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248 11. GUIDELINES FOR PRETREATMENT SYSTEM SELECTION
pretreatment. As indicated in Chapter 10, membrane pretreatment generates approximately two times less solids and therefore it allows for easier compliance with the turbidity limita- tions in the desalination plant discharge permit. In addition, use of membrane pretreatment does not discolor the discharge area and therefore often regulatory agencies allow spent filter backwash water from membrane pretreatment to be discharged directly without pretreat- ment. The latest regulations in most developed countries require that the backwash water from conventional granular media pretreatment systems using coagulant to be treated before discharge to remove the discoloration of the discharge and reduce the discharge turbidity.
The 2015 California Ocean Plan, in the United States, has requirements for reduction of impingement and entrainment of marine organisms and, therefore, it encourages the use of sub- surface intake systems (wells) instead of open intakes. Such regulations have a significant impact on the selection of pretreatment system because if intake wells are selected, they would likely result in more simplified pretreatment system that consists of cartridge filters only or com- bination of cartridge filters and pressure-driven granular filtration system. It should be pointed out, however, that some subsurface intakes may yield source waters of high content of iron and/or manganese in a reduced form, which would require a special type of granular media pretreatment filters (e.g., greensand filters) or conservatively designed oxidation and mem- brane pretreatment system to remove the iron and manganese (typically to below 0.1 mg/L) so that these compounds do not cause fouling of the downstream RO membranes.
11.3.2 Impact of Project Procurement on Pretreatment System Selection
Practical experience to date shows that the type of project procurementddesignebide build (DBB), designebuild (DB), or designebuildeoperate (DBO)dhas significant impact on the selection of a pretreatment system. When the designer and contractor are not respon- sible for the plant operation (e.g., under DBB or DB methods of project delivery), the designer may prefer to select the lowest capital cost granular media pressure-driven pretreatment sys- tem or to choose the use of membrane pretreatment with a very aggressive design flux.
Since performance guarantees and acceptance testing typically last only 1 month, the lowest-cost contractor with aggressive pretreatment system design could arrange the plant testing to occur during nonalgal bloom periods when the plant is not exposed to challenging performance and to pass the acceptance testing. This procurement approach is to the detri- ment of the project owner and operator, who will face project performance constraints during the period of challenging source water conditions (e.g., algal bloom season, monsoon season).
Another approach for project procurement is when the same contractor is responsible for design, construction, and operation of the plant. Under such procurement conditions (e.g., DBO), the designer may select a robust and conservative pretreatment system, which is capable of operating reliably and cost effectively under all conditions, including challenging water quality events such as algal blooms.
References
Jackangelo, J., Voutchkov, N., Badruzzaman, M., Wienrich, L.A., 2017. Pretreatment for Seawater Reverse Osmosis: Existing Plant Performance and Selection Guidance, Water Environment and Reuse Foundation, WRRF-14e07.
Mickley, M., Voutchkov, N., 2016. Database of Permitting Practices for Seawater Concentrate Disposal, Water Envi- ronment and Reuse Foundation, WRRF-13e07.
Voutchkov, N., 2010. Considerations for selection of seawater desalination system. Desalination 261, 354e364. Elsevier.