Little Falls Run Wastewater Treatment Facility began operation in 1991. As one of two wastewater treatment facilities owned by Stafford County, Va., the plant was constructed to accommodate the area’s growing population. The plant’s original treatment scheme included an activated sludge system with a secondary clarifier for secondary treatment and dual-bay traveling bridge sand filters, 44 ft long by 16 ft wide, for tertiary filtration.
In 2003, the plant began experiencing problems with the filters going into constant backwash mode when the flows exceeded 6 mgd. To remedy the problem, the plant operators were bypassing a portion of the flow. With flows expected to increase and the anticipation of tougher discharge limits, the Stafford County Department of Utilities initiated a project to resolve the situation.
Alternative solution
The initial design consideration was to double the filter capacity by duplicating the existing traveling bridge filter process, which involved constructing identical cast-in-place concrete tanks with matching filter underdrains, dual-media and traveling bridges. Significant drawbacks to this solution included very limited land space with the elimination of future expansion possibilities, concerns about the actual capacity of the expanded filters and expected downtime during the construction period.
The Department of Utilities decided to investigate an alternative solution, the AquaDiamond cloth media filter manufactured by Aqua-Aerobic Systems, Inc. Two diamond filters could potentially treat 12 mgd on average and 24 mgd during peak flows within the existing filter basin footprint with minor structural modifications and for approximately the same cost as duplicating the original filter process.
This benefit, combined with the successful operation of an AquaDisk cloth media filter at its sister facility, prompted the Department of Utilities to conduct a three-week pilot program to test the filter’s suitability for Little Falls Run. The pilot program demonstrated the filter’s ability to meet more strict design limits under extreme hydraulic and solids loading conditions.
Incorporating new filters
The expansion project was then redesigned to incorporate the cloth media filters. The traveling bridge sand filter tanks were retrofitted one at a time to reduce interference with plant operations. The first retrofitted cloth media filter went into operation in August 2005, within six weeks of the time the existing sand filter was taken out of service. A testing period followed to validate performance requirements prior to removing the second existing sand filter from service. Again, performance results were excellent, so the second filter was retrofitted with a cloth media filter in September 2005.
Filtration process
During filtration, the cloth media is completely submerged. Solids are deposited on the outside of the cloth as the influent wastewater flows through. The filtered effluent is collected inside the diamond lateral and flows by gravity to discharge. The filtration process requires no moving parts. Increased headloss due to the deposited solids automatically initiates periodic backwashing.
During backwash, a pump provides suction to the vacuum heads, allowing solids to be vacuumed from the cloth as the platform traverses the length of the diamond laterals. The platform operates only during backwashing and solids collection. Because of the vertical orientation of the media, some solids will settle to the basin floor during normal operation. Small suction headers provide a means for collecting and discharging the settled solids. The solids collection process utilizes the backwash pump for suction.
Design characteristics & operating data
Little Falls Run currently has a design average daily flow of 4 mgd and an actual flow of 3.45 mgd.
Current discharge limits for the plant include 9 mg/L BOD, 9 mg/L total suspended solids and 2 mg/L total phosphorus; however, the plant expects more stringent limits in the near future, with discharge requirements of 4.5 mg/L BOD, 4.5 mg/L total suspended solids and 0.5 mg/L total phosphorus. The filters are already consistently producing effluent total suspended solids below the expected future limits of 4.5 mg/L.
In fact, stringent testing has shown excellent results with average effluent total suspended solids of 2.4 mg/L with only one filter online. The low backwash waste volume generated has been especially impressive. The results were 0.1% of total throughput in initial start-up testing and 0.3% during the most recent one-month testing period, compared to typical rates of 1 to 3%.
A Little Falls Run Wastewater Treatment Facility representative said, “By selecting this technology, the facility has preserved valuable space for a later expansion, while preparing for more stringent discharge limits in the near future.”
