Ozone STP Treatment: Tertiary Disinfection and Colour Removal for Sewage Plants

What Ozone Does in an STP: A Direct Answer

Ozone in a sewage treatment plant (STP) is used as a tertiary-stage oxidant, applied after biological treatment and filtration to disinfect the final effluent, break down residual colour and odour compounds, and cut leftover COD before the water is discharged or reused. Unlike chlorine, ozone leaves no chemical residual, forms no trihalomethanes (THMs), and is generated on-site from ambient air or oxygen — there is no chemical storage, no delivery logistics, and no dosing-pump chemical handling risk. For plants sending treated water to flushing, gardening, cooling-tower makeup, or construction use under state reuse mandates, ozone is frequently the deciding technology because it meets colour and pathogen-removal targets that plain chlorination cannot reach economically.

STPs in India increasingly need tertiary polishing because secondary (biological) treatment alone leaves behind colour, faint odour, trace pathogens, and non-biodegradable COD from surfactants and personal-care residues. CPCB's reuse and discharge norms, and most state pollution control board consent conditions for STPs above a certain capacity, now expect treated water to be visually clear, low-odour, and microbiologically safe — targets that push designers toward ozone or ozone-plus-UV as the tertiary barrier. Our wastewater treatment solutions page covers the full STP process chain; this guide focuses specifically on where and how ozone fits.

Where Ozone Sits in the STP Process Train

Ozone is applied after biological treatment (activated sludge, SBR, MBBR, or MBR) and after filtration — not before. Feeding raw or partially treated sewage directly to an ozone contactor wastes ozone on the bulk organic load that biological treatment is designed to remove cheaply; ozone should only see the residual, harder-to-treat fraction. A typical sequence for a mid-size institutional or industrial STP looks like this:

• Screening and primary settling — removes grit, rags, and settleable solids.
• Biological treatment (ASP / SBR / MBBR / MBR) — removes the bulk of BOD and suspended solids.
• Filtration (sand, multimedia, or ultrafiltration for MBR-less trains) — removes remaining turbidity that would otherwise consume ozone demand.
• Ozone contact tank — disinfection, colour and odour removal, residual COD polishing; contact time typically 4–10 minutes at the design dose.
• Off-gas destructor — thermal or catalytic breakdown of unreacted ozone before venting, mandatory for occupational safety.
• Optional UV polishing or activated carbon — for reuse streams needing an additional pathogen or trace-organic barrier.

Ozone Dosing and Contact Time for STP Effluent

Dose depends on the treatment objective, not a fixed number. Disinfection-only duty on clean secondary effluent needs the least ozone; colour removal and COD polishing on effluent with higher residual organics need more, because ozone demand is consumed first by organic load before it reaches pathogens. Typical design ranges used across Lotus Ozone Tech STP installations:

• Disinfection only (clear, low-COD secondary effluent): 2–4 mg/L applied dose, 4–6 minutes contact time.
• Disinfection plus odour and mild colour removal: 4–8 mg/L applied dose, 6–8 minutes contact time.
• Colour removal and COD polishing on higher-strength or textile-influenced effluent: 8–15 mg/L applied dose, 8–10 minutes contact time, sometimes combined with hydrogen peroxide (ozone/H2O2 advanced oxidation) for recalcitrant colour.
• Control parameter: ORP (oxidation-reduction potential) in the contact tank, targeted at 300–500 mV for STP disinfection duty — ozone decays too fast for a downstream residual reading to be a reliable dosing proxy; ORP and periodic effluent microbiology are the correct verification points.

Ozone vs Chlorine for STP Tertiary Treatment

Most STPs upgrading their tertiary stage are choosing between ozone and chlorine (or comparing both against UV). The table below summarises the trade-offs specific to STP duty, where colour and odour removal matter as much as pathogen kill.

• Pathogen disinfection — Ozone: strong oxidant, effective against bacteria, viruses, and protozoa (Giardia, Cryptosporidium) at low contact times. Chlorine: effective against bacteria and most viruses, but requires very high CT for protozoa, often impractical at STP scale.
• Colour and odour removal — Ozone: directly oxidises chromophores and odour compounds (H2S, mercaptans, geosmin); visibly clarifies treated effluent. Chlorine: limited colour removal, can create chlorinous odour of its own.
• By-products — Ozone: no THMs or HAAs; bromate risk is minimal in typical Indian STP feed. Chlorine: forms THMs/HAAs with the organic load in effluent, a growing regulatory concern for reuse water quality.
• Chemical handling — Ozone: generated on-site from dried air or PSA oxygen; no chemical storage or delivery. Chlorine: requires sodium hypochlorite storage, dosing pumps, and periodic replenishment logistics.
• Residual in reused water — Ozone: none after ~10–20 minutes; suits closed-loop reuse (flushing, gardening, cooling makeup) with no lingering chemical taste or odour. Chlorine: leaves a residual, useful only if the reuse water travels through extended piping before use.
• Operating cost driver — Ozone: electricity only, roughly 8–10 Wh per gram of O3 for air-fed systems. Chlorine: recurring chemical purchase, with sodium hypochlorite pricing in South India running ₹25–35 per litre delivered and fluctuating with logistics costs.

Worked Cost Example: Ozone Tertiary Treatment for a 500 KLD STP

Consider a 500 KLD (kilolitres per day) STP adding an ozone tertiary stage for disinfection plus mild colour polishing at a 6 mg/L applied dose. Ozone required: 6 g/m3 × 500 m3/day = 3 kg O3/day. At 9 Wh/g (typical for an air-fed DSC ceramic-electrode generator), that is 27 kWh/day. At an industrial tariff of ₹8/kWh, daily electricity cost is roughly ₹216, or about ₹79,000 per year. Compare this with a chlorine-based tertiary stage delivering an equivalent disinfection and partial-colour result: matching effective oxidant demand typically needs 8–10 mg/L of available chlorine, meaning 4–5 kg/day of active chlorine, or 40–50 litres/day of 10% hypochlorite solution. At ₹28/litre, that is roughly ₹1,260/day, or about ₹4.6 lakh per year — six times the ozone electricity cost, before counting storage tank capital, dosing pump maintenance, and safety infrastructure for chemical handling. Ozone's higher upfront generator cost is typically recovered within 18–30 months at this scale purely from the chemical-cost differential, and continues delivering savings for the remaining service life of the plant.

Sizing Checklist: Specifying an Ozone System for Your STP

Work through these points before finalising a tender or purchase order — under-specifying any one of them is the most common cause of an underperforming or short-lived STP ozone system.

• Design for peak hourly flow, not average daily flow — ozone cannot be stored or buffered, so a generator sized only to daily average will under-dose during peak-load hours.
• Confirm feed-gas source: ambient air (dried to below -40°C dew point) is adequate for disinfection-only duty; PSA oxygen feed raises ozone yield per kWh by 20–30% and is worth it above roughly 5 mg/L design dose or where footprint is constrained.
• Size the contact tank for 4–10 minutes hydraulic retention time at peak flow, with baffled plug-flow geometry — a poorly baffled or short-circuiting tank wastes a large share of the dosed ozone.
• Specify a catalytic or thermal off-gas destructor rated for the generator's full output — this is a statutory requirement, not an optional extra, since ambient ozone above 0.1 ppm is a workplace hazard.
• Confirm upstream filtration is adequate — feeding turbid or high-TSS water into the ozone stage inflates ozone demand and reduces effective disinfection at the design dose.
• Specify ORP-based dose control with an alarm, rather than a fixed-rate dosing pump — effluent organic load varies through the day, and ORP control automatically compensates without operator intervention.
• Ask for in-house component manufacturing and local service support — ozone generators with imported cells or electronics can have long lead times for spares in India; Lotus Ozone Tech builds its DSC ceramic-electrode cells and control systems in-house in Chennai specifically to keep spares and service turnaround short.

Common Mistakes in Ozone STP Retrofits

These are the recurring design and commissioning errors seen when STPs add or retrofit an ozone tertiary stage:

• Dosing ozone directly into poorly filtered secondary effluent — high TSS and residual organics consume ozone before it reaches pathogens, forcing over-dosing to compensate.
• Undersized contact tanks retrofitted into existing civil structures without re-checking retention time at peak flow, leaving disinfection under-performing exactly when flow is highest.
• Treating ozone residual downstream as a control signal — it decays too fast to be meaningful; ORP in the contact tank and periodic microbiological testing of the final effluent are the correct checks.
• Skipping the off-gas destructor or under-rating it for the generator's peak output, creating a workplace ozone exposure risk and a compliance gap during inspection.
• Ignoring seasonal variation — monsoon dilution and inflow/infiltration change effluent character and hydraulic loading; a system tuned only for dry-season conditions can be under- or over-dosed the rest of the year.

Getting the Right Ozone System for Your STP

Lotus Ozone Tech has designed and manufactured ozone systems in Chennai since 2010, with more than 1,000 installations across STP tertiary treatment, ETP advanced oxidation, and related water and wastewater applications — all built on 100% in-house components, including DSC ceramic-electrode ozone cells engineered for consistent yield and long service life. Our engineering team has also delivered systems for demanding institutional projects, including a Department of Atomic Energy facility, and can size the correct dose, contact time, and generator configuration for your specific effluent quality, flow rate, and CPCB or state board consent conditions.

For background on the underlying chemistry, see our guide on how ozone water treatment works, or compare technologies directly in our ozone vs chlorine water treatment guide. Explore the full ozone technology overview and ozone generator product range, or contact our engineering team to get a quote sized to your STP's flow and reuse requirements.