Morrisville Tackles TSS with Cloth Media Filter Upgrade

The Municipal Authority of the Borough of Morrisville, Bucks County, Pa., recently completed the final piece for its effluent discharge quality upgrade with the installation of three AquaDiamond® Cloth Media filters by Aqua-Aerobic Systems. The Aqua-Diamond Filter is designed as a retrofit into existing traveling bridge filter basins.

The Morrisville Treatment plant, rated for 7.1 million gallons per day (MGD), serves a mix of domestic and industrial customers and discharges to the Delaware River. The average daily flow varies seasonally with a range of 4.0 to 5.5 MGD with wet weather flows up to 13 MGD. The treatment plant uses a Pure Oxygen UNOX biological process.

The Pennsylvania Department of Environmental Protection (PADEP) and the Delaware River Basin Commission (DRBC) regulate the plant’s effluent. The PADEP permit contains an average daily total suspended solids (TSS) limit of 30 mg/L. Under the DRBC allocation rules, the plant needs to reduce the influent biological oxygen demand (BOD) by 93 percent monthly average and the influent TSS by 85 percent monthly average. The treatment plant reliably met the BOD reduction criteria, but at times the effluent TSS reduction was not achieved.

Filter media, underdrain, and sand support porous plates were heavily fouled and FRP filter cell divider sheets were heavily worn and cracked.

As part of a 1985 capacity upgrade, traveling bridge sand filters were installed. After almost 30 years of service, the filters had lost their filtration capacity for both flow throughput and TSS solids capture. The plant received several violations for monthly average TSS limit overages. The operators were reporting that the traveling backwash bridges were jumping their tracks and there was an excessive amount of backwashing.

Filters one and two, complete and in service.

An inspection of the filters showed that the filter media, underdrain, and sand support porous plates were heavily fouled. There was a loss of filter media and the filter rails were heavily worn and cracked. The backwash pumps and backwash shoes were worn, and the porous plates’ fouling was beyond chemical cleaning. The inspection report recommended complete rehabilitation of the filters.

The UNOX process achieves required BOD removals with smaller tanks and a shorter sludge age than the standard activated sludge processes. Morrisville has a sludge age of 3 to 5 days as compared to a standard activated sludge process sludge age of 6 to 8 days. Low sludge age processes can experience fluctuations in the settleability of the biomass and generate a large amount of pin floc. Lower sludge age biomass bacteria can have more polysaccharides on their exterior surfaces, which can have a higher tendency to foul filter surfaces over time.

For the Morrisville filters, the decrease in settleability and increase in pin floc resulted in elevated TSS levels in the filter influent. Subsequently, the filters backwashed more often — sometimes continuously. The increase in backwash exacerbates the operating problems, as the backwash water is recycled to the head of the plant, putting more pressure on the clarifiers and filters.

As a sand filtration alternative, Morrisville investigated cloth media filtration. A bench top test demonstrated the feasibility of the cloth media filtration. The tested sample had a TSS concentration of 48 mg/L and the filtered effluent samples had an average TSS concentration of 6.8 mg/L.

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.

At Morrisville, the cloth media filters have met the expectations of the treatment plant and are critical in maintaining TSS effluent compliance. WW

About the Author: Tim Daily, PE, is a senior process engineer for Pennoni. His focus is on the design and retrofit of water and wastewater treatment systems. He is an instructor for wastewater operator certification and Drexel University On-line PE review courses.