Study On Stepwise Method Of Disinfection For Raw Water Polluted By Micro-Organism In Period Of Post-Earthquake Reconstruction

After the earthquake, the concentration of pathogen in raw water might significantly increase if garbage and dejecta were not disposed appropriately. Thus, the emergency measures should be taken to ensure the quality of water supplies. Based on the actual water quality and treatment process in Mianyang Third Water Treatment Plant, an emergent disinfection process controlling the sudden pollution of pathogen in slightly alkalescent (pH≈8.0) raw water was studied.First, a comprehensive comparison is made on three kinds of disinfectants: chlorine, chloramines and ozone. The effectiveness of E.coli removal, disinfectant persistence, economic and feasibility as well as the actual situation in Mianyang Third treatment plant was taken into account. Chlorine disinfection was chosen as the main disinfectant method, and pre-chlorination followed by conventional water purification process (flocculation + sedimentation + sand filter + disinfection) was studied as an emergent disinfection process to control the burst of high microbial contamination in slightly alkalescent raw water after earthquake.The basic law of chlorine disinfection process was studied in deionized water. Influencing factors such as chlorine dosage, contact time, temperature and pH were discussed, and the result showed that the inactivation of E.coli with the first three factors were positively correlated, while the alkaline condition was not conductive to the chlorine disinfection.Based on the theoretical law of the chlorine disinfection process, the pre-chlorination followed by conventional water purification process was conducted to simulate the actual water quality and treatment process in Mianyang Third Water Treatment Plant. First, the factors affecting water quality which could influence the disinfection efficiency were studied. The results indicated that the removal of E.coli decreased with the increase of TOC and turbidity in raw water. Based on a fixed total chlorine dosage, two-times addition is better than one-time addition in the removal of E.coli, TOC, UV254 and turbidity. Then, as for different levels of microbial contamination after earthquake, the corresponding contingency plans were developed and optimized: when initial E.coli concentration was 10CFU/mL or so, the conventional process (2mg/L chlorine after filter) could make the effluent to the standard; when the initial concentrations of E.coli in the treated water were 10~2,10~3,10~4 and 10~5CFU/mL, the optimal chlorine dosages were 6, 10, 15 and 25 mg/L, respectively. If the original pathogen and organic matter in raw water is high, ozone oxidation can be used as an alternative method: the corresponding ozone dosage to the initial bacteria concentration of 10~4 and 10~5CFU/mL were 2.0mg/L and 3.0mg/L.The process of E.coli inactivation by chlorine was investigated in morphology, structural parameters and mechanical properties, using atomic force microscopy (AFM) imaging technology and determination of force curve. The mechanism of chlorine inactivation of E.coli was dicussed. From the analysis results, the inactivation process was related to the cell wall damage.