Although from the title this should be a specific and straight forward topic, there are several design edges to consider when selecting the proper seawater pretreatment system. As most of you may imagine, seawater is not the same from one place to another even when having similar salinity levels. Other than the high salinity (30,000 ppm TDS and above) we have to deal also with: sand, marine fauna, algae, suspended solids, colloidal solids, contaminants like oil, micro-plastics, other organics, temperature variations, dissolved oxygen variations, and finally seasonal variations that also may change all previous mentioned parameters.
At last the pretreatment system shall be robust enough as to protect the seawater RO membranes during the most stringent design scenarios. The minimum capability we shall expect from pretreatment is to rid of suspended solids (of all kind) so that by keeping a Silt Density Index low enough (SDI<3), fouling is minimized. But be careful, biofouling is still an issue that for sure will try to hurt the RO membranes performance.
The first design consideration that makes a difference in the water quality and thus on the pretreatment system design is the type of seawater intake. The most common ones are the open intakes (offshore), typically for larger seawater demanding facilities, but also beach wells (onshore) for lower capacities are considered. At the same time open intakes can be open surface channel type, usually concrete made channel walls provided with bar screen machines, or open submerged pipe type running from tens to hundreds of meters into the sea (screening end). The seawater intake decision will be influenced by the coast topography, the required seawater feed flowrate, the anticipated seawater quality concerns, the Environmental Impact Assessment guidelines and the budget constrains.
Ultrafiltration (UF) systems are nowadays a standard as pretreatment for seawater RO, but in some cases it might not be enough as to deal with the sort of contaminants present in most of seawaters. UF membranes in general are sensitive to particle size, suspended solids concentration, organic carbon and strong oxidants presence. The point to highlight is that the type of seawater intake selected should help to mitigate some UF membranes undesired operating conditions. In case the water intake does not provide such protection, at least a two stage pretreatment system shall be considered. An additional pretreatment stage upstream will lead to have a smaller UF system (less UF area required) and a smoother membrane performance. Nonetheless, biofouling shall still be a remaining concern to solve.
To be continued…