Experiment on purified effect of ceramic membrane for digested slurry

Abstract: Due to the high concentration of suspend solids (SS) and colloidal particulate matter in digested slurry, it is difficult to pretreat with membrane concentrate treatment process. However, the pretreatment with organic membrane has some shortcomings in terms of pollution resistance and service life. Given to the advantages of ceramic membrane (CM), such as large flux, anti-pollution ability and other characteristics, it is gradually being applied to pretreat digested slurry. However, the key parameters that affect the performance of CM, such as membrane pore size and operating pressure, need to be determined based on the quality of the raw materials. Aiming at this problem, 50 and 200 nm CMs were used to pretreat digested slurry. In addition, the performance of CM, water quality characteristics of effluent, and membrane fouling cleaning strategies were investigated simultaneously. Membrane flux, COD (chemical oxygen demand) and turbidity removal effectivity were used to select the appropriate pore size and operating pressure. Then, a continuous batch test was conducted to examine the stability and the recovery of membrane flux with the appropriate pore size (200 nm) and operating pressure (0.3 MPa). The results showed that with the operating pressure of 0.3 MPa, both of the turbidity removal rates of 50 and 200 nm CM could reach more than 99%, and there was no significant difference on the COD removal between 50 and 200 nm CM, which was 36.2%±0.6% and 32.6%±1.5%, respectively. However, the maximum membrane flux could reach 100.6 L/(m2?h) for 200 nm CM with the operating pressure of 0.3 MPa, which was 52.8% higher than that found in 50 nm CM. However, when the operating pressure exceeded 0.3 MPa, the membrane flux increase rate reduced remarkably. While increasing the operating pressure of 200 nm CM to 0.35 MPa, the membrane flux rose to 105.0 L/(m2?h). Thus, considering membrane flux, turbidity and COD removal efficiencies, 200 nm ceramic membrane at 0.3 MPa operating pressure was selected as experimental condition, and further tests were carried out under this condition. Ceramic membrane operating temperature also seriously affects the membrane flux. As VRF (volume reduction factor) was up to 6, the water temperature increased from 13 to 29 ℃ and the membrane flux increased from 75.0 to 107.5 L/(m2?h). Therefore, on the basis of the further clarification of the correlation between the temperature and membrane flux, in order to improve the operation efficiency during the engineering design process, it is suggested to control the influent temperature by auxiliary heating and heat preservation. After pretreatment, the removal efficiencies of phosphorus, total organic carbon, ammonia nitrogen, total nitrogen and potassium were 61.2%, 35.0%, 3.8%, 6.2% and 3.0% respectively. Continual batch operation of 200 nm CM with a membrane flux of 51-122 L/(m2?h) and an average membrane flux of 84.4 L/(m2?h), which could meet the membrane flux requirements (≥ 40 L/(m2?h)), was performed. The cleaning strategies of sodium hydroxide (NaOH), citric acid, and NaOH + citric acid were compared. The maximum membrane flux recovery rate was achieved by the combination of 1% concentration of NaOH and 1% concentration of citric acid, which reached around 95.4%.