Application of low-pressure nanofiltration coupled with a bicycle pump for the treatment of arsenic-contaminated groundwater

A nanofiltration (NF) membrane process coupled with a bicycle pumping system was examined by using arsenic-contaminated tube well water in a rural area of Bangladesh. It was operated under the conditions of low recovery and low operational pressure. It was proposed using both permeate and concentrate of NF membranes for drinking water and smother purpose. Rejection characteristics of arsenic compounds such as arsenite [As(III)] and arsenate [As(V)] were also investigated. Arsenite, a neutral solute at pH 8, showed lower rejection than arsenate, which existed in ionized forms at pH 8. In this sense the pre-oxidation of arsenite to arsenate was necessary to obtain safer drinking water using the NF process. Furthermore, the reverse osmosis (RO) process could be operated under the conditions of high operational pressure at 4MPa by using a bicycle pedal pump. Rejection of arsenite and arsenate with the RO membrane was over 95%. However, treatment of concentrate with a high concentration of arsenic might be necessary for the application of the RO process where recovery was obtained at 66%. A high recovery of 66% was delivered at three times a higher concentration of arsenic in concentrate than feed water. Consequently, a low-pressure NF process with the pre-oxidation of arsenite to arsenate or the RO process coupled with a bicycle pumping system could be applied to the treatment of arsenic-contaminated groundwater in areas where electricity supply is not efficient or feasible.