Direct And Horizontal Roughing Filtration Technique In Bank Filtration

Abstract

Efficient application of the slow sand filter (SSF) treatment process requires raw water of low turbidity; the major draw back of SSF however, is its sensitivity to turbidity. The suspended solids of turbid water will rapidly clog SSF resulting in unacceptably short filter operation runs. Therefore pretreatment of surface water containing solid matter load is necessary. Prefiltration through horizontal roughing filtration (HRF) is not only a simple, efficient and chemical-free alternative treatment process for solid matter separation, it also makes ample use of local resources and hardly require mechanical equipment. Pre-filtration by HRF is thus a potentially alternative option for suspended solids removal although Its application is predominantly limited to rural areas. Experimental findings showed that HRF could achieve a particulate matter reduction of 90% or more. HRF requires a low filtration rate (0.5-2.7 m/h) and thus needs a large construction volume. In order to improve HRF performance at higher filtration rate the process is combined with direct filtration i.e. coagulant is added in a rapid mixing unit prior to HRF. The combined process is called direct HRF (DHRF). Comparing the performance of HRF and DHRF, it was found that DHRF systematically performed better, with higher removal efficiencies as well as high filtration rates. DHRF has a potential to substitute the conventional flocculation-sedimentation process widely used before rapid sand filters. The main process parameters of DHRF were optimized as 1.0 mg/l alum as coagulant dose effective over a wide range of pH, and a 4 m length for first compartment (grain size 12-9 mm) and filtration rate till 10 m/h. The short filter length prevents it from absorbing shock load, which could be expected in practice, and hence an adequate length is necessary (6-7 meters). HRF for rapid sand filters differs from that for slow sand filters. The results show mainly in the filtration rate, which should be very high to supply the rapid sand filter with sufficient flow rate (10-20 m/h). Little has been reported in the literature about HRF for rapid sand filter, this study addressed the problems relating to rapid sand filter by dealing with high filtration rate (24 m/h) and super-high suspended solids (20,000mg/l). The upper limits of raw water turbidity and flow rate are the two variables that limit the application of HRF to plants using rapid sand filtration. The present investigation suggests that only one type of grain could be used (12-9 mm) in the filter the polishing compartment could be cancelled. This will result in a reduction in the efficiency of the filter to only 57% in average, but sufficient flow is then attained (10 m/h). This is because the second and third compartments are generally used for polishing and hence increasing the efficiency, which is not first priority in this case. The distributor box, which distributes water to the numerous HRFs in the scheme, will act as a basin for plain sedimentation, and hence very high load of SS up to 20,000 mg/l could be dealt with, since 90% is got rid of. Bank filtration which is a natural process faces the problem of clogging, which results from the nature of the bank soil gives opportunity for interference to raise the efficiency of this natural process. One of the major parameters affecting the success of HRF is time run where the time between two hydraulic cleaning intervals of the filter could be as short as twice a day. Therefore constructing the HRF at the riverbank will allow repetitive cleaning to be readily and naturally done, and the raw water of the river will be used as the cleaning agent. For further studies it is recommended to construct a large-scale pilot plant in the field to be investigated to determine optimum design parameters compared with lab-scale studies. Alternative for large-scale pilot plant is to design a computer program to simulate the conditions that couldn’t be attained easily in the lab.