Bangalore-based researchers have developed a permeable reactive barrier (PRB) toilet which uses a mixture of sand and bentonite clay.

While thousands of toilets prop up across the country, there has been little attention given to an environmentally-friendly set up. The low-cost toilets being built rely entirely on a refuse-collection pit — a design that ends up leaching nitrates into the groundwater below. 
Researchers at the Centre for Sustainable Technologies at the Indian Institute of Science (IISc), Bengaluru, have developed a permeable reactive barrier (PRB) toilet which uses a mixture of sand and bentonite clay. The clay regulates the flow of leachates by swelling when in contact with water.
The study — which was published recently in theJournal of Water, Sanitation and Hygiene for Development — shows the barrier reducing nitrate concentration by 66 per cent within 12 hours and nearly 94 per cent within a day.
Denitrifying bacteria
Sudhakar M. Rao, lead author of the study, says the mixture of sand (95 per cent) and bentonite clay (5 per cent) ensures a suitable anaerobic environment for de-nitrifying bacteria which convert nitrates into gaseous nitrogen compounds.
The importance of curbing leachates was demonstrated in a 2013 study of Mulbagal town in Karnataka by Prof. Rao. The seepage of untreated water into groundwater resulted in an average nitrate concentration of 148 mg per litre, which was more than three times greater than the permissible limit. When consumed in large quantities, nitrites can cause anaemia and can even react with amines to form carcinogenic nitrosamines.
The researchers have taken the results into a “cost-effective” workable set-up: a top nearly 1 metre-thick gravel layer below the pit-base followed by a 32 cm-thick sand layer and a bottom 20 cm-thick layer of sand plus clay. A perforated vent pipe allows gas bubbles to escape.
While the study demonstrates the potential of the mixture, Prof. Rao is aware of the need to develop mixtures that can be locally sourced. “Bentonite is not easily available in rural areas or small towns. Instead, we have decided to use a mixture of cow dung and sand (1:1 ratio) that can be equally effective,” he said. His team has received funding for pilot trials
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