Page 93 - ro membanes

76 4. SALINE WATER INTAKES AND PRETREATMENT
The wave action near the ocean floor is the force that allows the solids separated from the source water by natural filtration to be dissipated in the ocean. If the bay area is not well flushed and the naturally occurring wave movement is inadequate to transport the solids away from the intake collection area at a rate higher than the rate of solids deposition and retention by natural filtration, then these solids would begin to accumulate on the ocean floor and would ultimately enter the filtration bed, reduce the well capacity, and deteriorate the source water quality over time. Since filtration process through the ocean bed is very slow, it usually takes 6e12 months until deterioration of well performance and capacity are observed. Therefore, if such conditions of poor flushing of the intake area occur, loss of well capacity over time is very likely and is difficult to predict.
Another important factor to consider when assessing the ability of subsurface intakes to pretreat the source water is the impact of heavy storms on the depth of the filtration layer through which the water passes before it enters the intake wells. For such natural filtration layer to perform comparably to dual media granular filters or membrane pretreatment filters, the depth of the natural filtration layer should be 10e30 m (67e98 ft). For example, if large storm or beach erosion due to tidal movement reduces this depth (distance) between the sur- face of the ocean bottom and the filter collectors to less than 10 m (33 ft), the source-water quality produced by the subsurface intake will deteriorate drastically. For comparison, man-made pretreatment systems for source water collected by open intakes are designed to handle the impact of storms and beach erosion on the source water quality, and under the same circumstances, their pretreatment performance would not be affected significantly.
Subsurface intakes whose area of influence (e.g., source water collection area) extends to nearby fresh/brackish groundwater aquifers may have a negative impact on the capacity and water quality of these aquifers, and in some cases their operation may result in enhanced seawater intrusion. In addition, if the coastal aquifer used for source water supply is hydrau- lically connected to nearby saltwater marches (coastal wetlands), the collection of large amounts of water may result in drainage or significant reduction in the water level in the marshes, which in turn may have a detrimental impact on the marsh ecosystem/drain the coastal wetlands. Such negative environmental impact of subsurface intakes is more pro- nounced in large desalination projects where the radius of influence of the subsurface intake on coastal aquifers could extend to 5 km (3 miles) or more inland.
If a nearby fresh/brackish groundwater aquifer hydraulically connected to the coastal intake aquifer is contaminated with pollutants, which are typically not present in the ambient open-ocean water in a measurable quantity (such as fuel oil contaminants, endocrine disrup- tors, heavy metals, arsenic, septic tank or landfill leachate, leachate of embalming solutions from cemeteries, etc.), then the RO plant may need to be provided with additional pretreat- ment and/or disposal facilities, which would erode the benefits of subsurface intake use.
The numerous factors that could impact subsurface intake-water quality described above point out to the fact that assessment of the potential sources of contamination of the source water in the zone of influence of the subsurface intake is a critical and integral part of the feasibility evaluation of using subsurface intakes and the complexity of source water pretreat- ment for the site-specific conditions of a given desalination project. The outcome of such eval- uation would allow determining whether the natural source water pretreatment provided by the subsurface intake would be worse, comparable, or better than the man-made pretreat- ment system threating ambient seawater collected by an open intake.