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156 8. GRANULAR MEDIA FILTRATION
The solids and microorganisms retained in the pore volume between the filter media grains reduce this volume over time and create hydraulic losses through the filter media (filter bed resistance). Most filters used for saline water pretreatment operate at constant filtration rate, which means that the feed pressure of these filters increases over the duration of the filtration cycle to compensate for the headlosses in the filter bed caused by accumulation of solids. Once the filter media headlosses reach a certain preset maximum level, the filtration cycle is completed, the filter is taken out of service and media backwash is activated to remove the solids accumulated in the media.
8.2.2 Filter Media Backwash
Granular media filters are configured in individual cells (units), which operate independently. Once a given filter cell reaches a preset target headloss through the filtration bed (for constant flow filters), this cell is backwashed using filtered saline source water or concentrate from the RO-membrane system. Backwashing is always completed in a direction opposite to the direction of the source water flow during the filtration cycle to remove the solids out of the media.
Filter-cell backwash frequency is usually once every 24e48 h. Deeper filters of larger surface area have higher capacity to retain solids and, therefore, usually have longer filtration cycles. The actual length of the filtration cycles will depend not only on the depth and loading rate (e.g., the retention time) of the filters but also on the content and size of solids in the saline source water.
Spent (waste) backwash volume is typically 2%e6% of the intake source water. Use of RO concentrate instead of filtered effluent to backwash filter cells allows reducing backwash volume and energy needed to pump source water to the desalination plant. However, because of the corrosive nature of the concentrate, backwashing with RO concentrate is not commonly practiced.
During backwash of downflow filters, the backwash water flows upward through the filter media, scours the filter grains, removes the solids accumulated in the filter bed, expands the bed, and transports the removed solids toward the backwash troughs for their evacuation out of the filter cell.
Practical experience shows that backwashing of filter media grains smaller than 0.8 mm with water only is difficult. Therefore, at present, a typical backwash cycle includes a combination/ sequence of air and water washing of the filtration media.
Typically, the backwash sequence includes three steps: (1) water wash; (2) air wash; and (3) second water wash of the media. The first water wash aims to mainly dislodge the coarser solids accumulated in the filter media pores. The following air wash creates greater turbulence and enhances particle scrubbing allowing to also dislodge the finer solids and some of the bacterial film accumulated on the grains of the filtration media. The air wash is followed by another water wash, which aims to remove the solids and air accumulated in the media and to prepare the filtration bed for another filtration cycle. The length of each of the water and air backwashing cycles is a function of the content of solids in the source water and the depth of the filtration media and typically it is between 5 and 8 min.
Filter backwash is initiated either at preset interval of time, or at predetermined maximum filter bed headloss. Sometimes, filter backwash is also initiated if the filtered water quality begins to deteriorate steadily and exceeds the acceptable threshold for RO-membrane