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Waste to Wealth: Reusing Sewage Waste for Industrial Input

Can India's dual problems of increasing water scarcity and sewage treatment be solved by using sewage treated water as the input water for industries?

Problem 1: Freshwater scarcity - Thermal power plants in India account for 88% fresh water use in Industry
India is increasingly becoming a water scarce nation. Even though it is home to 18% of the world’s population, India has only 4% of the world’s freshwater resources. An increasing population coupled with growth in agricultural and industrial demand has increased India’s freshwater extraction, propelling the nation on a path of acute water scarcity by 2050. 
Indian industry uses about 10% of the country’s water.1 Out of that, thermal power plants account for nearly 88% of water used.2 Most of this water is non-consumptive and used as cooling water, and not directly as an input in the processing of coal. However, many Indian power plants follow open-cycle cooling rather than closed-cycle cooling process, which causes Indian thermal power plants to be 2.5-3 times more water intensive than international standards.3 Mostly water is used as cooling water – some of this is lost by evaporation and the rest is discharged into streams or local water bodies. The water quality does not deteriorate when used for cooling. However, the temperature rise poses a threat to aquatic biodiversity of the water-bodies into which they are discharged.
The water needed in thermal power plants does not have to be high-grade water. For instance, gas-based thermal power plants use over 90% of their water for cooling, and coal-based plants use more than 70% of water for ash handling and cooling towers – all of which requires water at low-end industrial use standards.4 Only a fraction of water, 5 and 30 percent respectively, is required at the high-end level which requires de-mineralization (DM). DM is not only an expensive process but also needed even if freshwater is used. 
In light of increasing water scarcity, if water can be effectively recycled and reused, it would go a long way in easing the stress on India’s water resources. 
Problem 2: Untreated Municipal Sewage Waste
Municipal waste, particularly sewage waste is an issue plaguing all of India’s burgeoning urban centers. A CPCB (Central Pollution Control Board) study shows that in 2011, more than 78% of the sewage went untreated in India’s Class 1 (population greater than 100,000) and Class 2 cities (population between 50,000 and 100,000).5 This sewage is largely dumped into drains (or nullahs) and is finally discharged into rivers or other water bodies.  This water often forms the input water for municipalities and industries downstream and the cost of untreated water gets transferred to those industries and Urban Local Bodies (ULBs).
Figure 1: Status of Sewage Treatment in Indian cities: Source: CPCB
Process for treating sewage water

Wastewater is treated in stages to progressively improve the quality of water. The most important water quality characteristics are Total Suspended Solids (TSS), Biological Oxygen Demand (BOD), Chemical Oxygen Demand and Nutrients (Nitrates and Phosphates). 

There are several technologies to treat wastewater at the municipal level. One of the most popular treatments is the Conventional Activated Sludge Process (ASP) which comprises of 3 stages - prior to this,  heavy solids like wood and grit that could choke the plant are removed. ASP is suitable for Indian conditions as it can effectively treat both diluted and concentrated wastewater as well as mixed household and industrial waste. However it is highly energy intensive.
Figure 2: Stages of Sewage Water Treatment. All 3 stages of treatment are required by ASP, SBR and MBR
Sequential Batch Reactor (SBR) is an advanced form of ASP. Due to automated controls, it consumes 35-45 percent less power than conventional ASP, has lower chemical requirements and reduces manpower costs significantly. Another variant, Membrane Bio Reactor (MBR) requires further processing and is more energy and chemical intensive, but can directly be used for low-grade industrial purposes. 
Table 1: Characteristics of Sewage Water and Treated Sewage Water; Source: IDFC 6

Forging symbiotic action to create a solution: Using Municipal Sewage Waste as Input Water in Thermal Power Plants

A way to deal with India’s sewage waste problem as well as scarcity of water for industrial use is to treat sewage water for low grade industrial use instead of using large quantities of fresh water. 
The economics of the process would depend upon the local cost of water, and the operational model which is agreed upon between the industrial unit and the municipal corporation. The cost of water extraction from the ground or from municipal sources can vary across India, from around Rs. 15-Rs. 200/ kL.7 According to Municipal Corporations of various cities, the cost of collection and treatment of waste varies from Rs. 5-15/ kL.8  There is also additional cost involved for pumping the treated or untreated sewage to the industrial unit depending upon the distance of the industrial unit from the STP. 
The issue of water scarcity is only expected to intensify in the near future, especially since water for agricultural and municipal use would continue to be accorded higher priority as compared to industrial use. Thus, industries will have to look at alternative sources of water to continue operations.
In a case where water scarcity threatens the very existence of factories, the option of using STW is a simple and not very expensive way out. It might cost more than what it would have cost to extract fresh water, but when that alternative ceases to exist, this becomes the next best choice. 
Already, some power plants have been shut down in Maharashtra due to water scarcity. In 2013, all 6 units of the Parli power plant, with a 1100 MW capacity were shut down. In comparison to that many industries are willing to spend the additional Rs.5-10/kL to ensure that operations continue.
Some of the operation models in use are explored below:
Option 1- Sewage Treatment is done by the power plant
In this model, the Urban Local Body (ULB) supplies untreated sewage to the power plant. The Sewage Treatment Plant (STP) is set up by the ULB, but the STP is operated by the power plant. 
If the STP is on the premises of the thermal power plant, it uses energy generated by the plant (which costs less than commercial electricity) to treat the sewage water to a standard that is suitable for its use.  If the STP (Sewage Treatment Plant) is not on the premises of the power plant, the cost for sewage treatment would be higher. 
The operating conditions would depend on the terms of the agreement between ULBs and the power plant. In some cases, the power plant treats the sewage to generate water only in the amount it requires. The rest of the sewage would still be discharged into water bodies.  
Option 2- Treated sewage water is supplied to the power plant
An alternative model would be one in which Sewage Treated Water (STW) is sold to the power plants. The cost of installation and operation of the STP is met by the ULB, but the cost of operation can be recovered. 

Where can it be done?

From the above 2 maps 9,10, it can be seen that some of the regions where water stress coincides with the presence of thermal power plants and heavy urban population are: Tamil Nadu, Gujarat, Delhi and surrounding regions, parts of Karnataka, Madhya Pradesh, Uttar Pradesh, and West Bengal. These are the regions in which power plants face the risk of being shut down or not given permission for expansion due to the large quantities of water they consume. It is in these areas that a model for reusing municipal sewage waste for input to thermal power plants can be an effective solution, provided that these power plants are situated close to human habitation so that adequate quantity of sewage can be treated.
Other industries which are heavy water consumers are manufacturers of Engineering goods, Pulp and Paper, Textiles, Steel, Sugar and Fertilizers. These industries located in water stressed regions, close to large urban centers, can also use STW for their operations. 
With the government’s particular focus on “Swachh Bharat”, as well as “Clean Ganga”, the issue of Municipal sewage waste is one that needs to be tackled. It makes economic sense for the municipalities to tie up with neighboring industries and supply sewage treated water to them. This ensures a double boost to the local water quality because less fresh water is extracted and untreated sewage is not disposed into local bodies. 
Image Credit: Wikipedia


Author: Sustainability Outlook