Study On The Application Of Coagulation-microfiltration Process For Treatment Of Micro-polluted Water And Low-radioactive Wastewater

With the stringent water quality standards and the decreased cost of membrane, the coagulation-microfiltration process was increasingly concerned in the field of micro-polluted water and low-level radioactive wastewater treatment. Coagulation process could improve the treated water quality, and it could effectively retard membrane fouling because of the floc deposition on the membrane surface.Microfitration process could effectively exert liquid-solid separation function. Two pilot-scale devices of coagulation-mircofiltration processes, which were submerged coagulation membrane filtration (SCMF) with a capacity of 150 m3/d and submerged membrane coagulation reactor (SMCR) with a capacity of 300 m3/d, were constructed in a water treatment plant in Tianjin. The aim was to offer the basic data and the operational experience for new water treatment plant. A study on the resistance to radiation of poly(vinylidene fluoride) (PVDF) hollow fiber membrane was carried out. The aim was to design a device to treat the low-level radioactive wastewater for China Academy of Engineering Physics.The SCMF process, with a flux of 53.3 L/(h·m2), was backwashed after every 30 minutes'filtration. The membrane was chemically cleaned five times during 154 days'steady operation. The membrane fouling rate and the quality of treated water were remarkedly influenced by the quality of the raw water, which was relatively well during low temperature and turbidity period. The dosage of FeCl3 was 6 mg/L in this period. The dosage of polyaluminium chloride was 10~20 mg/L during normal temperature period, and a higher frequency of EFM was needed. The quality of the raw water was worst during high temperature and alga bloom period, so that further measures such as prechlorination and reduction of the flux were required. The SMCR process was operated under continuous aeration, with a 2 minutes'interval after every 8 minutes'filtration. The ratio of air to water was 15:1, and the sludge discharging period was 24 h firstly, then 48 h finally. The membrane flux was 16.7 L/(h·m2) and the dosage of FeCl3 was 3~4 mg/L. The SMCR, whose fouling rate was much lower than that of SCMF, was never chemically cleaned during 134 days'steady operation. Higher coagulation dosage and prechlorination were also required to control the membrane fouling and improve treated water quality when the raw water quality was poor.The quality of the treated water by both coagulation-microfiltation processes met the Standards for Drinking Water Quality (GB 5749-2006). Results showed that the turbidity was removed almost completely in this process. The treated water from SCMF and SMCR had the same average turbidity of 0.09 NTU, with 89.5% and 92.0% of turbidity less than 0.1 NTU respectively. The average CODMn of the treated water from two systems was 2.03 mg/L and 2.25 mg/L respectively. The quality of the treated water from two systems was excellent in microorganic indexes. Compared with SMCR water productivity of 98.8%, SCMF was only 90.9%. The power consumption of SCMF and SMCR process was 0.20 kW·h/m3 and 0.42 kW·h/m3 respectively. But the reagent cost of SMCR process was less than that of SCMF. As the total cost of the power and reagent was concerned, SMCR process was 0.247 yuan/m3,higher than that of SCMF during low temperature period and lower than that of SCMF during normal temperature period.A study on the remove of soluble organic compound and THMFP with coagulation-microfiltration process was carried out. The results of ultrafiltration test showed that the organic matters with low MW were the major THMs precursors and the organic matters with MW between 3k-1k Da had the greatest THMFP in raw water. The organic matters with high MW were removed efficiently, and that with low MW even had minus-increase reversely. UV254 had a better removal than DOC in various MW fractions. THMFP removal efficiency of the two systems is nearly 40%. Powered actived carbon (PAC) absorption test was performed to remove THMFP. By the jar test, the contact time and dosage were 45 min and 20 mg/L respectively, and the remove rate of THMFP was 57%.The effect of low-level doses of gamma radiation on the properties of the PVDF hollow fiber membrane was studied. In the experiment, different radiation doses were chosen with the certain dose rate. The membranes dipped in the sodium hydroxide solution with pH of 10 were irradiated and some performance tests were carried out. The radiant chemic reaction mechanism showed that–OH, -COOH and unsaturated chemical bond were appeared on membrane functional surface, so the flux, the maximal pore size and the mechanical property were affected. Based on the results of the tests, suggestion for the engineering application was proposed.