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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Results and Discussions

1. Inflow
The wastewater nitrogen budget is summarized in Fig. 2.  Of the total >500 MLD of water flowing from the Bellandur lake in the upstream region of the Varthur an additional ~95 MLD in contributed by the multiple drains from the vicinity into the feeder channel. The average inflow total N concentration was found to be 55 mg/l which gives a value of 11.94 Mg/yr of N entering the system at the rate of 32.72 t/d.  Out of these the inflow concentrations has an average NH₄-N concentration of 33 mg/l which is about 60% of the total N.  This shows that the predominant N_r form is NH₄-N at the inflows.

Figure 2 . N budget in Varthur lake

2. Water and sludge analysis

Water samples collected from five different zones of the lake showed a marked gradient in N_r forms especially NH₄-N.  The NH₄-N values ranged from 33 mg/l near the inflow to 15 mg/l near the outfalls.  The nitrate values ranged from 0.4 mg/l to 0.67 mg/l.  The nitrate values didn’t vary significantly between inflows to the outflows (<1 mg/l) as shown in Table 1.  Now considering <1 mg/l as the nitrate concentration at the outlets and taking the mean depth of the lake as 1.1 m; surface area of 220 ha and the outflow rate as ~600 MLD, the annual N_r in form of nitrates is 817.6 t NO₃ N/yr.  Therefore at the outfalls N_r in the form of nitrates escapes from the system is 0.6 t/d.  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 average outflow was higher by 0.2 % compared to average inflow during non-monsoon period. The N_r in the form of NH₄-N in water at outfalls was computed to be 8.85 t/d taking mean NH₄-N concentration of 15.05 mg/l.

Table 1.   Distribution of Nr forms in various zones of the lakes (Water and Sludge)

3. Algal count and N content

On the basis of periodic physico-chemical analysis and microalgae the lake could be divided into three zones – anaerobic; algal driven facultative and macrophyte (with algae) dominated aerobic zones depending upon functions. The microalgal analysis showed predominance of Chlorella (1-10 μm size).  The algal counts in the 3 zones mentioned earlier are inlet ~7,200 cells/ml; middle ~ 2, 00,000 cells/ml, outlets ~ 1, 24,500 cells/ml.  The N content in algae ranged from 4.7-7.4 g/100 g of dry wt. biomass. Therefore for a retention period of 5 days – 4 g of biomass was formed. Taking the euphotic zone of the lake as 20 cm (measured), the total algal N at the outlets was calculated to be 1.172 t Algal N/d. During the rainy and winter months the algae rapidly take up N_r in the form of ammonia for their growth which brings down their level in the lake water substantially.  This is facilitated because during, monsoon (rainy) and winter periods macrophytes are pushed out of the lake by strong winds and algae dominate the microflora.

4. Bacterial count and N content

The bacterial analysis showed higher abundance of bacilli near the inlets while higher abundance of spirillum was found near the outfalls. With the flowcytometer analysis, the bacterial counts were detected to be of the order of 10⁸ cells/ml. The total N escaping from the system as bacterial N was calculated to be 4.7 t/d.  The bacterial cell volumes measured in this study varied by two orders of magnitude (0.02-1.8 μm³).  Taking a mean bacterial volume as 1 μm³ which is equivalent to 0.85 X 10-13 gN (29).  The N fixed in bacteria escaping from outlets is found to be 4.7 t/d.

5. Macrophyte N uptake and accumulation

The macrophytes were regularly monitored for the growth and extent of coverage on the lake surface, water hyacinth (Eicchornia) and Alternanthera were the dominant floating macrophytes present.  The peak growth of macrophytes mostly comprising the water hyacinth was from Jan to April (4 months) which then started to deteriorate by pest infestation.  During growth, water hyacinth can store N up to 350 g/m².  In the peak growth period the macrophytes covers almost 75% of the surface area and becomes increasingly compacted by the effect of wind and rain in the following season. Therefore the total N in standing population of the macrophytes was calculated to be 577t/yr.  Hence the daily N accumulation in macrophytes works out to 1.58t/d.  Considering 10 % of the entire macrophyte biomass as standing crop annually, the macrophyte N content fixed in the system is 0.15t/d.  Eight macrophyte species were found during the study period and amongst them Lemna had a high N content (4.09%) followed by Alternanthera (3.87%) followed by Eichhornia (2.51%) compared to others.  During the summer the macrophytes forms the bulk of N uptake from water.

6. Outflow characteristics

The outflow comprises of the N_r as NH₄-N and NO₃-N escaping from the system, which is calculated to be 8.95t/d and 0.6t/d respectively.  In addition to this the outfall also helps in releasing a substantial amount of sludge in the windy and rainy period July-Oct.

Aided by the turbulence created by westerly monsoon winds and subsequent upwelling and rise of organic sludges, these floating small islands of sludge are pushed over the outfall and consequently need to be accounted as N release from the lake system.  Based on physical and visual measurements the N lost through this route is estimated to be 1.4t/d.  Thus with the addition of this component total N passing the outfall and that unaccounted work out to be as follows.

N_Inflow           =       N_Algae + N_Bacteria + N_Macrophytes + N_Sludge + N_unAcc + N_Outflows (NH₄-N and NO₃-N)

32.72 t/d        =      1.172 t/d (N_Algae) + 4.7 t/d (N_Bacteria) + 0.15 t/d (N_Macrophytes) + 2.97 t/d (N_Sludge)
(595MLD)                 + 8.95 t/d (NH₃-N) + 0.6 t/d (NO₃-N) + N_unAcc

 N_unAcc           =       14.18 t/d.

7. N unaccounted (Possible reasons for N loss)

A substantial portion of the N, 14.18t/d remained unaccounted in our assay. The various ways in which N may escape unaccounted from the lake are in the form of ammonia volatilization and denitrification.  It was observed that the major source of nitrogen loss apart from bacterial uptake was through sedimentation of Org-N, which accounted for 9.07% N removal.  This is in agreement with other findings (13).  In the present study 2.97t/d was removed through this process out of the total N load of 32.72t/d.  The settled org-N may be non-living organic matter or living cells of algae and bacteria.  The uptake of nitrogen by living microorganism’s cells was 5.872t/d.  

Denitrification process would have essentially contributed to removal of substantial amount of N_r since their rates are supposed to be high because of overloading.  The NH₃-N removal through volatilization would have been crucial and can be a significant component in unaccounted N.  NH₃-N concentration decreased from 33mg/l in the influent to 15mg/l in the effluent, the dissolved ammonia available can also be discharged to the atmosphere because of high pH.  The mean pH in the effluent at the outflow of the lake was 7.68.  The major transformation could be mineralization followed by ammonia uptake (13).  In this study it was observed that the most prominent form of N_r is ammonia and it is also the main N source of algae and macrophytes.  When the rate of cell uptake is more than the rate of mineralization then N starvation occurs which later leads to the succession of algae and macrophyte communities. In the current study the degree of mineralization of organic N (not studied) can bring to light the availability of N sources to the lake biota.  Throughout the lake the nitrification was found to be insignificant (~2%). About 45 % of N was unaccounted. The losses through seepages and infiltration can be further investigated for a systematic N balance.

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