While some of the more innovative methods of leachate treatment still combine biological and physical-chemical methods to meet effluent requirements, high-tech biological systems that use specialized bacteria are promising candidates for leachate treatment, according to industry experts.
Membrane biological reactors (MBRs) are fairly new innovations that are an improvement of classic biological treatment, says Frank DeOrio, technical manager for O’Brien and Gere based in Syracuse, N.Y. “MBRs provide an advantage in allowing for longer solids retention times than traditional biological systems,” DeOrio says. “It is this extra solids retention that allows the bacterial mass to acclimate to the leachate and select for bacteria more resilient and efficient in treating the biodegradable constituents.”
Membrane technology also plays a role and has made major advances in recent years regarding physical-chemical methods, he says.
“The ability to manufacture precise occlusion limits in membrane systems has made ultrafiltration and reverses osmosis (RO) a more common treatment approach in modern leachate treatment systems,” he adds. Up to 99 percent rejection rates for dissolved solids are possible with current membrane technology.
Kevin Torrens, vice president of Brown and Caldwell in Upper Saddle River, N.J., agrees that MBRs that use an ultrafiltration membrane as part of a biological treatment process are now being used fairly commonly and RO is being applied to meet low discharge limits.
“MBRs produce a high quality effluent and allow for a reduced footprint as compared to conventional … biological treatment systems… Polishing steps can be added to an MBR easily if needed to achieve improved effluent quality,” he says. Moving bed bioreactors (MBBRs) can also increase capacity of existing systems and provide treatment for dilute leachate.
Torrens says that there is research under way to also evaluate advanced oxidation processes and specifically what’s known as Fenton’s oxidation to target specific contaminants and in conjunction with landfill bioreactors. (Fenton's reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or wastewaster.)
“Full-scale systems are still down the road,” Torrens says. “Forward osmosis is also under evaluation. Enhancements to biological processes to reduce energy and chemical requirements such as Anammox are of great interest but have not yet been proven for leachate.”
William Soukup, senior hydrogeologist and client service manager for Cornerstone Environmental Group, a TetraTech company based in Middletown, N.Y., says that some facilities employ RO systems as one of the latest options to address difficulties in meeting effluent limits.
“RO takes advantage of the process of osmosis—where aqueous solutions of different concentration will ultimately equilibrate—but reverses it by application of pressure,” he says. In these cases, RO units are preceded by some form of treatment to reduce solids loading that can clog the membrane. The leachate is separated from the effluent by a semi-permeable membrane that is capable of removing dissolved species. “Thus it is more often considered when the total dissolved solids concentration in leachate is a problem for discharge,” he says.
More innovative treatment methods are still in the research phase.
For example, research is taking place on bioelectrochemical systems, Soukup says. “This type of system can use a microbial fuel cell (the leachate is the fuel), where a current is imposed across electrodes in the presence of bacteria and electricity can be generated. This type of system is intriguing with the potential for generating renewable electricity.”
However, more needs to be learned about microbe interaction with electrodes and if such systems can be scaled large enough for practical applications, he says.
Another method that has not yet been applied to municipal landfill leachate but Soukup says shows promise based on research associated with fracking fluids is non-thermal plasma.
“Non-thermal plasma has the potential to soften leachate thereby eliminating scaling problems and as a result provide for the possible application of vapor compression distillation—a highly efficient distillation technology—for leachate treatment,” he says.
Ronald Ruocco, project manager III of water resources practice for Charlotte, N.C.-based Civil & Environmental Consultants, says that innovative methodology is typically driven by need and is site specific.
“All can be quite successful if a systematic approach is followed by facility management and the … engineers and technicians,” he says.
The most successful technologies to date have been the aerobic biological systems in combination with chemical precipitation, membrane separation, or filtration, says Soukup. “These systems are tried and true, and if properly operated can typically meet discharge standards for pretreatment and discharge to a POTW or direct discharge.”
Torrens argues that MBRs have been very successful treating leachate in the U.S. and overseas.
“The humidification/dehumidification distillation process has also performed well although the operating history is only about six months,” He says. “RO has been challenged if used as a stand-alone system but is working successfully following pretreatment. MBBR has also been successful in a variety of applications,” he says. “It is important to note that the success of any technology depends on numerous factors beyond the specific technology. Some things that are critical for success are proper influent equalization, operations staff capabilities and commitment and owner support. Failure on any of these can make the best process fail.”
DeOrio says he is sticking with traditional biological systems, such as the sequencing batch reactor, that continue to be a successful treatment approach.
“Advanced biological treatment such as MBR coupled with RO has been able to meet even the most stringent requirements,” he says. “These combined systems provide a great deal of flexibility in the traditional biological treatment of leachate, and the performance of the membrane technology is often a tough combination to top.”