Every seawater RO application is unique and very site specific. What may be the optimum solution for one plant will not necessarily be the optimum solution for other plant. Variables include salinity, temperature, pretreatment requirements and finished water quality. Some parameters, such as materials, will be constant. Seawater is, after all, seawater.
Regardless of location, the predominant component of the seawater is sodium chloride, which experience has taught us requires materials that resist attack by the chloride ion.
Perhaps the most critical factor in the design of a successful seawater RO facility is acquisition and pretreatment of the source water.
There are two techniques for acquiring the raw water, and for a variety of reasons, both are probably not applicable to all cases. Seawater, of course, is predominately a coastal phenomenon and as such occurs alongside beaches, mudflats, swamps, cliffs and other assorted geologic features.
- In most cases, it is unlikely that sufficient seawater for a large plant can be acquired from one of the many options commonly classified as “beach wells.” However, if the proposed plant capacity is sufficiently small (<5 mgd), extracting the seawater feed from a beach well or through a constructed infiltration gallery is preferable to open intake because the pretreatment challenges will be less demanding.
- For large plants, surface water intake facilities are required. These are similar to intakes used for coastal power plant cooling water.
Assuming the feed source is an open seawater intake, the proposed pretreatment scheme must consider the following:
- Control of plugging and colloidal fouling.
- Control of organic fouling.
- Control of biological activity and fouling.
Traditionally, seawater pretreatment has consisted of flocculation and coagulation with a ferric salt, sometimes applied in conjunction with a polymer, sedimentation and filtration. Depending upon the circumstances, a continuous chlorination and dechlorination procedure sometimes is employed. Intermittent chlorination and dechlorination is also practiced.
Filtration has typically been practiced as a two-stage process using dual or multi-media beds. Traditionally, the first filter operates at a relatively low loading rate of 3 to 5 gpm/ft2, while the polishing filter operates at a higher 5 to 7 gpm/ft2. When sizing the filtration equipment, capacity for the production of backwash water must be included.