Secondary Pollution Control Of Water Supply And Purification Process At Pipe Network Terminals

In order to solve the problem of secondary pollution of drinking water,this paper carried out field sampling and investigation,laboratory purification experiments,process designing and case study etc.and aimed to provide practical measures to the secondary pollution control and useful solutions for the upgrading of urban water supply with better guaranteed water quality and safety.Current status of urban water supply security measures was reviewed from several aspects,such as water source water plants,transmission and distribution networks,water supply facilities,and terminal purification.Tap-water quality sampling and customer satisfaction survey were carried out in 15 typical communities of a coastal city in southeast China.Results showed that the quality indicators such as turbidity and total iron increased obviously to different extents compared with those of water treatment plant effluent and pipeline water samples.A positive correlation was found among the problematic water quality indicators.It has been confirmed that phenomena of secondary pollution exist in water supply system,usually with high frequency in the early morning and in old communities,mainly because of the corrosion of pipeline materials and turbulence of water flow.In view of the urban water supply system of the city,a series of suggestions are put forward to control secondary pollution of pipelines,improve the customer tap-water quality,and promote the quality of water supply services.Synthetic water of ferric trichloride（FeCl₃）and humic acid（HA）or bovine serum albumin（BSA）was prepared to simulate serious polluted urban drinking water with high concentrations of corrosion products and organic matter.Dead-end filtering of PVDF membrane was carried out with the rejection of the two kinds of pollutants measured and filtration flux during filtration recorded.The results indicated that the coexistence system of Fe（Ⅲ）and HA or Fe（Ⅲ）/BSA had synergistic removal effect during the ultrafiltration.The fact revealed the interaction between the two kinds of pollutants have beneficial influence for the removal of each other with the help of particle size distribution and three-dimensional fluorescence spectroscopy analyses.The coagulation reaction which transformed two pollutants from micro colloidal state to macro particulate state,contributed to the significant improvement in the interception effect of pollutants.In addition,the membrane fouling index was calculated and the reversible fouling index was found to be positively correlated with the concentration of Fe（Ⅲ）,concluding that the interaction effect of binary system could change the characteristics of membrane and reduce membrane fouling.Further experiments of simulated polluted drinking water were carried out with a self-designed apparatus which incorporated an ultrafiltration membrane module as the side-stream device and a static sedimentation unit.Results of showed that Fe（Ⅲ）and organic matter co-existing in concentrated water of ultrafiltration underwent obvious coagulation reaction which resulted average diameter of settled particles to increase from 2.8μm to 35.27μm in 3～4 h.The dynamic ultrafiltration test showed that the water quality of test water was steadily improved in 6 hours operation with the 80%of the metal ions and 50%of the organic matter removed from the original test water and the remaining pollutants concentrated in flush water of membrane were further removed in the sedimentation unit.A Complete-mix mathematical model was established to describe the concentration distribution with the time for water storage tank,which correlates the parameters of the inflow and outflow pollutant concentrations C₀,C_e,side-stream purification water flowrate Q_Rand water supply flowrate Q,etc.,providing a theoretical basis for future engineering application.Community water treatment methods were proposed to achieve the requirement of water quality improvement including full-stream filtration,side-stream filtration and multi-stream filtration etc.An optimum one with innovated water storage tank was designed for a specific community through reasonable selection of equipment and analysis of economic indicators.The case study confirmed that the ultrafiltration technology could be used as a side-stream purification device to reduce the adverse influence in case of sudden pollution and improve the drinking water quality of usual performance in daily time.