Despite directing all runoff water to a 120-acre holding pond, the 330-acre Lena Landfill in Bradenton, Manatee County, Fla., still faced potential water quality-related fines and penalties in 2005.
For residents’ safety and environmental preservation purposes, the Florida Environmental Protection Agency (EPA) requires that all water discharged to storm water ditches and drains meets a quality standard at or below 29 nephelometric turbidity units (NTU). Turbidity represents the use of reflected light to measure the size or density of solid particles present in a liquid. The Lena Landfill, at the time in question, operated with a turbidity of 35-40 NTU.
Manatee County officials had limited options in remedying this problem. The first involved building an additional 60-acre holding pond, but Lena buries over 350,000 tons of trash annually, so every useable square foot counts. Furthermore, obtaining a permit from the Department of Environmental Protection for a new hazardous waste site can take years of effort.
The second option, filtering the water in the existing settling pond, proved a better fit for Lena. This alternative allowed officials to save precious landfill acreage for burying future trash, a particularly significant factor considering the county’s rapidly growing population.
In an effort to avoid losing millions of dollars in revenue, Manatee County contracted with the engineering firm Post, Buckley, Schuh & Jernigan (PBS & J) to develop the solution. Taking into account the capital cost, efficiency and maintenance requirements of various filtration systems, PBS & J engineer David Weber specified the installation of an automatic disc filter system capable of handing water at a rate of two million gallons per day (gpd).
The automatic Turbo-Disc Filter system by Miller-Leaman Inc. proved less costly than a traveling bridge sand media filter, which the county also considered as an option. The system also uses a fraction of the backwash water used by other sand filters.
How the system works
Designed and manufactured to accommodate 1,400 gallons per minute (gpm) of contaminated water, the automatic Turbo-Disc Filter system has a pair of 24-pod systems with booster pumps. As the water in the settling pond rises to a predetermined level, submersible pumps activated by a float switch are engaged.
The water is then directed through the filters, which house stacks of color-coded polypropylene discs, each with grooves molded across its surface to provide three-dimensional filtration. When stacked, the grooves overlap and have a “cross-hatching” effect, creating a winding path through which dirty water must flow. Particles become trapped within the disc grooves, and clean water can then flow through the filter’s outlet and into adjacent waterways in accordance with Florida EPA standards.
In an effort to validate its technology and familiarize landfill personnel with the automatic Turbo-Disc Filter, Miller-Leaman transported a fully operational demonstration trailer on site. The demo trailer pumps water from the actual water source to determine important variables, including particle distribution size and NTU level. Once the appropriate micron size was determined to satisfy NTU requirements, the information obtained during the demonstration was used to properly size the system.
In the instance of Lena Landfill, filtering an existing storm water retention pond proved a cost-effective solution for managing water quality on a high-value real estate site with limited land availability.