This twin-pit toilet could help save India’s remaining groundwater from contamination

Spoorthy Raman July 13, 2017

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October 2, 2019 — the 150th birth anniversary of Mahatma Gandhi — is the deadline India has set for itself to eliminate open defecation in the country. To realise this goal, the government has allocated Rs 9,000 crore under the Swachh Bharat Abhiyaan for construction of new toilets and for repair of dysfunctional ones.

According to the Swachhta Status Report of 2015, more than half of India’s rural population (52.1%) defecates in the open, posing a major public health and sanitation problem. With insufficient sewage infrastructure, the 17.54 million households have pit toilets. According to 2011 census, India has about 248 million households.

In a bid to save what is left of groundwater in India, professor Sudhakar Rao and his team of researchers from IISc, Bengaluru, have designed twin-pit toilets that release treated leachate into the soil beneath the pit  

While these save people the embarrassment of open defecation and address hygiene problems to some extent, studies in the past have shown that the leachate (water that has percolated through a solid and leached out some of the constituents) from pit toilets cause havoc to groundwater.

“Seepage of sewage from onsite sanitation facilities is known to contaminate groundwater with nitrates and pathogens,” said professor Sudhakar Rao from the Department of Civil Engineering, Indian Institute of Science (IISc), Bengaluru. This in a country where nearly 80% of the population depends on groundwater for potable use.

IISC-Pit-toilet-Lab-members
The team of researchers — (L-R) Mogili Nitish V, Lydia Arkenadan and Ananth Nag R — from IISc, Bengaluru, who worked on the project with professor Sudhakar Rao

In a bid to save what is left of groundwater in India, Rao and his team of researchers have designed twin-pit toilets that release treated leachate into the soil beneath the pit. “Eventual migration of treated leachate to aquifers (a body of permeable rock that can contain or transmit groundwater) should cause minimal degradation of groundwater/surface water quality,” said Rao about the advantage of such toilets. Their study was recently published in the Journal of Environmental Engineering and Science, ICE Publishing, London, and is funded by the Arghyam Foundation, Bengaluru.

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But what makes sewage leachate so harmful? Sewage leachate contains nitrate, ammonia, faecal coliform and is rich in organic matter, all of which pose hazardous health risks. For instance, when water contaminated with nitrate is consumed, the nitrate ions get reduced to nitrite and bind with haemoglobin in the blood, causing the “blue baby syndrome” — a condition in which the oxygen-carrying capacity of haemoglobin is reduced in babies, leading to death in extreme cases. The conversion of nitrate to nitrite and carcinogenic N-nitroso compounds in the digestive tract can also cause gastric cancer.

Besides, drinking water contaminated with faecal coliform increases the risk todiarrhoeal infection. Also, when untreated organic matter in the leachate enters lakes or rivers, it undergoes aerobic decomposition and reduces the dissolved oxygen levels, thus spelling doom for the aquatic life.

Using bio-barriers to reduce contamination

The modified twin-pit toilet system proposed by Rao and his group aims to reduce organic, ammonium and nitrate loads in treated sewage. In this design, two pits are lined with stacks of concrete rings and the inner walls plastered to prevent lateral flow of sewage. The first pit serves as a septic tank, and anaerobically treated wastewater overflowing from this enters the upper half of the second pit, which serves as a nitrification chamber. Nitrified wastewater from the upper half of the second pit enters its lower half, which contains a bio-barrier made of sand, gravel and cowdung at its base.

The modified twin-pit toilet system proposed by Rao and his group aims to reduce organic, ammonium and nitrate loads in treated sewage. In this design, two pits are lined with stacks of concrete rings and the inner walls plastered to prevent lateral flow of sewage  

“Cowdung serves as an affordable organic carbon source; sand particles act as media for attached bacterial growth; and gravel improves the permeability of the barrier,” explained Rao. The cattle manure also serves as a consortium that contains several species of denitrifying bacteria.

Also read: Study shows eating red meat, small dietary changes can help India’s water crisis

The researchers have observed that the leachate emerging from the bio-barrier into the soil underneath the pit has reduced concentrations of nitrate (because of the denitrification reactions in the bio-barrier), ammonium and chemical oxygen demand (COD) levels. The team has tested the twin-pit toilets for nearly 75 days now and this testing is still on. So far, they have recorded significantly reduced levels of COD (the oxygen needed to oxidise the contents of sewage) and ammonium from 750 mg/litre and 400 mg/litre during the pre-treatment stage to 145 mg/litre and 20 mg/litre after passing through the bio-barrier.

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Lab equipment used in twin-pit toilet the project. (L-R) Titration equipment for chemical analysis; pH meter for measuring the pH of liquids

And the best part is these twin-pit toilets won’t dig a hole in the pockets of those looking to install them. While a conventional twin-pit system (without superstructure and sanitary fittings) costs about Rs 10,000, the proposed twin-pit toilet costs about Rs 15,000. While there is an increase of 50% in the cost, the benefits far outweigh the higher cost. “The higher cost of the modified twin-pit toilet system more than offsets the costs involved in treating nitrate-contaminated drinking water, which is removed by using reverse osmosis devices. Further, if one has to treat the aquifer source to remove pit toilet contaminants, costs are way beyond the realm of a community,” said Rao.

“Eventually, it is visualised that the technology will be incorporated in all pit toilets of the country as a ‘standard practice’ to protect groundwater resources from quality degradation” — Sudhakar Rao    

The researchers have already constructed such a twin-pit toilet in the town of Mulbagal in Kolar, Karnataka, serving one household. They periodically monitor the different chambers of the pit for quality parameters like ammonium, nitrate, COD, etc. “Under optimal conditions, the first pit should result in sewage with reduced COD; near complete removal of ammonium ions should occur in the nitrification chamber of the second pit; and near complete removal of nitrate and residual ammonium ions should occur in the bio-barrier at the bottom of the second pit,” said Rao.

While the design of the smart twin-pit toilets could just be the solution to decentralised sewage treatment at a household level, it will take some time for them to hit the market as the researchers are still monitoring the model. “After the system becomes optimal in treating wastewater, the design will be shared and publicised. Eventually, it is visualised that the technology will be incorporated in all pit toilets of the country as a ‘standard practice’ to protect groundwater resources from quality degradation,” signed off Rao.

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