WRG Arpley Landfill Passes 1/2 Million Tonnes of Leachate Treated

From the WRG Newsletter

During early 2008, WRG’s Arpley leachate treatment plant passed a total of 500,000m3 of leachate treated. Since it was commissioned in October 2001, the plant has been treating very strong leachates which routinely contain greater than 2000mg/l of ammoniacal-N, to standards which allow safe discharge of effluent into the River Mersey.

When WRG took over Arpley Landfill in the spring of 1999, leachate was not under control. Although heads of leachate on the liner were limited to a maximum depth of 1 metre, levels were typically 7 or 8 metres, and well out of compliance. A key part of site remediation was investment of more than £1 million in a dedicated pneumatic leachate collection system, to automatically manage leachate levels in nearly 100 boreholes. From the start of 2001 leachates then began to be tankered off-site for treatment, at rates of up to 10,000m3 per month.

This allowed detailed pilot-scale treatability trials to be carried out by Enviros, to enable an optimum treatment scheme to be developed, which would represent the largest load of contaminants to be treated at any landfill site in the UK. WRG were no stranger to leachate treatment. In 1994, working with Enviros, a large leachate treatment plant had previously been designed and commissioned at Buckden Landfill near to Huntingdon, where a discharge consent into the sensitive River Great Ouse demands that rainbow trout shall be unharmed after 96 hours exposure to the final effluent. That plant continues to perform well, nearly 15 years later.

The treatment process adopted at Arpley (Plate 1) includes aerobic biological treatment of up to 450m3 of leachate each day, in three identical large Sequencing Batch Reactors. The SBR process has been pioneered for leachate treatment by Enviros, and shown to provide very stable and robust treatment of the very high ammonia and COD values which characterise leachates from large landfills. Effluent from the SBRs then passes through a Dissolved Air Flotation (DAF) process, which removes any remaining suspended solids. The treated leachate receives final “polishing” by passage through the root zone of extensive engineered reed beds, before being discharged safely into the River Mersey which flows beside the site.

WRG and Enviros worked closely with the Environment Agency to plan and implement the leachate treatment solution, and by 2004 leachate levels had been brought back into compliance with the site licence in every monitoring borehole. The plant remains a key part of controlled landfilling at Arpley, and continues to treat leachates at typical rates of 8,000 to 9,000 cubic metres per month.

Operation of all functions of the plant is completely automated. A programmable logic controller (PLC) provides maximum reliability, and the plant operator interfaces with this using a PC. Many dozens of failsafe systems, alarms, and telemetry links provide enormous security of operation. Experiences at Arpley have also formed the basis for many other large leachate treatment plants around the world. For example, Taman Beringin is a very large landfill located close to the centre of the city of Kuala Lumpur, in Malaysia. Remediation of the site has included a new leachate treatment plant (see Plate 2). It achieves very high effluent quality standards, which allow discharge of treated leachate to be made into a nearby watercourse.

As environmental standards at landfill sites are raised around the world, the ability to treat leachates reliably, and to high standards, remains critical, and WRG and Enviros are presently designing several new plants that will be commissioned during 2009.

Enviros is a UK-based full-service environmental consultancy, with an international reputation in wastes management. In the specialised field of landfill leachate management, the company has unique expertise, having worked on every continent except Antarctica, and has designed and commissioned more than 80 full-scale
leachate treatment plants throughout the World.

Can Landfill Leachate be Treated by Anaerobic Digestion?

Every now and again the question of whether landfill leachate can be effectively treated by Anaerobic Digestion is raised.

My reply is yes, and in fact, it has already happened in the landfill before you usually see most leachate. After an initial aerobic (acetogenic) stage, modern landfills in effect become anaerobic digesters themselves. Once this has occurred the leachate produced has already been subjected to a form of anaerobic digestion, so there is little additional treatment which an AD Plant can provide to these mature leachates once leachate is removed from the landfill.

In a modern Municipal Solid Waste (MSW) landfill. as it is filled, each cell or area within it, will within 6 months to one year, or, at the most eighteen months, not only become airless, but methanogenic (methane producing). Once this happens, the decomposition process taking place in the landfill is broadly similar to, but slower than, that which occurs in an anaerobic digester.

So, modern lined and well regulated landfills these days do, almost without exception, produce a mature methanogenic leachate.

As a result the use of Anaerobic Digestion to treat landfill leachate is not normally a good choice and the use of the Anaerobic Digestion process to treat landfill leachate is not very effective. This can be readily deduced just by thinking about the processes which leachate undergoes within a landfill. The big problem with using AD on a mature leachate would be the lack of significant reduction of ammoniacal nitrogen in the discharge, and ammoniacal nitrogen is one of the most important contaminants to remove, for reduced toxity to water life.

However, the opposite does work. Now think of using aerobic reactors to treat Anaerobic Digestion concentrates, if these concentrates cannot for any reason be disposed as a fertiliser product and thus have to be treated as a waste material.

So, leachate treatment plant aerobic biological reactors can be used very effectively to treat AD liquid digestate, if that "product" ends up proving to be unsaleable locally. Indeed, on site aerobic digestate treatment might be essential in these circumstances if no sewage treatment works was available to accept tanker loads of liquid from an AD Plant.