Study On Removal Of As (Ⅲ) By Membrane Chemical Reactor And Treatment Of Filtered Water By Ultrafiltration Membrane

As (III) was a kind of toxic substance, but the traditional process could not remove it from water effectively. In order to develop the treatment technology to meet the new drinking water standards, As (III) removal efficiency was investigated by the process of oxidation-coagulation-microfiltration, through jar tests to study the kinetics and simulate the reactor, and the bench test with membrane chemical reactor (MCR). Besides, for upgrade of the traditional drinking water treatment processes, a pilot test on treatment of filtered water in a waterworks was carried out with the ultrafiltration membrane system.The kinetic results on the reaction between HOCl and As (III) showed that the reaction rate was fast when pH was neutral and weak alkaline. The specific rate constants for reaction between HOCl and H₃AsO₃, H2AsO3- and HAsO3_2- were (1.72±0.11)×103, (2.10±1.02)×107 and (3.66±0.79)×108 mol/(L·s) respectively. The overall rate constants were weighted sums of the specific rate constants. On the basis of Arrhenius equation, the apparent activation energy of reaction between HOCl and H₃AsO₃, H2AsO3- and HAsO3_2- was 65.3×103, 72.6×103 and 95.7×103 J/mol respectively. According to the primary salt effect, only the HOCl molecular was involved in the oxidation reaction in the NaOCl solution. Under acidic conditions, existence of Cl- could speed up the reaction rate. The reaction process could be simulated by the reaction rate expression.Jar tests simulating the reactor showed that As (III) removal efficiency was poor by direct coagulation with FeCl3. When the initial concentration of As (III) was 140μg/L, the total arsenic concentration of the treated water was 54.1μg/L. When adding 0.30 mg/L NaOCl solution, the total arsenic concentration of the treated water was only 0.5μg/L. When the pH value of raw water was 7.91, the oxidation reaction could be completed within 30 s, and the treated water could also meet the demand when NaOCl and FeCl3 were added at the same time.When the bench test was carried out by the MCR, the operation condition was optimized. When the concentration of the total arsenic in the raw water was 197.4μg/L and the dosage of NaOCl and FeCl3 were respectively 0.50 and 15 mg/L, the average concentration of the total arsenic in the treated water was only 1.3μg/L. The pH, turbidity, organic matter, iron and other ions in treated water all met the Standards for Drinking Water Quality. The membrane fouling resistance had a small proportion in the total membrane resistance, and the reduction of FeCl₃ dosage could slow down the growth of membrane resistance. The change of particle size had little effect on the membrane resistance, while the distribution width (Span) played a major role on the change of membrane resistance.The pilot test treating filtered water by the ultrafiltration process showed that this process was reliable and the treated water quality was stable. The filtration flux, prechlorination, method to discharge sludge and gas-water ratio all affected the change of membrane resistance. More than 96.6% turbidity of the treated water was less than 0.10 NTU. The average removal efficiency of CODMn and UV254 was 9.38% and 13.14% respectively. The membrane process could ensure the microbiological safety of the treated water. The average removal efficiency of TOC for the filtered water was 7.2%. The removal efficiency of THMs and HAAs precursors were unstable. Prechlorination will significantly increase the agent cost of the membrane system, and the energy consumption took up the major part of the total operation cost.