Prof. Sudhakar Rao Talks about Eco-friendly Toilets
Dr. Sudhakar M. Rao, is a Professor in the Department of Civil Engineering and Associate Faculty, Center for Sustainable Technologies, Indian Institute of Science, Bangalore. His research areas include, groundwater geochemistry, remediation of contaminated drinking water, hazardous waste management and re-use of waste materials.
He has handled/handling several sponsored research projects in Geoenvironment engineering funded by Department of Science and Technology, Bureau of Research in Nuclear Sciences, Atomic Energy Regulatory Board, Ministry of Drinking Water and Private Industries to examine diverse problems of monitoring contaminated groundwater, impact of anthropogenic contamination on groundwater quality, develop technologies to remediate contaminated groundwater, develop clay barriers to contain toxic wastes and re-use of hazardous waste in civil engineering applications. The water network research team had the pleasure to interview Prof. Rao and learn more about his research team's latest work, on creation of Eco-friendly toilets by using permeable reactive barriers.
Q1. What is a permeable reactive barrier (PRB) toilet?
Answer: The PRB toilet is conventional pit toilet with additional PRB layer at thebase of the pit to remediate the nitrate present in pit-toilet leachate and therebypreventing it from contaminating the groundwater.
Q2. Have there been pilot studies of such a toilet?
Answer: No pilot studies in the field are being planned. However, large scaleexperiments are in progress that is discussed latter.
Q3. Are these toilets suitable for both urban and rural environments?
Answer: Yes, such toilets are suited for both urban and rural environments.
Q4. What is the nitrate removal efficiency of such toilets? How does it compare with the nitrate concentration when untreated water is released directly?
Answer: Column experiments that simulate the pit-toilet scenario in the laboratoryindicated that pit toilets equipped with permeable reactive barriers can reduce thenitrate concentrations by 70 to 75 % from their initial value (100 mg/L). The nitrateconcentration in the untreated leachate can range from 85 to 300 mg/L (Rao et al.,2015).
Q5. How is the nitrate removed released into the atmosphere, is it in the form of nitrogengas? Does this nitrogen gas have environmental implications?
Answer: A recent study of Rao et al (2015) suggests that only 4.5 % of availablenitrate concentration in the effluent transforms to N2 and N2O gases; 36-55 % of thegases comprise N2Oand the remainder of N2. Further only 18 % of N2O formedescapes to atmosphere, while the remainder is retained in soil solution. The smallfraction of N2O gas that escapes to atmosphere does have environmental implications.
Q6. What are the costs involved in building such a toilet? How can the costs be minimizedby the usage of locally available materials?
Answer: Introduction of PRB layer in pit-toilets should not substantially add to thecosts; routine construction materials, such as, gravel, brick- bats and sand is used inthe construction of PRB layer. We have been experimenting with use of 1: 1 cow-dungsand mix (on mass basis) instead of sand-bentonite layer due to the ease of availabilityof cow dung in rural settings. Cow-dung was chosen as it was identified to hostseveral strains of denitrifying bacteria (Bacillus subtilis, Bacillus cereus, Micrococcus,Pseudomonas and Serratia) that is essential to remediate the nitrate in the leachate.
Large scale column experiments (Figure 1a) are presently underway for about 2months in the laboratory. 1000 g of 1:1 mix of sand and cow dung is sandwichedbetween two gravel layers in PVC tube (diameter = 14.9 cm), whose upper end is opento atmosphere and lower end is inserted in soil column (Figure 1b). The soil columnhas height, diameter, mass and porosity of 62 cm, 39 cm, 132 kg and 43 %respectively. 1000 mL of nitrate contaminated water (nitrate concentration = 100mg/L) is permeated through the cow dung - soil column on daily basis.
The passage of1 L of water needs 7-8 h to flow through the cow dung-soil column; the leachateemerging from the lower end of the soil column is collected and chemically analysed.
The soil column controls the flow rate of the leachate providing adequate time fordenitrification reactions to occur. The large soil column simulates the soil beneath thepit toilet that would regulate the leachate flow rate from the pit toilet to the sub-surfacebelow.
Nearly, for the entire 60 day period, permeation of nitrate contaminated waterthrough the cow dung- soil column combination reduced the nitrate concentrationbelow 45 mg/L, which is the permissible limit for drinking water (Figure 2).
Figure 1a: Experimental set up for large scale soil column experiment
Figure 1b: Top-view of PVC tube containing sand-cow dung mi
Figure2: Reduction of nitrate concentration during the ongoing test by the cowdung-soil column
Q7. Are there hygiene or odor issues associated with these types of toilets?
Answer: There are no hygiene or odor issues from installation of PRB in the pit toilet.
Q8.Do you consider these toilets an alternative to decentralized wastewater treatment plants or do you think both are necessary to prevent excess nitrate release into the surface and groundwater?
Answer: The PRB technology is pertinenttoallon-site sanitation systems. Off-site sanitation systems would not require the PRB.
Q9. What are the next steps planned in the near future for further research on this topic?
Answer: The next step is to install the PRB in pit toilets in field and monitor the quality of leachate emerging from the pits. We are seeking support to carry out such field studies.
Estimate of N2O release from pit-toilets. Environmental Earth Science, DOI10.1007/s12665-015-4203-3.