Two desalination plants in Chennai, India replace reverse osmosis membranes as daily output climbs back toward full capacity

With storage in the city’s reservoirs falling sharply, the local water supply agency in Chennai, India, is optimising its available water resources to maintain a stable daily supply. A key part of that effort is maximising production from the two desalination plants at Nemmeli. Driven by this, membrane replacement at both plants is being accelerated: work at the 110-million-litres-per-day (mld) plant is due for completion by the end of October, and at the 150 mld plant by the end of November.

By the numbers: the five reservoirs supplying drinking water to the city currently hold a combined storage of nearly 37 percent of total capacity, about 3,700 million cubic feet less than was recorded on the same day last year. Against a designed system capacity of 1,712 mld, Chennai is currently being supplied with 1,272 mld of drinking water, of which the two desalination plants contribute 175–180 mld — nearly 15 percent of total demand. At the 110 mld plant, 6,543 of a total 7,056 reverse osmosis membranes have been replaced over the past two years; the plant is now producing 95 mld and is expected to reach full capacity by the end of the month. The 150 mld plant is currently producing 85 mld and is expected to reach full capacity by the end of November once its membranes are replaced.

One detail worth watching is the supply chain. The membrane elements were originally to be sourced from Europe and North America, but could not be delivered on time because of geopolitical factors, which delayed the production ramp-up. Contractors have now switched to airlifting the reverse osmosis and ultrafiltration membranes to replace the old ones. Reverse osmosis membranes are the core consumable of seawater desalination; the unit price, service life and lead time of a single element directly determine a plant’s operating cost and the pace at which it can raise output. Membranes have a finite life — typically after three to seven years of operation under high pressure, high salinity and biofouling conditions, salt rejection declines and permeate flow decays, so performance has to be maintained through chemical cleaning before the elements finally enter the replacement cycle. The cost premium of air-freighting these membranes is essentially buying time with money: with the northeast monsoon yet to bring replenishment and reservoir storage continuing to fall, restoring desalination capacity early matters more than saving on freight.

At present the two plants serve south Chennai and the southern suburbs. Once the 150 mld plant runs at full capacity, the additional water will be conveyed into the network supplying the central parts of the city. The city is also drawing water from two abandoned quarries as part of its supply.

Commentary: Chennai is a textbook case of the water-scarcity crisis along India’s southeast coast. A city of well over ten million people, it has long depended on monsoon-fed reservoir storage, and falters whenever the monsoon underperforms — in 2019, with four reservoirs nearly dry, it was forced into strict water rationing. Since then the city has accelerated the build-out of desalination, trying to turn seawater into a “baseload” source that is immune to climate variability. But the real bottleneck at a desalination plant is usually not construction capacity; it is operations and maintenance — the funding arrangements for membrane replacement, the stability of the cross-border spare-parts supply chain, and capacity scheduling that matches the wet-dry cycle of source water and the decay curve of the membrane elements. All of that demands careful management.