The broad classes of end-uses that MSW-derived composts have been used for (CIWM 2002, Efstathios and Stentiford 2004, Newport 1990) are:
· soil improvement - enhancing soil structure, condition and fertility
· growing media – as a component of mixes used to grow crops in containers
· mulches – used to suppress weed growth and conserve water
· restoration – used for “soil forming” and soil improvement
· landfill applications – restoration and improvement of landfill covers and as a daily cover material
· other applications – such as manufactured top soils and “top dressings”, in turf production, as a fill material, for tree planting, and as a fuel
Composting as a process is being used, or considered for use, as a pre-treatment prior to landfill in several European countries. This is in part a response to the EC Landfill Directive.
These are discussed in turn in this chapter. In addition this chapter reviews current developments in standards for composts and advice about compost marketing.
An important facet of end-uses for MSW-derived composts is to ensure that the quality, and perhaps the perceived quality, of the compost is appropriate for the end-use envisaged. There has been a lot of debate, particularly in the wake of developing interest in MBT composting, about so-called “lower grade” uses (Centemero et al. 1999, DETR 1998, Godley et al. 2002, US EPA 1994, Walker and O'Donnell 1991, Wheeler et al. 1994 and1996). “Lower grade” uses are applications thought of as more tolerant of some of the contamination problems of composts derived from mechanically-segregated MSW. However, it is dangerous to make very simple assumptions about the market place for composts. “Lower Grade” compost may not be seen as suitable by those managing applications such as landscaping (Kendle 1990). Some feel it has no use on the land (Hammer 1992). “Lower Grade” is a contentious term. Some believe that any compost derived from mechanically segregated MSW should be described as lower grade. Others feel that distinctions between compost quality grades should be made on the basis of the compost product composition, rather than the feedstock it was produced from. The term “lower grade” is also though of as pejorative, and therefore one that should not be used.
In the UK the current regulatory situation appears to be that composts produced from source segregated composts will more readily seen as recycled products than composts produced from mechanically segregated composts, as discussed in the Critical Review Section, Operational and Strategic Issues - Regulations Standards and Guidelines for Compost Products. Consequently, taking into account both the current regulatory climate and the sensitivity over the term lower grade, the following might be a better broad classification of compost types, by application.
· Premium Grade - freely usable for agricultural and horticultural applications, or in the manufacture of formulated products such as “composts” for home use, turf, pot plants etc. These applications may still be subject to over-arching regulations such as those controlling the application of nitrogen to land, but can otherwise be freely traded by any organisation without specialist expertise.
· Regulated Grade – composts suitable for applications such as use in remediation, restoration, agriculture, forestry, short rotation coppice (SRC) and non food crops where either an element of specialist expertise is necessary in trading and use or there is ongoing regulation of the application or both. These applications can make beneficial use of recycling organic matter to land. However, biological, chemical or physical hazards remain a regulatory concern, for example controls on trace elements or animal pathogens.
· Engineering Grade – composts used where access is strictly limited, and other risk management measures are already in place, for example uses such as daily cover, or as engineering fill material - for example in bunds and sound barriers, or as pollution control measures such as biofilters.
MSW-derived composts are unlikely to meet requirements for “premium grade” compost as concerns over their contents of inerts trace elements and possibly organic pollutants (see Critical Review Sections: Composting: Past and Present and Product Quality and Environmental Impacts) Some degree of regulation of their use seems inevitable (see Critical Review Sections: End-uses - Standards and Guidelines and Operational and Strategic Issues).
Note: In addition dense reject fractions are often produced during MSW-derived compost refining, which may have some possible uses, for example as engineering fill or in drainage layers.