Coagulation-microfiltration Process For The Removal Of Arsenic In Drinking Water

In the year of 2006,the Chinese government promulgated the new Standards for Drinking Water Quality (GB 5749-2006) and set the maximum contaminant level (MCL) for arsenic at 10μg/L for the first time. However, the available methods for arsenic removal in drinking water are very few, and most of them are less cost-effective. There is therefore a great need to develop new methods for arsenic removal from drinking water.In recent years, membrane separation technology has been used for arsenic removal from drinking water in many countries. As to China, there are few research reports about this application. Considering that endemic arsenic toxicosis occurs mainly in the rural areas, the feasibility of membrane coagulation reactor (MCR) was investigated for removing arsenate from drinking water.The whole MCR test could be divided into three parts: the jar test, the bench test and the disposal test of arsenical sludge. The jar test has studied the arsenic removal efficiency by coagulation and microfiltration from synthetic tap water. In addition, the factors affecting arsenic removal were also explored. The bench test was conducted through operation of a MCR, in which the removal efficiency of arsenic and the membrane fouling characteristic were investigated. Furthermore, the influence of various factors in the jar test was verified in the bench test. In the disposal test of arsenical sludge, the characteristics of arsenical sludge were studied through a series of experiments.The results showed that arsenic removal efficiency ranged from 92.8% to 98.2%. Arsenic removal to low levels (<10μg/L) was achieved using a coagulant dose of 4 mg/L (calculated as Fe3+) when the concentration of arsenic in raw water was about 100μg/L, and the average concentration of arsenic in treated water was 4.40μg/L. The quality of treated water satisfies new Standards for Drinking Water Quality. Also, it was observed that MCR improved the quality of treated water by reducing the concentration of organics measured by TOC and UV254.Tests about the effects of different factors on the MCR showed that different co-occurring substances in raw water could exert different influence on the removal efficiency of arsenic. F-, Cl-, NO3- and SO42- had a negligible effect on the removal of arsenic in the test conditions. The increasing of HCO3- and HPO42- in raw water would decrease the removal efficiency of arsenic. When the concentrations of K+, Ca²⁺ and Mg²⁺ increased, no apparent changes in arsenic removal were observed. The removal efficiency of arsenic was significantly reduced with the increasing of Si in raw water. The effect of pH on arsenic removal was studied by applying a constant ferric dose of 2.0 mg/L (calculated as Fe3+). The result showed that arsenic removal performance improved obviously with decreasing pH.The results of membrane cleaning showed that the membrane specific flux could recover to 87.8% of the initial value after physical and chemical cleaning. The membrane fouling was mainly caused by organic matters in raw water which contributed to 67.2% of the total membrane resistance. Concentration polarization could be attenuated to some extent by air agitation in the submerged system.The disposal test of arsenical sludge showed that arsenic could be well removed using MCR process with an enrichment coefficient of 3668. The sludge volume was acceptable after free sedimentation, and the water-content of the naturally dehydrated sludge could be reduced to 95.4%. The crystalloid components of dried arsenical sludge were CaCO3 and FeO(OH).