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162 8. GRANULAR MEDIA FILTRATION
retention capacity; (2) when the second stage filters are designed as deep-bed (rather than shallow depth) filters, they can achieve enhanced TOC removal by biofiltration. While deep single-stage dual-media filters can typically reduce 20%e30% of the TOC contained in the source seawater, the two-stage systems with deep second-stage filters can achieve 40%e60% TOC removal, mainly due to enhanced fine particle coagulation and biofiltration.
8.4.3 Downflow and Upflow Filters
Most filters used in pretreatment of seawater and brackish water are downflow filters. This flow direction allows large algal particles to be retained at the top of the filter media and removed with the backwash water with minimum brakeage and release of organics. If upflow filtration is used, algae contained in the source water are pressed against the filter media and unwanted dissolved organics are released from the broken algal cells into the filtered water, which is undesirable because it enhances biofouling of the downstream RO membranes.
8.4.4 Filters Combined With DAF Clarifiers
In cases where the saline source water contains a large amount of algal particulates, and/ or oil and grease, and space is at premium, DAF and granular media filtration processes can be combined in one structure, where the DAF clarifier is located is located above the filter cell (see Fig. 8.2).
FIGURE 8.2 Combined DAF clarifier and granular media filter.