Most apartment complexes, hotels, hospitals and tech parks in South Indian cities run their own sewage treatment plant (STP), and many reuse the treated water for flushing and gardening. The biology does most of the work, but a handful of chemicals keep the plant within its consent limits and the treated water clear and safe to reuse.
Where chemicals are used in an STP
- Tertiary clarification: a coagulant such as PAC or alum, often with a polyelectrolyte, removes the fine suspended solids that the biological stage leaves behind, before the pressure sand filter.
- Disinfection: sodium hypochlorite or bleaching powder kills pathogens before the water is reused or discharged.
- Sludge dewatering: a cationic polyelectrolyte helps the filter press, screw press or drying beds separate water from sludge.
- pH correction: lime or caustic soda when the influent is too acidic for the biology.
- Odour and colour polishing: activated carbon filters for reuse water.
Disinfection: hypochlorite or bleaching powder?
Liquid sodium hypochlorite is easy to dose with a pump and suits plants with an operator on site. Bleaching powder stores longer and costs less to transport, but has to be dissolved and leaves some insoluble residue. Whichever you use, check the residual chlorine in the treated water daily: too little means incomplete disinfection, too much wastes chemical and can harm gardens.
Hypochlorite loses strength over time, faster in heat and sunlight, so buy quantities you will use within a few weeks and store drums in the shade. Our guide Bleaching Powder vs Sodium Hypochlorite compares the two in detail.
Coagulant and polymer for clear reuse water
If treated water looks cloudy or the sand filter clogs quickly, the usual fix is a small coagulant dose ahead of the tertiary clarifier or filter. PAC is popular in STPs because it works over a wide pH range and makes less sludge than alum. Run a quick jar test to find the dose instead of guessing.
Keeping the paperwork right
STPs run under consent from the state pollution control board and are inspected against the limits in that consent. Keep a record of chemicals bought and dosed, with the batch test reports and safety data sheets, so you can show inspectors and residents' associations how the plant is run.
Buying tips for facility managers
- Buy the pack size that suits your storage: bags and drums for small STPs, larger loads for big campuses.
- Store hypochlorite and bleaching powder away from acids. Mixing them releases chlorine gas.
- Keep the SDS for each chemical near the dosing area and train the operator on spills and PPE.
We supply STP chemicals across Tamil Nadu, Karnataka, Telangana and the rest of South India.