Development of Design Criteria for Multi Stage Filtration Units for Surface Water Treatment

Abstract

Surface water with proper treatment can be an effective alternative to groundwater in salinity and arsenic affected areas since surface water sources i.e. ponds, canals and rivers are available in many places of the country. However, exposed to direct contamination, surface water is a vulnerable carrier of water-borne diseases. Multi Stage Filtration (MSF) is a community based low cost and simple technology for treating surface water. The thesis work has been done in quest of determining appropriate design criteria for Multi Stage Filtration (MSF) units for use in Bangladesh. The MSF system considered under the study comprised of three units, i.e. Dynamic Roughing Filter (DyRF) unit, Up-flow Roughing Filter (URF) unit and Slow Sand Filter (SSF) unit. In the study, one MSF model unit was set up in the laboratory and three field MSF units were installed in three districts of the country. Necessary design data were collected regarding roughing pre-filtration and slow sand filtration system by reviewing the relevant literatures. The performance of different units of MSF were monitored and investigated regarding improvement of different physical, microbial and chemical water quality parameters both in the laboratory and field. For this the effect of different process variables i.e. bed material, rate of filtration, role of different filtration stages, flow condition, exposure condition of filter bed etc were considered in several experimental runs. Finally the optimum criteria of different design parameters have been developed through rigorous analysis of the performance of MSF units which are summarized below. ‘Coarse media size range’ and ‘depth of bed’ in DyRF and URF are more important design parameters for the reduction of turbidity. A coarse media size range from 4.75 mm to 25 mm for DyRF, and 6.3 mm to 25 mm for URF placed in three layers have been found suitable. Slow sand filter bed materials size range and grading, particularly on the top layer of filter bed, are very important design parameters for efficient microbial removal performance. Filter sand having following characteristics have been found appropriate: FM = 1.8-2.0, D10=0.21-0.22 mm, D60 = 0.45-0.47 mm, U = 2.14 -2.16 and Filter Media Size Range = 0.15 mm to 1.1 mm. A moderate influent turbidity limit of around 20 NTU may be proposed for SSF. In case of raw water turbidity level greater than 150 NTU, pre-settling arrangement should be made and if the raw water level turbidity remains within 60 to 150 NTU both DyRF and URF units will be required. However, for turbidity values below 60 NTU, DyRF unit may be omitted. To maintain slow sand filter influent turbidity value within 20 NTU and Color value within 25 Pt.Co.Unit, slow sand filtration rate should be within 0.20-0.25m/h. To maintain this flow rate at SSF, the corresponding flow rates at URF and DyRF were in a range of 0.43-0.54 m/h and 1.6-2.00 m/h respectively. Beyond slow sand filtration rate of xiv xv 0.20-0.25 m/h, microbial quality deteriorates significantly. For a maximum slow sand filtration rate of ≤ 0.1m/h, an acceptable level of microbial quality of water may be obtained and at a filtration rate up to 0.15 m/h, TTC and E.Coli may appear occasionally. Reduction of Dissolved oxygen level inversely related to the flow rate of water and average reduction of organic matters is approximately independent of rate of flow. Intermittent operation affected the color removal performance, not the turbidity removal performance. Effluent of consistently satisfactory microbial quality was obtained in MSF system operated without interruption and a definite deterioration in microbiological quality was noticed in intermittent operation. Therefore, to obtain a better removal performance uninterrupted flow condition should be maintained. For a slow sand filtration rate of around 0.20 m/h, a maximum of 40 cm head loss may be permitted before cleaning of bed within 6-8 weeks operation period. Therefore, provision of total height of water level of about 60 cm (10 cm Initial Water Depth + 40 cm Head Loss + 10 cm Free Board) should be kept over sand bed. For an up-flow coarse media filtration rate of 0.43 m/h and a down-flow coarse media filtration rate of 1.6 m/h, a maximum of 10 cm and 2 cm head loss may be permitted, respectively before cleaning of bed after about 8 weeks operation period. Approximately 7 to 10 days are required to improve the removal performances for ripening of the “Schmutzdecke” on filter sand (SSF). Twin bed filter chambers may be used and cleaning may be performed alternatively after 7 to10 days interval using the method known as “harrowing” after every 6-8 weeks period. However, complete removal of filter sand, washing and replacing may be necessary after every 5 to 6 months of operation. Filter bed should be kept covered to avoid the unnecessary growth of algae particularly on slow sand filter bed. The scouring action of falling water on sand bed can be reduced by installing homogenous flow distribution arrangement.