Page 200 - ro membanes

8.8 CONSTRUCTION COSTS OF GRANULAR MEDIA FILTRATION SYSTEMS 183
compounds do not have impact on filter design. Because the pretreatment system consists of two-stage filtration, the overall volume of backwash water is expected to increase, with the first-stage filter daily backwash water volume equal to 6% of the total daily plant intake flow and the second-stage pressure filters using 4% of the intake flow.
Taking under consideration that the plant is designed for the same 43% recovery, the total volume of filtered water that will need to be produced for the RO-system operations is still 50,000 m3/day/0.43 1⁄4 116,279 m3/day. However, to accommodate the 10% (6% for first-stage filters þ 4% for second-stage filters) of additional flow for backwashing of the two stages of the pretreatment filters, the intake source water volume will be 116,279m3/day
1.1 1⁄4 127,907 m3/day (33.8 MGD).
In this case as with the other two examples, the source water is preconditioned using
ferric chloride for coagulation, polymer for flocculation, and sulfuric acid for pH adjustment to optimize the use of coagulant. While provisions for addition of coagulant are usually incorpo- rated ahead of each of the two filtration stages, most of the particle precipitation, coagulation, and flocculation occur in the coagulation/flocculation chambers upstream of the first-stage filters. Therefore, coagulant addition upstream of the second-stage filters is not typically practiced. However, facilities for in-line coagulation and flocculation of the feed to the second- stage filters are sometimes installed, because for certain periods of the year, if the water quality is very good, the first-stage filtration can be bypassed. In this case, coagulant and flocculant are fed directly into the inlet to the second-stage filters or coagulation, and flocculation facilities are installed upstream of the fist-stage filters, and bypass line is provided from these facilities directly to the feed of the second-stage filters.
It should be pointed out that two-stage filtration systems are not very commonly used because in most existing desalination projects, the location and depth of the intake are typically selected such that the collected source water is of good quality and requires single-stage filtration only. However, if the intake for the desalination plant is built in a shallow water body (e.g., the Persian Gulf) or the RO desalination plant is colocated with thermal desalination plant and/or power plant with onshore intake, then the use of two-stage filtration system is warranted.
An alternative solution for very large desalination plants [400,000 m3/day (106 MGD) or more] would be to construct deep tunnels under the ocean bottom, which extend outside of the tidal zone at locations where intake can be built at depth of 15 m (50 ft) or more below the water sur- face. Experience with construction of such intakes in Australia indicates that the costs for such structures could be much higher than the costs for construction of second-stage pretreatment sys- tem. In such cases, two-stage filtration systems could become a more attractive and cost-effective solution to minimize the biofouling potential of the saline source water than deeper intakes.
Tables 8.6 and 8.7 present key design criteria of the first- and second-stage of a two-stage pretreatment system of 50,000 m3/day plant with shallow or onshore intake.
8.8 CONSTRUCTION COSTS OF GRANULAR MEDIA FILTRATION SYSTEMS
Fig. 8.9 depicts construction costs for the year 2017 for granular media gravity and pressure filters as a function of the desalination plant intake flow they pretreat. As seen from this figure,