What Does It Cost to Treat One Kilogram of COD?

Everyone in an ETP talks about COD in the abstract: inlet 4,000 mg/L, outlet under 250, load 300 kg/day. But ask a plant manager the more useful question — what does it actually cost, in rupees, to remove one kilogram of that COD? — and the room usually goes quiet. This article answers it with a transparent worked example, building the cost up from its parts so you can re-run the numbers with your own tariff and effluent. If the COD number itself still feels fuzzy, start with our spoon-of-sugar explainer and come back here to put a price on it.

Turning COD Load into Rupees

COD load is just the concentration multiplied by the flow. An effluent of 4,000 mg/L (4 kg/m³) flowing at 100 m³/day carries 400 kg of COD every day. To comply, most of that organic matter has to be oxidised away in the biological stage — and oxidation is not free. It costs electricity, chemicals, and sludge disposal. Divide the money spent on those things by the kilograms of COD removed and you get a single, portable number: rupees per kilogram of COD. It is the wastewater equivalent of “cost per unit produced”, and once you have it, budgeting, benchmarking, and technology choices all get easier.

A quick orientation before the arithmetic: for conventional aerobic biological treatment in India, the marginal operating cost of removing COD typically lands in the ₹15–30 per kg range. The rest of this article shows exactly where that band comes from, and why a high-strength stream on anaerobic treatment can sit far below it — sometimes at net-zero cost.

What You Actually Pay For

The cost of biological COD removal breaks into four buckets. Three of them scale directly with the load; the fourth (aeration) dominates:

• Aeration energy — the power to run blowers that force oxygen into the aeration tank so microorganisms can oxidise the COD. Almost always the single biggest item.
• Other electrical loads — feed and recirculation pumps, mixers, dewatering equipment, and instrumentation. Smaller, but real.
• Chemicals — nutrients (urea and DAP to supply nitrogen and phosphorus for the biomass) plus acid or alkali for pH correction, and any coagulant or polymer.
• Sludge handling and disposal — biological treatment converts part of the COD into new cells (waste sludge), which must be dewatered and hauled away, often to a TSDF.

Notice what is not on this list: the capital cost of the plant, operator salaries, and land. Those are largely fixed — they do not rise and fall with each extra kilogram of COD — so we keep them out of the per-kg figure and revisit them at the end.

Aeration Energy: The Biggest Line

Aeration deserves its own section because it usually accounts for 50–70% of an ETP's total electricity bill. The chain of reasoning is short and worth internalising:

• Oxidising 1 kg of COD requires roughly 1 kg of oxygen. (Some COD leaves as sludge instead of being fully oxidised, so field oxygen uptake is often 0.7–1.0 kg O₂ per kg COD removed — we use ~1 kg for a clean, slightly conservative example.)
• A modern fine-bubble diffused aeration system delivers about 1.0–1.5 kg of oxygen per kWh of blower power under real field conditions (its standard-rated efficiency is higher, but field water is warmer, dirtier, and less efficient at transferring oxygen).
• Divide oxygen demand by aeration efficiency and you get roughly 0.5–1.5 kWh per kg COD removed, with about 1.0 kWh/kg as a sensible central assumption. Worn coarse-bubble systems and surface aerators sit at the high end; well-tuned fine-bubble systems with dissolved-oxygen control sit at the low end.

At an industrial power tariff of around ₹9 per kWh (typical Indian industrial tariffs run ₹8–10/kWh), 1.0 kWh/kg works out to about ₹9 of aeration energy per kg of COD. This is also why energy efficiency is the highest-leverage lever on operating cost — halving your aeration energy roughly halves the largest line item, the theme of our ETP OPEX reduction guide.

The Worked Example: ₹ per kg COD

Now assemble the four buckets for a representative aerobic ETP treating a medium-strength organic effluent (food, beverage, or similar). Every figure below is indicative and typical, not a quotation — treat them as a template to re-run with your own numbers:

So a defensible headline figure is about ₹19 to remove one kilogram of COD aerobically, comfortably inside the ₹15–30/kg band you will see across Indian plants. Push the tariff to ₹10, run worn coarse-bubble aeration at 1.5 kWh/kg, and add hazardous-sludge disposal, and you climb toward the top of the range. Tight dissolved-oxygen control, fine-bubble diffusers, and non-hazardous sludge pull you toward the bottom.

When Anaerobic Flips the Maths

Everything above assumes you spend energy adding oxygen. For high-strength streams — distillery spent wash, starch, brewery, dairy — there is a fundamentally different route that turns the cost equation on its head: anaerobic treatment. Instead of blowing air in, an anaerobic reactor (such as a UASB) lets bacteria break the COD down without oxygen, releasing the energy as biogas.

The stoichiometry is favourable: each kilogram of COD removed yields about 0.35 m³ of methane (at standard conditions), carrying roughly 3.5 kWh of thermal energy. Burned in a boiler or engine, that biogas has real fuel value — several rupees per kg of COD — which offsets operating cost instead of adding to it. Anaerobic systems also generate far less sludge (roughly 0.05–0.10 kg per kg COD, versus 0.3–0.5 kg for aerobic) and need no aeration blowers at all.

The honest caveat: anaerobic treatment alone rarely reaches discharge limits, so an aerobic polishing stage is still needed. But that polishing step only has to treat the residual COD — perhaps 20–30% of the load — while the anaerobic stage strips out the bulk cheaply, or even at a net credit. The blended cost per kg COD for a well-designed anaerobic-plus-aerobic train on a strong effluent can therefore fall well below the pure aerobic ₹19/kg. This is the core reason high-COD industries invest in biogas recovery: above a certain strength, treating COD stops being purely a cost and starts partly paying for itself.

Ready Reckoner: Daily Load to Annual Cost

Because the cost is (roughly) linear in load, you can scale the ₹19/kg figure straight up to whatever your plant handles. The table below uses the aerobic central value; treat the totals as indicative operating cost, not a quote:

That 1,000 kg/day row is the eye-opener: nearly ₹70 lakh a year of pure operating cost, most of it electricity, just to keep one high-load plant compliant. It is exactly this figure — not the capital cost — that makes energy optimisation and, where the strength allows, anaerobic pre-treatment so financially attractive at scale. It also explains why biological processes such as MBBR are chosen for their oxygen-transfer efficiency and compact footprint.

What This Number Does and Doesn't Include

The per-kg COD figure is a marginal operating cost — the extra money spent on power, chemicals, and sludge for each additional kilogram of COD removed. To keep it honest and portable, it deliberately excludes:

• Capital depreciation — the amortised cost of the tanks, blowers, and civil works.
• Manpower and overheads — operators, lab staff, and management, which are largely fixed regardless of load.
• Land, financing, and downtime — site-specific and not load-linked.

When you are sizing a plant or comparing technologies, those fixed costs matter just as much as the per-kg operating figure — which is why the right lens for a purchase decision is total cost of ownership, not any single number. Our CAPEX vs OPEX guide shows how to combine them, and the wastewater treatment cost overview puts typical capital and operating benchmarks side by side. Used together with the ₹/kg COD figure here, they let you both budget the running cost and justify the investment.