In Mumbai alone the demand for potable water is as high as four billion litres per day while the city can only supply 3.5 billion litres per day. The solution, we posit, lies in recycling the wastewater generated from the city’s domestic sewage, which has not yet been considered by municipal authorities. It should be noted however that the city is currently not in a position to recycle sewage water because of space constraints. Sewer water, after due treatment, is simply disposed into the sea.
The potential uses for recycled water include the following:
- Construction Sites
- Flushing
- Washing
- Cooling Towers
- Industrial Applications
Cooling Tower Applications
Most facilities nowadays treat wastewater through direct aeration – a process during which soap is dissolved into the water leading to high TDS, total hardness and alkalinity. This further leads to scaling in the cooling tower and higher energy consumption as the Chiller temperature has to be maintained in order to attain the required temperature to cool the facility. The norm is to then follow this aeration with biocide treatment – a process by which chlorine is converted to chloramines and forms weak hydrochloric and hypochlorous acids, leading to different forms of corrosion within the cooling tower.
To counter these effects, we suggest the use of media such as microbes and algae to break down the wastewater, followed by a biocidal treatment with chlorine dioxide. Such media help reduce TDS, total hardness and alkalinity in the water, subsequently reducing scaling and corrosion in the cooling tower.
At a facility which was treating water only by the aeration method, a high presence of bacteria in the water was noticed. This would eventually damage their cooling tower. The facility engineers initially did not take the use of the eco-friendly algae-microbe-biocide media seriously, eventually leading to the collapse of their cooling tower. A new cooling tower had to be installed, incurring Capex and forcing facility engineers to consider our recommendation.
Trials were carried out on the facility’s raw sewage water with a combination of Algae and Microbes. The findings were as follows:
Although there are no standards for recycling wastewater in India, the recommendations in the USEPA guidelines state that the BOD should be 10 mg/litre. Although the reading was 23mg/litre, the 10mg/litre could have been easily achieved, if wastewater had been aerated just a little more than needed to attain the target BOD.
This method is also highly energy-efficient and cost-reductive as it is a self-sustaining cycle – the algae provides the microbes with oxygen in order for them to grow and break down the waste, and the microbes in turn release carbon dioxide required for the growth of algae. The facility was previously using 30kWh as energy needs on the aeration; therefore by estimation the savings would be around 300kW per day, and 9MW per month, translating into savings of `54,000/- per month.
Sewage Treatment Plants on larger scales such as for communities, gated townships or cities offer many additional advantages in the form of multiple revenue streams. For example, Carbon Credits can be easily earned as the Algae / Microbe treatment is a carbon negative process. Besides this, the harvesting of the Algae and its oil can be used for the production of Biodiesel, forming another revenue stream. Lastly, the algae cake can be used as feedstock as biomass, feed for dairy, poultry, aqua or as a fertilizer for agriculture.
Several other trials are being carried out at the moment on the treatment of chemical effluents, soap water, combination of sewage, soap wastewater and many others.