Quick answer: Size hydraulic equipment on peak hourly flow and biological volume on daily organic load (COD kg/day). These are two different numbers and using one for both is the most common sizing error. Add equalisation to convert a peaky discharge into a steady feed — it is the cheapest reliability you can buy.
The two numbers that size a plant
Almost every sizing mistake comes from collapsing two separate requirements into one figure. They are:
- Peak hourly flow — sizes screens, pumps, pipework, clarifiers, flotation units and every hydraulic component
- Daily organic load, in kilograms of COD or BOD per day — sizes biological reactor volume, aeration capacity and sludge production
A factory discharging 480 m³/day steadily and one discharging the same 480 m³ in three shift-end surges need very different hydraulic equipment and identical biological volume. Size the first plant on 20 m³/h and the second on 20 m³/h, and the second one overflows.
Step 1: Build a real flow profile
Do not accept “about 500 cubic metres a day”. Ask for, or measure, the flow in hourly blocks across a normal production day and across the worst day of the week. What you need out of it:
- Daily total (m³/day)
- Peak hourly flow (m³/h)
- Peak factor = peak hourly ÷ average hourly
- Whether production is continuous, two-shift, or batch
A peak factor of 1.5 is comfortable. A peak factor of 4 or more — common where tanks are dumped at shift end — means equalisation is not optional.
Step 2: Size the equalisation tank
Equalisation converts a spiky discharge into a steady feed, and it does two jobs at once: it protects the biology from hydraulic washout, and it damps out pH and concentration shocks that would otherwise inhibit the biomass. As a working approach, size it to hold the volume discharged during the peak period so that the downstream plant can be fed at close to the daily average rate.
It needs mixing. An equalisation tank without agitation stratifies and settles solids, and then releases them as a slug when it is emptied — which is exactly the shock it was built to prevent.
Step 3: Size the biological stage on load
Convert concentration and flow into load before anything else:
COD load (kg/day) = COD (mg/L) × flow (m³/day) ÷ 1000
So 500 m³/day at 2,000 mg/L COD is 1,000 kg COD/day. That number, not the 500, drives reactor volume, oxygen demand and sludge production. Two further decisions follow from it:
- Whether you need an anaerobic stage. High-strength effluent is usually treated anaerobically first, because aerating that much load is expensive and produces a large amount of surplus sludge. Anaerobic treatment removes the bulk of the load at far lower energy cost and produces much less sludge; aerobic treatment then polishes what remains.
- Whether nutrients need adding. Biological treatment needs nitrogen and phosphorus in rough proportion to carbon. Some industrial effluents — sugar, starch, some chemical streams — are carbon-rich and nutrient-poor, and the biology will not perform until nutrients are dosed.
If you are choosing between process types at this point, see our comparison of complete wastewater treatment systems and the five process stages.
Step 4: Check the limiting parameter, not just COD
COD usually sizes the plant, but not always. Check each of these against the discharge limit and size for whichever is hardest to meet:
| Parameter | Sized by | Often the real constraint when |
|---|---|---|
| COD / BOD | Biological volume and aeration | Food, brewing, chemical, palm oil effluent |
| Ammonia nitrogen | Nitrification volume and sludge age | Limits are strict; nitrification needs a long sludge age and is temperature-sensitive |
| Suspended solids | Clarifier or flotation area | Fine or slow-settling solids, poor floc |
| Oil and grease | DAF sizing | Slaughterhouse, dairy, refinery, palm oil |
| Phosphorus | Chemical dosing and sludge production | Discharge to sensitive waters |
| Heavy metals | Precipitation and filtration | Electroplating, surface finishing, mining |
Ammonia is the parameter that most often surprises people. Nitrifying bacteria grow slowly and are sensitive to temperature and to toxicity, so a plant sized comfortably on COD can still fail its ammonia limit in winter.
Step 5: Size the sludge stage — and do it honestly
Sludge production follows from load, not from flow. Both the biological stage and any chemical dosing produce solids, and chemical sludge from coagulation can easily exceed biological sludge in a physical-chemical plant.
What you need to fix at design stage:
- Estimated dry solids produced per day
- Whether the sludge is thickened before dewatering — it almost always should be, because dewatering dilute sludge wastes capacity
- Target cake dryness, driven by how disposal is charged
- Whether dewatering runs continuously or in batches, which decides between a screw press and a filter press
For the dewatering stage specifically, our filter press sizing calculation guide works through the volume-per-cycle arithmetic, and filter press vs screw press covers the continuous-versus-batch decision.
Common sizing mistakes
- Sizing everything on daily average. The plant works on paper and washes out at shift end.
- Omitting or undersizing equalisation. The cheapest tank on the site protects the most expensive stage on the site.
- Sizing biology on flow instead of load. Produces a plant that is hydraulically generous and biologically starved of volume.
- Ignoring temperature. Biological rates fall with temperature, and nitrification falls fastest. A plant sized for summer can fail in winter.
- Designing for today’s production only. If output is planned to grow, reserve footprint and hydraulic profile now; retrofitting into a running plant is far more expensive.
- Leaving sludge until last. It is the stage you pay for every month.
Frequently asked questions
Should I size a wastewater plant on flow or on COD load?
Both, for different parts. Hydraulic equipment — screens, pumps, pipework, clarifiers, flotation — is sized on peak hourly flow. Biological reactor volume, aeration capacity and sludge production are sized on daily organic load in kilograms of COD per day. Using one number for both is the most common sizing error.
How do I calculate COD load?
COD load in kg/day equals COD concentration in mg/L multiplied by flow in m³/day, divided by 1000. For example, 500 m³/day at 2,000 mg/L COD is 1,000 kg COD per day. That load figure, not the flow figure, drives the size of the biological stage.
How big should the equalisation tank be?
Large enough to hold the volume discharged during the peak period so the downstream plant can be fed at close to the daily average rate. Where production is batch-based or shift-based with a peak factor of four or more, equalisation is not optional. The tank must be mixed, or it stratifies and releases a slug of settled solids when emptied.
Why does my plant meet its COD limit but fail on ammonia?
Nitrifying bacteria grow slowly, need a long sludge age, and are sensitive to temperature and toxicity. A plant sized comfortably on COD can still be short of the reactor volume and sludge age that nitrification needs, and performance falls further in cold weather. Ammonia has to be sized for explicitly, not assumed to follow from COD removal.
Do I need an anaerobic stage?
It depends on strength. High-strength effluent — food processing, brewing, palm oil, some chemical streams — is usually treated anaerobically first, because aerating that much load costs a great deal of energy and produces a large volume of surplus sludge. Anaerobic treatment removes the bulk of the load at much lower energy cost, then aerobic treatment polishes what remains.
How much sludge will the plant produce?
It follows from organic load and from any chemical dosing, not from flow. In physical-chemical plants the chemical sludge from coagulation frequently exceeds the biological sludge. Estimate it at design stage, because it sizes the thickening and dewatering equipment and it sets your monthly disposal cost for the life of the plant.
Send your flow profile, water analysis and discharge limit and we will size the plant stage by stage: request a sizing proposal.