--- title: "ETP and STP Process: Stages, Flow Diagram and Difference | Filtsap" description: "How an effluent treatment plant (ETP) and a sewage treatment plant (STP) work, stage by stage, and the difference between them. With typical equipment at each step." source: https://www.filtsap.com/guides/etp-stp-process/ updated: 2026-10-23 --- Buyer guide · Updated 23 Oct 2026 · By the Filtsap technical team # ETP and STP process explained, stage by stage **Short answer** An ETP (effluent treatment plant) treats industrial wastewater, and an STP (sewage treatment plant) treats domestic sewage from toilets, kitchens and bathrooms. Both screen and equalise the flow, treat it biologically and settle the solids. An ETP adds chemical steps such as neutralisation and coagulation because industrial effluent carries chemicals, colour, oil or high COD. ## ETP vs STP at a glance | | ETP | STP | | --- | --- | --- | | Treats | Industrial effluent from process lines | Domestic sewage from buildings and townships | | Typical problems | Variable pH, COD, colour, oil, heavy metals, high TDS | BOD, suspended solids, nitrogen, bacteria | | Main treatment | Physico-chemical plus biological | Mostly biological | | Reuse | Often followed by RO and evaporation for ZLD | Treated water reused for flushing, gardening, cooling | ## ETP process, step by step - **Screening and oil removal.** A bar screen removes rags and solids; an oil and grease trap skims floating oil. - **Equalisation tank.** Mixes batches from different process lines so the plant sees a steady flow and load. - **Neutralisation.** Acid or alkali dosing brings pH to the 6.5 to 8.5 range that biology and chemicals need. - **Coagulation and flocculation.** Alum, ferric or PAC with polyelectrolyte binds fine solids and colour into flocs. - **Primary clarifier.** Flocs settle out as chemical sludge. - **Biological treatment.** Aeration tank (activated sludge or MBBR) where bacteria break down dissolved organics and reduce COD and BOD. - **Secondary clarifier.** Settles the biomass; part of it returns to the aeration tank. - **Tertiary treatment.** [Pressure sand and activated carbon filters](https://www.filtsap.com/build/water-treatment-systems/) , sometimes UF, polish the water for discharge or reuse. - **Sludge handling.** A thickener and filter press or drying beds dewater the sludge for disposal. Where pollution control boards ask for zero liquid discharge, the treated water goes on to [RO](https://www.filtsap.com/build/membrane-systems/) and the RO reject to an evaporator. See [MLD and ZLD systems](https://www.filtsap.com/build/wastewater-recovery/) . ## STP process, step by step - **Bar screen and grit chamber** to remove solids and sand. - **Equalisation tank** with coarse-bubble aeration to stop septic conditions. - **Biological treatment** using MBBR, SBR, activated sludge or MBR, with fine-bubble [diffusers](https://www.filtsap.com/supply/water-treatment-spares/) supplying oxygen. - **Tube settler or clarifier** to settle the biomass. - **Chlorination** or UV for disinfection. - **Sand and carbon filtration** before reuse for flushing or landscaping. - **Sludge holding tank** and filter press or drying beds. ## What we supply for both We build complete [ETP, STP, MLD and ZLD plants](https://www.filtsap.com/build/wastewater-recovery/) sized to your effluent analysis, and supply the spares that keep existing plants running: diffusers, dosing pumps, filter media, multiport valves, filter bags for oil and solids, and online pH and TDS meters. ## FAQs What is the full form of ETP and STP? ETP is effluent treatment plant, for industrial wastewater. STP is sewage treatment plant, for domestic sewage. What is the main difference between ETP and STP? An ETP treats industrial effluent and needs chemical steps such as neutralisation and coagulation. An STP treats domestic sewage and relies mainly on biological treatment. What size of ETP or STP do I need? It is sized on daily flow in KLD and on the effluent analysis (pH, COD, BOD, TSS, TDS). Send us both for a design. ## Not sure what to order? Send your application and current filter. We reply on WhatsApp. [Ask an engineer](https://wa.me/918160852169?text=Hello%20Filtsap%2C%20I%20read%20your%20guide%20%22ETP%20and%20STP%20process%20explained%2C%20stage%20by%20stage%22%20and%20need%20advice.) [Request a quote](https://www.filtsap.com/get-quote/) More guides - [Filter micron rating guide](https://www.filtsap.com/guides/filter-micron-rating-guide/) - [Spun vs wound vs pleated filter cartridge](https://www.filtsap.com/guides/spun-vs-wound-vs-pleated-filter-cartridge/) - [How to choose a bag filter](https://www.filtsap.com/guides/how-to-choose-bag-filter/) - [When to replace a filter cartridge](https://www.filtsap.com/guides/when-to-replace-filter-cartridge/) ## Need help choosing? Send your application details and our team will recommend the right filter, at no cost. [Get a quote](https://www.filtsap.com/get-quote/) [WhatsApp](https://wa.me/918160852169?text=Hello%20Filtsap%2C%20I%20need%20a%20quote.)