Sewage Treatment Plant Process Explained

A sewage treatment plant (STP) takes the used water from toilets, kitchens, bathrooms and laundries, collected by the sewer network, and cleans it in a fixed sequence of physical, biological and sometimes chemical steps. Each unit removes a different kind of pollutant, and each one protects the units downstream. This article walks through the whole train from the inlet screen to the outfall, explains what every stage removes, and works through a primary clarifier sizing example. The biological stage itself is covered in depth in the activated sludge process article, and drinking water plants, which treat much cleaner raw water, are covered in the water treatment process.

What sewage contains

Raw municipal sewage is more than 99% water. The remaining fraction is what the plant has to deal with:

• Coarse solids: rags, plastics, sanitary products and debris that would block pumps and pipes.
• Grit: sand, gravel and other heavy inorganic particles that wear pumps and fill tanks.
• Suspended solids (TSS): organic and inorganic particles, part of which settle under gravity.
• Biodegradable organic matter: measured as BOD (biochemical oxygen demand) or COD. If discharged untreated, it consumes dissolved oxygen in rivers and kills fish.
• Nutrients: nitrogen and phosphorus, which cause algal blooms (eutrophication) in lakes and coastal water.
• Pathogens: bacteria, viruses and parasites from human waste.

Metcalf & Eddy classify medium strength untreated domestic wastewater as having BOD and TSS concentrations in the low hundreds of mg/L. The plant is designed to bring these down to the limits set by the discharge regulator.

Sewage treatment plant process flow

Download diagram (WebP)

1. Screening

Sewage first passes through bar screens. Coarse screens with openings of several millimetres up to about 150 mm catch rags and large objects, and fine screens with openings below about 6 mm catch smaller debris (Metcalf & Eddy). Mechanically raked screens lift the screenings onto a conveyor for washing, compaction and disposal. Screening protects every pump and pipe downstream, which is why it always comes first.

2. Grit removal

In the grit chamber, the flow is slowed so that heavy sand and gravel settle while lighter organic solids stay in suspension and continue to the next stage. Horizontal flow channels hold the velocity near 0.3 m/s; aerated and vortex chambers use a spiral flow pattern for the same purpose. Removed grit is washed and sent to landfill. Without this step, grit abrades centrifugal pumps and builds up in digesters.

3. Primary sedimentation

The primary clarifier is a large circular or rectangular tank where the sewage stands quietly for about 1.5 to 2.5 hours. Settleable solids sink to the floor as primary sludge, and grease and scum float and are skimmed off. Metcalf & Eddy report that well designed primary tanks remove about 50 to 70% of suspended solids and 25 to 40% of BOD. The main design parameter is the surface overflow rate, the flow divided by the tank’s plan area.

4. Secondary (biological) treatment

The settled sewage still contains dissolved and colloidal organic matter that gravity cannot remove. In the aeration tank, a dense population of bacteria, the activated sludge, consumes this organic matter using oxygen supplied by diffusers or surface aerators. The mixed liquor then flows to the secondary clarifier, where the biomass settles. Most of it is pumped back to the aeration tank inlet as return activated sludge (RAS) to keep the bacterial population high, and the surplus is removed as waste activated sludge (WAS).

Activated sludge is the most common choice, but CPHEEO lists other secondary technologies used in Indian STPs, including sequencing batch reactors (SBR), moving bed biofilm reactors (MBBR), trickling filters, upflow anaerobic sludge blanket (UASB) reactors and waste stabilisation ponds.

5. Tertiary treatment and disinfection

Where the receiving water is sensitive or the effluent is reused, the secondary effluent is polished by sand or cloth filtration, nutrient removal or membranes. Before discharge, it is usually disinfected with chlorine in a contact tank or with ultraviolet (UV) lamps to reduce pathogens. Chlorinated effluent may need dechlorination to protect aquatic life. The treated water then flows to the outfall in a river, lake or sea, or to a reuse network for irrigation, cooling or flushing.

6. Sludge treatment

Primary sludge and WAS are mostly water. They are thickened by gravity or flotation, stabilised by anaerobic digestion (which produces methane rich biogas) or aerobic digestion, and dewatered in centrifuges, belt presses or drying beds. Liquid removed during sludge processing is returned to the head of the plant. Handling sludge is a large share of an STP’s operating cost.

Sewage treatment plant effluent standards

Plants are designed to meet a legal discharge limit. Two widely cited examples:

The EU directive adds nitrogen and phosphorus limits for discharges to sensitive areas. In India, the CPHEEO manual and the pollution control boards set the effluent norms that a new STP must meet. Monitoring of these parameters at the outlet, often with online analyzers, is how operators prove compliance.

Equations used in STP design

The surface overflow rate of a clarifier and the hydraulic retention time of any tank are:

$$SOR = \frac{Q}{A} \qquad t = \frac{V}{Q}$$

The removal efficiency of a unit or the whole plant for BOD or TSS is:

$$E = \frac{C_\text{in} - C_\text{out}}{C_\text{in}} \times 100$$

Key formulas for sewage treatment plant design

Operating a sewage treatment plant well

Good performance depends on steady operation as much as on design. Operators check the screens and grit system daily, keep primary sludge pumped out before it turns septic, control dissolved oxygen in the aeration tank, adjust the RAS and WAS rates to hold the target sludge age, and watch the sludge blanket in the secondary clarifier. Flow and level measurement at the inlet, accurate material balances across the plant, and regular laboratory BOD and TSS tests reveal problems early. Energy use is dominated by aeration, so blower control based on dissolved oxygen is one of the most effective savings measures.

Sewage treatment plant interview and viva questions

1. Why is screening the first stage of a sewage treatment plant? Screens remove rags, plastics and large debris that would clog or damage pumps, pipes and valves in every later unit.
2. What is the purpose of a grit chamber? It slows the flow so heavy inorganic particles such as sand settle while organic solids stay suspended, protecting pumps from abrasion and tanks from filling with grit.
3. What is surface overflow rate? It is the flow divided by the plan area of a settling tank, in m³/m²·d, and it equals the settling velocity of the smallest particle that the tank removes completely.
4. Why is activated sludge returned to the aeration tank? Returning settled biomass keeps a high concentration of microorganisms in the aeration tank, so organic matter is removed in a short retention time.
5. What does the US EPA secondary treatment regulation require for BOD5? Under 40 CFR Part 133, the 30 day average effluent BOD5 must not exceed 30 mg/L and removal must be at least 85%.

Related reading

• Activated sludge process
• Water treatment process
• Reverse osmosis
• Solid waste management