1 Centre for Sustainable Technologies,
2 Centre for infrastructure, Sustainable Transportation and Urban Planning,
3 Energy and Wetlands Research Group, Centre for Ecological Sciences, Indian Institute of Science, Bangalore 560 012, India Introduction Reactive nitrogen (Nr) accumulation and pollution in urban water bodies is ubiquitous and has serious consequences. Rapid and unplanned urbanization accompanied by enormous sewage discharges has resulted in reactive nitrogen overloads fed to water bodies leading to eutrophication. Nitrogen is frequently a limiting nutrient in urban waters, but excessive inputs of N can result in an overabundance of algae with deleterious impacts (anoxia of bottom waters; thick green algal mats, etc). This process is accompanied by high macrophyte cover and potential algal bloom but at the same time they offer simple and sustainable Nr recovery and storage option. Nr manifestations in these polluted water bodies govern the fate of the water bodies. These water bodies also remove organic loads offering a purifying function. Among the different forms of Nr the surface of the urban lakes contains high amount of NH4-N than oxidized forms of Nr because of a very high organic load. Elevated concentrations of ammonia (>0.5mg NH3–N/l) are toxic to aquatic organisms (1). Nitrogen removal and recovery from wastewater has been subject of many studies. The uncontrolled release of Nr to the environment is known to cause serious pollution problems. A form of Nr like nitrates in surface water has been attributed to wastewater outfalls (sewage ingress) and agricultural runoff (2,3). N in wastewater is a potential nutrient resource which can be effectively reused in myriads of ways (4). Current technologies for wastewater treatment with N removal are too costly and energy intensive to answer the growing wastewater problems in developing regions (5). The manmade lake/lagoon in developing countries are effective means of Nr transformations in their ability to stabilize reactive N components with the help of algae, bacteria, macrophytes, settled sludge and achieves a superior removal of organic pollutants and inorganic nutrients (6). However, N budgeting and quantitation of Nr has not been studied in great detail in these urban water bodies. Low prevailing nitrate concentrations in lake/waterbodies systems, shows no nitrification and hence denitrification does not seem to play a major role in nitrogen removal (7). However many studies have also demonstrated that denitrification is one of the major N-removal mechanisms (8,9). In several studies, ammonia volatilization was proposed as a major mechanism for nitrogen removal from lagoons (10-12). Yet, in many other studies the ammonia volatilization accounts for only a small fraction of total N-removal and the main removal was via sedimentation and denitrification (13). In other studies ammonia volatilization in algae-based ponds represents a minor fraction of nitrogen removal (14). An attempt for mass balances and N budgeting was studied which indicated plant uptake to be the major removal mechanism compared to detrital settling (15–20). The unaccounted N was attributed to denitrification. So far, no study has been undertaken to measure the Nr budget in quantifying the N content and thus compare the main nitrogen influxes in urban water bodies receiving sewage and address issues of sustainability. The present communication develops a daily nitrogen budget for 220ha sewage fed manmade lake Varthur receiving 595 million liters per day (MLD) to examine their functioning incorporating the main nitrogen fluxes, quantifying threats and risks. The N accumulation in bacteria, algae, macrophytes, sludge-sediment and Nr forms present in the system was put into perspective of a whole-system N balance to determine how wastewater management practices affect whole-system nitrogen export and accumulation and issues related to sustainability. The waste-water N balance also points to ways to improve utilization of wastewater N and find a better way to manage the system to enhance the N recovery mainly through rapid uptake of ammonia after the anaerobic stage and subsequent conversion to algal biomass. Citation: Durga Madhab Mahapatra, Chanakya H. N and T. V. Ramachandra, 2010, NR Budget and sustainable n-recovery in a sewage fed urban waterbody – case study of Varthur lake, Bangalore. Proceedings of the 5th International Nitrogen Conference on Reactive N Management for Sustainable Development - Science, Technology and Policy, 3-7 December 2010, New Delhi, India.
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