Cities are not only dense human habitations engaged in non-agricultural activity but also can be seen as being part of a vast water flow arrangement. Take the city of Bengaluru with a population of approximately 11 million people as of 2018. The draft revised master plan for the year 2031 prepared by the Bengaluru Development Authority estimates that the population of Bengaluru could be as high as 20 million and above by the year 2031. This means that the water demand for the city would be as high as 3,000 million litres per day. The city then becomes like a diffused dam, receiving and leaving out large volumes of water on a daily basis.
Currently the city gets 1,400 million litres per day from the Cauvery. It is estimated that at least an additional 400 million litres per day could be pumped from groundwater through borewells. Of the 1,800 million litres, 80 % or about 1,440 million litres could be converted to waste-water and flows through the three major valleys of the city. This waste-water is nutrient rich but full of pathogens. This waste-water will be collected in 24 Waste-Water Treatment Plants (WWTPs) which have been built or are under construction. This treated waste-water is led into natural waterways for now, though some of it is being reused by industries.
The Koramangala and Challaghatta valley waste-water treatment plants are located close to Bellandur and Varthur Lakes, now fully contaminated with untreated waste-water. This combination of treated and untreated waste-water flows from these lakes into the Dakshina Pinakini. Farmers all along the river have been using it, even pumping it far distances, for irrigation purpose.
Looking at the availability of large volumes of treated waste-water and its possibility for agriculture reuse, the project by the Minor Irrigation Department seeks to pick up 440 million litres per day secondary treated waste-water from the K & C treatment plants and pump it in stages to the drought-prone districts of Kolar and Chickballapur. Pumping will be in stages and will fill up 137 tanks in these two districts. It is expected that the groundwaters surrounding the lakes will be recharged and thus become available through wells and borewells for the farmers to use for their fields. The industrial zone near Narsapura in Kolar district will also get access to some of this groundwater for industrial purpose.
This project is one of the first in India to formally use secondary treated waste-water at such a volume to fill the tank ecosystem and provide water for agricultural use in the hinterland of the city.
It sees the obligation of a city to the farmers who grow food and vegetables for it, understands the distress that they are facing due to drought and water shortage and seeks to provide nutrient rich treated waste-water for their irrigation needs, thus ensuring livelihood security for the farmers and food security for the city.
With the tanks being full, an enhancement in the bio-diversity of the locale can be expected. The two rivers, Dakshina Pinakini and Palar, can expect minimum flows and up-to 500 tanks in the basin can get filled eventually with the treated waste-water .
The way ahead and precautions
The project will need to make sure that proper monitoring systems are in place to ensure that waste-water gets treated effectively and meets the norms set for irrigation of the crops proposed to be grown using them. Online monitoring at the sewage treatment plants and the first pumping stations would help in ensuring mitigation of health and environmental risks.
A water balance study for each lake with aquifer mapping would help in understanding the recharge zone and the benefit area as well as overflow design required during rains.
An intensive farmer education programme should be undertaken and a recommended crop palate to be grown should be developed. The recommended dose of fertilizers will change since these waters will be nutrient rich. The assistance of the University of Agricultural Sciences should be taken to ensure this.
As far as possible root vegetables and greens which can come in contact directly with the waste-water can be avoided. Crops such as mulberry which are non-edible can be considered along with horticultural crops which are less likely to face contamination issues with the application of the waste-water to be grown.
Drinking water sources near all these tanks, both public and private, will need to be monitored regularly.
A multi-disciplinary monitoring system should be in place for the project results to get maximum benefit.