Effects of Bed Top Layer Mixing on Slow Sand Filter Performance

Document Type : Technical Note


1 M.Sc., Fars Higher Education and Research Complex, Shiraz

2 Assist. Prof., Dept. of Civil Engineering, Shiraz University


"Schmutzdecke" is developed by particle deposition of a few centimeters deep on the top of the sand layer in slow sand filters. It plays an important role in removal mechanisms and is responsible for the significant increase of head loss during operation. After several weeks of filter operation, the surface area of the bed becomes clogged due to deposition of suspended solids. Cleaning of the bed top is required when the final head loss occurs. Since the operation costs of slow sand filters depend on cleaning intervals, the operation cost decreases by increasing the time of filtration run. The objective of this study was to evaluate the effect of bed top layer mixing on filter performance. Two experimental pilots (one test and one control) were built from 240 mm PVC pipe 170 cm high. The sand media depth in each filter was 60 cm with a uniformity coefficient of 2.3 and effective size of 0.2 mm. During the 149 days, 30 cm of the top layer of the test filter bed was mixed every other day and filter performance was evaluated and compared with the unmixed control filter. This research indicated that the rate of head loss decreased with bed top layer mixing and that filtration run increased to more than two times the typical value without notable effects on effluent quality.


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