Ozone Treatment for Water and Wastewater: Applications and Design

Ozone treatment is one of the most powerful oxidation and disinfection technologies available for water and wastewater applications. As a triatomic oxygen molecule (O₃), ozone has a higher oxidation potential than chlorine and faster CT-based inactivation kinetics for some organisms, making ozone water treatment an increasingly attractive option for facilities facing stringent pathogen reduction and contaminant destruction requirements.

Ozone Fundamentals

Ozone is generated on-site by passing dry air or oxygen through a high-voltage electrical discharge (corona discharge). The energy splits O₂ molecules, and the resulting atomic oxygen recombines to form O₃. Because ozone is unstable with a half-life of 15–30 minutes in water, it must be generated and applied continuously.

Key properties relevant to water treatment:

• Oxidation potential: 2.07 V (compared to 1.36 V for chlorine)
• Solubility: 3–10 mg/L depending on temperature and pH
• Half-life in water: 15–30 minutes at pH 7, shorter at higher pH
• Decomposition product: Reverts to molecular oxygen—no persistent residual
• Generation concentration: 6–12% by weight from oxygen feed; 1–3% from air feed

Ozone Generation Systems

Modern ozone generators use corona discharge technology. The two primary feed gas options affect both generation efficiency and system design:

Most municipal installations above 1 MGD use oxygen-fed systems for their superior efficiency. Liquid oxygen (LOX) delivery is common for larger facilities, while smaller systems may use on-site pressure swing adsorption (PSA) oxygen generators.

Contacting and Dissolution

Ozone must be efficiently dissolved in water to be effective. Common contacting methods include:

• Fine bubble diffusion: Ozone-enriched gas is diffused through porous diffusers in a baffled contact chamber. Transfer efficiency: 85–95%. The most common method for municipal applications.
• Side-stream injection: A portion of process water is pressurized, mixed with ozone via a venturi injector, and returned to the main flow. Higher transfer efficiency (>95%) but more complex.
• Turbine contactors: Mechanical mixers create fine bubbles and high turbulence for rapid dissolution. Used in some industrial applications.

Contact chambers are typically designed with 4–6 cells and over/under baffling to approach plug flow conditions, with a total contact time of 10–20 minutes at design dose.

Drinking Water Applications

Ozone serves multiple treatment objectives in drinking water:

Primary disinfection: Ozone achieves required CT (concentration × time) values for Giardia and virus inactivation at lower doses than chlorine. As a temperature- and pH-dependent example, roughly 1.0 mg/L residual and 5 minutes contact time can achieve on the order of 3-log Giardia and 4-log virus inactivation; required CT values rise at lower temperatures, so design to the applicable CT tables for site conditions.

Taste and odor control: Ozone effectively oxidizes geosmin and MIB (2-methylisoborneol), the two most common taste and odor compounds in surface water supplies. Doses of 1–3 mg/L typically achieve >90% removal.

Micropollutant destruction: Ozone degrades pharmaceuticals, personal care products, and endocrine disruptors. Many European utilities have adopted ozonation specifically for micropollutant control.

Color removal: Effective for degrading natural organic matter that causes color in surface water. Often combined with biologically active filtration (BAF) to remove biodegradable oxidation byproducts.

Iron and manganese oxidation: Ozone rapidly oxidizes dissolved iron and manganese for removal by downstream filtration.

Wastewater Applications

Ozonation wastewater applications are expanding as discharge requirements tighten:

• Tertiary disinfection: Where chlorine residual toxicity is a concern for receiving waters, ozone provides disinfection without persistent residual.
• Advanced oxidation processes (AOP): Combining ozone with hydrogen peroxide or UV generates hydroxyl radicals for destruction of refractory compounds such as 1,4-dioxane. Note that ozone/AOP does not destroy terminal (fully fluorinated) PFAS such as PFOA and PFOS; it can transform some PFAS precursors, sometimes increasing measured PFOA/PFOS, so specialized destruction technologies are required for PFAS.
• Odor control: Low-dose ozone in headspace or scrubber systems controls hydrogen sulfide and other malodorous compounds.
• Sludge reduction: Pre-ozonation of return activated sludge can reduce excess sludge production by 30–50%.
• Reuse applications: Ozone provides an additional barrier for pathogen and contaminant removal in water reuse trains.

Design Considerations

Dose requirements: Municipal drinking water: 1–4 mg/L applied dose. Wastewater disinfection: 5–15 mg/L. Industrial oxidation: varies widely based on contaminant loading.

Materials of construction: Ozone aggressively attacks natural rubber, certain plastics, and mild steel. Contact chambers require 316L stainless steel or concrete. Piping must be PVDF, Teflon-lined, or stainless steel. Seals and gaskets require ozone-compatible elastomers (Viton, PTFE).

Off-gas destruction: Undissolved ozone in contact chamber exhaust must be destroyed before discharge. Thermal catalytic destruct units convert O₃ back to O₂ at temperatures above 300°C.

Safety systems: Ozone is toxic at concentrations above 0.1 ppm in air. Ambient ozone monitors, automatic shutdowns, and adequate ventilation are mandatory. OSHA PEL is 0.1 ppm (8-hour TWA).

Ozone vs. Chlorine vs. UV

Each disinfection technology has trade-offs. Ozone provides the broadest oxidation capability and best taste/odor control but requires higher capital investment and has no distribution system residual. Chlorine is lowest cost and provides residual protection but forms disinfection