AquaDiamond® Cloth Media filter laterals with the 5-micron cloth media.
The advantages of the cloth media filter include a 2.7-times higher solids loading capacity than a traveling bridge sand filter; and the percentage of forward flow used for backwash water by a cloth media filter is much less than a traveling bridge sand filter. In the event of an upstream upset, the cloth media filters can temporarily treat TSS excursions up to 200 mg/L. Typically sand filters are designed for influent TSS levels of 20 mg/L and can only accommodate 50 mg/L for short periods of time.
The Authority visited a local treatment plant that had recently retrofitted its traveling bridge sand filters with cloth media filters. During the visit, the operator described the cloth media filter performance during a large storm event, which occurred with one clarifier out of service and a filamentous bacteria outbreak. The filter influent total TSS level was greater than 200 mg/L, which threw the filters into almost constant backwash. During the storm, the filters did not overflow and the plant was able to maintain compliance with its NPDES Permit. After the plant recovered, the plant personnel did not need to manually remove solids from top of the sand beds, as they had done for previous high solids events.
Aqua-Aerobic Systems offered to provide Morrisville with a full-scale, 35-gallon-per-minute pilot filtration unit plant for an extended five-month pilot study to evaluate the ability of cloth media filtration to consistently achieve the TSS discharge limit, verify sustainable cloth media filter performance, and compare the performance of 10-micron and 5-micron pore size filter cloths.
The pilot study showed that both filter cloths could accommodate the effluent TSS limit, but the 5-micron cloth media provided additional removal of the pin floc. During the pilot study, the plant experienced several sustained plant upsets. Both filter cloths showed recovery after the plant upsets and did not show evidence of immediate polysaccharide fouling. The 10-micron cloth backwash volume was on average 1.5 percent of the forward flow and the 5-micron cloth backwash volume was on average 7.0 percent. This difference in backwash rates was expected due to differences in cloth pore size. The pilot study demonstrated that cloth media filtration was a viable alternative to retrofitting the traveling bridge filters.
The cost to rebuild the filters was $2.1 million dollars while the cloth media filter upgrade estimate was $3.1 million dollars. However, it was anticipated that without increasing clarifier solids settling capacity (another $8.0 million dollars), and dealing with the unique operating issues associated with the UNOX process, rebuilding the traveling bridge filters would eventually lead to similar operating issues. Facing over $10 million dollars in costs, Morrisville decided to install the cloth media filters with the smaller pore size filter cloths.
The three filters were installed in a staged sequence beginning in October 2016. As shown in Fig. 1, the treatment plant effluent showed immediate improvements. Prior to installation of the filters, removal had slipped below 90 percent. With the installation of the cloth media filters, the treatment plant removal has increased to over 95 percent at times. During May 2017, the operators tested the effectiveness of the filters. The average filter influent TSS was 40 mg/L and the discharge averaged 11 mg/L with a 73 percent removal. On the maximum day during May, the influent to the filters was 139 mg/L and the overall removal was 91 percent, maintaining the plant’s compliance with its effluent permit.