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

Abstract

Introduction

Materials and Methods

Results and Discussions

Conclusion

References

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Materials and Methods

1.       Study area and its characteristics
Varthur Lake (12°57’24.98”-12°56’31.24” N, 77°43’03.02”-77°44’51.1”E), is the second largest fresh water body in Bangalore. It covers a water-spread area of 220 ha (mean depth 1.1m, residence time - 5 days Figure 1). It is part of a series of connected and cascading water bodies. At present the lake receives inadequately treated sewage of about 595 million liters per day (MLD). The average annual rainfall of Bangalore is 859 mm and temperatures vary from 14°C (December to January) to 33°C (Maximum during March to May). There are two rainy periods i.e. from June to September (South-West Monsoon) and November to December (North-East Monsoon). During the rainy periods, fresh water also enters the lake as runoff.

Figure 1. Study Area: Varthur lake-with sampling locations, Greater Bangalore, India

2.        N budget
2.1      Inflow and outflow characteristics and rates
The water body was monitored during the monsoon and the non-monsoon periods to study the nitrogen transformations and removal mechanisms based on variations in organic loading rate (OLR).  The first Period [1] was from December 2008 to April 2009 during cold season (average water temperature=18°C) and the influent wastewater was of low strength (21) (BOD=98 mg/l). Period 2 was from April to August 2009 during warm season (average water temperature = 28°C) using the same wastewater source as in period 1 (BOD=186 mg/ l).

The outflow channel from Bellandur lake upstream was the main source of inflow to the lake which receives sewage from the southern part of Bangalore and effluent and sludge from two treatment plants of inadequate capacity.  All illegal drains linked to the channel connecting to Varthur lake were identified and the wastewater flow rate was calculated.  The average N content of raw wastewater was 55 mg/l.  This concentration was used to represent raw wastewater throughout the southern part of the metropolitan area.  Data on flows and chemical concentrations were calculated/determined on site. The effluent N was tested for the presence of N_r forms as NH₄ and NO₃ by standard methods (22).  The N embedded in the bacterial, algal, macrophytes and in sludge escaping out of the system through the outlets was also determined and quantified.

Inflow was calculated to be 595 MLD with ~500 MLD getting into the lake from the major upstream lake Bellandur and an additional 95 MLD from illegal sewage discharges through various drains connecting to the feeder channel. The outflows were found to have a little higher flow rates due to many drainages connecting the lake. Outflow measurements during period 1 (rainy season) was increased by average rainfall (859 mm) distributed evenly over period.

2.2     Nitrogen budget and nitrogen removal

Nitrogen removal from the urban water body was calculated by the N mass balance approach (19).  The N budget of the Varthur Lake in given by

N_Inflow = N_Algae + N_Bacteria + N_Macrophytes + N_Sludge + N_unAcc + N_Outflows

where, N_Inflow  is total Nitrogen influx into the lake in t/d, N_Algae  is the N fixed in algal biomass, N_Bacteria  is the N fixed in bacteria, N_Macrophytes is N uptake by macrophytes, N_Sludge is N content of sludge/sediment, N_unAcc  is the N unaccounted and N_Outflow is the N in effluent which comprise of NH₃-N and NO₃-N.

2.2. 1       Algal sampling and analysis
Algal sampling was periodically performed with the help of plankton net (23) and algal cell density was calculated by cell counting per ml under microscope (24).  Algal N content was determined with the help of CHN analyzer.  The total nitrogen in algae was calculated based on the assumption that the fraction of nitrogen in algae is 7.2 % (25) and 60% of the total biomass is algae (26).

2.2.2       Bacterial sampling
Water samples collected from specific sites were analyzed for bacterial quantification.  Shapes of the bacteria were determined with the help of microscope.  Flow cytometer was used for size measurement, separation into different groups and calculation of cell no. and density (27).  The total nitrogen in bacteria was calculated based on the assumption that the fraction of nitrogen in bacteria is 10.7 % (25) and 40% of the total biomass is bacteria (26). 

2.2.3        Macrophytes sampling
Macrophytes specimens were sampled during the study period and their spread and extent was recorded. Quadrate method (1m X 1m) was adopted to sample macrophytes (28).  They were weighed in the field conditions and the dried in the oven and dry wt. was measured.  CHN analyzer was used for the determination of the total N content fixed in all the macrophyte species biomass.  Remote Sensing imageries were used to see the extent of growth and cover of macrophytes associated with its phenology.

2.2.4      Sediment/Sludge and water analysis

For the study the lake was divided into five zones and sludge and water were sampled from these zones during the all seasons.  The sludge collected was analyzed for the moisture content and the total N content as a function of residence time by Standard Methods.  The water samples collected were analyzed for physico-chemical parameters (APHA, 1998). The N_r forms as NH₃-N and NO₃-N were analyzed based on Standard Methods.

Corresponding Author :
	Dr. T.V. Ramachandra Energy & Wetlands Research Group, Centre for Ecological Sciences, Indian Institute of Science, Bangalore – 560 012, INDIA.		Tel : 91-80-23600985 / 22932506 / 22933099, Fax : 91-80-23601428 / 23600085 / 23600683 [CES-TVR] E-mail : cestvr@ces.iisc.ernet.in, energy@ces.iisc.ernet.in, Web : http://wgbis.ces.iisc.ernet.in/energy