A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge

Abstract

The content, behaviour and significance of heavy metals in composted waste materials is important from two potentially conflicting aspects of environmental legislation in terms of: (a) defining end-of-waste criteria and increasing recycling of composted residuals on land and (b) protecting soil quality by preventing contamination. This review examines the effects of heavy metals in compost and amended soil as a basis for achieving a practical and sustainable balance between these different policy objectives, with particular emphasis on agricultural application.

All types of municipal solid waste (MSW) compost contain more heavy metals than the background concentrations present in soil and will increase their contents in amended soil. Total concentrations of heavy metals in source-segregated and greenwaste compost are typically below UK PAS100 limits and mechanical segregated material can also comply with the metal limits in UK PAS100, although this is likely to be more challenging. Zinc and Pb are numerically the elements present in the largest amounts in MSW-compost. Lead is the most limiting element to use of mechanically-segregated compost in domestic gardens, but concentrations are typically below risk-based thresholds that protect human health.

Composted residuals derived from MSW and greenwaste have a high affinity for binding heavy metals. There is general consensus in the scientific literature that aerobic composting processes increase the complexation of heavy metals in organic waste residuals, and that metals are strongly bound to the compost matrix and organic matter, limiting their solubility and potential bioavailability in soil. Lead is the most strongly bound element and Ni the weakest, with Zn, Cu and Cd showing intermediate sorption characteristics. The strong metal sorption properties of compost produced from MSW or sewage sludge have important benefits for the remediation of metal contaminated industrial and urban soils.

Compost and sewage sludge additions to agricultural and other soils, with background concentrations of heavy metals, raise the soil content and the availability of heavy metals for transfer into crop plants. The availability in soil depends on the nature of the chemical association between a metal with the organic residual and soil matrix, the pH value of the soil, the concentration of the element in the compost and the soil, and the ability of the plant to regulate the uptake of a particular element. There is no evidence of increased metal release into available forms as organic matter degrades in soil once compost applications have ceased.

However, there is good experimental evidence demonstrating the reduced bioavailability and crop uptake of metals from composted biosolids compared to other types of sewage sludge. It may therefore be inferred that composting processes overall are likely to contribute to lowering the availability of metals in amended soil compared to other waste biostabilisation techniques.

The total metal concentration in compost is important in controlling crop uptake of labile elements, like Zn and Cu, which increases with increasing total content of these elements in compost. Therefore, low metal materials, which include source-segregated and greenwaste composts, are likely to have inherently lower metal availabilities overall, at equivalent metal loading rates to soil, compared to composted residuals with larger metal contents. This is explained because the compost matrix modulates metal availability and materials low in metals have stronger sorption capacity compared to high metal composts.

Zinc is the element in sewage sludge-treated agricultural soil identified as the main concern in relation to potential impacts on soil microbial activity and is also the most significant metal in compost with regard to soil fertility and microbial processes. However, with the exception of one study, there is no other tangible evidence demonstrating negative impacts of heavy metals applied to soil in compost on soil microbial processes and only positive effects of compost application on the microbial status and fertility of soil are reported. The negative impacts on soil microorganisms apparent in one long-term field experiment could be explained by the exceptionally high concentrations of Cd and other elements in the applied compost, and of Cd in the compost-amended soil, which are unrepresentative of current practice and compost quality.

The metal contents of source-segregated MSW or greenwaste compost are smaller compared to mechanically-sorted MSW-compost and sewage sludge, and low metal materials also have the smallest potential metal availabilities. Composting processes also inherently reduce metal availability compared to other organic waste stabilisation methods. Therefore, risks to the environment, human health, crop quality and yield, and soil fertility, from heavy metals in source-segregated MSW or greenwaste-compost are minimal. Furthermore, composts produced from mechanically-segregated MSW generally contain fewer metals than sewage sludge used as an agricultural soil improver under controlled conditions. Consequently, the metal content of mechanically-segregated MSW-compost does not represent a barrier to end-use of the product. The application of appropriate preprocessing and refinement technologies is recommended to minimise the contamination of mechanically-segregated MSW-compost as far as practicable.

In conclusion, the scientific evidence indicates that conservative, but pragmatic limits on heavy metals in compost may be set to encourage recycling of composted residuals and contaminant reduction measures, which at the same time, also protect the soil and environment from potentially negative impacts caused by long-term accumulation of heavy metals in soil.

Introduction

Heavy metals are naturally present in the environment, soil and food and are widely used in manufacturing processes and in the built environment and, consequently, they transfer to and are present in composted organic residuals (Lineres, 1992). There are many sources of heavy metals in compost and particularly products derived from household municipal solid waste (MSW). For example, these include household dust, batteries, disposable household materials (eg bottle tops), they are present in plastics, paints and inks, bodycare products and medicines and household pesticides (NHHWF (National Household Hazardous Waste Forum), 2000, Bardos, 2004). Consequently, composts derived from source-segregated waste streams or greenwaste are generally reported to contain smaller amounts of heavy metals compared to mechanically-sorted products (Epstein et al., 1992, Sharma et al., 1997, Amlinger et al., 2004). In relation to the application of composted residuals to soil, the main elements generally of concern include: Zn, Cu, Ni, Cd, Pb, Cr and Hg (CA, 2001) because they are potentially present in compost in amounts that may be greater than the background values in the receiving soil. The concentrations of conservative elements like heavy metals increase during the composting process (García et al., 1990, Ciavatta et al., 1993) due to the microbial degradation of part of the organic matter and loss of volatile solids (Smith and Hall, 1991). The long-term accumulation of heavy metals in the soil environment is a concern because they potentially have important consequences for the quality of the human food chain, toxicity to plants and soil microbial processes and once applied they have very long residence times in soil. The principal environmental end-points for heavy metals applied to soil in all organic residual materials (Smith, 1996), including compost (Chaney and Ryan, 1993, Ryan and Chaney, 1993, Déportes et al., 1995), are:

• Reduced plant growth (phytotoxicity) due to phytoaccumulation in plant tissues above tolerable threshold values (Zn, Cu, Ni; Cr may also be listed here, but there is no evidence of crop damage due to Cr in sewage sludge or compost-amended soil);

• Human foodchain via crop uptake (Cd);

• Human foodchain via direct child ingestion of compost, eg applied to home gardens (Cd, Pb, Hg);

• Human foodchain via offal meat from animals ingesting compost-treated soil (Cd and Pb);

• Animal health (Cu, Pb);

• Soil microbial processes (Zn).

A comprehensive receptor and environmental pathway analysis for contaminants in sewage sludge was developed by US EPA (1992) as a basis to regulate the use of sewage sludge on land in the US (US EPA, 1993). As MSW-compost has similar properties to sewage sludge and is treated and utilised by application to land in a similar manner to sludge, the pathway analysis may also provide a technical basis for evaluating the significance of contaminants, including heavy metals, in composted residuals (Chaney and Ryan, 1993, Logan et al., 1999). However, the limit values derived from a quantitative environmental pathway analysis and risk assessment of heavy metals are much larger than the concentrations that are considered acceptable in most European compost standards (Hogg et al., 2002).

The majority of research on the fate and behaviour of heavy metals in MSW-compost amended soil is focussed on mechanically-sorted material (eg Smith, 1992, Gigliotti et al., 1996, Pinamonti et al., 1997), or compost prepared by, for example, manual sorting mixed refuse (eg Pichtel and Anderson, 1997, Baldwin and Shelton, 1999, Jordão et al., 2006). This is because compost derived from these feedstocks is recognised as a potentially important source of heavy metals entering soil and the environment (Epstein et al., 1992). Relatively few studies have specifically examined source-segregated products because these are not considered a priority with regard to metal contamination; those referring to source-segregated compost identified in this review include, for example: Zinati et al. (2001), Liu et al. (2003) and Zheljazkov and Warman, 2004a, Zheljazkov and Warman, 2004b. A number of field experiments report long-term effects of MSW-compost additions to soil (eg Gigliotti et al., 1996, Baldwin and Shelton, 1999, García-Gil et al., 2000, Crecchio et al., 2004). An extensive database of information is available considering the fate and impacts of heavy metals in sewage sludge-amended soil (eg see Smith, 1996).

Quantifying the significance of heavy metals in composted residuals derived from municipal wastes has increased in importance recently, especially in the European context, where, on the one hand environmental policies aim to increase recycling biodegradable wastes and composts to land, as an alternative to landfill disposal, whilst at the same time prevent inputs of contaminants entering the soil. This discussion is therefore relevant to: (1) the expansion of composting as a means of diverting biodegradable waste from landfill disposal (Nikitas et al., 2008), (2) increasing mechanical biological treatment of MSW and production of ‘compost-like’ outputs (Archer et al., 2005), (3) increased recycling of these materials to land (Nikitas et al., 2008), (4) compost quality standards (BSI, 2005), (5) revision of the European Waste Framework Directive and development of associated ‘end-of-waste’ criteria for compost and related product protocols (ComEC (Commission of the European Communities), 2005a, ComEC (Commission of the European Communities), 2005b, WRAP/EA (Waste and Resources Action Programme/Environment Agency), 2007), and (6) development of policies to protect soil quality and the environment (ComEC (Commission of the European Communities), 2006a, ComEC (Commission of the European Communities), 2006b).

This article gives an overview of the environmental significance and bioavailability of heavy metals in MSW-derived compost and amended soil and in sewage sludge for comparison with a well established soil amendment material, with particular emphasis on agricultural application.