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Scientists design a twin-pit toilet system that protects ground water from getting contaminated by pit-toilet sewage

  • Prof. Sudhakar Rao's research team: Mogili Nitish V, Lydia Arkenadan & Ananth Nag.R (from L to R)
    Prof. Sudhakar Rao's research team: Mogili Nitish V, Lydia Arkenadan & Ananth Nag.R (from L to R)

Photo: Siddharth Kankaria / Research Matters

India aims to eliminate open defecation by 2nd October 2019, marking the 150th birth anniversary of Mahatma Gandhi. To realise this goal, the government has allocated ₹ 9000 crores under the ‘Swachh Bharat Abhiyaan’ to construct new toilets and repair 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, those 17.54 million houses that have a toilet resort to using pit-toilets. While pit-toilets save the embarrassment and dispel hygiene woes to some extent, studies in the past have shown that the leachate from pit toilets cause havoc to the ground water.

“Seepage of sewage from on-site sanitation facilities is known to contaminate groundwater with nitrates and pathogens”, remarks Prof. Sudhakar Rao from the Department of Civil Engineering, Indian Institute of Science, Bangalore. Prof. Rao and his team of researchers have now designed ‘twin-pit’ toilets that release treated leachate to the soil beneath the pit. In a country where nearly 80% of its population depend on groundwater for potable use, such toilets are necessary to save what is left of the ground water. “Eventual migration of treated leachate to aquifer should cause minimal degradation of groundwater/surface water quality”, says Prof. 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.

But what makes sewage leachate so harmful? Sewage leachate contains nitrate, ammonia, faecal coliform and are rich in organic matter, all of which pose hazardous health risks. When nitrate contaminated water is consumed, the nitrate ions gets reduced to nitrite and bind with haemoglobin, causing ‘blue baby syndrome’ – a condition resulting in decreased oxygen carrying capacity of haemoglobin in babies, leading to death. The conversion of nitrate to nitrite and carcinogenic N-nitroso compounds in the digestive track can possibly lead to the development of gastric cancer. In addition, drinking water contaminated with faecal coliform, increases the risk to diarrheal 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.

Prof. Rao and his group of researchers have proposed a modified twin-pit toilet system that aims to reduce organic, ammonium and nitrate loads in treated sewage, which seeps into the soil beneath the pit toilet. In this design, two pits are lined with stack of concrete rings with inner walls plastered to prevent lateral flow of sewage. The first pit serves as a septic tank; and anaerobically treated wastewater overflowing from the septic tank enters the upper half of second pit that serves as a nitrification chamber. Nitrified wastewater from upper half of the second pit enters the lower half containing the bio-barrier at its base, which is constructed using sand, gravel and cow dung. “Cow dung serves as affordable organic carbon source, sand particles act as media for attached bacterial growth, and gravel improves permeability of the barrier”, explains Prof. Rao. The cattle manure also serves as a consortium that contains several species of denitrifying bacteria.

The researchers have observed that the leachate emerging from the bio-barrier to the soil underneath the pit have reduced concentrations of nitrate (because of the denitrification reactions in the bio-barrier), ammonium and chemical oxygen demand (COD) levels. The researchers have tested this design of ‘twin-pit’ toilets for nearly 75 days now and this process is still on-going. So far, they have recorded significantly reduced levels of oxygen needed to oxidise the contents of sewage, also called chemical oxygen demand (COD), and ammonium significantly reduced from 750 mg/L and 400 mg/L, to 145 mg/L and 20 mg/L respectively.

If you thought these ‘twin-pit’ toilets might dig a hole into your pocket, think again! Today, a conventional twin pit system (without superstructure and sanitary fittings) could cost about ₹ 10,000 while the proposed twin-pit toilet costs about ₹ 15,000. While there is an increase of 50% in the cost, the benefits far outweigh the initial cost say the researchers. “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”, says Prof. Rao, advocating the ‘prevention is better than cure’ approach.

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, chemical oxygen demand, etc. “Under optimal conditions, the first pit should result in sewage with reduced chemical oxygen demand (COD), near complete removal of ammonium ions should occur in nitrification chamber in 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”, says Prof. Rao.

While the design on smart-pit toilets heralds a new beginning in terms of decentralized sewage treatment at a house-hold level, there is some distance to go before they become commonplace as the researchers are monitoring the model. “After the system becomes optimal in treating wastewater, design would be shared and publicised. Eventually, it is visualized that the technology will be incorporated in all pit toilets of the country as a ‘standard practice’ to protect groundwater resources from quality degradation”, signs off Prof. Rao.