No two leachates are the same. Since each landfill has its own constituents, the liquid that passes through the solid waste takes on its own unique properties. Leachate quality also changes over time, so a treatment system must be sized according to individual parameters and should be flexible enough to treat a varied influent stream.
The quality of the leachate stream is an important management issue. Leachate is categorized by water chemistry, heavy metals and volatile and semi-volatile compounds.
Historically, leachate was ignored or siphoned off to an evaporation pond. Subtitle D will substantially increase the amount of leachate collected, and the Clean Water Act (CWA), which is still on the congressional docket for reauthorization, will regulate its treatment.
Options for disposing leachate include discharge to a remote facility such as a publicly owned treatment works (POTW) plant; discharge to the environment; pumping and hauling to a POTW; land application and treatment; and natural or mechanical evaporation.
Leachate can either be treated or pretreated, depending on the discharge options and state regulatory standards. Pretreatment usually applies before the leachate is discharged into a POTW plant.
The typical processes used for pretreatment include equalization, aeration, pH adjustment and metals removal.
Leachate treatment technologies fall into two basic types: biological and physical/chemical. In large systems, these types often can be combined to meet high discharge standards.
The most common biological treatment is activated sludge - a suspended-growth process that uses aerobic microorganisms to biodegrade organic contaminants in leachate. With conventional activated-sludge treatment, the leachate is aerated in an open tank with diffusers or mechanical aerators. After the aeration phase, the mixed liquor of microorganisms and leachate is pumped to a gravity clarifier.
The rotating biological contactor (RBC) is an attached-growth, aerobic, biological treatment process in which a series of discs are partially submerged in a tank of leachate. The disks eventually develop a slime layer, then rotational shear forces strip off the excess solids and carry them with the effluent to a clarifier, where they are settled and separated from the treated waste.
The carbon technique removes dissolved organics from the leachate. Although carbon systems may be useful with some older leachates, the cost of the carbon in the regeneration stage can make the process one of the most expensive treatment options.
Advanced Treatment The new landfill regulations have made some treatment systems obsolete. Many landfill operators are now choosing new systems that produce a cleaner effluent and can reduce capital and operating expenses. Such systems include:
* Recirculation and Injection. Direct recirculation distributes the leachate onto the landfill in a semi-closed loop process. While promising, this system has limitations of recirculating 100 percent of the leachate without literally soaking the landfill.
* Membrane Solution. Membrane technology can be adapted to many steps of purification and keep clean-up standards at a high level. Membranes can remove contaminants without extensive biological infrastructure or toxic chemicals.
* Reverse Osmosis (RO). Prior to 1988, reverse osmosis wasn't able to treat leachate successfully due to the core membrane design of spiral-wound modules, which were state-of-the-art at that time. While this method produced efficient results, it also promoted bio-fouling and premature clogging.
Disc Tube technology, developed by the Rochem Group, has been installed in more than 35 European landfills to treat feed waters that would foul conventional RO configurations. After the contaminated water is fed into the tubular chamber, its flow is controlled as it passes through a system of discs and over flat membrane cushions, removing clean water and concentrating the waste material. The turbulent flow reduces the membranes' tendency to scale or foul and requires cleaning less frequently.
The system removes heavy metals, suspended solids, ammonia and hazardous non-degradable organics including pesticides and herbicides without extensive pre-treatment systems. The pure water is clean enough for direct discharge into the environment and accounts for 75 to 92 percent of the leachate. The remaining concentrate can then be recycled to the landfill or further processed.
When installing a leachate treatment system, choose a plan that will provide the maximum amount of long-term flexibility to assure compliance with future regulations and discharge standards.