Why flush water does not need to be drinking water
Much of the treated water a home uses goes down the toilet. Here is how on-site treatment plants let housing societies flush with recycled water instead.
Sai Naman Group5 min read
Every time a toilet is flushed in most Indian homes, water that was treated to drinking standard, pumped across a city and stored in a tank is used to carry waste away. It is used once and then leaves the building as sewage. In a city where tankers arrive every summer, that is an expensive way to use the cleanest water available.
The scale of the problem is measured. In its guidelines for reusing treated sewage, the Central Pollution Control Board cites the National Inventory of Sewage Treatment Plants: urban India generates an estimated 72,368 million litres of sewage a day, while the capacity actually being used to treat it is 20,235 million litres a day. The remaining 52,133 million litres a day is let out untreated. The same document notes that about 80 per cent of the water supplied to municipalities flows back into the environment as untreated wastewater.
Treating sewage where it is made
A sewage treatment plant inside a housing project deals with that society's own sewage on site. Screens remove solids, biological treatment lets bacteria consume the organic load, and filtration and disinfection finish the job. What comes out is not drinking water and is never plumbed as such. It is clean enough for jobs that do not need drinking water at all.
The CPCB guidelines list the uses that treated sewage is already put to in India, including horticulture, irrigation, washing of roads, vehicles and trains, fire-fighting, industrial cooling, toilet flushing, gardening and construction. For quality, they point to norms recommended by the Central Public Health and Environmental Engineering Organisation for treated sewage at the point of use, set separately for each activity rather than as one number.
There is need to realize sewage as a resource which can be treated as per requirement and utilized for non-potable purposes.
What this means inside a building
Reuse only works if the recycled water can reach the cisterns, which is a decision taken long before anyone moves in. It needs a second set of pipes running through the building, separate storage, and clear marking so the two supplies are never confused.
- Dual plumbing: one set of pipes for drinking-quality water at taps and showers, a second for treated water at flush cisterns and garden taps.
- Separate tanks for treated water, kept apart from the drinking water storage.
- Clear labelling at every outlet that carries treated water, so no one drinks from it.
- Room for the plant itself, and a maintenance contract, because a plant that is not run properly is worse than none.
Retrofitting this into a finished building is difficult and expensive. Planning it into the drawings costs far less and lasts the life of the building.
What the society gains
Flushing, gardening and washing are a large share of what a home and its grounds use water for. Serving them with treated water means the municipal supply and the borewell are left to do what only they can do: drinking, cooking and bathing. In a dry month, that is the difference between a normal week and a queue for a tanker.
The city gains too. Sewage treated and reused inside the project is sewage that does not travel to an overloaded municipal plant, and does not join the 52,133 million litres a day that reach drains and rivers untreated.
At Suburbia Estate, the treatment plant and the second set of pipes are part of the design, not an upgrade offered later. It is one of the quieter decisions in a project, and one of the few that keeps paying for itself every single day.