Monday, November 06, 2006 Speak Out: A second look at desalination By Engineer Sokearno Tecson Ch.E. Class 1975 University of San Carlos
THE scarcity on the supply for potable water has been a continuing issue of serious concern. As Metro Cebu continues to be one of the fastest growing cities in the country, the groundwater source, its only source for potable water, is also fast becoming a dying treasure.
The recent move of MCWD to develop nine more exploratory water wells to support the current water supply, with or without the Carmen Bulk Water Supply, could not be considered a strategic investment. While the need to meet the current water demand is apparent, increasing pumpage is not an environmentally compatible solution. There is alternative water source that can be found in the desalination of brackish water or seawater by reverse osmosis (RO) but this has always been regarded as a costly option.
At first glance, seawater desalination ranks as the costliest source when judged against other available sources. However, the comparison is deceiving because many costs are not accurately computed into the estimates for groundwater source. Environmental impacts are not factored in, along with the need for rotational capacity to back up these groundwater systems during periods of drought and emergencies such as contamination of the source site with chlorides from seawater intrusion and nitrate from cesspit seepage or from agricultural operations. There are also major regulatory, legal and political hurdles on groundwater supply across political boundaries, and transferring water supply from adjacent towns is very contentious and promotes ill will between neighbors.
The rationale for suggesting seawater desalination is obvious when one observes the location of high growth rate in coastal communities of Metro Cebu, which have easy access to seawater. Significant advantage that drives seawater desalination is the proximity of the point of use from the point of withdrawals. This will have an edge in SRR (Systems Recovery Rate) value against the 60 percent or less MCWD usually achieves with the present supply sources situated farther inland. In addition, seawater source has a limitless potential for supply without causing permanent environmental damage in the host area of withdrawals and the apparent spark of infamous water war.
RO technology has already matured and is now used extensively in other countries, especially those who have their share of environmental overloads. Getting an additional 1,000 cubic meter per day of potable water through the seawater desalination RO system instead of from the proposed nine more water wells is believed to have long term benefits. RO system design is modular and subsequent increases in capacity can be done in less than a year. Also, the bulk components of an RO system can easily be fabricated locally. On energy usage which is contributing 55-60 percent of the operating costs, innovated energy recovery schemes are now readily available that can reduce power cost by 30-40 percent through the recovery and re-utilization of residual pressure of the reject stream prior to discharge.
Seawater RO system has a limited recovery rate of 35 percent. This means, it can produce 35 percent potable quality water and rejects 65 percent of the seawater to be fed as it becomes 1.54 times saltier. Acceptable method in treating seawater desalination system reject stream is the pre-discharge dilution method. RO reject stream is first neutralized then diluted with fresh seawater prior to discharge in dilution ratios of two parts ambient seawater to one part RO reject stream. Final salinity of the outfall in 2:1 dilution ratio will only be 10 percent higher than the normal seawater salinity level, which is permissible.
Estimated production cost for desalinated seawater is about P45-P50 per cubic meter. It is, of course expected: MCWD will give the realistic “blended water costs” to the consuming public. A 20 percent blend of desalinated water to the P42 MCWD is charging for commercial rate, the blended water costs will be P43.60. The P1.60 per cubic meter increase in consumers’ water bill may not be noticed if the standard of sufficiency in potable water supply is attained.
All things considered, seawater desalination by RO process is a technically and economically viable alternative to supply the requirement for potable water and must be considered a serious option. It is now the right time to take advantage of the continually developing membrane technology, which has been regarded as the water treatment technology of choice in a wide variety of situations around the world.
