STPs are costly, why not treat the grey water?

Grey water treatment. This cheaper alternative to full-scale Sewage Treatment Plant (STP) could be the way out for flat owners in small apartments, says water expert and Thippeswamy, former chief engineer of BWSSB.

The black water from toilets and oily waste from kitchen sinks could be left to the BWSSB sewage lines. But this amounts to only 30 per cent of domestic waste water. As Thippeswamy says, the rest 70 per cent that emanates from washing machines, bathrooms and wash basins could be recycled through grey water treatment (GWT).  
As with STPs, the treated water from GWT could be used for flush tanks, gardening or car-wash. “Being economical to set up and maintain, this could be ideal for apartments with 10 flats or less. The system does not occupy much space either,” says the water expert.

He estimates that the GWT system will not cost more than Rs. 1.5 lakh. The running cost will not exceed Rs 6,000. To make it even more beneficial, the plant could be integrated to a rainwater harvesting system.

Grey water, the waste generated from laundry, dishwashing and bathing, typically breaks down faster than black water since it has lower levels of nitrogen and phosphorus. It is easier to treat due to the lower level of contaminants. Black water cannot be treated without a STP due to its high concentration of chemicals.  


The grey water could be collected in any water tank. The treated water is stored in another tank. Preferably, the collected greywater should be treated the same day as the high bacteria content could raise a big stink. The treatment system could be based on aeration or freshening, filtration and polishing to reach the quality requred for home use.   
Besides filtration, the process also includes settlement of solids, flotation and separation of ligher solids, anaerobic/aerobic digestion and chemical/UV disinfection. Disinfecting greywater requires two chemicals, chlorine and iodine. A settling tank is used to let solids and large particles settle at the bottom. Grease, oils and smaller particles float. The liquid that remains could be reused.

Large suspended particles are filtered right when the greywater enters the tank through a screen. It is then pumped into an aeration tank, where chlorine is added for chemical treatment. Further filtering through a multigrade pressure filter removes the finer suspended solids. A final polishing at the collection tank through re-chlorination makes the water fit for reuse.

To avoid confusion with the main drinking water pipes, the supply points on the greywater system should be clearly labelled. A separate internal waste water drainage system is required to collect and distribute the greywater.

Water experts estimate that greywater systems can actually help flat owners save about 40 per cent on the annual water bill. Here’s how: By educating themselves and non-residents such as maids who wash utensils and clean apartments, gardeners, and drivers who wash cars, on the value of conserving greywater for treatment.

Perhaps, BWSSB could strike a deal with apartment owners associations and greywater system providers to make the equipment even more affordable. That might be a better way to ensure that small apartments treat their own waste water than costly STPs.

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