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8.5 FILTER PERFORMANCE 169 8.5.3 Removal of Microorganisms
8.5.3.1 Algae
The rate of algae removal by the filters will mainly depend on the size of the algae in the source water and the size of the filter media. Most algae larger than 100 mm are typically retained on the surface of the top media (anthracite/pumice). Practical observations indicate that the closer the desalination plant is located to the equator, the larger the percentage of micro- and picoalgae in the source seawater. Such algae are not well removed by conventional sand media of sizes 0.4e0.6 mm (400e600 mm) and require the installation of a third layer of finer filter me- dia or use of two-stage filtration system. Depending on the size of media and size of the algae dominating in the source water, the algal removal could typically vary between 20% and 90%.
8.5.3.2 Bacteria and Viruses
Desalination pretreatment filters would typically remove 99% (2 logs) of pathogens, but sometimes may have lower removal rates in terms of marine bacteria because these bacteria are very small in size and would pass through the filters.
8.5.4 Monitoring of Filter Performance
Besides filter effluent quality flow, length of filtration cycle, and maturation time, granular media filters are also monitored for the depth and condition of their media, the filter under- drain, and the volume and quality of the backwash they generate.
Loss of filter media is usually assessed annuallyd2e4 cm of media loss from the filter cells per year is considered normal. Media is topped off when its depth is reduced by approximately 10 cm below design level.
Filter media condition is usually inspected every 6 months to 1 year. Over time, pretreatment filters accumulate debris, coagulant residuals, and dead organic matter, which tend to deteriorate their performance. Therefore, many plants worldwide practice periodic 1-day soaking of their granular media pretreatment filters in citric or sulfuric acid in the winter months and in caustic soda (sodium hydroxide) in the summer/algal bloom season.
Low-pH sulfuric, hydrochloric, or citric acid filter media soak is equivalent to the perfor- mance of clean-in-place (CIP) of the RO membranes and aims to loosen calcite deposits and ag- gregates that may be blocking the filter media and to maximize the performance of the filters in terms of removal of solids, organics, and silt, and to extend the filter cycle length. High-pH (caustic soda) filter-media soaking aims to dissolve patches of organic matter, sand, silt, and anthracite cemented with organic residuals from the source water. The caustic-soda soak mainly targets filter-media solid patches of organic origin, while the acidic soak aims to dissolve filter blockages formed from calcium and magnesium precipitates and overdozing of coagulant.
Depending on the flow distribution configuration of the pretreatment filtration system, filter soak should be completed on individual filter cells rather than on all cells at the same time. The recommended low-pH/high-pH filter-soak procedure is as follows:
1. Drain down the filter cell that is planned to be cleaned.
2. Refill the cell with filtered water that contains sulfuric (hydrochloric or citric) acid of
concentration of 80e90 mg/L for acid soak, and with sodium hydroxide of dosage of