Examination of plant performance and filter ripening with particle counters at full-scale water treatment plants

Filter performance has traditionally been assessed using turbidity as the main water quality parameter. However interest in process optimization and the ability to accurately monitor filtration and particle removal has led to particle counting becoming increasingly more popular. The objective of this research was to evaluate filtration performance and filter ripening at four full-scale water treatment plants. Most of the plants investigated in this study were capable of achieving 2 log total particle removal as well as an effluent turbidity of 0.1 NTU. In some cases 2 log removal was achieved in the lower particle size ranges of 2-5 and 5-10 microm. Log removals in this study compare particle counts in the filtered and raw water. For water treatment plants that do not filter to waste, the ripening period following a filter backwash represents a time where the plant may be most vulnerable to breakthrough of waterborne pathogens. For the plants studied, the ripening period which was defined as the time required to reach peak particle removal and a turbidity of 0.1 NTU, were very similar in duration. The ripening period generally lasted for one hour after the backwash event had been completed. Because the times to reach 2 log total particle removal and 0.1 NTU are similar, the practice of using 0.1 NTU turbidity as a measure of filter ripening may be a useful benchmark for utilities that do not have particle counters.