CASE STUDY

Cooling water in Indian Thermal Power Plants (TPPs)

According to CPCB's report on Water Quality in India 1990-2001 status and trends of the total wastewater discharged from all major industrial sectors, 80.3 per cent is cooling water generated just from TPPs. Therefore, closing the cooling water cycle should be the priority of Indian industry and the regulators alike.


Two cooling technologies are in use today:
Once-through cooling system: This system requires the intake of a continual flow of cooling water. The water demand for the once-through system is 30 to 50 times that of a closed cycle system. Most Indian TPPs operate this system.
Closed-cycle systems: This system discharges heat through evaporation in cooling towers and recycles water within the power plant. The water required to do this is comparatively small since it is limited to the amount lost through the evaporative process. Because of the expense associated with closed-cycle cooling, once-through systems are far more common. Some recently commissioned Indian TPPs employ this technology.

In once-through cooling system approximately 100 litres of water is required to produce 1 Kwh electricity. In badly managed TPPs this could go up to 200 litres. In comparison in a closed-cycle system, about 2-3 litres water is required to generate 1 Kwh electricity.

By converting all Indian TPPs to closed-cycle cooling system, by rough estimation almost 65,000 mld or 24 billion m3 fresh water can be saved. This is roughly equivalent to India's total domestic water requirement.

In closed-cycle cooling towers water is lost due to evaporation, windage and drift and intentional blow down. These losses are about 1.5-2 per cent of the recirculation rate. Currently fresh water is used as makeup in Indian industry. But with proper treatment of process wastewater and effective chemical treatment to control corrosion and fouling, wastewater can be easily reused in the cooling towers, reducing the freshwater intake for cooling to zero.

In general, combined primary (sedimentation) and secondary (biological oxidation, disinfection) treatment of wastewater is sufficient to make it suitable for cooling towers. Currently most large and medium scale Indian companies are required to treat their wastewater till secondary treatment to meet the pollution norms. Therefore, in these companies no additional investment is required for treating the wastewater and reusing it in cooling towers.

A wide range of chemicals are available today which can reduce the danger of corrosion and scaling in the cooling tower equipment thereby enabling the use of treated effluent as cooling water. Many companies outside India are using treated effluent as cooling water quite successfully. In places where fresh water is quite costly, use of treated effluent as cooling water presents substantial financial gain for the companies.

WATER RECYCLING AND REUSEWastewater treatment and recycling at industrial level with RO
Agency/project Technology used Capital cost (Rs crore) Operation and maintenance costs (Rs/kl)
Madras Fertilizers Limited, Chennai Size: 15.12 MLD 12.24 MLD is recycled to the cooling towers
and from this plant 3.0 MLD fresh water is
being supplied to Chennai City
Reverse 14.5 40-50
Chennai Petroleum Corporation Limited (earlier known as Madras Refineries Limited) Size: 11.25 MLD
Treated water used for cooling towers
The quality of treated water is BOD-2 mg/l;
TDS- 30 mg/l; COD- 5.0 mg/l
Reverse Osmosis 20 43
GMR Power corporation, Chennai Size: 7. 2 MLD Treated water used for cooling towers Secondary and tertiary treatment followed by Reverse Osmosis 17.5 25
Source: i Showing the world the way, EverythingAboutWater, May-June, 2001
ii Conserving Water at MRL, EverythingAboutWater, May-June, 2001
iii Generating Clean Power, EverythingAboutWater, May-June, 200
Membrane Technologies
A semipermeable membrane is a thin layer of material separating substances when a driving force is applied across it. Once considered a viable technology for desalination, membrane processes are increasingly employed for removal of bacteria and other microorganisms, particulate material, organic and inorganic chemicals and colour and other contaminants. As advances are made in membrane production and module design, capital and operating costs continue to decline. The pressure-driven membrane processes are essentially of four different kinds: micro filtration, ultra filtration, nano filtration and reverse osmosis.

Reverse osmosis (RO): RO use for wastewater recycling and reuse process has become quite common. O systems are compact, simple to operate, and require minimal labor, making them suitable for all systems. RO can effectively remove nearly all inorganic contaminants, nearly all contaminant ions and most dissolved non-ions from water. RO is particularly effective when used in series. Water passing through multiple units can achieve near-zero effluent contaminant concentrations.

The pre-treatment section, where the feed is treated by chemical clarification (precipitation, coagulation/flocculation or flotation) and subsequent filtration, or by filtration and subsequent ultra filtration

The membrane section, where high pressure is applied and the waste water is cross-flowed across the membrane

The post-treatment section, where the permeate is prepared for reuse or discharge, and the concentrate brine is collected for further work-up or for disposal

The capital and operating and maintenance cost of RO systems are become quite competitive with the increasing cost of buying water in water-scarce areas. For instance, the cost of treating municipal sewage water by RO in Chennai is in the range of Rs 25-50 per m3 (See: Wastewater treatment...). This is similar (in cases even lower) to the cost of fresh water charged by the Madras Water Supply & Sewage Board.