Hydrodynamic cavitation in water and wastewater treatment

Where cavitation sits in a treatment plant

In these studies, cavitation was a module added to an existing treatment train, not a plant of its own. It appeared in the sludge line before anaerobic digestion, before or after biological treatment to target compounds that biology breaks down poorly, and on concentrated industrial streams with oxidants. A 2024 review lists aeration, activated sludge treatment and anaerobic digestion as plant processes that cavitation has been used to intensify Yeneneh et al., 2024.

One finding runs through the studies below: where cavitation was tested both alone and in combination, most of the removal came from the added oxidant or from biological treatment. Cavitation made those steps work harder; it rarely did the job by itself. The physics is in What hydrodynamic cavitation is.

Sludge disintegration before digestion

Cavitation breaks up flocs and microbial cells in waste-activated sludge, moving organic matter into solution where digesting microbes can reach it. The biology is in Biogas and sludge hydrolysis.

• A pilot. On a 400 L pilot bioreactor plant, 20 passes through a rotation generator raised soluble chemical oxygen demand (SCOD) from 45 to 602 mg/L, and biogas production by 12.7% Petkovšek et al., 2015.
• Energy input. In a laboratory comparison, cavitation used 60–1,200 kJ per kilogram of total solids (TS), against 180–3,600 kJ/kg TS for ultrasound and 72,000 kJ/kg TS for thermal treatment. Methane yield rose as disintegration proceeded, and adding alkali gave a synergistic effect Lee & Han, 2013.
• A process window. In a 2026 laboratory study, a pinned-disc rotary generator raised the methane potential of municipal sludge by up to 27%. Hydrolysis was fastest after 15–30 passes, but ultimate methane yield peaked after 100. The sludge flowed more easily but filtered worse, and the authors called for a "constrained process window" balancing methane, filterability and energy Blagojevič et al., 2026.

Pharmaceuticals and other micropollutants

Cavitation with hydrogen peroxide (H₂O₂) removed only part of the pharmaceuticals tested, and less in real wastewater. The highest removals came when cavitation was one step in a train with biological treatment and UV.

• With peroxide only. In laboratory tests on six pharmaceuticals, cavitation with H₂O₂ removed 3–70% under optimal conditions Zupanc et al., 2013.
• In a train. Attached-growth biological treatment, cavitation with H₂O₂ and UV together removed more than 90% of clofibric acid and more than 98% of carbamazepine and diclofenac; for four compounds, the biological step did most of the work Zupanc et al., 2013.
• Spiked water versus effluent. A shear-induced generator removed 47–86% of the same drugs from spiked deionized water at 50 °C, 15 min and 340 mg/L H₂O₂. Real effluent lowered removal, which the authors offset with 3.4 g/L H₂O₂ and 30 min of cavitation Zupanc et al., 2014.
• Before or after biology. Cavitation before biological treatment gave higher removal of diclofenac and carbamazepine, 54% and 67%, than after it, 39% and 56% Zupanc et al., 2014.

The same group later concluded that different pollutants need different types of cavitation, each removing pollutants by a different mechanism Dular et al., 2016.

Hybrid processes: cavitation plus oxidants

Adding an oxidant raised removal well above cavitation alone in every study here. The radical chemistry behind this is in Advanced oxidation processes (AOPs).

• Carbamazepine, laboratory. Cavitation alone reached 38.7% degradation at 4 bar and pH 4; 52.9% with UV, 58.3% with H₂O₂, 91.4% with ozone, and 100% with H₂O₂ and ozone together Thanekar et al., 2018.
• Industrial wastewater, 70 L pilot. Cavitation alone gave minimal chemical oxygen demand (COD) removal in 180 min. Adding air gave 15.5%, oxygen 42%, and oxygen plus Fenton's reagent, with H₂O₂ held at 15 g/L, 63% Joshi & Gogate, 2019.
• Dyes, pilot-scale reactor. Cavitation alone did not fully degrade the dyes; H₂O₂ helped, with an optimum of 1.0 g/L. Color removal reached 68% for a black dye but only 15% for printing-ink wastewater, whose COD fell by 55%, and the authors recommended a post-treatment step Zampeta et al., 2021.

Energy per unit treated

Studies report cavitation energy in units that do not convert into one another, so comparisons hold only within a study. Sludge studies use kilojoules per kilogram of solids Lee & Han, 2013. Pilots use cost per cubic meter: 398 US$/m³ for the best case of the 70 L pilot, a complex industrial wastewater treated with Fenton's reagent Joshi & Gogate, 2019. Pollutant studies use cavitational yield, the amount degraded per unit of energy, defined in the advanced oxidation lesson Thanekar et al., 2018.

The cavitation-number reporting problem

The cavitation number is often used to describe how strongly a device cavitates, but it cannot do that alone. One research team found that many published results were poorly repeatable because studies put so much weight on this single value Šarc et al., 2017.

Their experiments showed that geometry, flow velocity, water temperature and water quality all changed the size, dynamics and aggressiveness of cavitation, and that the number's definition varied between studies. They proposed parameters that every report should include Šarc et al., 2017. A paper that gives only a cavitation number cannot be compared with another.

From bench to pilot scale

The largest systems here are the 400 L sludge pilot, the 70 L industrial pilot and a pilot-scale orifice reactor for dyes. Scaling up is not straightforward. A 2026 modelling study of published vortex-diode data, covering throat diameters of 6–38 mm and nominal flows of 5–200 L/min, found that hydroxyl-radical yield fell as throat diameter increased, and noted that a quantitative relationship between scale and radical generation is not yet established Pang et al., 2026. A 2016 review summed it up: the path to routine use in water treatment "is still long" Dular et al., 2016.