How an STP Works: Sewage Treatment Plant Process Explained Step by Step

A sewage treatment plant converts raw domestic wastewater — which contains human waste, food scraps, soaps, and all the waterborne discharge from a building or community — into treated water that is safe for discharge to a water body or reuse for non-potable purposes. The process uses physical separation, biological degradation, and disinfection in sequence. Understanding each stage helps plant operators diagnose problems, helps designers specify systems, and helps building managers ask the right questions of their STP contractors.

STP Stages: The Full Treatment Train

A complete STP passes sewage through four functional stages:

1. Preliminary treatment — removes gross solids and grit that would damage equipment
2. Primary treatment — physical settling removes suspended solids and floating materials
3. Secondary treatment — biological degradation removes dissolved organic pollution (BOD/COD)
4. Tertiary treatment — polishing removes residual suspended solids, nutrients, and pathogens

Sludge is produced at multiple stages (primary settling, biological sludge) and requires separate handling — thickening, dewatering, and disposal or composting. All stages must be designed and sized as an integrated system — a high-capacity biological stage cannot compensate for an undersized primary treatment stage or inadequate sludge handling.

Primary Treatment: Screening, Grit Removal, and Settling

The first stage of an STP protects biological treatment from the materials that would impair it:

Bar screen / Fine screen: Metal bars or a rotating drum screen intercept rags, plastics, hair, and large food particles from the sewage stream. Screenings are collected on a conveyor and disposed of as solid waste. Screen openings: 5–20 mm for coarse screens, 1–3 mm for fine screens before MBR systems. A blocked or bypassed screen is the most common cause of downstream pump failures and media/membrane damage.

Grit chamber: A slowed-flow channel or vortex separator where dense inorganic particles (sand, grit, eggshell, fine gravel) settle out. Grit that reaches the biological reactor settles in aeration tanks, reducing effective volume and damaging aeration diffusers. Grit is collected and removed for disposal.

Primary clarifier: A large, quiescent settling tank where settleable organic solids settle to the bottom as primary sludge and floating scum is skimmed from the surface. HRT of 1.5–2.5 hours; removes 50–65% of TSS and 25–40% of BOD. Primary clarifiers are omitted in some modern compact STPs (particularly SBR-based designs) to reduce footprint and cost — but their absence increases the organic loading on the biological stage.

Secondary Treatment: Biological Degradation

Secondary treatment is the heart of the STP — the stage that removes dissolved and colloidal organic matter (BOD and COD) that settles would not remove. Aerobic microorganisms consume organic compounds and convert them to CO₂, water, and new biomass. The biological stage requires:

• Adequate oxygen: DO maintained at 2.0–3.0 mg/L by aeration system
• Stable biomass: MLSS 2,500–4,000 mg/L in suspended growth systems
• Sufficient contact time: HRT 6–16 hours depending on inlet BOD and technology
• Temperature: Mesophilic bacteria operate optimally at 20–35°C; below 15°C, biological activity decreases significantly

Common secondary treatment technologies used in Indian STPs:

• SBR (Sequencing Batch Reactor): Most common for municipal STPs in the 0.5–10 MLD range; combines aeration and clarification in a single tank in timed cycles
• MBBR (Moving Bed Biofilm Reactor): Plastic media carriers support biofilm; more robust to load variation; preferred for industrial and commercial ETPs
• Extended Aeration ASP: Long HRT (18–30 hours) simplifies operation; common in residential complex STPs
• MBR (Membrane Bioreactor): Ultrafiltration membrane replaces clarifier; produces high-quality effluent (BOD <5 mg/L, TSS <1 mg/L) directly suitable for reuse

Tertiary Treatment and Disinfection

Tertiary treatment polishes the secondary effluent for discharge or reuse:

Pressure sand filter (PSF): Removes residual suspended solids and turbidity to below 5–10 NTU. Standard in most Indian residential STPs. Backwash with clean water every 1–2 days to prevent clogging.

Activated carbon filter: Adsorbs remaining colour, odour, and trace organic compounds. Included in residential STPs where treated water is used for toilet flushing (where odour is perceptible in confined spaces). Carbon requires periodic replacement (12–24 months) as adsorption capacity is exhausted.

Disinfection — UV: Ultraviolet radiation inactivates pathogenic bacteria and viruses without adding chemicals. Effective only in clear water (TSS <10 mg/L, turbidity <5 NTU) — always placed after filtration. UV lamps require annual replacement and quartz sleeve cleaning every 3–6 months.

Disinfection — Chlorination: Sodium hypochlorite dosing kills pathogens and provides residual disinfection in distribution pipes. Used for discharge to water bodies; avoided for garden reuse (chlorine phytotoxic above 1 mg/L).

Nutrient removal: STPs above 1 MLD discharging to sensitive water bodies increasingly require nitrogen and phosphorus removal. See our guide on nitrogen removal in wastewater treatment for the technology options.

Sludge Handling

Sludge is generated at two stages: primary settling (primary sludge — raw, odorous, easily digested) and biological treatment (secondary sludge — biological floc, lower odour). Combined sludge must be:

1. Thickened: Gravity thickener or dissolved air flotation (DAF) thickener increases sludge solids from 0.5–1% to 3–5% — reducing volume for dewatering
2. Dewatered: Belt press, filter press, or centrifuge reduces moisture to 70–80% (20–30% dry solids) — reducing transport and disposal cost
3. Disposed: Dewatered sludge cake goes to composting (if pathogen-free), co-processing in cement kilns, or authorised landfill

For residential STPs, sludge accumulation is slower (domestic wastewater sludge yield: 0.05–0.08 kg dry solids per kg BOD removed). A 100 KLD residential STP generates approximately 50–80 kg dry sludge per day — typically 0.5–1 m³/day as dewatered cake.

Common STP Technologies in India

Technology selection for STPs in India is shaped by plot availability, operator skill, capital budget, and reuse requirements:

• SBR: Dominant for municipal STPs (0.5–10 MLD); compact, can achieve nutrient removal; requires reliable PLC control
• MBBR: Growing use in commercial and residential complexes for operator robustness; requires secondary clarifier
• MBR: Premium choice for water reuse in residential complexes and commercial buildings; higher CAPEX but highest reuse water quality
• Constructed wetlands: Very low-cost but requires large land area (200–500 m²/KLD); slow treatment; used in rural community STPs and where land cost is negligible
• Biofilter/Trickling filter: Older technology, still found in small community STPs; simple operation but limited performance; rarely specified for new installations