Effectiveness of multistage filtration in removing iron, manganese and arsenic from groundwater of Bangladesh

Abstract

The presence of iron, manganese and arsenic in ground water beyond the permissible limit is now considered to be a major problem throughout the world and produce numerous adverse effects. These problems are severe in the context of Bangladesh as groundwater is a vital source for the safe drinking water supply. In the present study seven numbers of multistage filtration units (MSFU) have been constructed in Sirajgonj,Comilla and Jessore ( three different hydro-geological conditions) to investigate the effectiveness of multistage filtration in removing iron, manganese and arsenic from groundwater of Bangladesh adopting the technique of adsorption and co-precipitation of arsenic and manganese onto the flocs of ferric hydroxide, making use of the naturally occurring iron of groundwater. The MSFU, which is attached to a tube well, has three chambers, 1st chamber (Aerator plus Down-flow Flocculator), 2nd chamber (Sedimentation plus Up-flow Roughing Filter) and 3rd chamber (Down-flow Roughing Filter) .The flocculation and roughing filtration processes in the MSFU were accomplished through the use of brick chips. The MSFU is connected to the spout of tube well with a short piece of 75 mm PVC / flexible pipe. Water entering the first chamber is distributed uniformly over the whole bed of course media through a porous thin ferro-cement plate placed on the top, resulting strip out of CO2 and increase of pH value for the oxidation of soluble iron. In the Down-flow Flocculator oxidation and subsequent precipitation of iron oxy-hydroxides occurs respectively on the top and within the interstices of coarse media which adsorbs arsenic oxy-anions as well as manganese ions. Sinusoidal flow across the coarse media enhance collisions for the flocculation of precipitated particles. Comparatively larger flocculated precipitates settle at the bottom of the sedimentation chamber. Significant removal of precipitated particles occurs by sorption on to iron oxy hydroxides and mechanical straining during up-flow through the comparatively finer coarse media bed in the 2nd chamber. Final removal of precipitated particles occurs through sorption on to iron oxy hydroxides and mechanical straining during down-flow through the comparatively finer coarse media bed in the 3rd chamber. Water samples collected weekly from different location of the Multi-Stage Filtration Units (MSFU) were tested in the laboratory for determining the concentration of iron, arsenic and manganese. Around 97 % of iron reduction was achieved through the MSFU. Arsenic removal efficiency upto 91 % was achieved through the MSFU without using any chemicals. Two different equations have been developed to express the effect of tube well water iron concentration on iron and arsenic removal performance respectively. Using these equations it will be easy to determine the residual iron concentration in the final effluent achieved through properly designed MSFU treating tube well water of different initial iron concentration. Manganese removal was observed as a function of raw water manganese concentration. Higher the manganese concentration, greater was the removal performance. Up to 85 % manganese removal performance was achieved through the MSFU. Contribution of DRF alone in removing manganese was observed very significant (around 37%). For tube well water having manganese concentration around 1.5 mg/l and iron concentration around 15 mg/l, a residual Mn concentration below maximum permissible limit (WHO health risk guide line value for Mn) of 0.4 mg/L could be maintained through the MSFU. Performances of Iron Chips Column (filled with iron chips and iron coated sand.) attachment with the MSFU were also monitored and necessary modifications in the design have been recommended. Operation and maintenance procedure (cleaning) were determined .The initial effluent flow from the URF were around 85 % of tube well water flow. Length of filter run between cleaning should be maximum 3 - 4 weeks. MSFU will be cleaned when flow from the outlet of URF chamber will reduce by 45-50% of the tube well flow i.e. flow from the URF chamber = 9-10 L/min. Numbers of users and water consumption was increased by about 10 folds after the installation of the MSFU.