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Desalination: Answer to Chennai's water woes

M. S. Srinivasan

A reliable and low-cost desalination-based water generation and distribution system has the potential to overcome the shortage, provided there is an agreement on the payment of usage charges.

CHENNAI city is solely dependent on ground water resources to meet the water needs of the citizens. The ground water availability depends on rains and as every resident has experienced, the water availability situation is deteriorating steadily. Though successive administrations have tried several alternatives to alleviate the problem, none has turned out to be satisfactory.

An alternative, that has potential to overcome the scarcity in a reliable and low cost manner, has been outlined. A robust water generation and distribution system can be set up to supply water at reasonable cost provided there is an agreement on the payment of usage charges.

How much water does the city need? A literature survey shows that in Florida, the US, where the climatic conditions are similar to Chennai and where a high degree of water conservation efforts exist, the average per capita consumption is around 600 litres/day.

Taking a very conservative figure of 100 litres/day/person for Chennai residents, with a population of nearly 4.2 million, Chennai city requires 420 million litres per day (MLD). This represents just the need for individual consumption and does not include community needs such as parks, zoos; commercial establishment needs such as hotels; industrial and agricultural needs.

In the absence of any more details, assuming that these categories of water use may account for similar order of magnitude consumption, the total water consumption totals about 840 MLD.

How much water does the city have? The average rainfall for Chennai is 1,276 mm, giving the quantity of rainfall over the city area of 174 sq. km. approximately 2,22,000 million litres. Since 100 per cent of rainwater does not go into recharging the ground water and at more realistic 25 per cent collection efficiency, the gap between the availability and consumption is huge, approximately 690 MLD.

Without in anyway underestimating the utility of attempts at rain water harvesting, it is futile to depend on rain and ground water alone for meeting the city's water needs.

The alternatives: There are several alternatives such as:

Interconnecting of rivers: Though this is theoretically feasible, political realities are likely to pose time delays. The experience with Cauvery water and Krishna water is a testimony to the kind of issues involved.

Pumping water from far away places to the city: This is the alternative that the Metro Water Board has been attempting over the last few years. As the water sources have moved further away, the initial investment and pumping costs have escalated. Also, there are serious political issues. People at the source of water are raising concerns about depletion of water resources.

Setting up desalination plants in the outskirts of the city: In several parts of the world, such desalination projects have reliably and, in a cost-effective manner, solved the availability of water for large urban population.

Desalination of seawater: Broadly, there are two types of processes:

a) Thermal processes where heat energy is used;

b) Membrane processes where electrical energy is used.

Both are reliable and proven methods with several hundred such units in operation for decades in operation.

Is it expensive? A comparison is made between two alternatives:

a) Pumping water from a distant ground water source;

b) A desalination plant that may be set up near Chennai.

The cost data for the first alternative is taken from the published information on the recently-approved Veeranam project to bring 180 MLD of water to Chennai.

The cost data for the second alternative is based on a recently-contracted seawater desalination project to supply water to the city of Ashkelon in the Mediterranean Sea in Israel. This project, which is the world's largest seawater desalination plant, with a capacity of 330 MLD, is being built on a BOOT basis in cooperation with the Government of Israel.

The cost estimates have been adjusted to reflect Chennai site conditions based on the author's experience with similar projects.

  • For the Veeranam project, the only variable cost considered is cost of pumping of water @ 0.5 kwh/m3 and @ Rs 3.5 per unit. Other operational costs are not known.

  • For the SWRO-based desalination plant, the power consumed within the plant is six units/m3 and 0.05 units/m3 for pumping to a point 30 km away.

  • For desalination project, the variable cost includes cost of a 50 MW captive power plant @ Rs 2.75 per unit variable cost (liquid fuel based) and the cost of chemicals used.

  • The capital cost of desalination project includes a captive power plant based on liquid fuels. This plant supplies power requirements for the desalination plant.

  • Capital-related charges are taken at 7 per cent for depreciation and 12 per cent for cost of capital and 5 per cent other costs. The percentages refer to installed capital cost in each case.

  • What do the numbers convey? On the face of it, it would appear that the Veeranam project supplies water at a lower cost. This conclusion however is erroneous. The moot point is: If there is likely to be 657-lakh m3 of water available at Veeranam every year. Taking annual rainfall at 1,276 mm and 25 per cent catchment efficiency, a catchment area of nearly 200 sq. km is needed. One does not know if such is the case.

    On the other hand, there is no doubt whatsoever on the reliability of the desalination plant.

    The Ashkelon project is so financially structured that it guarantees water at Rs 25 per thousand litres. The capital-related charges are probably lower for this project as it enjoys the high credit rating of the Israeli Government.

    What are pre-requisites to make such a project feasible? This can be answered by addressing two issues, namely:

    Will the user pay for water at Rs 35-40 per thousand litres?

    The monthly consumption of a typical family is about 12,000 litres. Thus, the outgo at the above prices will be nearly Rs 400. There are now many families that spend almost an equal amount in buying water only for drinking purposes in tankers or in smaller packs delivered to their homes.

    Though they pay such a price, there are uncertainties with respect to the quality and availability. A desalination plant, using the infrastructure of piping already built up, would overcome these two problems. It is well-documented that a large percentage of the health problems are directly linked to the drinking water quality. Therefore, there is likely to be a saving for the user if one accounts for the avoidance of water-borne hazards.

    Typical households spend about Rs 120 on newspapers and about Rs 150 on cable TV per month. An expenditure of Rs 400 for meeting a critical need for good quality water is acceptable. Who will come forward to invest about Rs 400 crore and operate the desalination plant? Once there is clarity and agreement on the issue of pricing and collection mechanism, many investors would come forward to put their money. After all, water is a commodity for which there is no demand recession. The technicalities of project financing, selections of most competitive technology/vendor are important but not critical. There is adequate expertise available in these areas within India and globally.

    What could be done? It is clear that urgent action is needed to set up a large desalination plant in the outskirts of the city.

    The State government would do well to initiate a discussion forum in which the contours of the public policy issues concerning such a project could be debated and a consensus arrived at.

    (The author, a chemical engineer experienced in desalination plants, can be reached at siddhu@giasmd01.vsnl.net.in)

    Article E-Mail :: Comment :: Syndication

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