Clean water out
Sewage Treatment Plants (STP)
A building’s sewage, treated well enough to use again.
A sewage treatment plant treats the wastewater a building produces — from toilets, kitchens, bathrooms and laundries — so it can be safely discharged or, better, reused. Most of the work is done by bacteria.
What it means
Sewage is mostly water carrying organic matter. Left alone, that organic matter consumes the oxygen in any water it reaches. An STP removes it under control, in tanks, before the water leaves.
The heart of most plants is biological: air is blown into a tank so that bacteria thrive and feed on the organic load. The water is then settled, filtered and disinfected.
Treated water is a resource. Flushing, gardening and cooling towers do not need drinking-quality water, and using treated water for them cuts what a building has to buy.
How it works
Step by step, as drawn in the diagram above.
- 1
Screening
A bar screen catches rags, plastics and grit before they reach pumps and tanks.
- 2
Equalisation
A collection tank evens out the morning and evening peaks so the plant sees a steady flow.
- 3
Aeration
Blowers push air through diffusers; bacteria in the aerated tank consume the organic load.
- 4
Settling
In the clarifier the bacteria settle out as sludge; part returns to the aeration tank, the rest goes to sludge handling.
- 5
Filtration and disinfection
Sand and carbon filters polish the water and disinfection makes it safe to reuse.
Where the treated water goes
Reuse is the payoff. Typical uses for treated water:
- 1 Toilet flushing Often the largest single use in a residential building.
- 2 Gardening and landscape Lawns, plants and green areas.
- 3 Cooling towers Make-up water for air-conditioning plant.
- 4 Washing Driveways, car parks and common areas.
What it takes out
- Organic load (BOD and COD)
- Suspended solids
- Pathogens, after disinfection
- Much of the colour and odour, after polishing
What it does not
- Dissolved salts — treated water is not drinking water
- Some nutrients, depending on the process
Where it is used
- Apartment complexes
- Hotels and resorts
- Hospitals
- Malls and offices
- Campuses and townships
What we size it on
- Occupancy, and the sewage flow per person per day
- Peak flow and how it varies over the day
- The discharge or reuse standard to meet
- Space available, and the noise and odour limits around it
Looking after it
- Keep the blowers and aeration running continuously
- Remove sludge regularly
- Clean screens and check the clarifier
- Test treated water against the required standard
Questions, answered
Does my building need an STP? ＋
Larger residential and commercial buildings are often required by local regulations to treat their own sewage — check with your local authority. We size the plant to the occupancy and the norms that apply.
Can treated sewage be used for drinking? ＋
No. A typical STP treats water for reuse in flushing, gardening and cooling — non-potable uses — not for drinking.
Will an STP smell? ＋
A well-designed, well-run plant should not cause a nuisance. Odour usually means aeration has stopped or sludge has been left too long.
The rest of the treatment train
Clean water in
Water Softeners
Hardness out. Scale, white spots and failing geysers with it.
Clean water in
Sand & Carbon Filters
Particles out with sand. Chlorine, odour and colour out with carbon.
Clean water in
Ultrafiltration (UF)
Particles and microbes out. Minerals stay in.
Clean water in
Industrial RO Skid Systems
Dissolved salts out — the one thing no filter can touch.
Clean water out
Effluent Treatment Plants (ETP)
Industrial effluent, treated to meet discharge or reuse norms.
Send us your water report
Or tell us where your water comes from — we design the treatment train, install it and stand behind it.