Seawater desalination isn't feasible.

Byline: F.H. Mughal

In an interview with Dawn published on Sept 9, the CEO of Hub Power Company (Hubco) shared his plans to provide Karachi with clean drinking water. The CEO should be careful in what he is wishing for. He might just get it.

He said that a plant of 50 million gallons per day (MGD) will be set up at Site for the treatment of industrial wastewater. Its hydraulic capacity will later be increased to 150MGD, he added. His company has many other water-related plans, including a desalination plant of 5MGD for DHA Karachi.

In the future, Hubco may consider taking over the water distribution and bill collection functions for the entire city and provide it with drinkable water from its seawater desalination plant in Hub.

Desalination technologies are of three categories: thermal, membrane-based and charge-based.

The company intends to opt for the reverse osmosis desalination, a membrane-based technology, for its DHA-based initiative.

The disposal of brine is expensive. It harms the environment and threatens the future use of seawater in desalination

In reverse osmosis, raw water is passed through a membrane that rejects substances meant to be removed. The system splits the water in two streams: the concentrate stream, which contains the substances, and the permeate stream, which passes through the membrane. The process takes place when high pressure is applied across the membrane, which means it is highly energy-intensive.

The use of high energy means higher emissions greenhouse gases, principally carbon dioxide. According to one estimate, desalination plants on average use about 15,000 kilowatt-hours of power for one million gallons of fresh water produced.

The water recovery rate in reverse osmosis plants is low. Only 25 per cent of the applied water is recovered. This means that for every four cubic meters of saline water fed into the system, only one cubic meter of fresh water is produced. The rest of the three cubic meters of water are discarded as brine waste. The low water recovery rate is a major problem.

The membrane fouling is also a major problem. Fouling occurs when microorganisms and suspended solids clog the small pores of the membrane. It is necessary to carry out extensive pre-treatment on feed water because fouling affects the performance of membranes. The pre-treatment process is costly.

Additionally, anti-fouling chemicals and antiscalant solutions end up in the discarded water, causing sea pollution. Membranes are...