Experimental Research On High-Arsenic Source Water And Low-Level Liquid Radioactive Waste Treatment By Coagulation/Microfiltration Process

Coagulation/microfiltration(C/MF) process is of good potential in water treatment application due to its advantages of simple operation, low energy consumption, excellent treated-water quality and less sludge production. The treatment of high arsenic and fluoride source water by C/MF process, its influence factors and its sludge disposal were studied. An on-board low-level liquid radioactive waste treatment device was developed and a cold experiment was conducted to verify the design. The application field of C/MF process was enlarged.The results of jar tests and lab-scale C/MF membrane reactor tests showed that C/MF process with ferric chloride as a coagulant could be used for treatment of high-arsenic source water with different source water quality. Arsenic removal to low level (<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. The average turbidity of treated water was 0.05 NTU and the average concentration of total iron in treated water was 0.03 mg/L. pH value of treated water was increased about 0.48 compared with source water. Appropriate pretreatment was needed when the concentration of silicon in raw water was higher than 3.50 mg/L.Arsenic concentration of the treated water could also be lowered to less than 10μg/L from about 100μg/L by C/MF process with aluminium sulphate as a coagulant when coagulant dose was 50 mg/L. When coagulant dose was 150 mg/L, arsenic and fluoride were removed simultaneously by C/MF process with aluminium sulphate as a coagulant. The concentrations of aluminum and sulphate of the treated water were entirely satisfied the new Standards for Drinking Water Quality.Comparing the characteristics of two processes, it is concluded that C/MF process with ferric salt as a coagulant should be used preferentially when only high concentration arsenic existed in source water and appropriate measures to retard membrane fouling is needed. However, C/MF process with aluminum salt as a coagulant should be used to remove arsenic and fluoride simultaneously from high arsenic and high fluoride coexisted source water.The results of membrane resistance calculation indicated that inorganic fouling resistance occupied little proportion of the total resistance. Organic fouling was the main reason which induced the decrease of membrane flux of C/MF process with ferric salt as a coagulant. Irreversible membrane fouling of C/MF process with aluminium sulphate as a coagulant was much severe and was the main component of membrane fouling.The experimental results of sludge disposal showed that the arsenic-bearing sludge produced in the treatment of high-arsenic source water did not belong to the hazardous waste with leaching toxicity. When the arsenic-bearing sludge was solidified with cement, arsenic concentration in the leaching solution reduced about 90%. The compression strengths of road-application concretes were increased about 12% when minor part of ferric salt arsenic-bearing sludge was added. It provides a method for the efficient utilization of arsenic-bearing sludge. At the same time, the possible environmental problems are avoided.Oxidation-C/MF hybrid process was employed by the on-board low-level liquid radioactive waste treatment device which contain two same membrane reactors. The main membrane module with effective filtration area of 20 m² and the auxiliary membrane module with effective area of 1.0 m² were installed in the two membrane reactors. The device with on-line monitoring equipments was run automatically under the control of a programmable logic controller. The device has the advantage of compact structure and can be removed flexibly and safely on the existing roads. The treatment capacity of the device is 500 L/h. The results of the cold test showed that the condensing factor could be reached to 1010.