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Critical flux has been defined as the permeate flux of a membrane system under which little or no fouling is observed. Exceeding critical flux results in rapidly increasing transmembrane pressure and fouling. This article describes bench-scale techniques that can be used to measure critical flux and determine the effects of changing water quality on membrane performance. The bench-scale technique also demonstrates the effectiveness of coagulation on low-pressure membrane system performance. The critical flux test for lowpressure filters could be considered the operational equivalent of jar testing in conventional water treatment plants. It is proposed that utilities and consultants should use similar benchscale critical flux measurements to assist in low-pressure membrane system design, to modify operation during periods of changing water quality, and to determine the effects of coagulation or other treatments on low-pressure membrane system performance.

In this study unidirectional flushing (UDF) was conducted to improve the effectiveness of deposit removal from internal pipe walls and to provide information on pipe networks to enable operators to implement flushing in field. Four ductile cast iron pipes with epoxy-lined and cement-lined were selected for the practice of UDF. Water pressure was measured using a data logger equipped with a pressure gauge. The normal flow velocities of the networks were found to be 0.12 m/sec or below in the study area. Flushing velocities ranged from 1.07 m/sec to 2.78 m/sec by using a blow-off valve for discharge. The discharged water was taken in five-minute intervals to measure water quality parameters: turbidity, residual chlorine, pH, temperature and conductivity in the field; metals, inorganic ions, total suspended solids and heterotrophic plate count bacteria in the laboratory. The practice of periodic UDF as preventive maintenance is expected to contribute to enhancement of safety and stability of the water supply. It was indicated that part of the biofilm could be removed with the sediments in the pipe by UDF since the removal of HPC bacteria was correlated with total suspended solids (TSS) (R 2 ¼ 0.90). The coefficient of determination (R 2 ) between turbidity and TSS is 0.90 except for one outlier. Sediment removal could be estimated from the turbidity data of flushed water in the field using the equation of the relationship between turbidity and TSS.