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
Rapid urbanization (3000 cities) accompanied by high sewage discharges has resulted in reactive nitrogen (Nr) overloads fed to water bodies resulting in their eutrophication accompanied by high macrophyte cover – yet they offer simple and sustainable Nr recovery and storage option. These water bodies also remove organic loads offering a purifying function. A Nr budget was prepared for a 220ha 595 MLD sewage fed man-made lake to examine its functioning and quantifying threats and risks. For the study the lake was divided into five zones and water quality, various N components, microbial and macrophyte biomass, etc. was monitored at monthly intervals. The lake could be divided into three zones – a.anaerobic, b.algal driven facultative and c.macrophyte (with algae) dominated aerobic zones depending upon functions. Loadings were higher during the monsoon due to the intrusion of storm-water. Multiple point sources contributed significantly in the increase in the overall loading to the lake. The major N forms in the system were in the form of NH4-N (60%, 20 t/d) of the total N entering the system. Estimates made from such periodic sampling reveal that on a daily basis the water body received about 32tpd Nr which was then partitioned into bacterial, micro-algal, macrophyte biomass and that into sludge and NO3-N, NH4-N in effluent after a 5d retention time (estimated). The balance (unaccounted) was considered to be losses from the system through ammonia volatilization and de-nitrification processes. While bacterial uptake remained fairly constant throughout the year, micro-algae was the major player during monsoon and winter and macrophytes dominated Nr capture during summer. From the estimates we find that nearly 55% Nr was recovered and recycled when this water is used for irrigating fodder crops and the remaining 45% was lost from the system - thereby showing a reasonably high potential for sustainability. Based on these findings a better way to manage the system to enhance the N recovery is suggested – mainly through rapid uptake of ammonia after the anaerobic stage and conversion to algal biomass.
Keywords: Reactive nitrogen; N budget; lake; macrophytes; algae; nitrification