Quick answer: Municipal plants produce roughly 70–90 g of dry solids per person per day. Industrial plants vary far more, because chemical sludge from coagulation often exceeds biological sludge. Size on dry solids per day, then convert to cake volume — never on influent flow.
Flow tells you almost nothing
Two plants treating the same cubic metres per day can produce sludge quantities that differ several times over. Flow sizes pumps and pipework. Sludge quantity follows from the solids and organic load that arrive, plus whatever chemicals you dose to remove them.
So the question has to be rebuilt as three separate numbers:
- Dry solids per day — biological plus chemical, in kg DS/d
- Cake dryness — what percentage of that cake is solids after dewatering
- Cake volume — what you actually pay to store, haul and dispose of
Municipal: start from people
For domestic sewage, per-capita design loads are the most reliable starting point. Minnesota Pollution Control Agency design guidelines use 0.20 lb TSS (91 g) and 0.17 lb BOD₅ (77 g) per person per day as design minimums.
Working those through primary capture and secondary yield gives roughly 70–90 g dry solids per person per day in total, split about 60/40 between primary sludge and waste activated sludge. A 50,000-person plant therefore produces on the order of 3.5–4.5 tonnes of dry solids per day.
Two warnings on per-capita figures. They assume genuinely domestic sewage — industrial discharges into the sewer break the assumption immediately. And published national figures are not comparable between regions, because some divide by connected population and others by total population.
Industrial: start from load, then add the chemicals
There is no per-capita shortcut for industrial effluent. Biological solids come from the organic load actually removed, and the yield depends heavily on how long sludge stays in the system.
Sludge age is the whole story. Pennsylvania DEP operator training gives sludge retention times of 3.5–10 days for conventional activated sludge against 20–30 days for extended aeration. Longer retention means more of the biomass is consumed by endogenous decay, so extended aeration produces roughly one-half to one-third the waste solids per kg of BOD removed.
As an operator-level check, Michigan EGLE activated sludge training uses 0.7 lb of solids per lb of BOD removed on a total suspended solids basis for conventional operation. Use it as a sanity check on a design figure, not as a design figure itself — it is short-retention, TSS-basis, and includes inert solids carried in with the influent.
Chemical sludge is the one that surprises people
This is where industrial estimates go wrong. Coagulant does not disappear — it precipitates and reports to the sludge.
The stoichiometric floor is straightforward. Alum converting to aluminium hydroxide yields about 0.26 kg of dry solids per kg of alum; ferric chloride converting to ferric hydroxide yields about 0.66 kg per kg of FeCl₃. Those are minimums: they count the hydroxide only, not co-precipitated phosphate, adsorbed organics or enmeshed influent solids.
Measured values confirm the order of magnitude. A 2016 study in Desalination and Water Treatment measured 0.40–0.82 mg of sludge per mg of FeCl₃ dosed across aerobic and anaerobic effluents. A practical design band of 0.4–0.8 kg DS per kg FeCl₃ is defensible.
Run the comparison and the point becomes obvious. At 80 mg/L of ferric chloride you generate around 53 mg/L of chemical solids. A biological stage removing 200 mg/L of BOD at an observed yield of 0.3 produces around 60 mg/L. At that dose the two are already the same order of magnitude — and industrial dosing frequently runs far higher.
The same 2016 study found that above roughly 80 mg/L of FeCl₃ the volatile-to-total solids ratio falls to about 0.5. The sludge becomes half mineral. That matters twice over: mineral sludge is heavier, and it dewaters differently from biological sludge.
From dry solids to cake volume
Dry solids per day is an engineering number. Cake volume is the number you are billed on.
The conversion is:
Cake volume (m³/d) = dry solids (kg/d) ÷ [ cake bulk density (kg/m³) × solids fraction ]
Cake bulk density is commonly taken in the range 1,050–1,250 kg/m³, toward the higher end for lime- or iron-conditioned cake with a high mineral content. Using 1,100 kg/m³, one tonne of dry solids occupies roughly:
- 4.5 m³ at 20% dry solids
- 3.6 m³ at 25%
- 3.0 m³ at 30%
- 2.6 m³ at 35%
Moving cake from 20% to 30% dry solids cuts haulage volume by about one third, for the same mass of solids. Where disposal is charged by volume, that is the commercial case for pressure filtration stated as arithmetic rather than as a claim.
What cake dryness can you actually reach
The US EPA fact sheet on recessed-plate filter presses reports cake solids of 45–50% for primary plus waste activated sludge, and up to 55% for lime-treated tertiary sludge.
Read the conditioning note before you plan around those figures. The same document specifies 10–30% lime and 5–7.5% ferric chloride on a dry weight basis to reach them. Lime and iron report to the cake as inert mass, so part of that high percentage is added solids rather than removed water. With polymer conditioning only, the same source gives about 45% for primary solids and 35% for primary plus waste activated sludge.
For comparison, the EPA belt filter press fact sheet gives 20–35% for raw primary plus waste activated sludge, and 12–22% for anaerobically digested waste activated sludge. The gap between belt and pressure filtration is real, and it is widest on the sludges that dewater worst.
One more citable line worth knowing: the EPA fact sheet states recessed-plate filter presses are cost-effective for plants below roughly 4 MGD (about 15,000 m³/d).
What to send for a sludge estimate
- Population served, or process flow and influent COD/BOD and TSS
- Whether the sewer receives industrial discharge, and from what industries
- Coagulant type and dose, and any lime addition
- Sludge age or aeration mode, if the biological stage already exists
- Whether sludge is digested or thickened before dewatering
- How disposal is charged — by weight or by volume
Related: how many filter plates you need, filter press sizing for municipal sludge by population, and filter press versus screw press. Related equipment: Chamber Filter Press and Membrane Filter Press.
Frequently asked questions
How much sludge does a wastewater treatment plant produce per day?
Municipal plants produce roughly 70 to 90 grams of dry solids per person per day. A 50,000-person plant produces about 3.5 to 4.5 tonnes of dry solids daily. Industrial plants vary far more, because coagulant dose and organic load drive the figure rather than population.
Can I estimate sludge from flow rate?
No. Flow sizes pumps and pipework, not sludge quantity. Two plants treating identical volumes can produce several times different sludge masses depending on influent solids, organic load and chemical dosing. Estimate from dry solids per day instead.
How much sludge does ferric chloride produce?
About 0.4 to 0.8 kg of dry solids per kg of ferric chloride dosed, based on measured values. The stoichiometric floor is 0.66 kg per kg, counting only the hydroxide precipitate. Alum yields about 0.26 kg per kg on the same basis.
Does chemical sludge weigh more than biological sludge?
Often yes, in industrial plants. At 80 mg/L ferric chloride, chemical solids are already comparable to biological solids from 200 mg/L BOD removal. Above that dose the sludge becomes roughly half mineral, which changes both its mass and its dewatering behaviour.
How do I convert dry solids into cake volume?
Divide dry solids per day by cake bulk density multiplied by the solids fraction. Using 1,100 kg per cubic metre, one tonne of dry solids gives about 3.0 cubic metres of cake at 30% dry solids, or 4.5 cubic metres at 20%.
Does extended aeration produce less sludge?
Yes, roughly one-half to one-third of conventional activated sludge per kg of BOD removed. The reason is sludge age: extended aeration runs 20 to 30 days against 3.5 to 10 days conventional, so endogenous decay consumes more of the biomass.
Send your population or process load, coagulant dose and disposal basis, and we will work the sludge quantity and cake volume through with you: request a sludge and sizing estimate.
References: US EPA — Recessed-Plate Filter Press Fact Sheet (832-F-00-058), US EPA — Gravity Thickening Fact Sheet (832-F-03-022), Minnesota Pollution Control Agency — Design Flow and Loading Determination Guidelines.