Mombasa County gearing up to purify sea water

Tampa Bay Seawater Desalination Plant

Tampa Bay Seawater Desalination Plant

The county of Mombasa has stepped up its efforts to purify ocean water after it signed a deal with a Japanese company for the process to begin.
Wednesday’s memorandum of understanding with Toyota allows the firm to carry out a feasibility study to determine whether desalination of sea water is viable.

The agreement comes at a time when the county is at the centre of a row with other regions over the supply of piped water to its residents.

Governors John Mruttu (Taita Taveta), Amason Kingi (Kilifi) and Salim Mvurya of Kwale have demanded part of the revenue generated from water supplied from their counties.

Mombasa has no fresh water sources and depends on water supply from Mzima Springs in Taita Taveta, Baricho Water Works in Kilifi, Tiwi boreholes and Marere Water supply in Kwale County.

The three water sources supply to Mombasa residents between 50 and 60 million litres of water a day.


But on Wednesday, Mombasa governor Ali Hassan Joho signed an MoU with Toyota in a bid to find a solution to the water crisis in the county.

“My county is determined to introduce the desalination technology to produce clean water for domestic use, industries and other purposes,” he said.

Toyota Kenya chairman Dennis Awori said the study will take six months, adding, the company’s recent research had shown that desalination of sea water in Mombasa was viable.

He added: “Toyota Company has diversified to the desalination of sea water, and that is why we are set to carry out a study in Mombasa on the matter.”

Toyota will look into the possibility of financing the building of desalination plants in Mombasa.

The county is expected to provide land for putting up the plants while Toyota will fund the project.



Tampa Bay Seawater Desalination Plant

How the Plant Works

The Tampa Bay Seawater Desalination Plant uses a process called reverse osmosis (RO) to produce drinking water from seawater.

The desalination plant is located next to Tampa Electric’s (TECO) Big Bend Power Station, which already withdraws and discharges up to 1.4 billion gallons a day of seawater from Tampa Bay, using it as cooling water for the power plant. The Tampa Bay Seawater Desalination plant “catches” up to 44 million gallons per day (mgd) of that warm seawater, separates it into drinking water and concentrated seawater and dilutes the twice-as-salty seawater before returning it to the bay.


Before the RO process, seawater entering the desalination plant flows through screens that remove debris, then goes through a traditional treatment process called coagulation and flocculation. In this process, chemicals are added to the seawater to make algae, organic materials and particles clump together so they can be removed more easily in the sand filtration stage.

After sand filtration, the salty water goes through diatomaceous earth filters to remove silt and fine particles. Cartridge filters just before the RO membranes serve as a backstop, removing any particles that may be remaining after the diatomaceous earth filters.

Reverse Osmosis

Next is the RO process. High pressure forces the pretreated water through semi-permeable membranes to separate the freshwater, leaving twice-as-salty seawater and other minerals behind.

The size of each RO membrane pore is about .001 microns, which is about 1/100,000th the diameter of a human hair.

Post-treatment, Blending and Delivery

After the RO process, chemicals are added to stabilize the desalinated seawater. The high-quality water is delivered to Tampa Bay Water’s regional facilities site where it is blended with treated drinking water from other supply sources before being delivered to Tampa Bay Water’s members.

Concentrate Return

At full capacity, the RO process leaves about 19 mgd of twice-as-salty seawater behind which is returned to Big Bend’s cooling water stream and blended with up to 1.4 billion gallons of cooling water, achieving a blending ratio of up to 70-to-1. At this point before entering and mixing with any bay water, the salinity is already only 1.0 to 1.5 percent higher, on average, than water from Tampa Bay. This slight increase falls within Tampa Bay’s normal, seasonal fluctuations in salinity.

The cooling water mixture moves through a discharge canal, blending with more seawater, diluting the discharge even further. By the time the discharged water reaches Tampa Bay, its salinity is nearly the same as the Bay’s. And, the large volume of water that naturally flows in and out of Tampa Bay near Big Bend provides more dilution, preventing any long-term build-up of salinity in the bay.

Tampa Bay Water’s comprehensive hydrobiological monitoring program collects thousands of samples including continuous salinity measurements every 15 minutes near the desalination facility. This and other water quality monitoring since 2003 shows no measurable salinity changes in Tampa Bay related to plant production




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