Getting to Grips with… landfill leachate
Liquid that oozes from landfills can be some of the most challenging of wastewaters to treat, but technology is now available that can deal with it in an energy-efficient manner, writes LAT Water director Mark Hardiman
What is landfill leachate and how is it created?
Leachate is the liquid that is formed in landfills and drains or “leaches” to the bottom of the landfill. It is formed from the natural rotting and decomposition of the organic matter that is buried combined with water that enters the landfill together with dissolved material from other solid matter. It is always present in a landfill and gradually seeps to the bottom. Modern landfills are sealed at the bottom so the leachate has to be extracted to prevent it from saturating the landfill. The make-up varies depending on the composition of the landfill but comprises in general a mix of dissolved salts, organic compounds, frequently quite high levels of dissolved ammonia and various levels of metal ions. Basically anything that has been buried in the landfill can end up dissolving and being present in the leachate. The volume that is created varies depending on the size and age of the landfill but even after the landfill is closed and sealed it remains biologically active, hence producing leachate that requires management for decades after being sealed.
What particular treatment challenges does it present?
Leachate is one of the most challenging contaminated liquids to treat. Treatment at the site creates problems for conventional water treatment methods as the volume produced varies by season depending on the heat and hence biological activity, thus sizing of a plant is the first challenge. A bigger problem is the sheer mix of different contaminants – traditional water treatment processes can deal with specific contaminants but struggle with a varying mix of different contaminants. Current methods involve a series of processes involving biological and membrane filtration but can only partially reduce the volumes involved. In many cases the leachate has to be trucked away to a hazardous liquid treatment site many kilometres away from the landfill creating further negative environmental impacts. In a typical site LAT Water is working with in the UK which involves five tanker movements a day, each over 60 km. The ultimate energy consumption to dispose of a cubic metre of leachate is typically over 75 kWhr and can cost upwards of £25+ per m³. It is one of the biggest costs of managing a landfill even ignoring the environmental impacts.
What are the consequences of failing to deal with leachate properly?
Leachate is many times stronger and toxic than conventional foul sewage. Discharges to the environment where it can enter the water system either through aquifers or river water are thus totally unacceptable and can result in massive contamination. A typical active landfill site in the UK produces around 100-250 m3 of leachate with smaller closed sites producing 15-25 m3 per day. A landfill operator must monitor the production and ensure systems are in place to remove and dispose of the leachate on a continuous basis. Today the Environment Agency maintains strict monitoring to ensure that there are no discharges to the environment. In the past though in Europe, much leachate was allowed to discharge into the environment and water systems. This has now ceased but many emerging economies are still only beginning to tackle this problem. China is noteworthy in now taking strong regulatory steps to ensure proper handling and disposal of leachate.
What solutions have traditionally been used to deal with leachate?
As mentioned above, a combination of systems involving typically a series of biological and membrane processes to concentrate the liquid coupled with high temperature controlled incineration have been used to dispose of the leachate. These of course require large investment in plant and equipment and can be very inflexible as far as volume and make-up of the liquid to be handled. The consequence is that much leachate is trucked, frequently long distances to suitable disposal sites. This of course adds to the environmental impact and cost of handling the leachate problem.
What innovative technology is now available in this field?
The LAT (Low temperature Ambient pressure Technology) process is a novel approach utilising waste heat from the onsite biogas generators that would be otherwise discharged into the environment. The process uses an interlinked pair of humidification and condensing columns at ambient pressure, thus reducing the costs of manufacture, operation and maintenance. Importantly LAT can simultaneously handle high levels of organics, dissolved solids, metals and ammonia together with variable feed volumes. Typically LAT can recover up to 90 per cent of the water from the leachate leaving a small residue to be removed to the hazardous chemical disposal process. This enables installation at the landfill site reducing the trucking requirements and potentially reducing the costs to the landfill operator by up to 50 per cent, even after including the capital costs of the LAT system.
How much of an issue is landfill leachate in the UK, and what is the potential for improvement?
Leachate management is a major cost and environmental problem for landfill operators, but because the amount of waste being land filled is declining it has been ignored by many of the waste water companies and so has not benefitted from new products or major innovation. As mentioned above though even if landfills are closed the leachate problem continues hence leachate management is a vital part of the waste water industry. Today the costs in the UK of handling leachate are estimated at over £250 million pa. Even more significantly the lessons learned in handling leachate can be applied to many other contaminated industrial waters – overall there remain many opportunities for improved processes and technologies to reduce costs and improve the environmental impact of the treatment.
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