by Michele Monroe
One of the most important problems with designing and maintaining a landfill is managing the leachate that is generated when water passes through the waste. The leachate consists of many different organic and inorganic compounds that may be either dissolved or suspended. Regardless of the nature of the compounds, they pose a potential pollution problem for local ground and surface waters.
Many factors influence the production and composition of leachate. One major factor is the climate of the landfill. For example, where the climate is prone to higher levels of precipitation there will be more water entering the landfill and therefore more leachate generated. Another factor is the site topography of the landfill that influences the runoff patterns and again the water balance within the site.
VSEP and Landfill Leachate
Current leachate treatment options include recycling and re-injection, on-site treatment, discharge to a municipal water treatment facility or a combination. However, with stricter regulations regarding ground and surface water contamination, landfills are having to find new treatment alternatives. One treatment approach is to use VSEPTM (Vibratory Shear Enhanced Processing). Developed by New Logic International, Inc. of Emeryville, California, this advanced membrane technology has made it possible to filter streams containing a variety of components without the fouling problems exhibited by conventional membrane systems. This new system does not only filter the suspended solids but also reduces or eliminates dissolved organics and inorganics. The result is a crystal-clear water stream and a concentrated sludge.
The main difference between VSEP and traditional crossflow membrane filtration is the mechanism by which the foulants are prevented from accumulating on the membrane surface. A traditional crossflow system relies on the fluid velocity of the feed material to reduce fouling. The high feed velocity creates shear forces in the liquid which help maintain the motion of the solids and slow their deposition onto the membrane surface. However, a thin, stagnant boundary layer remains on the surface of the membrane that allows the solids to accumulate and eventually deteriorate the throughput rate. On the other hand, a VSEP system utilizes a patented vibratory drive mechanism that creates a high shear force on the surface of the membrane in order to repel foulants. This mechanism enables the filter to maintain higher, sustained throughput rates and process larger volumes of material economically without the pretreatment costs of conventional crossflow systems.
Western Landfill Using VSEP for Nickel Reduction
Many tests have been performed on VSEP laboratory and pilot units with leachate from various landfills. Depending on the location of the landfill, the chemistry of the leachate and the discharge specifications have differed. The most extensive test work has been completed on leachate from a landfill located in the Western U.S. The discharge specification at that location requires the reduction of chelated nickel.
Membrane Selection and Testing
The initial testing was completed on a small laboratory VSEP unit and was used primarily to identify a membrane that would meet the discharge specifications. A thin-film nanofiltration membrane with a 50% sodium chloride rejection was selected. This membrane was used to construct a 1.393 m2 (15 ft2) filter pack for pilot test work on a slightly larger VSEP unit. The pilot testing confirmed the information that was collected during the laboratory test work, including nickel rejections and flux data (see Figure 2).
Figure 2. Pilot test data showing the relationship between flux and amount of filtrate which was recovered from the feed stream.
System Design and Processing
After both the preliminary test study and additional pilot testing, VSEP was selected as the appropriate treatment system for the leachate. The single VSEP unit will be filtering 113.6 l/m (30 gpm) of the leachate in order to reduce the nickel by 85-90%. The clean effluent stream from the VSEP will be discarded to the local POTW. The concentrated stream or sludge will remain on-site for further treatment, including volume reduction by evaporation and some chemical treatment. The site hopes to evaluate the possibility of reusing the clean water but at this time the goal was to meet the local discharge specifications.
VSEP offered many advantages compared to alternative treatment systems that included large amounts of chemical addition or required extensive space for installation. The vibrating membrane units are compact, cost effective and easy to maintain. VSEP units are flexible in that they can be manufactured with almost any membrane media. This feature enables the systems to be customized to fit the processing requirements for different leachates given the chemical composition of the stream. The membrane filter packs are modular and can be replaced to meet changing processing requirements.
Though membranes have experienced great advances in the past twenty years, their use in leachate treatment has only been explored recently. With more stringent regulations placing greater emphasis on leachate treatment, the industry is seeking new technologies to solve the problem. Offering economic and operating advantages, VSEP is a leading technology for treating landfill leachate and will continue to revolutionize the use of membranes in the industry.
1. Diaz, L. F., G. M. Savage and C. G. Gouleke. “Resource Recovery from Municipal Solid Wastes”, Volume II, Final Processing, CRC Press, Inc., 1982
2. Farquhar, G. J., “Leachate Production and Characterization in Civil Engineering”, pp. 317-325 (1989)
3. “Procedures Manual for Ground Water Monitoring at Solid Waste Disposal Facilities” “A Current Report on Solid Waste Management”, United States Environmental Protection Agency, EPA/530/SW-611, August 1977