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11.3 ADDITIONAL CONSIDERATIONS FOR SELECTION OF PRETREATMENT 247
The most popular pretreatment configuration at present is the use of two-stage system, the first stage of which includes DAF clarifiers and the second stage consists of granular media or MF/UF membrane filters. Such pretreatment can handle up to 1 mg/L of THC. If the source water contains higher THC levels, the desalination plant will need to be shut down until the intake area is decontaminated.
Another pretreatment alternative applied at the 300,000 m3/day Adelaide desalination plant in Australia is feeding powdered activated carbon (PAC) into the source seawater up- stream of the plant’s disk filters for the duration of the oil spill event. PAC of grain size smaller than the microscreen openings is fed upstream of the microscreens in order to facil- itate thorough mixing and adsorption of the THC by the PAC before its removal by the plant’s vacuum-driven membrane filtration system. The key advantage of this pretreatment approach is its cost efficiency. The construction and O&M costs for the PAC feed system are less than 10% of the cost of construction and operation of DAF. Taking under consideration that the Adelaide desalination plant has a very deep intake (22 m/73 ft) and that the amount of THCs that could reach the intake at this depth is very small, the use of PAC instead of DAF is much more cost effective and provides equal protection of the RO membranes.
11.3 ADDITIONAL CONSIDERATIONS FOR SELECTION OF PRETREATMENT
Intake type and associated source water quality are the key technical factors used for the selection of pretreatment configuration of the desalination plant. However, depending on the site-specific circumstances, other nontechnical factors may influence the pretreatment selec- tion process. Such factors include environmental project permitting, plant site footprint re- quirements, waste stream quantity and quality, coagulant dose, chemical costs, energy costs, and economy of scale (Jackangelo et al., 2017).
11.3.1 Permitting Issues
Environmental permitting for desalination plants is controlled by various governmental entities responsible for enforcement of the country’s environmental regulations. Regulatory requirements associated with the desalination plant pretreatment system mainly focus on the discharge of the waste streams generated during the pretreatment process such as intake screenings, spent filter backwash, filter-to-waste water discharged during the maturation of the filtration cells, and spent MF/UF membrane cleaning chemicals. These requirements vary significantly worldwide (Mickley and Voutchkov, 2016). However, two key approaches used in the environmental regulations of many countries are to either allow direct discharge of the pretreatment waste streams along with the plant concentrate or to require treatment of the pretreatment waste streams by sedimentation prior to discharge of the clarified stream after blending with the desalination plant concentrate. In such cases, the solids separated from the pretreatment waste stream are dewatered by centrifuges or belt filter presses and are disposed as sludge to a landfill after their dewatering.
Numerous regulatory limitations for turbidity of the discharge and/or direct specific permitting requirements for treatment of the concentrate usually impact the selection of