orleans, Louisiana; 2013.

[18] Korniluk M, Ozonek J. Application of hydrodynamic cavitation for leachate of municipal landfill site. Proceedings of the 8th International Conference Environmental Engineering; Vilnius, Lithuania; 2011. p. 584-587.

[19] Badve M, Gogate P, Pandit A, Csoka L. Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry. Separation and Purification Technology. 2013;106:15-21. doi:10.1016/j.seppur.2012.12.029.

[20] Sayyaadi H. Enhanced cavitation-oxidation process of non-VOC aqueous solution using hydrodynamic cavitation reactor. Chemical Engineering Journal. 2015;272:79-91. doi:10.1016/j.cej.2015.03.024.

[21] Bremner DH, Di Carlo S, Chakinala AG, Cravotto G. Mineralisation of 2,4-dichlorophenoxyacetic acid by acoustic or hydrodynamic cavitation in conjunction with the advanced Fenton process. Ultrasonics Sonochemistry. 2008;15:416-419.

[22] Raut-Jadhav S, Pinjari DV, Saini DR, Sonawane SH, Pandit AB. Intensification of pollutant degradation using hydrodynamic cavitation and advanced oxidation. Chemical Engineering Journal. 2016;295:326-335.

[23] Gągol M, Przyjazny A, Boczkaj G. Wastewater treatment by means of advanced oxidation processes based on cavitation: a review. Chemical Engineering Journal. 2018;338:599-627.

[24] Thanekar P, Gogate PR. Application of hydrodynamic cavitation reactors for treatment of wastewater containing organic pollutants: intensification using hybrid approaches. Fluids. 2018;3:98. doi:10.3390/fluids3040098.

[25] Dölle K, Van Bargen M. Phosphor removal from wastewater using hydrodynamic cavitation. Journal of Scientific Research and Reports. 2017;14(2):1-11. doi:10.9734/JSRR/2017/33285.

[26] Michel FC Jr, Kozyuk O. Hydrodynamic cavitation processing. In: Letcher TM, Scott JL, Patterson DA, editors. Chemical Processes for a Sustainable Future. Cambridge: Royal Society of Chemistry; 2015. p. 84-142. doi:10.1039/BK9781849739757-00084.

[27] Gómez E. Biodegradation of Bio-Based Plastics and Anaerobic Digestion of Cavitated Municipal Sewage Sludge. Doctoral dissertation. Columbus, Ohio: The Ohio State University; 2013.

[28] Grübel K, Suschka J. Hybrid alkali-hydrodynamic disintegration of waste-activated sludge before two-stage anaerobic digestion process. Environmental Science and Pollution Research. 2015;22:7258-7270. doi:10.1007/s11356-014-3705-y.

[29] Abdelrahman AM, Tebyani S, Rokhsar Talabazar F, et al. The flow pattern effects of hydrodynamic cavitation on waste activated sludge digestibility. Chemosphere. 2024;357:141949. doi:10.1016/j.chemosphere.2024.141949.

[30] Son H, Na S, Guo M, Le DK, Yoon JY, Sun X. Sludge disintegration through advanced rotational hydrodynamic cavitation reactor for improvement of biogas production. Journal of Environmental Chemical Engineering. 2025;13:116116. doi:10.1016/j.jece.2025.116116.

[31] Vilarroig J, et al. Design and optimization of a semi-industrial cavitation device for pretreatment of anaerobic digestion of excess sludge and pig slurry. Water Environment Research. 2020. doi:10.1002/wer.1366.

[32] Lloyd’s Register. Ballast Water Treatment Technology: Current Status. London: Lloyd’s Register; June 2007.

[33] International Maritime Organization. International Convention for the Control and Management of Ships’ Ballast Water and Sediments, 2004. IMO website.

[34] International Maritime Organization. Implementing the Ballast Water Management Convention: D-1 and D-2 standards and implementation schedule. IMO website.

[35] International Maritime Organization. Code for Approval of Ballast Water Management Systems, resolution MEPC.300(72), together with the 2026 Guidelines for ballast water management and development of Ballast Water Management Plans, resolution MEPC.409(84). IMO.

[36] Cvetković M, Kompare B, Krivograd Klemenčič A. Application of hydrodynamic cavitation in ballast water treatment. Environmental Science and Pollution Research. 2015;22:7422-7438. doi:10.1007/s11356-015-4360-7.

[37] Sangave PP, Mukherjee AC, Pandit AB. Ballast water treatment using hydrodynamic cavitation. Indian Journal of Geo-Marine Sciences. 2014;43(11).

[38] Sawant SS, Anil AC, Krishnamurthy V, Gaonkar C, Kolwalkar J, Khandeparker L, et al. Effect of hydrodynamic cavitation on zooplankton: a tool for disinfection. Biochemical Engineering Journal. 2008;42:320-328. doi:10.1016/j.bej.2008.08.001.

[39] Jyoti KK, Pandit AB. Water disinfection by acoustic and hydrodynamic cavitation. Biochemical Engineering Journal. 2001;7:201-212. doi:10.1016/S1369-703X(00)00128-5.

[40] Jyoti KK, Pandit AB. Hybrid cavitation methods for water disinfection: simultaneous use of chemicals with cavitation. Ultrasonics Sonochemistry. 2003;10:255-264. doi:10.1016/S1350-4177(03)00095-6.

[41] Arrojo S, Benito Y, Martínez Tarifa A. A parametrical study of disinfection with hydrodynamic cavitation. Ultrasonics Sonochemistry. 2008;15:903-908. doi:10.1016/j.ultsonch.2007.11.001.

[42] Gogate PR, Mededovic-Thagard S, McGuire D, Chapas G, Blackmon J, Cathey R. Hybrid reactor based on combined cavitation and ozonation: from concept to practical reality. Ultrasonics Sonochemistry. 2014;21(2):590-598. doi:10.1016/j.ultsonch.2013.08.016.

[43] Zupanc M, Kosjek T, Kompare B, Blažeka Ž, Ješe U, Dular M, et al. Hydrodynamic cavitation: a technique for augmentation of removal of persistent pharmaceuticals? Conference paper; University of Ljubljana and Jožef Stefan Institute.

[44] Liu P, Calcio Gaudino E, Lee J, Cravotto G. A review of pharmaceutical removal from wastewater via hydrodynamic cavitation. Ultrasonics Sonochemistry. 2026;129:107843. doi:10.1016/j.ultsonch.2026.107843.

[45] Verdini F, Desogus N, Kamler AV, Mikhalev E, Cosenza B, Concari S, et al. Addressing the challenge of wastewater upcycling through cavitation-plasma synergy: from pilot to semi-industrial scale. ChemSusChem. 2026;19:e202502393. doi:10.1002/cssc.202502393.

[46] Gandhi R, Gogate PR. Degradation of ofloxacin using hydrodynamic cavitation coupled with advanced oxidants. Environmental Quality Management. 2026;35:e70266. doi:10.1002/tqem.70266.

[47] Kommagalla PK, Timmapuram SS, Guru Prasad P, Vadthya P. Sustainable and energy-efficient cefixime mitigation by hydrodynamic cavitation, peroxide, and ozonation. Chemical Engineering Communications. 2025;212:1506-1520. doi:10.1080/00986445.2025.2478414.

[48] Huang X, Yang D, Song L, Jiang Y. Degradation of antibiotics in aquaculture seawater: a treatment based on ozone assisted with hydrodynamic cavitation. Water. 2025;17(4):566. doi:10.3390/w17040566.

[49] Khiadani M, Taheri E, Bobade V, Fatehizadeh A, et al. Enhanced degradation of triclosan using aerated hydrodynamic cavitation: turbulence-based modelling and economic evaluation. npj Clean Water. 2025;8:89. doi:10.1038/s41545-025-00520-z.

[50] Ye YF, Zhu Y, Lu N, Wang X, Su Z. Treatment of rhodamine B with cavitation technology: comparison of hydrodynamic cavitation with ultrasonic cavitation. RSC Advances. 2021;11:5096-5106. doi:10.1039/D0RA07727E.

[51] Kozyuk OV. Water treatment processes and devices utilizing hydrodynamic cavitation. US Patent 7,247,244 B2. 24 July 2007.

[52] Gogate PR, Pandit AB. Cavitation reactors: efficiency assessment using a model reaction. AIChE Journal. 2001;47:2526-2538. doi:10.1002/aic.690471115.

[53] Sivakumar M, Pandit AB. Wastewater treatment: a novel energy efficient hydrodynamic cavitational technique. Ultrasonics Sonochemistry. 2002;9:123-131.

[54] International Maritime Organization. Two international treaties to protect the ocean reach 100 Contracting States. IMO Media Centre; 17 December 2025. Accessed 28 July 2026.

[55] Kalumuck KM, Chahine GL. The use of cavitating jets to oxidize organic compounds in water. Journal of Fluids Engineering. 2000;122(3):465-470. doi:10.1115/1.1286993.

[56] Machnicka A, Grübel K, Suschka J. The use of hydrodynamic disintegration as a means to improve anaerobic digestion of activated sludge. Water SA. 2009;35(1):129-132. doi:10.4314/wsa.v35i1.76715.

[57] Fulton R. The APV INature Sludge Treatment Process. Proceedings of the 35th Annual Queensland Water Industry Operations Workshop; Rockhampton, Australia; 2010. p. 18-24.

[58] Nicholas D, Vella P. Wastewater digester enhancement and sludge volume reduction through cell lysis with controlled hydrodynamic cavitation. WEFTEC 2008. Alexandria, Virginia: Water Environment Federation; 2008.

[59] Siemens Water Technologies. Crown Disintegration System: Combining Systems for Minimal Waste and Maximum Gas. Technical brochure; 2009.

[60] Mezule L, Tsyfansky S, Yakushevich V, Juhna T. A simple technique for water disinfection with hydrodynamic cavitation: effect on survival of Escherichia coli. Desalination. 2009;248(1-3):152-159. doi:10.1016/j.desal.2008.05.051.

[61] Sun X, Park JJ, Kim HS, Lee SH, Seong SJ, Om AS, Yoon JY. Experimental investigation of the thermal and disinfection performances of a novel hydrodynamic cavitation reactor. Ultrasonics Sonochemistry. 2018;49:13-23. doi:10.1016/j.ultsonch.2018.02.039.

[62] Thomas CC, Butler AD, Medina VF, Griggs CS, Katzenmeyer AW. Physicochemical Treatment of Cyanobacteria and Microcystin by Hydrodynamic Cavitation and Advanced Oxidation. ERDC/EL TR-19-2. Vicksburg, Mississippi: U.S. Army Engineer Research and Development Center; 2019.

[63] Bodurova D, Angelov M. Intensification of the process of water purification by hydrodynamic cavitation. In: Manufacturing and Management in the 21st Century. Ohrid, Republic of Macedonia; 2004. p. 189-194.

[64] Schmitt W. The operation of the Crown Disintegration System for the disintegration of return sludge at WWTP Wiesbaden. Technical presentation; date not stated.

[65] Siemens AG. Getting More Value from Anaerobic Digestion with the Crown Solids Disintegration System. Technical presentation; 2009.

[66] Kirpalani DM, Singla A, Lotfi S, Mohapatra DP. Cavitation Technology Development: A Paradigm Shift in Mining Effluent Treatment. Québec Mines 2016. Ottawa: National Research Council Canada; 2016.

[67] Kosel J, Gutierrez Aguirre I, Racki N, Ravnikar M, Dular M. Efficient inactivation of MS 2 virus in water by hydrodynamic cavitation. Accepted author manuscript; 2017. doi:10.1016/j.watres.2017.07.077.

[68] Chahine GL, Kalumuck KM. Fluid jet cavitation method and system for efficient decontamination of liquids. US Patent 6,200,486 B1. 13 March 2001.

[69] Chakinala AG, Gogate PR, Burgess AE, Bremner DH. Industrial wastewater treatment using hydrodynamic cavitation and heterogeneous advanced Fenton processing. University of Abertay Dundee.

[70] Arisdyne Systems. Applying the Power of Controlled Cavitation. Technical brochure.

[71] Yoshimura T, Kubota S, Seo T, Sato K. Development of ballast water treatment technology by mechanochemical cavitation. Proceedings of the 7th International Conference on Cavitation, CAV2009; Ann Arbor, Michigan; 2009. Paper 37.

[72] Van der Walt E. The effect of ultraviolet light, cavitational flow and ultrasound on protozoan cysts and oocysts, bacteriophages and Clostridium. Biennial Conference of the Water Institute of Southern Africa; Durban, South Africa; 2002.