Experimental Study On Membrane Technology For Drinking Water Treatment

Low pressure membrane separation technology is essential for the preparation of safe drinking water and is universally recognized in the field of drinking water treatment. To investigate the feasibility of coagulation - sedimentation - ultrafiltration (CSU) and coagulation - sedimentation - sand filtration - ultrafiltration (CSSU) processes, a pilot-scale of membrane system was set up, and the quality and stability of treated water, removal of organics, stability of system, operation costs of system and other indicators were analyzed and compared. In addition, in order to improve the productivity of the pilot-scale membrane system, a scheme of recovering membrane backwash water (MBW) was applied,which was to pre-treat MBW and recycle the effuent into the equalization tank. Two lab-scale coagulation microfiltration (MF) membrane systems were installed. The raw water of one membrane system was a blend water (pre-treated MBW blended with Luan River water (LRW) in a 1:9 ratio), and a separate membrane system was used to treat LRW.In the operation process of pilot-scale membrane system, the mode of pre-chlorination plus chemically enhanced backwash (CEB) was more effective to maintain system stability. The quality of treated water from the CSU and CSSU processes was safe and reliable, and met the requirement of the Standards for Drinking Water Quality (GB5749-2006). The mode of pre-chlorination with a low concentration of NaOCl was not cause security problems. The value of CODMn and UV254 in the treated water and costs by CSU were higher than those by CSSU, but turbidity in the treated water was lower. The productivities of CSU and CSSU processes were 88.2% and 91.8% respectively.Membrane fouling could slow down by pre-chlorination process which the CSU and CSSU process used. CEB could recover the membrane specific flux effectively. The effect of chemical cleaning with oxalic acid was better, and iron and dissolved organic matter were the main factors which leading to the pilot-scale membrane fouling.Compared to LRW, the concentration of DOC and CODMn in the two MBWs from CSU and CSSU processes was higher, but the value of UV254 was lower. Organic matter characterized by DOC in MBWs consisted primarily of compounds with a MW greater than 30 kDa and substances with a MW less than 1 kDa, whereas UV₂₅₄ represented compounds with a MW less than 1 kDa. The removal rate of CODMn, DOC and UV254 in the MBWs was improved due to addition of powdered activated carbon (PAC) in the pre-treatment. In the process of coagulation/PAC, the organics characterized by DOC with a MW greater than 30 kDa was removed substantially, but an increase in the removal efficiency of DOC with MW<10 kDa were not observed due to the enrichment of low MW organic matter during the coagulation process, which adsorbed more efficiently to PAC.Compared to the coagulation pre-treatment, the quality of finished water in the mixed water system had been improved by coagulation/PAC process. There was no obvious difference in water quality of the finished water form two lab-scale membrane systems. And the membrane fouling was not deteriorated after recovering MBW which was pre-treated by coagulation/PAC. The productivity of CSU and CSSU was reached 99% by recovering MBW.