Desalination thrust

The world's largest sea water hybrid desalination plant to be coupled to a nuclear power station is coming up at Kalpakkam.

THE nuclear power complex at Kalpakkam, about 50 km from Chennai, will soon have a nuclear desalination plant, which will be the world's largest sea water hybrid desalination plant to be coupled to a nuclear power station. It will produce 63 lakh litres of potable water a day using a thermal method and a reverse osmosis (RO) system. While the thermal method will produce 45 lakh litres of drinking water a day, the reverse osmosis system will produce 18 lakh litres. The Rs.40-crore Nuclear Desalination Demonstration Project (NDDP) is being built by the Desalination Division, Bhabha Atomic Research Centre (BARC), Trombay.

Dr. Anil Kakodkar, Chairman, Atomic Energy Commission; Dr. B. Bhattacharjee, Director, BARC; and Dr. B.M. Misra, Head, Desalination Division, BARC, visited the desalination project at Kalpakkam on November 17 and saw the work under way.

According to Misra, the desalination project aims to demonstrate safe and economical production of good quality water by nuclear desalination of sea water; establish indigenous capability in the design, manufacture, installation and operation of such plants; generate necessary design inputs for large-scale nuclear desalination plants; and serve as a demonstration project to the International Atomic Energy Agency (IAEA), welcoming participation from interested member-states.

Misra said that desalination would become inevitable by 2025 since the demand for quality drinking water would exceed availability. "That is why the Desalination Division of the BARC has been concentrating its research on this hybrid technology, that is, both thermal/MSF, and RO desalination," he said. BARC was a pioneer in research in desalination and has been engaged in research and development activities in desalination since early 1970s.

THE thermal process is also called multi stage flash (MSF) technology. The RO is called membrane technology as well because it uses a membrane to filter sea water. A nuclear desalination plant is called so because it is erected in a nuclear power station to use sea water, steam and electrical power from the latter.

In the MSF process, evaporated sea water at above atmospheric pressure is led to a lower pressure unit, resulting in the release of vapour which is condensed to get potable water. Reverse osmosis is a membrane process where saline water or effluent water is forced through a semi-permeable membrane at pressure in excess of osmotic pressure and permeate water (passing through the membrane) of potable quality is produced. The semi-permeable membrane is made of polyamide which will reject salt and permeate water. The membrane also rejects micro-organisms.

Since the thermal method requires steam, it is advantageous to erect a desalination plant at a power generating station. Misra said: "Although most of the desalination plants are erected in a power station, they can be constructed at nuclear power stations from which we get sea water, steam and electrical power. It was more economical to site them at nuclear power stations than thermal power stations because the former produces more waste steam that can be used."

Since 1975 the BARC has set desalination plants all over the country, including one on the BARC premises at Trombay. There are four operational plants at the BARC now. While the first plant produces one lakh litres of water a day using the RO method, the second one produces four lakh litres of water a day using the MSF method. The third plant uses the low evaporation technology (LET) method to desalinate water and produces about 30,000 litres of water a day. The fourth plant uses the multiple effect distillation (MED).

According to M.S. Hanra, Coordinator, NDDP (Kalpakkam), BARC, the RO plant at the BARC converted sea water with 35,000 parts per million (ppm) of salt into drinking water with less than 500 ppm of salt. The water was treated further to match the standards prescribed by the Bureau of Indian Standards (BIS). The BARC had earlier erected desalination plants using RO that produced 5,000 litres and 40,000 litres of water a day. The capacity was gradually stepped up. "Using the same design, we are now building an RO plant at Kalpakkam that can produce 18 lakh litres of drinkable water a day," Hanra said. The BARC was doing research to reduce the energy consumption in desalination plants and get more output through membranes.

The BARC also erected desalination plants in Andhra Pradesh, Gujarat, Rajasthan, Tamil Nadu, the Andaman and Nicobar islands (Port Blair) and Lakshadweep. (All of them used the RO technology.) The aim was to demonstrate the technology in a rural setting. The first plants came up at a village about 40 km from Nellore in Andhra Pradesh, and at Maliga village, Surendra Nagar district, Gujarat. Both produced 30,000 litres of drinking water a day from brackish water. But the plants could not be sustained owing to infrastructural problems, especially because of lack of assured power supply. In Gujarat, while public acceptability of desalination plants was limited, they were well accepted in the industrial sector.

In Tamil Nadu, 12 desalination plants were operated by Bharat Heavy Electricals Limited (BHEL) in the coastal Ramanathapuram district. Thus membrane distillation using RO technology has already been established in the country as one of the most reliable processes for the production of potable water from brackish and sea water.

S.R. Jayaraman, Project Engineer (civil), NDDP and Prototype Fast Breeder Reactor, Indira Gandhi Centre for Atomic Research, Kalpakkam, said that 80 per cent of the work in the RO part of the NDDP had been completed. The RO would go on stream in March or April 2002. The RO section has huge tanks called modules. There are pressurised filter tanks, three each in two rows. The sea water first undergoes pre-treatment in these tanks. Three pressurised tank filters have three layers of pebbles of different sizes and graded sand inside. There are three other activated carbon filter tanks that also have three layers of pebbles and carbon.

First, sea water will be fed into a clarification system where, with the addition of chemicals, collided and suspended particles in the water will be removed. When this clarified water is fed into pressurised tank filters, all the suspended particles of up to 25 micron would be screened. In activated carbon filter tanks, the organics present will be removed. (Pebbles, sand and carbon remove suspended salt particles.) This water will then be fed into cartridges for filtering and will be chemically treated and fed into the membrane by high pressure pumps. The membrane, made of polymeric material, has pores big and small. When pressure is applied on sea water and sent through the membrane, quality water is produced. That is, when the pressure on the fluid becomes more than the osmotic pressure, sea water loses its salinity. The filtered water undergoes post-treatment with minerals and lime to make it pure water.

Jayaraman said the foundation work for the MSF part of the plant had been completed and the erection of modules would follow. The pump pit, which would house pumps of varying capacity, was getting ready. The pumps would pump the sea water directly from the Madras Atomic Power Station (MAPS) to the modules.

In the MSF system, the sea water is evaporated by using steam. The sea water supply will be met from MAPS. The NDDP will receive sea water from the MAPS' process cooling water outfall. The cooling water is clean as it is filtered through trash track and travelling water screens and it has less biofouling potential. The MSF system will make use of the low pressure steam obtained from the turbines of the MAPS. (The fissioning of the uranium in PHWRs produces intense heat. Sea water is used as the secondary coolant and it cools the heavy water, the primary coolant. The resultant steam drives the turbine to generate power.) The blending of the product water from the RO and MSF plants will produce drinking water.

According to Misra, the MSF technology could be made totally indigenous, commercially attractive and reliable by using innovative design and engineering practices coupled with a robust control system. He added that the NDDP at Kalpakkam would give the BARC the technological confidence to construct nuclear desalination plants that would produce two to five crore litres of pure water a day.