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A permeable reactive barrier to remove harmful nitrates from pit toilet leachate

While the Indian government is promoting the No toilet, No bride campaign, especially in rural areas, little attention is given to the design and construction of pit latrines. Many studies have shown evidences of nitrate contamination into groundwater through infiltration of pit toilet leachate; not many, however, have worked on effective solutions. In this context, a study from the Department of Civil Engineering and Centre for Sustainable Technologies at IISc proposes using a permeable reactive barrier (PRB) containing Bentonite-Enhanced Sand (BES) for removing nitrates in pit toilet leachate. 

Approximately 1.77 billion people globally use pit latrines as their primary means of sanitation. “The National Sample Survey (NSS 65th Round 2010) has estimated that pit toilets cater to 12% of rural and urban Indian households and 49% of the population (rural and urban households) has no access to toilet facilities. Moreover, evidence of increased nitrate contamination in groundwater by sewage seepage is reported in 20 states of the country,” said Prof. Sudhakar Rao from the Department of Civil Engineering, who is the lead author of the study. The idea was to reduce nitrates to gaseous nitrogen compounds which will no longer be a hazard to groundwater, through anaerobic processes carried out by de-nitrifying bacteria, he added.

To aid the conversion of nitrate to nitrogenous gas and provide a suitable anaerobic environment for the de-nitrifying bacteria to act, a medium/barrier was developed. The medium is a mixture of bentonite and sand (95% sand + 5% bentonite), which will act as a permeable membrane for leachate to pass through and also support the growth of the de-nitrifying bacteria that remove the nitrates. Bentonite is a type of clay which swells when it comes into contact with water and hence regulates the flow of leachate to match bacterial kinetics.

“Several batch experiments and PRB experiments were conducted to understand the rate at which de-nitrification is performed by the bacteria supported in the BES media and simulate the performance of the barrier when installed at the base of pit toilet respectively,” said Prof. Rao. The batch experiments showed that the nitrate concentration decreases from 23.5 mg/L to 7.9 mg/L after 12 hours and to 1.56 mg/L after 24 hours. The PRB experiments showed that at a slow permeation rate of 33 mL/h in the PRB column, complete nitrate removal is achieved in 9.6 hours.

The researchers also recommend an installation design for pit latrines with PRB. They propose that a 0.2m-thick PRB layer can be introduced at a depth of 1.3m below the pit-base preceded by a 0.975m-thick gravel/brickbat layer and 0.325m-thick sand layer. In addition, they suggest that a perforated vent pipe passing through sand and PRB layer can remove the accumulated gas bubbles.

Several studies have been conducted to understand groundwater contamination through pit leachate, yet very few mitigation measures have been suggested. “With 80% of our country dependent on groundwater as drinking water source, preventing nitrate infiltration is essential. The nitrate concentration in pit leachate is much larger than the permissible limit for nitrate in drinking water, namely, 45 mg/L. With the thrust in the country to provide sanitation facilities to the uncovered population, steps should be taken through appropriate technologies to mitigate nitrate contamination emerging from such on-site facilities, or else nitrate contamination of groundwater will become a challenging problem in the near future,” cautioned Prof. Rao.

The next step after successful laboratory experiments is to observe the results in the field. “We appreciate the fact that bentonite clay used in the manufacture of Bentonite Enhanced Sand (BES) may not be available in rural and small towns across the country. Therefore, we have identified that a mix of cow dung and sand is equally efficient in removing nitrates from leachates and plan to install this barrier of cow dung + sand in a few pit toilets as part of pilot studies,” said Prof. Rao.

 

About the authors:

Dr. Sudhakar M. Rao is a Professor in the Department of Civil Engineering, Indian Institute of Science, Bangalore and R. Malini is a graduate student at the Centre for Sustainable Technologies, Indian Institute of Science, Bangalore.

 

This paper has been published online in the Journal of Water, Sanitation and Hygiene for Development, on 3 April, 2015. http://www.iwaponline.com/washdev/up/washdev2015159.htm

 

Contact information:

Prof. Sudhakar M. Rao, , Department of Civil Engineering, IISc.

Email: msrao@civil.iisc.ernet.in;

Website: http://civil.iisc.ernet.in/~msrao/

Phone: +91 80 2293 2812 (Office)