What the plant is trying to do
Sewage from flats, offices, hotels and hospitals carries food waste, soaps, grit, organic matter and germs. If it goes straight into a drain or river, it uses up the oxygen that fish and plants need and spreads disease. A sewage treatment plant, or STP, removes most of that load so the water can be discharged safely or used again.
Engineers track three main numbers. BOD and COD show how much organic matter is left to break down, and TSS shows how much solid material is floating in the water. Every stage of the plant is there to bring one or more of these down, step by step, until the water meets the standard set for the site.
The stages, in order
Most plants follow the same basic route, whatever their size, from a packaged unit in a basement to a large township plant. Water moves from one stage to the next by gravity or pumps, and each stage hands over cleaner water than it received. If one stage is skipped or poorly run, the stages after it have to work much harder.
- Screening: bar screens catch plastic, cloth and other large objects that would damage pumps.
- Equalisation: a holding tank evens out the morning and evening peaks so the rest of the plant gets a steady flow.
- Biological treatment: bacteria, fed with air, eat the dissolved organic matter.
- Settling: in a clarifier or settling tank, the bacteria and solids sink to the bottom as sludge.
- Tertiary filtration: pressure sand filters and activated carbon filters polish the water and remove remaining particles, colour and odour.
- Disinfection: chlorine or UV light kills the germs that are left.
- Sludge handling: settled sludge is thickened and dewatered so it can be dried and disposed of or used as manure.
Choosing the biological process
The biological stage is where the real cleaning happens, and there are several ways to build it. They all rely on aerobic bacteria that need oxygen, supplied by blowers and diffusers. The differences lie in how the bacteria are held in the tank, how much space the plant needs and how clean the final water is.
- Extended Aeration (EA): a simple, long-running tank where bacteria float freely. Easy to operate, but needs more space.
- SAFF (Submerged Aeration Fixed Film): bacteria grow on fixed media inside the tank, which keeps the process stable.
- SBR (Sequential Batch Reactor): filling, aeration, settling and decanting all happen in the same tank, in timed cycles.
- MBBR (Moving Bed Bio Reactor): small plastic carriers move around the tank with bacteria growing on them, giving a compact plant.
- MBR (Membrane Bio Reactor): membranes replace the settling tank, giving very clear water in a small footprint, with higher running care.
Sludge is part of the job
An STP produces two outputs, not one. Alongside the treated water there is sludge, which is mostly water mixed with spent bacteria and solids. Some of it is returned to the biological tank to keep the bacteria population healthy. The rest is removed regularly, otherwise the plant slowly fills up and treated water quality falls.
Excess sludge is usually thickened and then dewatered with a filter press, centrifuge or drying beds. The dried cake is much smaller in volume and easier to handle. Many small plant problems, such as cloudy outlet water or bad smells, trace back to sludge that was not removed on time.
What treated water can be reused for
Well-treated sewage is not drinking water, but it is useful. Common reuse includes toilet flushing, garden and landscape irrigation, cooling tower make-up, floor and vehicle washing, and construction work. Using it this way cuts the amount of fresh water a building or township needs to buy or draw from the ground.
The right process depends on the incoming load, the space available, the power budget and the standard the outlet must meet. A site survey and water analysis by an engineer, such as one from Zeal, is the sensible first step before choosing between them.