Study On Disinfection Characteristics Of Membrane Bioreactor Treating Hospital Wastewater And Related Chlorination By-products

Membrane bioreactor (MBR) used in hospital sewage treatment may serve as a pre-disinfection unit, in addition to its solid/liquid separation and biological degradation of pollutants functions, to produce the effluent with high quality. The aim of this study was to evaluate the pre-disinfection characteristics of MBR, chlorination disinfection by-products and the bio-toxicity for the effluent from a MBR treating hospital wastewater.The experimental results show that the removal rates of total bacteria and fecal coliform by this MBR are 2.0³.9 log and 2.8⁴.0 log, respectively, due to the rejection by membrane and the visible cake layer developed on the membrane surface. The disinfection efficiency decreases after the MBR having been operated 22-month continuously, for the membrane fibers'ageing and leakage.In this time-course study, sodium hypochlorite (NaClO) was used as a disinfectant to inactivate the residual indicator microorganisms in the raw wastewater and the effluent from MBR. The total bacteria and fecal coliform in the raw wastewater behaved"trailing effect"obviously, and their kinetic fitting accords with the Collins-Selleck Model, which can express both"lag and trailing". The results indicated that the total bacteria and fecal coliform in the MBR effluent were completely inactivated when the concentrations of available chlorine were 3.0 mg/L and the contact time was 2 min, meanwhile the concentrations of chlorine residual were less than 1.58 mg/L. With the nonlinear fitting for the experimental data, the Chick-Waston model incorporating a first-order disappearance term for chlorine residual was found to best describe the observed inactivation of total bacteria in the MBR effluent.The ion chromatography combined with solid phase extraction (SPE) method was developed for the analysis of sub-μg/L haloacetic acids (HAAs) concentrations, a class of disinfection by-products formed as a result of chlorination of wastewater. Linearity, repeatability and detection limits of the method were evaluated. The detection limits of monobromoacetic acid and dibromoacetic acid are 2.61μg/L and 1.30μg/L respectively, and the other three rang from 0.48 to 0.82μg/L under 25-folds preconcentration. When the above optimization procedure is applied to three hospital wastewater samples with different treatment processes in Tianjin, and the results indicate that the dichloroacetic acid is the major compound, and the growth ratios of the HAAs after disinfection by sodium hypochlorite are 91.28%, 63.61% and 79.50%, respectively.The chlorine-demands of the raw wastewater are 5¹0 times higher than that of the MBR effluent under the same value of CRT. Also the increasing rate of chlorine-demands in the raw wastwater is quicker along with the increasing of CRT value.To meet the requirement for indicative microorganism (fecal coliform) in the Discharge Standard of Water Pollutants for Medical Organization (GB18466-2005), the CRT value required for the MBR effluent is about 0.0075 mg·h/L, meanwhile, the concentration of trihalomethanes (THMs), haloacetic acids (HAAs) and adsorbable organic halogen compounds (AOX) are 8.01μg/L, 19.05μg/L and 102.5μg/L, respectively. And correspondingly the bio-toxity by acute toxicity test with Daphnia magna is 8.96μg/L (K₂Cr₂O₇), which is much lower than the raw wastewater.