Plant growth on the landfill surface requires nutrients and organic matter in the top soil. Topsoils can be emulated by adding compost to non-humic subsoils.
The surface capping system of a landfill has to ensure proper plant growth. This requires special soil characteristics, such as good water holding and cation-exchange capacity, as well as an adequate nutrient supply. Commonly, these requirements are met when loamy subsoils and organic-rich topsoils are applied. In case of lack of topsoil supply the subsoil can be ameliorated by compost in order to emulate a topsoil. The compost has to fulfil several requirements regarding the pollutant and foreign matter content (e.g. plastics). The thresholds are defined in the German Biowaste Ordinance (BioAbfV). The rotting process of the compost has to be completed. The nutrient content / availability and the nutrient demand of plants determine the mixing ratio of compost and soil since nutrient-leaching has to be eliminated. The soil has to be pollutant free as well and ensure a proper water holding capacity. Both materials have to exhibit a constant quality. When mixing ratio is defined, the construction of the topsoil can proceed. Quality control has to be conducted on the landfill, so it is just permitted to mix the two input materials onsite as quality control may fail, when mixed outside the landfill site by a third party (black box of mixing process). Vegetation has to be introduced immediately to prevent from nutrient leaching. Topsoil with compost should be installed within the vegetation period.
The cost for the constructing a top soil and the installation as part of the capping system is about 30,000 Euro per hectare.
Evidence of success
The practice opens the possibility to construct an artificial topsoil by using subsoil and organic waste in form of compost. The construction of topsoil may avoid excessive transport distances. The quality of topsoil achieved can sustain proper plant growth. The practice is also applied in Cordoba / Andalucía.
Quality control is crucial and must be conducted at least once each 2000 Mg for the compost. Since input material is inhomogeneous quality will shift, however the quality has to be in a certain range and fulfil the requirements set by the regulations and the quality management plan.
Potential for learning or transfer
Fertile soils can be sparse in some regions of Europe. Topsoil just accrues during construction works, linked with excavation and sealing. It is not reasonable to extract topsoil from elsewhere as the very vulnerable function of the soil system will be destroyed. The practice can be applied in regions, where the supply of topsoils is scarce but where a supply of organic wastes of high quality is given. The possibility to use organic waste such as compost may also support to establish or improve its collection system. A separated disposal spot for organic waste from greenery, e.g. at the civic amenity site, and from municipal waste is favourable.
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