Long term behaviour of compost-like-output and its associated soils

Abstract

In 2000, the re-interpretation of the Waste Strategy (WS2000) for England and Wales was published. This document clearly laid out the Government's new strategy for managing waste and resources more efficiently, and detailed the transformations required to deliver more sustainable human development. Compost-like-output (CLO), a material derived from optimally composting municipal solid waste (MSW), is one potential avenue through which these targets may be achieved. When applied to agricultural land, or, as it is currently used, as a topsoil cover for landfill and reclaimed Brownfield sites, the adsorption of inorganic metal ions can involve several processes. The extent to which these processes occur in a soil is difficult to predict, but it is these processes that govern the concentrations of metal ions and complexes in soil solution and therefore ultimately determine the availability to plant root systems, access to groundwaters and thus the success of this waste management practice. To authenticate and potentially improve this procedure, a comprehensive Environmental Impact Assessment (EIA) is needed. The aim of this study was to identify, quantify and characterise, under specific conditions, the release behaviour of a number of leachate components emanating from CLO and its associated soils, and to compare this performance against those of other waste materials and an agricultural soil. It was found that, although leachates from all treatments were weakly alkaline, substantial metal dissolution had occurred. Numerous heavy metals, nutrients and water-soluble metal salts were identified as being in major breach of landfill and drinking and surface water directives, as well as being significantly higher in concentration than a typical agricultural soil for the Durham region. In particular, extremely high conductivities (> 20mS m(^-1)) were recorded in undiluted and mixed CLO treatment leachates. Poor aesthetic quality and an extremely heterogeneous nature were also highlighted as major detrimental properties. In view of this, a simple pre-washing procedure using tap-water was devised which showed that significant remediation value would be gained from this economical pre-treatment technique. However, for the most part, active remediation was found to be insignificant, and mixing significantly reduced the favourable high organic matter (OM) content characteristic of CLO. This suggests that the amending materials chosen offer little more amelioration than that achieved by passive dilution. Further sequential extraction experiments and principal component analyses revealed that prevailing environmental conditions are of central consequence to the levels of contamination incurred. For the CLO-containing treatments, major releases were found to occur under field conditions experienced during the lysimeter trials - an oxic environment. The potential for further substantial contamination has, however, been shown to be immanent if an anaerobic environmental setting prevails. These anoxic releases are also true for sewage sludge, but additionally this material has been shown to intermittently leach components, under the same field conditions and to a significantly greater degree, that are characteristic of acidic, reducing and oxidising environments. It is imperative, therefore, that if CLO or sewage sludge is to be used in agriculture, either as a basis for bio-fuel production or as a soil amendment, the prevailing conditions should be continuously monitored and any necessary action swiftly taken.