Landfill leachate meets ammonia target

Water pollution at landfill sites poses a grave risk to the environment, but, writes to Dominic Hamblin of Enso, a Welsh council has found a solution

EnSo nitrification tank working with reed beds

Most people do not give a second thought about what happens to their domestic rubbish once the refuse collector has taken it away but for local authorities, processing household waste is a very complicated business. Disposing of rubbish to landfill is one common method of addressing the disposal problem, but this often creates harmful side effects such as the production of methane gas, ammonia and the leaching of hazardous and often toxic chemicals and heavy metals into sensitive groundwater or surface water assets.

The Environment Agency often imposes tight discharge constraints on such sites to protect environmental watercourses and drinking water assets and tip leachate will often require biological treatment to bring the contaminants down to acceptable levels. Addressing such issues head on is of prime importance and this is where EnSo International has recently helped Carmarthenshire County Council.

The Nant Y Caws landfill site comprises a number of landfill ‘cells’ (areas for depositing waste) with each cell designed on the basis of engineered containment. The site accepts the majority of municipal waste from the county of Carmarthenshire.

‘Phase 1’ is the only cell that has a discharge license – the other and future cells currently have all site produced leachate (contaminated liquid caused by the presence of water on the site) removed by suction tanker with post treatment off-site. The Phase 1 cell has been capped within the original site and is under aftercare and restoration. This is where EnSo have applied their wastewater treatment expertise.

Wastewater collection

The leachate from this cell is collected from the waste through a series of gravity drains to a pumping station, which delivers liquor to a trickling filter tower to encourage biological oxidation of carboniferous and ammonia elements within the wastewater. Gravity flow from the filter then enters a settling pond which contains a floating reed bed and further aeration arrangements.

Excess volume within the pond then spills forward via a controlling weir to further reed beds before gravitating to a sensitive watercourse. The pond is concrete-lined and the contaminated sludge including heavy metals, which sink to the bottom of the pond, will be removed periodically for disposal elsewhere.

When EnSo became involved in the project, the site had not been meeting with the ammonia consent limit for some time, with the result that large volumes of effluent had to be removed daily by suction tanker. This reflected the lack of removal efficiency for the whole of the biological filtration, settlement and reed bed arrangements.

Following detailed and lengthy site analysis by the specialist industrial chemist WEBS, EnSo International provided one of their nitrification tanks as a temporary pilot plant to prove the benefit of operating an above-ground biological submerged aerated filter (SAF) tank. A more permanent treatment solution would be provided once the process was proven.

EnSo recommended use of the Ocean Professional Nitrification Plant. The tank was manufactured in glass reinforced plastic (GRP) to a heavy duty specification and installed above ground, seated on galvanised steel cradles on a reinforced concrete pad. The air blower was installed within a kiosk and located on top of the tank for ease of access.

The temporary unit dealt successfully with the ammonia problem and consent levels are now being met. As a result, the County Council have placed an order for a permanent solution which will be rolled out in early 2012.