Located off of Highway 64 in Bradenton, Fla., the Lena Landfill in Manatee County has faced many obstacles in an effort to control its waste. Manatee County has a population of more than 310,000 residents and collects over 350,000 tons of trash per year. Occupying more than 330 acres of land, the landfill must bury the trash while preserving and protecting the environment in the surrounding area.
One way of protecting the environment and the surrounding population is to keep leachate, or potentially hazardous liquids containing contaminants from decomposing trash within the landfill, from entering groundwater or surrounding rivers and ponds. To accomplish this, the landfill has a system to direct all runoff water and storm water to a 120-acre holding/settling pond. In 2005, Manatee County’s Lena Landfill was facing fines and penalties if some water quality issues were not addressed.
The Florida Environmental Protection Agency (EPA) requires that all water discharged to storm water ditches and drains meet a quality standard of 29 Nephelometric Turbidity Units (NTU) or less. Turbidity represents the use of reflected light to measure the size or density of solid particles present in a liquid. The Lena Landfill was operating with a turbidity of 35 to 40 NTU. There were limited options on how to remedy the problem. One option was to build an additional holding pond that would occupy an additional 60 acres of the landfill’s valuable land. This land, however, is considered to be extremely valuable on a square-foot basis. Receiving a permit from the Department of Environmental Protection for a new hazardous waste site takes many years and can be very difficult to obtain. Another option was to filter the water in the existing settling pond while preserving the unused acreage for future use to bury trash from the rapidly growing population in the county.
Developing a Solution
In an effort to avoid millions of dollars of lost revenues, Manatee County contracted with the engineering firm Post, Buckley, Schuh & Jernigan (PBS&J) to develop a solution. There are many factors to consider when choosing a filtration system, including the capital cost, efficiency and maintenance. David Weber, an engineer at PBS&J, specified the installation of an automatic disc filter system capable of handling 2 million gal of water per day.
The automatic Turbo-Disc Filter system was designed and manufactured by Miller-Leaman, Inc. of Daytona Beach, Fla. The system consists of two, 24-pod systems, complete with booster pumps. As the water in the leachate, or settling pond, rises to a pre-determined level, submersible pumps activated by a float switch are engaged. The water is then directed through the fully automatic, self-cleaning Turbo-Disc Filter systems and discharged into adjacent waterways, meeting the Florida EPA requirements. The automatic filters were designed to accommodate 1,400 gal per minute of water, which is contaminated with particles, sediment and organic contaminants. The Turbo-Disc Filter system was more cost-effective than a traveling bridge sand media filter that was also being considered for the project. In addition, the Turbo-Disc Filter uses a fraction of the backwash water used by competing sand filters.
Now it was time for the test. In an effort to validate its technology and familiarize the landfill personnel with the automatic Turbo-Disc Filter, Miller-Leaman transported its fully operational demonstration trailer to the site. The demo trailer has the ability to pump water from the actual water source to determine the solids’ loading, the particle distribution size, NTU level and other important variables. The filter housings in the demo trailer are transparent, allowing the customer to visualize the effectiveness of the filter, both in filtration and backwash modes (click here to see a diagram). Each micron size was tested and water samples were taken to check the NTU level. Once the appropriate micron size was determined to satisfy the NTU requirement specified, the information learned from the demo was extrapolated to properly size the Turbo-Disc Filter system.
Each filter housing (pod) utilizes a stack of color-coded polypropylene discs with grooves molded across the surface of the individual discs, offering three-dimensional filtration capability. When stacked, the grooves overlap in a “cross hatching” effect, therefore creating a tortuous path through which the dirty water must flow. As the dirty water passes through the disc media, particles are trapped within the disc grooves, allowing clean water to flow to the outlet manifold of the filter and downstream. Disc filtration technology was developed decades ago by a company contracted by the Boeing Corp. and was originally used to filter fine metal particles from hydraulic fluid on aircrafts.
An automatic backwash cycle is initiated by an onboard Maxim backwash controller. The Maxim controller comes complete with a differential-pressure switch-gauge that continuously monitors the differential pressure across the stainless steel inlet/outlet manifolds of the filter. When the differential-pressure set-point is reached, an automatic backwash cycle is initiated. First, the filters’ booster pumps engage and a sequence of three-way backwash valves are actuated, allowing a flow of filtered water to sequentially reverse through each filter pod. The filter system was designed with booster pumps to minimize the operating costs of the system; the booster pumps only engage during the backwash cycle when a higher pressure is necessary for an optimal cleaning of the disc media.
Compressed air is also utilized to actuate the backwash valves. In addition, a small burst of compressed air is used to evacuate the dirty water out of each filter pod, just prior to the pod being backwashed. During the backwash cycle, clean filtered water is sprayed from four spray bars that are located on the inside of the discs and spray outward. Each of the four spray posts have multiple spray nozzles that are tangentially orientated relative to the disc media, therefore causing the discs to spin at a high velocity during the duration of the backwash cycle. Each pod takes approximately 20 to 30 seconds to backwash using as little as 8 to 10 gal of water. The dirty water comes out of the 2-in. backwash line.
Maintenance of the automatic Turbo-Disc Filter system is performed approximately once a year (this varies depending on water quality of a given application) and is relatively simple. For example, the top piston of each disc cartridge has a lubricated o-ring, allowing a piston to open up to decompress the disc media during the backwash cycle. The maintenance staff can simply unthread (no tools necessary) the entire piston cap to lubricate these o-rings.
Filtering existing storm water retention ponds is a cost-effective solution for other high-value real estate developments as well, such as shopping centers and commercial properties, where the availability of land is scarce or non-existent.