Assessment of iron, manganese and arsenic removal efficiencies of conventional iron removal plants

Abstract

The most widely reported groundwater-quality problems in Bangladesh include excessive concentration of As and Fe. However, available groundwater quality data suggests that excessive concentration of Mn is also a significant problem in many areas of the country. The iron problem has long been recognized, and many technologies have been developed for Fe removal at municipal, community and household levels. After the detection of As, many municipal IRPs are now being used for removal of both and Fe and As, and many community plants (AIRPs) have been developed for removal of both Fe and As from tubewell water. The Mn issue, however, has attracted less attention and there is little data available on the performance of these plants, especially with regard to Mn removal. The study assessed the Fe, As and Mn removal efficiencies of six municipal IRPs, three ironcum- hardness removal plants, and fifteen community IRPs/ AIRPs. The municipal IRPs have been found effective in removing Fe from groundwater. Only one municipal IRP had relatively high As concentration (108 Ilgll) in the raw water, and the performance of this plant suggests that good As removal could be achieved in municipal IRPs. However, very little removal of Mn could be achieved in the conventional IRPs. The hardness-cum-iron removal plants, with flocculation and sedimentation chambers, have been found to be very effective in removing Mn from groundwater. Pilot studies may be carried out to see if such designs could be used for simultaneous removal of Fe, As and Mn. Fecal coli forms were detected in the treated water reservoir in most of the municipal plants; better disinfection processes need to be employed in these plants to safeguard against bacterial contamination. With few exceptions, the community IRPs/ AIRPs appear to be very effective in removing Fe from groundwater. The AIRPs have been found to be effective in reducing As concentration in raw water below the Bangladesh drinking water standard (50 Ilgll) when As concentrations in the raw water are relatively low (around 100 Ilgll). The community IRPs/ AIRPs differed significantly in terms of their ability to remove Mn from groundwater. The removal efficiency appears to depend on a number of factors, e.g., provision for aeration, depth of filter bed, and filtration rate (face velocity). Currently, typical designs are used for IRPs/AIRPs and no specific design criteria (e.g., detention time, face velocity, surface over-flow rate, media size range and grading) are followed . . Variation in raw water quality is also not taken into consideration in the designs. These issues need to be assessed in more details in order to develop designs of community plants for simultaneous removal of Fe, As, and Mn. Bacteriological contamination of treated water from the community IRPs/ AIRPs is common. Effective disinfection process, considering the field situation in Bangladesh, need to be developed to make the treated water bacteriologically safe.