ADVANCED LEACHATE TREATMENT AT THE STEWARTBY LANDFILL SITE

Abstract

Landfill liquor (leachate) is produced by complex microbial processes within a landfill site. The long retention period, typically in excess of many years, ensures that easily-biodegradable materials disappear rapidly, whereas intractable (hard) Chemical Oxygen Demand (COD) substances decay slowly. The resultant liquor is often difficult to treat biologically due to high concentrations of Ammoniacal N (amm.N) and low concentrations of nutrient Phosphorus (P) and Biochemical Oxygen Demand (BOD). The advanced treatment process adopted at the Stewartby landfill site has been based on extensive laboratory trials by WRG Ltd (formerly Shanks Ltd), with technical input from Birse Process Engineering Ltd. This has significantly reduced the risks involved in plant scale-up. The particular choice of treatment process reflects the stringent discharge consent and the wide variety of waste received at the site, in particular, the presence of hazardous waste producing a leachate which is especially difficult to treat. Treatment is based on the activated sludge process, assisted by Powdered Activated Carbon (PAC) dosing. The leachate is heated to improve oxidation of amm.N and the process stream also includes Dissolved Air Flotation (DAF) and Sand Filtration to remove suspended solids (SS) and P. Odour control is provided by a two-stage activated carbon unit. Results from the full-scale treatment plant have been excellent, averaging 70% COD removal and almost 100% amm.N removal and fully satisfying the sewer discharge consents imposed by Anglian Water and the Environment Agency. This paper describes the laboratory trials, process design and commissioning of the leachate treatment plant at the Stewartby landfill site.