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Sewage Treatment Plant (STP)

Domestic wastewater from buildings and towns treated for safe discharge and everyday reuse.

What an STP is

A sewage treatment plant treats domestic wastewater — toilets, bathrooms, kitchens, laundries — from residential complexes, hotels, hospitals, offices, campuses and municipalities. Unlike industrial effluent, sewage is predictable: BOD 200–350 mg/L, COD 400–700 mg/L, TSS 200–400 mg/L, high faecal coliform. The challenge is not chemistry but consistency — daily peaks, weekends, holidays, low-flow nights, and an operator who is usually not a chemist.

Modern STPs are built for reuse: treated water for flushing, gardening, road washing and cooling-tower make-up. Many state PCBs and municipal bye-laws mandate this for larger buildings, and it directly reduces fresh-water bills.

Treatment stages

Process flow — typical sequence
Bar screen andgrease trap01Equalisation /collection tank02Biologicalreactor03Secondarysettling04Tertiaryfiltration05Disinfection06Treated waterstorage andreuse07Sludge handling08
  1. Bar screen and grease trapRags, plastics and kitchen grease are removed before they choke pumps and reactors.
  2. Equalisation / collection tankAbsorbs the morning and evening peaks; submersible pumps feed the reactor at a constant rate.
  3. Biological reactorBacteria consume organic matter with oxygen supplied by fine-bubble diffusers — MBBR, SBR, MBR or extended-aeration ASP.
  4. Secondary settlingTube settler or clarifier separates biomass; sludge is recycled and the excess wasted. MBR plants use membranes here.
  5. Tertiary filtrationPressure sand filter and activated carbon filter polish turbidity, colour and odour; UF for high-grade reuse.
  6. DisinfectionSodium hypochlorite dosing, UV or ozone to kill pathogens and meet faecal coliform limits.
  7. Treated water storage and reuseSeparate flushing and gardening network with level automation and residual chlorine control.
  8. Sludge handlingSludge holding, filter press or screw press; dewatered sludge to disposal or composting.

Technology options

MBBR

Moving-bed biofilm reactor — compact, forgiving, easy to operate. The default for most buildings.

SBR

Sequencing batch reactor — fill/react/settle/decant in one tank; good for municipal and variable flows.

MBR

Membrane bioreactor — smallest footprint and reuse-grade outlet without tertiary filters.

Extended aeration ASP

Simple, low-sludge; suited to institutions with land.

Underground / basement

Civil tanks below podium with odour control and forced ventilation.

Containerised / packaged

Factory-built units for sites without civil works or with a short timeline.

Where STPs are needed

Residential complexes and townships
50 KLD to several MLD; low noise, odour control, minimal operator attention.
Hotels and resorts
Sharp occupancy swings; laundry and kitchen loads; landscape reuse.
Hospitals
Disinfection-critical, isolation of laboratory and infectious streams, dedicated handling.
Malls, IT parks and offices
High peak factor, weekend lows, cooling-tower make-up reuse, basement installation.
Schools, hostels, institutions
Simple, robust plants that survive vacations and low flow.
Municipal / ULB
SBR and ASP schemes, decentralised packaged STPs, AMRUT-type works.

How we deliver an STP

Population and peak-factor based sizing → technology selection against space, noise and reuse targets → civil and mechanical design → supply and erection → seeding and commissioning → operator training and O&M. Reuse-grade plants include UF and disinfection with online monitoring.

Frequently asked questions

What capacity STP does my building need?

Sewage generation in residential buildings is about 100–135 litres per person per day (85–90 % of water supply). A 300-flat society at 4.5 persons per flat needs roughly 150 KLD. Hotels are sized per room, hospitals per bed, offices per person. We size from your occupancy, not a thumb rule.

MBBR, SBR or MBR — which is best?

For most housing societies and hotels a well-designed MBBR is the best balance of cost, robustness and outlet quality. MBR earns its premium where land is very tight or the treated water has a high-value reuse. SBR suits institutional and municipal plants with trained operators. See our comparison article in the Knowledge section.

Can STP-treated water be used for flushing and gardening?

Yes — that is the standard reuse. With tertiary filtration and disinfection the outlet meets reuse norms for flushing, landscaping and cooling-tower make-up. MBR outlet can go further.

Does an STP smell?

A correctly designed and operated plant does not. Odour comes from undersized aeration, poor sludge removal or open tanks in the wrong place. We design covered inlet works, adequate aeration and, where needed, biofilters.

What is the power consumption of an STP?

Typically 0.6–1.5 kWh per m³ treated depending on technology — MBBR at the low end, MBR at the high end. We state the design power consumption in writing with every proposal.

Need an STP for a building or town?

Share your inlet analysis, capacity and location — we reply with a preliminary scheme and budget range within one working day.