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An Overview of Leachate Treatment Processes

The following Table lists processes that have been used for full strength leachate treatment from modern landfills, and are listed in descending order of popularity:-

Primary Stage Tertiary Polishing Comments Grit and silt removal by settlement if collection system risks entraining these materials Specially designed & developed extended aeration activated sludge biological SBR process, providing nitrification, with a denitrification stage - if needed. May require further suspended solids removal (eg by DAF Unit), depending on discharge consent requirements and this is often achieved by the use of a reed bed This is the most common form of on-site leachate treatment plant installed in the UK. Examples have been operational for over 20 years. Removal rates for any dangerous substances present in the raw leachate are good, however, additional stages are applied very cost effectively to remove any residual chemicals after polishing. Such final processes range from Ozonation to Granulated Activated Carbon treatment. Ditto, plus pH adjustment. Aeration may be included to reduce COD & BOD. Reverse osmosis Lagooning or ponding Examples installed in the UK have met with only limited success in the author's view. This method of treatment is common in many EU states and elsewhere, particularly where effluent salinity or AOX must be reduced. Grit and silt removal. Trickling filter Settlement, with lagooning or ponding to further remove suspended solids Frequently suffer blockage due to high calcium carbonate and iron fouling build-up rates and sludge generated within the media. The problem is most severe for high strength leachates, however, even weaker methanogenic leachates tend to give similar problems, and require a re-commissioning period after cleaning - which requires plant downtime, and makes operational use difficult in the interim. Although the system is low in energy use, maintenance costs mount after some years and few if any of these plants have remained remain in use after say 5 years. Grit and silt removal by settlement if collection system risks entraining these materials. Membrane Bioreactor (MBR) Reed bed or lagooning The process principle is usually as for the extended aeration activated sludge biological nitrification process. The use of an effluent membrane filter allows a more compact (sometimes portable) plant design. Concerns remain that the inherent stability (robustness under stress), is less than for the extended aeration activated sludge biological process. The cost of the membrane is greater than the cost saving in reduced reactor size. Grit and silt removal by settlement if collection system risks entraining these materials. Air/Ammonia Stripping Further COD removal is normally required. This is a "text book solution", often attempted - rarely successful. Over the years a great deal of money has been wasted on such installations - which no longer exist. Ammonia stripping requires pH elevation to pH 11 or similar. Heat assisted ammonia stripping has more going for it - prior to biological aeration treatment but the stripped flow should be a minor flow contribution alongside untreated leachate to avoid biological treatment problems. Grit and silt removal. High recycle rate activated sludge process and BAF plants. Settlement (for activated sludge), possibly with a DAF Unit, plus reed bed etc dependent on final quality required. Can be seen to work in theory, major concerns remain that the inherent stability (robustness under stress) is less than for the larger tank volume, of the extended aeration activated sludge biological process. Versions have been offered which have been modified sewage works designs. Due to high recycle rates for these modified sewage works systems, they have poor at retaining biomass. They are alos not good at handling the shock loads characteristic of operational landfills. More recent versions have been trialled which have been specifically designed for leachate, but none are known by us to have been built at full scale. Grit and silt removal. Chemical Precipitation Activated sludge, membrane bioreactor etc Unfortunately, capital costs are high for the chemical dosing system required, plus settlement tanks, clarification, or filtration. Also, the precipitation tends not to cease within the settlement tanks, and continues through the piecework. These plants are reported to suffer from high maintenance costs due to pipe furring, and eventual blockage if not regularly cleaned.The chemical dosing required is also a high cost. “Beneficial Use” of Leachate, as a method of treatment and disposal In principle, leachates can sometimes be re-classified as a product rather than a “waste” if scientifically the leachate can be quality-controlled, and if it can be established scientifically that the leachate contains a nutrient or in some other way will provide a benefit to a crop.

This is not an exhaustive list, many more processes and process variants have been used.

While anaerobic biological treatment has been used for leachate treatment it should be recognised that, while anaerobic treatment will effectively reduce BOD and COD, it will not be effective in ammoniacal nitrogen removal. Aerobic biological processes are capable of ammoniacal nitrogen removal and do also require BOD/COD to maintain a suitable bio-mass to do so, therefore anaerobic treatment is not normally selected as a process suitable for leachate treatment.

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