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Deep Bed Filtration Models
11 Deep Bed Filtration K. J. Ives Professor of Public Health Engineering, University College, London
11.1 INTRODUCTION The principles of deep bed filtration are quite well-known. It is a clarification process using a deep bed of granular media, usually sand. As a sewage tertiary treatment process it can frequently produce filtrates containing only 5 m g l _ 1 or less of suspended matter. In conjunction with coagulation processes, either directly or with sedimentation, filtration may produce filtrates of exceptional clarity, with the suspended matter scarcely detectable by turbidimetric instruments. The penalty paid for this clarification is that the filter becomes clogged, requiring more and more energy to sustain the required flow. When the energy required reaches the maximum available, the filter media have to be cleaned, usually by reverse flow backwashing, assisted by air scouring or auxiliary wash jets. Rates of filtration are normally in the range of 5 to 15 m h _ 1 (100 to 300 gall ft - 2 h - 1 ) , with a trend towards the higher rates. The media may range in size from about 0.4 to 2.5 mm, and be graded in various layers (dual media or multi-media) or be size-graded continuously, as in up-flow filters. The thicknesses of the layers may vary, but overall depths are normally about 0.6 to 1 m (2 to 3.3 ft), except for up-flow filters which are considerably deeper. Open gravity filters usually operate to a head loss limit of 2.5 m (8 ft) water gauge, but pressure filters may use up to 2 or 3 times this value. Backwashing is normally employed at a rate sufficient to fluidize the media, and to ensure sufficient separation for multiple layer filters. Wash rates of up to 0.6 m min" l (2 ft min~ l ) can achieve this, although this depends on the media, and the water temperature. If air scour is used, typical rates are 0.4-0.6 m 3 m " 2 m i n - 1 . Normally, backwash water would represent about 1% of filtrate production ; 3 % would be high and quantities over 5 % would be excessive. Filter runs of about 24 h between washes are normal. Where the water to be filtered does not contain a high organic load, runs may be up to 100 h, but in sewage tertiary treatment filtration a daily washing is desirable to prevent anaerobic decomposition of the deposits retained in the filter pores. If filter runs fall to below 8 h it becomes difficult to maintain production, and wash water demands become excessive. 358
11.1 INTRODUCTION The principles of deep bed filtration are quite well-known. It is a clarification process using a deep bed of granular media, usually sand. As a sewage tertiary treatment process it can frequently produce filtrates containing only 5 m g l _ 1 or less of suspended matter. In conjunction with coagulation processes, either directly or with sedimentation, filtration may produce filtrates of exceptional clarity, with the suspended matter scarcely detectable by turbidimetric instruments. The penalty paid for this clarification is that the filter becomes clogged, requiring more and more energy to sustain the required flow. When the energy required reaches the maximum available, the filter media have to be cleaned, usually by reverse flow backwashing, assisted by air scouring or auxiliary wash jets. Rates of filtration are normally in the range of 5 to 15 m h _ 1 (100 to 300 gall ft - 2 h - 1 ) , with a trend towards the higher rates. The media may range in size from about 0.4 to 2.5 mm, and be graded in various layers (dual media or multi-media) or be size-graded continuously, as in up-flow filters. The thicknesses of the layers may vary, but overall depths are normally about 0.6 to 1 m (2 to 3.3 ft), except for up-flow filters which are considerably deeper. Open gravity filters usually operate to a head loss limit of 2.5 m (8 ft) water gauge, but pressure filters may use up to 2 or 3 times this value. Backwashing is normally employed at a rate sufficient to fluidize the media, and to ensure sufficient separation for multiple layer filters. Wash rates of up to 0.6 m min" l (2 ft min~ l ) can achieve this, although this depends on the media, and the water temperature. If air scour is used, typical rates are 0.4-0.6 m 3 m " 2 m i n - 1 . Normally, backwash water would represent about 1% of filtrate production ; 3 % would be high and quantities over 5 % would be excessive. Filter runs of about 24 h between washes are normal. Where the water to be filtered does not contain a high organic load, runs may be up to 100 h, but in sewage tertiary treatment filtration a daily washing is desirable to prevent anaerobic decomposition of the deposits retained in the filter pores. If filter runs fall to below 8 h it becomes difficult to maintain production, and wash water demands become excessive. 358