Rehabilitation Services of Old Landfills and Dumpsites
When landfill is near to capacity and reaching the end of its useful life, municipalities or landfill owned factories divert all collected waste to an alternative site and lightly cover all landfill or partly close it. All landfills must eventually receive closure when filled to capacity. This closure operation comes the need for an effective leachate and gas management systems.
- The wastes at old dumpsites can be on fire because of gas control system deficiency, resulting in minor flame outbreaks and some smoky emissions. The fires have to be extinguished, but with only minimal use of water as this would cause pollution of the nearby watercourses. Randomly deposited waste along the site is both unsightly and a potential hazard to the general environment. This waste is collected together and places it in the burial pit or lay on the existing landfill waste.
- It is necessary to reshape the waste surface on taking over the old landfills. Wastes will be excavated, moved and placed in order to achieve the required profile, to receive the capping system and to provide a suitable surface to collect landfill gas, and shed incident rainfall. A minimum roller weight of 5 tonnes/m width of roller should be used to compact waste.
- Horizontal gas extraction wells are drilled and installed in the landfill body to collect exempt gas which vertical gas wells disabled.
- Due to the topography of the restored surface it is probable that surface water running off land, outside the landfill boundary, will flow into the landfill area under gravity because of the slope. This runoff, together with runoff from the cap, will be intercepted by the peripheral ditch and drained to the lowest point of the site.
- Geotextile is manufactured from polypropylene or polyethylene virgin, non recycled, stable fibres and be in the form of permeable membranes. Geotextile is used to separate waste from upper layers. All geotextiles is overlapped and continuously sewn or thermally bonded without spot sewing. Any holes or tears in the geotextile are repaired using a patch made from the same Geotextile thermally bonded and sewn into place overlapped in all directions.
- The regulating soil layer materials are comprised site won or imported soil materials which are free from organic matter, and sharp objects. The materials to be placed as nearly 10 cm regulating layer are placed and minimally compacted in a single layer, generally working from the toe of any slope upwards.
- Geocomposite drains are used for gas drainage and circulation below the capping geomembrane, and for rainwater infiltration drainage above the geomembrane. Also Geocomposite layer supplies a stabile base layer under the cap. It comprises a double layer of geotextile, separated by a geogrid or a wave shaped monofilament core.
- Low Density Polyethylene (LDPE) Geomembrane is used in rehabilitation sites to isolation of the rehabilitation site from liquids. The subgrade material is placed and compacted such that the geomembrane will be in continuous contact with the layer and the geomembrane is not stretched or bridged over hollows or humps. Where a geotextile is used below the membrane, it should be flat and free from debris of any kind which could damage the membrane. Individual sections are joined by means of extrusion or hot wedge welding in an overlap seam. Seams should be made only by skilled and experienced operators using methods specified by the manufacturer. Prior to installation of the geomembrane, trial welds are carried out to confirm the suitability of the proposed welding techniques and equipment.
- This additional Geocomposite layer is used to protect Geomembrane with extra stability.
- Prior to the commencement of the permanent gas management Works, the characteristics of the landfill gas generation regime at the site is ascertained by on-site pumping trial. Further to the results and conclusions drawn after completion of these trials, the number and positioning of the landfill gas abstraction wells are arranged to suit the characteristics of the gas generated by the decomposing waste at the site. The design of the gas control system at the site is elaborated to suit the conditions pertaining at closure.
- Landfill gas extraction wells are drilled and installed in the landfill body. The wells are expected to comprise (but subject to the result of the pumping trials) the installation of a Æ 250 mm PN 10 vertical HDPE collection pipe in the centre of large diameter (typically 800 to 900 mm dia) and surrounded by a 16/32 mm particle size (i.e. no fines) non-calcareous gravel annular pack. The well granular pack is sealed below and into the capping system by a bentonite plug and the well capped with a proprietary, factory assembled wellhead, all protected by a steel, lockable wellhead chamber installed flush with finished ground level.
- The landfill gas extraction system interconnecting pipework is connected to the gas extraction facility (pump and flare unit or energy recovery plant). Regarding to the Pipe Arrangement Plan (for instance a herringbone arrangement), perforated HDPE pipes are laid in the top of the waste and immediately below the geocomposite gas drainage geosynthetic. These pipes are laid in appropriate sand.
- Appropriate sand is laid to construct top soil cover.
- The final restoration surface is generally grass seeded. The locally appropriate grass seed species mix are determined in discussion with the related Municipality. At least six inches of topsoil or suitable plant growth material is spread over the site. Topsoil nutrient is tested. Soil pH, nitrogen, potassium, phosphorous, conductivity, bulk density, and organic matter are suggested parameters. Based on this analysis, appropriate fertilizers and organic matter may be added to the topsoil to increase fertility. The site is then ready for re-vegetation.
- Flayer station or Energy Recovery Plant are completely installed with a manifold to connect each gas well separately. The gas flow of each connection is adjustable in a wide range.
- Proprietary condensate traps are required to ensure that condensate can drain out of (and not block) the gas extraction pipework. Additionally a Condensate Tank is constructed before the gas evaluation or flaring system.
- Flare is a gas flaring system that ensures non-hazardous gas outflow. The flare stack has sufficient capacity suitable for maximum amount of gas to be produced by the gas well field. The design of this unit is based upon the results of the pumping trials. In general, the Unit should comprise a rugged, containerized, with all external supplies and control systems needed for safe long term remote operation.