The down-flow hanging sponge (DHS) reactor was used for continuous As removal treatment of As-contaminated water. The treatment scheme was: (1) As(III) in contaminated water is oxidized by arsenite-oxidizing bacteria fixed in the sponges in the reactor; (2) Fe(II) naturally existing in the water is oxidized by dissolved oxygen; (3) Fe(III) is precipitated as iron hydroxide and As(V) is co-precipitated with the iron hydroxide; and finally (4) the co-precipitates are fixed in the sponges. This system could remove As from As-contaminated water on a small scale and at low cost. The results showed that, after using the DHS reactor, As and Fe concentrations in the treated water were lower than water quality standards for drinking water when Fe(II) concentration in the influent was lower than 10 mg/L and the Fe/As ratio was higher than 6.67–8.42, with dependence on the Fe concentration. Additionally, even if Fe concentration is higher than 10 mg/L, the treatment system is still applicable if the pH of the influent is higher than 7 or the retention time is longer than 2 h.

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