Development of activated alumina based household arsenic removal unit

Abstract

The present study deals with the development of a technology based on activated alumina adsorption process for arsenic removal from groundwater. The people of Bangladesh have been facing arsenic contamination of drinking water supplied by hand pump tubewells. At present, the most feasible short-term solution of this problem appears to be treatment of tubewell waters through suitable and low-cost technologies. Activated alumina adsorption process is very efficient in removing As(V) from water; while As(III) is poorly removed. In developing an arsenic removal unit based on activated alumina, groundwater chemistry and its effect on alumina column performance, performance of upflow and down flow systems, effective measures to enhance the column performance, effect of contact time, and ease of operation and maintenance are to be considered. The groundwaters of Bangladesh contain both As(III) and As(V). High concentration of iron is also present in most of the groundwaters. The naturally occurring iron degrades the performance of an alumina bed by fouling and clogging the bed. However, the iron can be beneficially used to remove part of the arsenic in the water through co-precipitation and adsorption. A system has been developed to improve the performance of alumina columns through efficient pretreatment of natural groundwater. The pretreatment steps include (i) chemical oxidation of As(III) using potassium permanganate and (ii) removal of iron through aeration, flocculation, sedimentation and sand-filtration. Through the pretreatment steps the problems of iron have been eliminated; while its beneficial use has been ensured. It has been found that the down-flow system is preferable to the up flow system. A design of a household arsenic removal system based on the experimental results and other available information has been presented and a number of arsenic removal units have been constructed. An assessment of the performance of the household arsenic removal unit has been carried out both at laboratory and field. The findings reveal that the arsenic removal efficiency of the technology is excellent. For feed water with 300 ppb of arsenic and negligible iron, the breakpoint (50 ppb arsenic in treated water) bed volume is 5300. For natural groundwater having arsenic concentration in the range of 450 to 600 ppb and iron content of 4.0 to 5.0 mglL, the breakpoint bed volume is between 19000 and 26000. The technology removes iron efficiently - the average removal efficiency is 85% for feed water iron content in the range of 4.0 mg/L to 5.0 mglL for alkaline water (alkalinity greater than 350 mglL). An appreciable amount of manganese is also removed by the technology. The effect of other water quality parameters is insignificant. However, unhygienic practices of the users may result in bacterial contamination of the treated water. Users' acceptability of the technology is good in terms of capital cost and chemical costs, and treated water quality. But the social acceptability seems to be not so good mainly due to the inadequate flow rate, height of the unit, and difficulty in flow control. The problems identified regarding users' acceptability of the technology can be minimized significantly with a few modifications of the present design.