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8.2 THE FILTER OPERATION CYCLE 157
processing in terms of filtered water turbidity (usually set to 0.5 NTU) and/or SDI15 (usually established at 4 or 5).
Once a given filter cell/vessel is taken out of service for backwash, the rate and length of the watereairewater backwash sequences is determined based on the length of the filtration cycle and the source water turbidity, which occurred for the duration of the filtration cycle. The shorter the filtration cycle and the higher the feed water turbidity, the longer the filter backwash cycle.
The optimum length of the entire filter backwash cycle is determined by collecting grab sample of spent backwash water from the surface of the filter (for gravity filters) or from the spent-filter backwash pipe (for pressure filters) every 2 min after the initiation of the filter backwash and measuring the turbidity of this waste stream with handheld turbidimeter. Based on a rule of thumb derived from practical experience, the filter backwash cycle should be discontinued when the turbidity of the spent filter backwash water is reduced down to less than 15 NTU. Further continuation of the backwash cycle will result in ovescrubbing of the filter media and increased maturation period which are undesirable.
After the backwash of a given filtration cell is completed and the cell is returned to normal filtration cycle, the filtered water produced in the first 15e45 min usually exceeds the levels of turbidity and/or SDI acceptable for processing of this water through the downstream RO system. During this initial period after backwash, the filter media rebuilds its ability to filter out fine solids via the internal accumulation of solids around the media grains. This time for recovery of the filter cell performance after backwash is often referred to as maturation period. Because the quality of the filtered water produced during the maturation period is not adequate for desalination, typically, this water is wasted (e.g., released via the desalination plant discharge). To optimize the filtration and backwash cycles, it is imperative that the maturation period is reasonably shortdpreferably not longer than 15e20 min. The length of the maturation period typically depends on the length and mode of filter backwash.
If the filters are over-backwashed (e.g., the backwash is too long and/or the backwash rate is too high), the filter cell will take longer to mature, and vice versa. On the other hand, if the backwash cycle is too short, some of the solids accumulated in the media during the filtration cycle will not be removed and the filtration cycles will become shorter and shorter over time. Therefore, the backwash time has to be adjusted to fit the amount of solids in the source water and these retained in the filter mediadthe lower the source water turbidity the shorter the backwash cycle should be. Optimizing the backwash schedule usually results in long (24e48 h) filtration cycles and short (15e20 min) maturation times, and ultimately in improved overall pretreatment filter performance.
For the filters to operate well, the suspended solids in the feed water have to be well coagulated and aggregated into large particles (typically >20 mm), which can be retained by the filtration media. As indicated in the previous section of this chapter, coagulation and flocculation process is dependent on many factors and requires careful monitoring and optimization to achieve satisfactory pretreatment. Overdosing of coagulant hinders filter performance and results in accelerated fouling of the downstream cartridge filters and RO-membrane elements by unreacted coagulant and flocculant. On the other hand, under- dosing of coagulant often results in poor retention of fine particles and silt, and causes particulate fouling of the RO elements.
The applied bed expansion depends on the size of the filter mediadthe smaller the size, the larger media expansion is needed. For example, media of diameter of 1.2 mm requires