One of the biggest challenges landfill operators must deal with is the treatment and disposal of leachate. Given the environmental and health threats that can arise from untreated or improperly treated leachate, as well as the significant quantities of leachate that today’s landfills generate, superior leachate management is becoming an increasingly important priority in the waste management industry. Designing and implementing an effective treatment process, however, can be a complicated and sometimes costly endeavor. Read on to learn more about what leachate is, and the various methods different operators are using to treat it.
What is leachate?
The broad definition of landfill leachate is any water or liquid that has been in contact with landfill waste. Most leachate comes from rainwater seeping through the body of waste in a landfill, but it can also be produced by moisture in the waste itself, such as water or juices in food waste. It also can arise from groundwater ingression, though this only occurs in landfills that are inadequately or improperly sealed.
The exact composition of landfill leachate depends on what types of waste are contained in the landfill, the weather and other environmental conditions, and how long the waste is held in the landfill body. However, most leachate contains high levels of persistent organic pollutants like soluble nitrogen and sulfur compounds. For this reason, it can be a significant hazard to human health and the environment if it comes into contact with nearby land or water supplies.
How is leachate treated?
Traditionally, discharging leachate to local publicly-owned treatment works (POTW) or other off-site wastewater treatment facilities for processing and disposal has been the most popular method of dealing with leachate. Under this method, leachate is transported via the local sewer system or by truck to these facilities, which then incorporate leachate treatment into their regular sewage treatment processes. Landfill operators have typically preferred this approach because they themselves are not in the business of treating wastewater (which is what leachate essentially is), but POTWs are. The consensus has been that it makes more sense to let the experts handle the task.
However, POTWs are increasingly facing tighter environmental regulations and more stringent limits on discharge quantities, and this is slowly but surely transforming the landscape of leachate management. Many treatment plants no longer accept any landfill leachate at all, leaving landfill operators rushing to find new ways of treating leachate on-site.
Fortunately, a number of possible alternative techniques already exist that have proven effective at leachate management. These include:
Biological treatments have long been used in leachate management and have proven themselves effective, particularly when combined with some kind of precipitation or filtration system. The idea behind these treatment methods is to use natural biological processes as tools in breaking down the harmful matter contained in leachate, which is largely organic. Aerobic (with air) and anaerobic (without air) systems are used: aerobic methods include the activated sludge process, in which a mass of microorganisms contained in a special reactor is used to break down the organic matter in leachate into carbon dioxide, water, and inorganic compounds. Anaerobic methods include the use of anaerobic lagoons, in which the process of anaerobic respiration converts leachate’s volatile organic compounds into carbon dioxide and methane.
While biological treatment methods are used most often, a number of other methods based on physical or chemical processes are also employed in leachate management. In fact, common practice is to combine biological treatments with a physical or chemical process in order to achieve better quality. Of these physical-chemical processes, reverse osmosis is one of the most commonly used. In this membrane-based process, the leachate diffuses through a semi-permeable membrane, resulting in high-quality effluent on one side, while the concentrate contained in the leachate is trapped on the other side of the membrane and then disposed of separately.
Other methods include chemical precipitation or coagulation, in which leachate is mixed with chemical reagent to coagulate the solids, which then settle and are disposed of; and ion exchange, which purifies and decontaminates leachate through an exchange of ions between it and another solution.
In addition, a number of even more innovative, higher-tech methods are currently undergoing research and development. If successful, they could begin playing a role in leachate management quite soon. Examples include:
In this cutting-edge technology, leachate actually serves as a fuel with the potential for generating a source of renewable electricity. A microbial fuel cell is used to impose a current across electrodes in the presence of bacteria in the leachate, thus producing electricity. However, further research is still needed to determine if these types of systems can operate on a large enough scale to have practical applications in leachate management.
Currently used in association with fracking fluids, non-thermal plasma is a new tool that could be used to “soften” leachate. This would allow the leachate to be treated through vapor compression distillation, an extremely efficient distillation technology that is not possible to use on “raw” leachate.