Quick answer: An STP (sewage treatment plant) treats domestic wastewater and is sized on population or building occupancy; an ETP (effluent treatment plant) treats industrial wastewater and is sized on the specific pollutant load of the process. STPs are almost always biological. ETPs usually need a physico-chemical stage first, produce more sludge per cubic metre, and the sludge is often hazardous. Both end in a sludge dewatering step, and that step is where most operating cost sits.
Why the two terms matter
In India, Southeast Asia, the Middle East and Africa, buyers rarely say “wastewater treatment plant”. They say STP when they mean a plant for sewage from a housing complex, hospital, hotel, campus or town, and ETP when they mean a plant for a factory. The distinction is written into permits: consent-to-operate conditions in India, for instance, are issued against the general effluent standards for industry and separately against sewage treatment norms, and the two carry different limits and different monitoring.
The engineering difference is larger than the names suggest, and it decides what equipment you actually need.
STP: designed on people, not on chemistry
Domestic sewage is remarkably consistent. Whatever the country, it carries roughly 50–80 g of BOD and 60–90 g of suspended solids per person per day, plus nitrogen and phosphorus in predictable proportions. That is why an STP is sized from the number of people it serves and a per-capita water use figure, and why STP designs are standardized: screening and grit removal, a biological stage, secondary settling, disinfection, and sludge handling.
The biological stage is where the choice lies. Extended aeration and SBR are the common choices for small and medium plants; MBBR is used where space is tight or an existing plant is being upgraded; MBR is used where the treated water will be reused for flushing, gardening or cooling towers, which is increasingly required for larger buildings. Our guide to choosing between MBR, MBBR and SBR covers the trade-offs.
STP sludge is biological: waste activated sludge at 0.5–1% solids, non-hazardous in most jurisdictions, and often permitted for land application or composting once it is dewatered and stabilized.
ETP: designed on the process
Industrial effluent has no per-capita shortcut. A textile dyehouse, an electroplating shop, a dairy, a pharmaceutical plant and a pulp mill produce effluents that differ in every parameter, and a single factory can produce several streams that must not be mixed. An ETP is therefore designed from a characterization of each stream: flow, COD and BOD, pH, suspended solids, oil and grease, dissolved salts, and any specific pollutants such as heavy metals, colour, ammonia or cyanide.
Most ETPs have three blocks:
- Physico-chemical treatment. pH correction, coagulation and flocculation, and either sedimentation or dissolved air flotation. This removes suspended solids, oil, colour and heavy metals, and produces a chemical sludge.
- Biological treatment. Aerobic treatment for the remaining dissolved organics; anaerobic treatment first when the COD is high, as in distilleries, dairies, breweries and palm oil mills. Some effluents, such as electroplating rinse water, have so little organic load that this block is omitted.
- Tertiary treatment. Sand and carbon filtration, and where reuse or zero discharge is required, ultrafiltration and reverse osmosis. See where RO and evaporation fit in a ZLD plant.
ETP sludge is where the two plant types diverge most. The chemical sludge from the first block is mineral, often exceeds the biological sludge in mass, and where it carries heavy metals it is classified as hazardous waste and disposed of by weight at a secure landfill. Our article on how much sludge a wastewater plant produces works through the estimate.
Side-by-side
| STP | ETP | |
|---|---|---|
| Feed | Domestic sewage | Industrial process effluent |
| Sized on | Population and per-capita flow | Measured flow and pollutant load per stream |
| Core process | Biological | Physico-chemical, then biological if organics remain |
| Design variability | Low; standard packages | High; every plant is engineered |
| Sludge type | Biological, usually non-hazardous | Chemical plus biological; often hazardous |
| Sludge per m³ treated | Lower | Higher, sometimes several times higher |
| Dewatering | Screw press or filter press | Filter press; membrane press where disposal is charged by weight |
| Typical reuse target | Flushing, gardening, cooling | Process water, or ZLD where mandated |
Choosing the sludge dewatering equipment
For an STP the sludge is light, biological and steady in quantity, so the decision is mainly about scale and attention. A multi-disc screw press runs continuously with very little supervision and suits plants up to a few tonnes of dry solids per day. A chamber filter press gives a drier cake, typically 20–30% solids against 15–20% from a screw press, which matters where sludge is hauled some distance.
For an ETP the filter press is the default, for three reasons. Chemical sludge is mineral and compressible and reaches 30–40% solids in a press, which a screw press or belt press cannot match. Hazardous sludge is paid for by the tonne, so every point of cake dryness is money. And ETP sludge production is intermittent — a batch of coagulant sludge from the morning shift, a tank clean-out on Friday — which suits the batch nature of a press. Where the sludge is hazardous, a membrane filter press pays for itself in disposal savings.
Two mistakes we see in enquiries. The first is sizing an ETP press on the biological sludge and forgetting the chemical sludge, which can be the larger fraction. The second is specifying an STP press on peak population and running it at a fraction of capacity for years; a smaller press on a longer daily schedule costs less to buy and to run.
What to send for a quotation
For an STP: the design population or occupancy, the per-capita water use you design on, whether treated water will be reused and for what, and the discharge norm you must meet. For an ETP: the industry and process, the daily flow of each stream, a laboratory analysis (COD, BOD, TSS, pH, oil and grease, TDS, and any metals or specific pollutants), the discharge or reuse requirement, and how sludge will be disposed of. From that we can size the treatment blocks, estimate dry solids per day and select the dewatering equipment, cloth and feed pump.
Related: industrial wastewater treatment equipment, heavy metal wastewater treatment, when you need dissolved air flotation.
Frequently asked questions
What is the difference between ETP and STP?
An STP treats domestic sewage and is sized on the number of people it serves; an ETP treats industrial effluent and is designed from the measured flow and pollutant load of each process stream. STPs are biological plants; ETPs usually need a physico-chemical stage first and produce more, and often hazardous, sludge.
Can an STP treat industrial wastewater?
Only if the effluent is similar to sewage in strength and composition, such as canteen and washroom waste from a factory. Process effluent with high COD, oil, metals, colour or salt will upset a biological STP and must go through an ETP first.
Which produces more sludge, an ETP or an STP?
An ETP, per cubic metre treated, in almost every case. Coagulant sludge from the physico-chemical stage adds to the biological sludge and is often the larger fraction, and in metal-bearing effluents it is hazardous waste.
What size STP needs a filter press?
There is no fixed threshold. Small STPs often use a screw press or sludge drying beds; a filter press becomes attractive when the sludge must be hauled a long distance or landfilled by weight, because it produces a drier cake. Plants above a few tonnes of dry solids per day usually justify a press on disposal cost alone.
Is ETP sludge hazardous?
Frequently, especially from electroplating, tanneries, textile dyeing, pharmaceuticals and chemicals. Classification depends on the local rules and on what the sludge contains. Hazardous sludge must go to an authorized disposal facility and is charged by the tonne, so dewatering to the highest practical solids matters.
What is a CETP?
A common effluent treatment plant that serves a cluster of small factories, typically in an industrial estate. Each member pre-treats to an inlet standard and the CETP does the rest. Sludge handling is centralized, which is why CETPs are among the largest users of filter presses in industrial estates.
Tell us whether you need an STP or an ETP, the flow and the load, and we will size the plant and the sludge dewatering together: request an STP or ETP proposal.
This article is part of our guide to wastewater sludge dewatering.