How Arabian Sea water may be ...
Published by MUHAMMAD ILYAS KHAN, DGM, Manager (Purification), Director New Ventures at (BST). at SSGC, PPL & BST
How Arabian Sea water may be treated to use as Industrial/domestic purpose?
An expert Prem Baboo replied:
In view of the ever-increasing threat of marine pollution along Karachi coast and its possible danger to the coastal and marine environment. Although there have been a number of overviews on status of marine pollution in Pakistan, to quantify the level of oil pollution in the coastal environment of Karachi, Pakistan. Water and sediment samples were collected from various locations along Karachi coast bordering northern Arabian Sea. Water quality was assessed using the parameters salinity, dissolved oxygen, suspended load, organic matter (in particulate carbon) and total oil and grease. The salinity in Karachi Harbour ranged between 25.062–36.042 ppt in the Down Stream Lyari River Mouth and outside Manora Channel to open sea respectively. The dissolved oxygen values observed at all stations ranged between 0–6.01 mg/l . Complete anoxic condition with zero oxygen value was found at Lyari River Mouth the discharge point of mostly untreated industrial and domestic wastes. The very high concentrations of suspended load have been found at Korangi creek and Gizri creek Tip which are 177.6 ppm , and 100 ppm respectively. Concentrations of the suspended load in Karachi Harbour were in the range of 28.30–61.60 mg/l however, higher values have been found in the Manora Channel and its surrounding area. The level of organic matter ranged between 2.35–11.50 mg/l, with high values detected in Korangi creek and Gizri creek, which were 11.5mg/l and 7.15 mg/l respectively. The concentration of total oil and grease in seawater was detected in the range of 0.9–49.9 mg/l . The highest concentration of total oil and grease (49.9 mg/l) in seawater had been observed in Manora Channel. Similar results were also observed for total oil and grease in sediments. The highest concentration in sediment was found in Manora Channel, and Korangi creek which were 115 mg/kg, and 81 mg/kg respectively. The lowest value 0.9 mg/l was detected in water sample collected out side Manora Channel in the open sea, which clearly showed that the pollution level decreased from Karachi Harbour towards open sea, due to tidal flushing and mixing of pollutant with seawater. Karachi the largest industrial and thickly populated city of Pakistan which is situated at the north western boundary the Arabian Sea is highly vulnerable to flood damages either due to flash floods during SW monsoon or impact of sea level rise and storm surges coupled with cyclones passing through Arabian Sea along Pakistan Coast.
Comments From Mr.Anjum Alvi: An Ex Employee of Subject Plant.
To begin seawater desalination water treatment, a stable, high quality feed water is required – this optimises membrane durability. Depending on the water purification system, from here water heads either to the distillation solution or membrane solution.
What the writer is writing is true for R.O. plants. The coastal sea water along Karachi beach is even worst.
In the urban belt, even raw sewage is dumped directly into sea as no sewage treatment plant is operational for 14-15 million people. This sea water is NOT suitable for R.O. intake unless extraordinary pre-treatment is designed and installed, raising the capital and production cost. That is why we have seen and know very well that big R.O. plant is not and has never been feasibly successful for Karachi beach water when it is meant for bulk supply to the consumers. Small size R.O. units for bottling mineral water or supplying for any industrial needs may be technically or financially feasible but when comes to plants for supplying in bulk to any community of say ¼ to many million GPD, then none of the desalination plants are profitable. In Europe and Middle East and all over the world it is Government’s responsibility to subsidize the cost and supply Desalinated Water to public where natural streams are in scarcity. That is why there is no Desalinated Plants supplying bulk water to communities in private sector all over the world. In Pakistan, the production cost alone comes out around Rs.650-700 per 1000 gallon which is too much for a consumer to pay. Depending upon the quality of feed water, the capital and production cost of R.O. water may vary further.
For distillation processes, the technology used to vaporise is Multiple Effect Distillation(MED), a low temperature sea water distillation process used to evaporate seawater in numerous stages – up to 14 stages in some instances – at temperatures of 70° C or lower to produce clean distillate water.
Mr.Anjum Alvi : Replied.
That is why the Cogen Plant was considered and installed with MED unit, which is a hot distillation process. It has a series of evaporation and condensation process and hence does not require very stringent feed water quality. It operates on waste heat available from any source at 65-70 degree C. The Cogen plant gets spared steam from a combined cycle power plant. The MED plant produces distillate and also in return supplies condensate and make up water to the HRSG of power plant to continue its operation. That is the operation of both the plants are dependent to each other, if one is shut down the other will also not work. However, it is a wrong perception that Cogen Plant is shut down due to problem in Desalination /MED Plant. There has NOT been serious issues with the MED Plant. The Plant is shut down due to problems in the main prime mover (GT) of the power plant which has nothing to do with quality of sea water. The plant is still technically doable and can be rehabilitated if some investor comes forward and invest money. However, ever rising liabilities on the company due to long shutdown are getting enormous, that is making the financial model unattainable for getting allowable power tariff from NEPRA.
Make the water suitable for drinking purposes the following procedures should e carried out:
Removal of suspended matter.e
Decolourisation and oxidisation of the inorganic material and killing of all pathogenic micro-organisms by adding chlorine (pre-chlorination).
Aeration of water.
Oil removal by gravity settler Or disc oil system.
Flocculation of colloidal/organic matter with aluminium sulphate and anionic polyelectrolyte.
Sedimentation.
Filtering.
Addition of lime. .
Post chlorination
Mr.Anjum Alvi Replied:
In April, 2008 when the plant was commissioned and operating for 9 months there has been some problem in operating MED unit due to excessive silt coming in with the sea water feed. At that time, DHA was dumping thousands of tons of sand/earth every day to reclaim area for the Emaar housing project located next to Cogen. With the result the heavy silt ingress in feed was requiring excessive cleaning and/or replacement of filters. Considering the silting issue as a long term problem for this area, a sedimentation tank at the sea water intake was planned and a model testing was carried out successfully in 2009 in Germany to execute the same. That proposal is still available to resolve the silting issue, if it reappears in future.
If you let me know the intention of your interest in this field, maybe I can assist you further.
Regards.
Anjum