Rapid urbanisation coupled with industrialisation in urban areas has greatly stressed the available water resources qualitatively and quantitatively in India. This has also resulted in the generation of enormous sewage and wastewater after independence. Unplanned urbanisation and ad-hoc approaches in planning are evident everywhere, from settlements to sanitary systems and networks. Urban areas in India lack adequate infrastructure for sanitation, leading to inappropriate management of the wastewater and sewage generated in the locality. Most of the sewage and wastewater generated is discharged directly into storm water drains that ultimately link to water bodies. Bangalore is located on a ridge with natural water courses along the three directions of the Vrishabhavaty, Koramangala–Challaghatta (KC) and Hebbal–Nagavara valley systems along the foothills of the terrian, and these water courses are today being misused for the transport and disposal of the city’s untreated sewage. The shortfall or lack of sewage treatment facilities has contaminated the majority of surface and ground waters. These aquatic resources are now unfit for current as well as future use and consequently pose critical health problems.

Bangalore is the principal administrative, cultural, commercial, industrial and knowledge capital of the state of Karnataka. Greater Bangalore, an area of 741 km2 including the city, neighbouring municipal councils and outgrowths. Bangalore is one of the fastest growing cities in India, and is also known as the ‘Silicon Valley of India’ for heralding and spearheading the growth of Information Technology (IT) based industries in the country. With the advent and growth of the IT industry, as well as numerous industries in other sectors and the onset of economic liberalisation since the early 1990s, Bangalore has taken the lead in service- based industries, which have fuelled the growth of the city both economically and spatially. Bangalore has become a cosmopolitan city attracting people and business alike, within and across nations (Sudhira et al., 2007; Ramachandra and Kumar, 2008).

The undulating terrain in the region facilitated the creation of a large number of tanks in the past, providing for the traditional uses of irrigation, drinking, fishing and washing. This led to Bangalore having hundreds of such water bodies through the centuries. In 1961, the number of lakes and tanks in the city stood at 262. A large number of water bodies (locally called lakes or tanks) in the city had ameliorated the local climate, and maintained a good water balance in the neighbourhood. A current temporal analysis of wetlands, however, indicates a decline of 79% in Greater Bangalore during 1970- 2016, which can be attributed to unplanned intense urbanisation processes. The undulating topography, featured by a series of valleys radiating from a ridge, forms three major watersheds, namely the Hebbal Valley, Vrishabhavathi Valley and the Koramangala and Challaghatta Valleys. These form important drainage courses for the interconnected lake system which carries storm water beyond the city limits. Bangalore, being a part of peninsular India, had the tradition of storing this water in these man-made water bodies which were used in dry periods. Today, untreated sewage is also let into these storm water streams which progressively converge into these waterbodies and results in a) algal bloom b) proliferation of exotic aquatic weeds and macrophytes c) large scale fish kill due to asphyxia (zero dissolved oxygen levels) and d) frothing due to P enrichment (Fig. 1.1)

Fig. 1.1: Consequences of nutrient enrichment (algal bloom, macrophytes infestations, fish mortality, frothing..)

Of the three major catchments, the Koramangala–Challaghatta (KC) valley houses the largest of the water to dowsteam and finally takes the water out of the city limits with the drainage channels. Bellandur and Varthur lakes are historic that were originally built to store water for drinking and irrigation purposes (Government of Karnataka, 1990). Today, large-scale developmental activities in recent times due to unplanned urbanisation in the lake catchment has resulted in reduced catchment yield and higher evaporation losses.  Inefficient primary feeder channels feeding the lake have also contributed to water shortage and reduced catchment flow. However, this shortage has been supplemented by an increased quantum of sewage inflow. Due to the sustained influx of fresh sewage over several decades, nutrients in the lake are now well over safe limits. Bellandur and Varthur lakes being the end of the lakes series in the KC valley Varthur Lake has been receiving about ~45% of the city sewage for over last 60 years resulting in eutrophication. There are substantial algal blooms, Dissolved Oxygen (DO) depletion and malodour generation, and an extensive growth of water hyacinth that covers about 70–80% of the lake in the dry season. Sewage brings in large quantities of C, N and P which are trapped within the system. A similar situation prevails in most of the other lakes and water bodies in Bangalore due sewage influx and associated increase in productivity. Such instances have been recurring despite the fact that a certain part of the sewage undergoes at least primary treatment in most cities of India. Thus, any solution to this problem can go a long way in restoring thousands of such water bodies in India.

The various forms of nitrogen influent in sewage are organic N (protein N), urea, ammonia, nitrites and nitrates through processes like nitrification, de-nitrification and ammonification. Autotrophic nitrification consists of two consecutive aerobic reactions, the conversion of ammonia to nitrite by nitrosomonas and then from nitrite to nitrate by nitrobacter (Koops and Pommerening-Roser, 2001). Nitrite-Oxidising Bacteria (NOB) use CO2 and bicarbonate for cell synthesis and ammonium or nitrite as the energy source (Hooper et al., 1997). Ammonia-Oxidising Bacteria (AOB) a β-Proteobacteria comprises of two genera, nitrosospira and nitrosomonas (Purkhold et al., 2003). The availability of any oxygen in the system helps in nitrification chemistry. Complete nitrification stoichiometry requires 4.6 kg oxygen per kg NH4+ (ammonia N). Dissolved oxygen concentrations of 1 mg/l are sufficient for the oxidation of ammonium (Hammer and Hammer, 2001). However, at DO concentrations lower than approximately 2.5 mg/l, nitrite oxidation is inhibited, leading to its accumulation (Paredes et al., 2007). In such conditions, the oxygen transfer rate may be as important as the actual O2 concentration. In natural shallow aquatic vegetation set up, the plant species as Typha and Phragmites provides as oxygenated zone around the roots which enhances nitrification (Munch et al., 2005). In less-aerated systems, however, the transfer rate varies according to the plant species and other environmental and operational factors (Faulwetter et al., 2009). Higher concentrations of nitrates and phosphates primarily contribute to the nutrient enrichment of these urban water bodies resulting in higher productivity and consequent ecological instability. Though there are large number of studies on nutrient enrichment in urban water setup but  studies on various forms of nitrogen and its partitioning into protein, urea, ammoniacal-N, nitrate, nitrite and nitrate denitrified into di-nitrogen are scarce. More importantly N flux and the conversion rates, uptake/release rates by various biological agents and their quantification are often not carried out.

Carbon and nutrient (N and P) recycling are very important for sustainable development (Janzen, 2004) and there has been significant accumulation of these nutrients into urban water bodies, lakes and tanks, thus causing phenomenal alteration to the aquatic integrity of the system resulting in deterioration of these systems at the same time resulting in degradation of water quality and higher GHG emissions. This has not only rendered these systems useless but also has impacted the microclimate of the regions and the ground water resources and is certainly a potential threat to existence. Bangalore has been witnessing a manifold increase in its population, due to IT and BT boom in the city for the last 3 decades. This has resulted in a serious change in both the land cover consequent to the land use and alteration to existing use of water resources and the local hydro-geology of the region. Due to a large population in the city there has been a unprecedented demand for resources and basic amenities but incidentally the city has been witnessing a huge deterioration of the vital landscape elements as vegetation (7.53 %) and water bodies (0.98 %), air (RSPM: 190-347 μg/m3; 39-83 μg/m3 NOx; 8-20 μg/m3 SO2; KSPCB under NAAQM at Graphite) and water resources (90 % surface water (Ramachandra et al., 2015) and 60 % ground water (CG), noise (>100 db; permissible 60-70 db, KSPCB). Rampant urbanisation without any fundamental understanding of the assimilative capacity of the city region and the hydro-geological and climatic variability’s, have lead to an unsustainable city profile. This has not only resulted in a disturbance in the water balance of the region but also has affected the micro-climate of the region immensely. 

Historically the lakes, tanks and water bodies were created basically for maintaining a hydro-geologic regime in water flows for checking floods, recharging, and maintaining the ground water table. They also act as sediment traps, prevent clogging up of natural valleys and reduce erosion by regulating run off. Lakes and Tanks belong to wetland ecosystem and have a larger biological and ecological role that acts as natural filters to myriads of pollutants and maintains a balance between the biota and the aquatic environment. Due to unplanned urbanization most of the tanks/lakes/water bodies in the districts have been transformed to built-up area wrecking the drainage patterns and the hydrologic regime in the city. Therefore, measures for rejuvenation of tanks and lakes in the city will definitely build up ground water resources and safeguard our future water availability for domestic use. In requires an integrated effort made by State authorities (BBMP) in consultation with various stakeholders to conserve and rejuvenate major tanks in Bangalore city.