First leachate treatment plants
Consultancy Enviros has followed two years of detailed treatability trials in South Africa by designing and commissioning the country's first two leachate treatment plants, in Cape Town and Durban. Directors Howard Robinson and Steve Last describe these market-leading environmental projects.
Vissershok Landfill near Cape Town receives some 2,000 tonnes of the city’s municipal solid wastes, and low to medium hazardous wastes every day. The decomposing wastes produce up to 80 m3 of highly polluted leachate per day, which since July 2003 has been treated to high standards by a state-of-the-art, on-site leachate treatment plant.
In the past this very strong leachate was collected from within the site’s lined disposal cells, and transported by road tanker to the City of Cape Town’s wastewater treatment works, at a substantial cost to the City of Cape Town (CoCT). In addition, large quantities of potable water were being used every day to control dust at the landfill site, in accordance with permit conditions.
In 1999, the CoCT appointed Enviros, the UK-based leachate management specialist, to assist in developing an appropriate and cost-effective leachate treatment scheme at Vissershok. The appointment initially involved the design, construction and operation of pilot-scale leachate treatability trials, in experimental units that ran for 30 months. These trials investigated both nitrification and denitrification processes in leachates being generated from various sections of the site.
The trials were extremely successful, and demonstrated that biological treatment processes could reliably and consistently achieve high effluent quality. They also provided detailed process data, which were used for the design of a full-scale plant.
Enviros was then appointed to work with local consultants to design, construct and commission the full-scale plant. Contractors for the civil, mechanical and electrical engineering works were appointed in August 2002, and the plant was ready for biological commissioning to take place during July 2003.
The treatment scheme adopted includes pumping of leachate from the various tipping cells into a large lined storage lagoon, which provides buffering of flows and quality for treatment by the plant. The main aerobic biological treatment process takes place in a sequencing batch reactor (SBR), which comprises a buried concrete tank (6 metres deep, 18 metres in diameter) that is aerated, mixed, and automatically settled each day to provide a clarified effluent. This effluent is then passed through a subsurface flow reed bed, containing phragmites australis plants, which provide final polishing of water to a quality suitable for use in dust suppression on the roads – replacing potable water previously used for this purpose.
The whole treatment process is controlled automatically by a PLC system, including all operations of the plant, and maintenance of optimum pH-values for treatment. The part-time plant operator is able to programme the PLC, and interrogate all operations of the plant, using a software control and data acquisition (SCADA) system, on a desktop computer.
Leachate being treated has been strong, with concentrations of ammoniacal-N up towards 1,200 mg/l, which have consistently been reduced to below 1 mg/l in effluent. COD values in leachate have generally been above 2,000 mg/l, and are typically reduced by 50 or 60 %. The remaining relatively inert organic compounds have no detectable toxicity when effluent has been subjected to the sensitive Microtox test.
Reed plants are expected to grow to more than two metres in height. As the reed bed becomes fully established, some further improvements in effluent quality may well be achieved.
The treatment plant at Vissershok played an important part as the City of Cape Town was recently awarded the 2003 award for the “Cleanest Metropole in South Africa”, recognised as a significant milestone in development of wastes management by the Council.
Durban Solid Waste (DSW) is responsible for managing three municipal landfill sites within the City of Durban area, which collectively receive about 1.1 million tonnes of waste per year, primarily domestic and commercial refuse. The newest is the Mariannhill Landfill Site, which was opened in 1997, and receives some 200,000 tonnes of waste each year, within an area of outstanding natural beauty and ecological interest.
DSW is nationally recognised for the high standard of operation of its landfill sites, and an interesting aspect of this has been a commitment to retain and restore the natural flora and fauna of the Mariannhill area. This has been accomplished by use of indigenous plants, rescued from a variety of other sites being developed in the Durban region – which enjoys a sub-tropical climate.
Part of the project has involved development of a wetland reserve, that is able to act as a silt trap and erosion control point (essential in an area of tropical storms), which can also act as a plant rescue area for wetland species. It is intended that leachate be treated to a standard that will allow discharge of effluent into this ecosystem.
Accordingly, following two years of pilot-scale treatability trials, a full-scale treatment system has been designed and constructed, and will be commissioned by Enviros during March 2004. The plant again comprises an SBR biological treatment system, with a reed bed polishing system, and will treat up to 50m3/d of very strong methanogenic leachate to very high standards.
Both projects represent critical protection of water resources in South Africa, and provide a clear message that the country is moving forward rapidly in the application of state-of-the-art waste management practice and technology.