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DHA Cogen Power & Desalination Project

An Overview of Fine Filtration System

By M. Samiuddin Mirza, Senior Project Manager DHA Cogen Ltd.

The water and electricity shortage have been some major issues in our city and to overcome such insufficiencies a high quality desalinated water and power technology has been set up in collaboration with DHA Cogen Ltd. and Siemens to provide quality potable water to DHA residents. An update on this Hi-tech plant is described below:

To ensure the provision of safe, reliable, efficient and affordable potable water and electric power for the comfort of the prestigious residents of Defence Officers Housing Authority, Karachi is in unison with the broad economic and social policy objectives of the Government of Pakistan.

It is time again to give our dear readers an update on the progress of Pakistan's first sea water Desalination Project. The major site activities related to civil engineering discipline and works of the foundations to place the prime equipment of water production is reaching completion stage.

Next stage is the mechanical erection and preliminary activities are in full swing. Heat Recovery Steam Generator mechanical erection is in progress under the supervision of foreign OEM expert from Japan. The progress of work from off shore piling for seawater intake structure to on shore works is monitored continuously on a daily basis. It must be appreciated that what is to follow is a concise overview of some of the major activities that are taking place at project location.

Here DCL shares the technical knowhow with its readers expressing the importance of the feed (sea) water quality to be maintained and continuously supplied to the desalination plants required to produce WHO recommended quality potable water for the residents.

Quality Assurance
Feed water (sea water) quality is of prime importance in selecting the design parameters for a Desalination Plant. The feed water quality varies from location to location in the sea. Various samples of sea water are collected for complete chemical analysis prior to establishing the best possible location of sea water intake to desalination plant. The feed water quality is analyzed for presence of various salts (TDS) chemical elements, total suspended solids (TSS), microbiological organisms and traces of metals.

The analysis decisive factor for the type and design of feed water pre. treatment for desalination units to avoid scaling/fouling of desalination plants and equipment. This is very important from operational point of view to minimize the down time of the plant. The sea water samples taken for DHA Cogen Plant showed high TSS values i.e. up to 60 ppm (parts per million) which requires special pre-filtration system titled as "Fine Filtration system" mesh size 30 microns, to supply quality sea water as feed water to desalination plants for processing.

Fine Filtration System
Fine Filtration of sea water is achieved by carrying out micro filtration, which is suitable for the filtration of pollutant (suspended solids) mechanically through micro-screen drum type filters especially manufactured by a reputable foreign company. The fine filtration unit consists of a rotation drum sieve through which the sea water feed passes. The micro sieves stop and control the presence of suspended solids by filtration to micron level.

The feed water from the intake system passes through the rotating drum leaving the intercept solids on the inner surface of the screen sieve basket. The screen baskets are made up of fabric of high tension resistant polyester fiber with high durability measuring mesh size from 5 micron to 200 micron. The micron giant filters are also provided with back wash system to clean the collected suspended solids by spraying water through pump and jet pipe mounted on the top of screen baskets. The debris are collected by scrappers and flushed into lateral water collecting channel. The filtered water is discharged over an effluent over flow weir to a pump pit for further pumping it to the plant. Penstocks and stand by arrangements are also provided to avoid shut downs during maintenance of the equipment.

DHA Cogen Ltd., has made all necessary arrangements for timely procurement of the F.F. plant, designed and manufactured by M/s Passavant of Germany with performance guarantee on expected sea water inlet worst condition of TSS 100 ppm, grain size 500 micron to achieve the filtered water outlet condition of 28 micron TSS less than 30 ppm acceptable conditions. This F.F. plant is in addition to the seawater intake system.

DCL will continue to remain in touch with dear readers of "DHA NEWS” updating them with Process, Progress, & Project (PPP).