Processes for Landfill Leachate Treatment
Landfill leachate is defined as all water that has been in contact with waste stored in a landfill. It arises primarily as rainwater seeps through the landfill body, but also from the moisture inherent to the waste itself or – in the case of inadequately sealed landfills – groundwater ingression.
It is caught in the drainage system and undergoes special wastewater treatment. Depending on the substances present, appropriate wastewater technology must be employed.
Landfills generally contain a highly inhomogeneous mixture of materials, which include both a very high organic component as well as soluble mineral substances. Some of the organic substances decompose naturally in the landfill body.
Properties of Landfill Leachate
Due to these exothermal processes inside the landfill, the temperature of the leachate is usually higher than typical groundwater in the area. Landfill leachate is usually quite turbid, has a very strong odour and a brownish colour.
The composition of landfill leachates differs depending on the type of waste stored, the weather and the holding time in the landfill body. As the landfill holding time increases, so too does the degree of persistent organic pollutants.
After two to five years, the initially aerobic decomposition processes give way to anaerobic processes. Initially, anaerobic decomposition consumes only short-chain fatty acids, and the organic compounds entering the leachate are still reasonably biodegradable. As the landfill holding time increases, anaerobic decomposition progresses to methane production. In addition to a range of soluble nitrogen and sulphur compounds, sulphates and chlorides, the leachate then contains a high degree of persistent organic pollutants.
Untreated Leachate is an Environmental Hazard
Untreated leachate is a hazard to the environment if it is allowed to enter a body of water. Municipal wastewater treatment plants are often not capable of processing the high organic and nitrogen loads in the leachate. It is often sufficient to treat the leachate to the extent that it can be passed off to the next municipal wastewater treatment plant for further processing. If this is not possible, then it is necessary to treat the leachate up to a quality meeting the requirements for direct discharge. In this case, the remaining contaminant load is so low that the treated water may be released into a river, stream or lake.
Technologies are available for the treatment of landfill leachate, proven in many diverse cases. These include, for example, biological processes for wastewater treatment such as MBBR, TFR, activated sludge processes, anammox and loop reactors as well as reverse osmosis.
If persistent biological compounds remain in the wastewater, activated carbon filters and/or ozonisation are employed to remove these contaminants. These processes are deployed both as stand-alone solutions and in combination. The composition of the landfill leachate plays a decisive role in selecting the appropriate wastewater technology.
Technologies for Landfill Leachate Treatment
A range of technologies are available for the treatment of landfill leachate, proven in many diverse cases. The following section introduces selected technologies that have predominantly been employed for landfill leachate treatment. The highlighted technologies are deployed both as stand-alone solutions and as part of a process combination. The composition of the landfill leachate plays a decisive role in selecting the appropriate technology.
Landfill Leachate Treatment with Biological Processes as the First Step
Biological treatment has proven itself in many cases as a first step in treatment, and is also useful for nitrogen removal. MBBR, TFR, activated sludge processes, anammox and loop reactors are deployed. A downstream ultrafiltration step is used in a great deal of cases to deal with sludge arising from biological processes. If persistent biological compounds remain in the wastewater, activated carbon filters or ozonisation are employed to remove the biorefractory contaminants.
Reverse Osmosis for Landfill Leachate Treatment
The discharge values obtainable using reverse osmosis are beyond what may be achieved with any other procedure. The applicability of reverse osmosis is limited by osmotic pressure, and thus ultimately by the salt content of the landfill leachate. A high salt content results in very high transmembrane pressures, and thus requires very large amounts of electrical energy. The presence of inorganic impurities with encrusting tendencies also limits the applicability of reverse osmosis. Despite use of anti-scaling reagents, this often still leads to films accumulating on the membranes, which then require chemical removal or may even destroy the membranes.
The concentrate resulting from reverse osmosis must be conveyed for separate treatment or disposal, which may include the procedures described below. A growing problem is that although the problematic compounds have been removed from the water, they are eventually disposed of as waste anyway.
Chemical-Physical Processes for Landfill Leachate Treatment
Wet oxidation processes, such as ozonisation, are used if it is possible to oxidise organic contaminants either completely or to convert biorefractory contaminants into biodegradable contaminants. Activated carbon adsorption is used for cases in which organic pollutants in the leachate cannot be degraded either biologically or using wet oxidation processes. The contaminants are first bound to the carbon through adsorption and then destroyed by incineration. Precipitation/flocculation and ion exchange processes are less widespread in the field of landfill leachate treatment. Both technologies are used to reduce inorganic ionogenic contaminants.