INTRODUCTION

Rapid urbanization coupled with industrialization 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 urbanization and ad-hoc approaches in planning are evident everywhere, be they settlements or sanitary systems and networks. Urban areas in India lack the infrastructure for sanitation, leading to inappropriate management of the wastewater generated. Most of the sewage and wastewater generated is discharged directly into storm water drains that ultimately link to water bodies.  Since Bangalore is located on a ridge with natural water courses along the three directions of the Vrishabhavaty, Koramangala-Challaghatta (K&C) and Hebbal-Nagavara valley systems, these water courses are today being used for the transport and disposal of the city’s sewage. The shortfall or lack of sewage treatment facilities have contaminated the majority of surface and ground waters. These aquatic resources are now unfit for current as well as for future use and consequently pose critical health problems. Central Pollution Control Board (CPCB, 2006; CPCB, 2009) estimate indicates that about 26,254 million liters per day (MLD) of wastewater are generated in 921 Class I cities (Population >1, 00,000) and Class II (Population 50,000 -1, 00,000) towns in India (housing more than 70% of the urban population). However only 27% (7044 MLD) (27%) of wastewater is treated.

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, neighboring municipal councils and outgrowths, was ‘notified’ (established) in December 2006 (Figure 1).  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 liberalization 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 Uttam 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 neighborhood. A current temporal analysis of wetlands, however, indicates a decline of 58% in Greater Bangalore which can be attributed to intense urbanization processes. This is evident from a 466% increase in built-up area from 1973 to 2007 (Ramachandra and Uttam Kumar, 2008). 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 water bodies. Varthur lake is one such lake at the end of a chain of lakes.

Varthur lake, situated in the south of Bangalore, was built to store water for drinking and irrigation purposes (Government of Karnataka, 1990). Today, large scale developmental activities in recent times due to unplanned urbanization 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. However, this shortage has been supplemented by an increased quantum of sewage inflow. Due to the sustained influx of fresh sewage over a decade, nutrients in the lake are now well over safe limits. Varthur lake has been receiving about 40% of the city sewage for over 50 years resulting in eutrophication. There are substantial algal blooms, dissolved oxygen depletion and malodor generation, and an extensive growth of water hyacinth that covers about 70 to 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 many other cities such as Bhopal (Shahpur lake), Jabalpur (Sardar lake). The Sihora, Gosalpur, Kundam and Seoni towns of Madhya Pradesh (Ghosh et al., 2008); Udaipur, Rajasthan (Chaudhury and Meena, 2007); Hussain Sagar  (Hyderabad), Nainital Lake (Region Special Area Development Authority, 2002) and Kandy lake in Sri Lanka (Silva, 2003). 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 extent of N (nitrogen) flowing through the Belandur-Varthur lake system is large (16.4 t/d; Chanakya and Sharatchandra, 2008) and is about 20-40 mg/l. The various forms of nitrogen influent in sewage are organic N (Protein N), urea, ammonia, nitrites and nitrates through processes like nitrification, denitrification 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 (Hooper et al., 1997; Koops and Pommereing-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) belongs to β-Proteobacteria which includes two genera, nitrosospira and nitrosomonas. (Stephen et al., 1996; Purkhold et al., 2000; Purkhold et al., 2003). Complete nitrification stoichiometry requires 4.6 kg oxygen per kg NH₄⁺ (Ammonia N). Dissolved oxygen (DO) concentrations of 1 mg l⁻¹ are sufficient for the oxidation of ammonium (Hammer and Hammer, 2001). However, at DO concentrations lower than approximately 2.5 mg l⁻¹, 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. Plants provide an oxygenated zone around the roots which enhances nitrification (Zhu and Sikora, 1994; Johnson et al., 1999 and Munch et al., 2005). In less aerated systems, however the transfer rate varies by plant species and other environmental and operational factors (Faulwetter et al., 2009).

Higher concentrations of nitrates and phosphates primarily contribute to the eutrophication of urban water bodies. Higher values of NO₃ N were observed during the post monsoon season (Srivastava et al., 2007; Bharali et al., 2008; Dhanalakshmi et al., 2008; Edokpayi et al., 2008). There is, however, scant mention about the various forms of nitrogen being observed and analyzed in all these studies. In most of these studies, the N forms have not been partitioned into protein, urea, ammonia, nitrate, nitrite and nitrate denitrified into di-nitrogen. The conversion rates from one form to another as well as their uptake/release by various biological agents and their quantification are often not carried out. Higher P values were recorded in July (Heron, 1961); premonsoon (Bharali et al., 2008; Kapil et al., 2009). Moderate to high values of BOD were reported in the pre-monsoon (Solanki et al., 2007; Raveen et al., 2008; Dhanalakshmi et al., 2008).

In all the cases above it is not clear what extent of the input water (influent into the lake) is sewage and therefore the contribution of sewage to the C, N and P loads have seldom been estimated. Earlier estimates indicate that Varthur lake receives about 500 MLD of sewage (Chanakya et al., 2006). This also serves as a water source for crop irrigation to downstream farmers. There were indications that the sewage passing through such a lake system was being partially treated. In this study, we examined the nature and extent of changes in water quality (treatment levels) during various seasons. It is of interest to determine whether such a lake could be converted to a sustainable and passive sewage treatment system adaptable to other locations, considering that water and energy are fast becoming scarce in the developing world.