Feedstocks and Composition

Feedstocks and composition

MSW is one of many feedstocks that have been or are composted.  In fact the dominant compostable wastes are agricultural wastes (Bardos et al. 1991).

Composts have been produced from unprocessed MSW and from MSW that has been processed in some way to increase its relative content of biodegradable material, and/or render the refuse more quickly degradable (typically by wetting and/or size reduction). When these materials are used as the input source for a composting process they are often referred to as "feedstocks".

The aims of applying composting to MSW encompass one or more of the following: producing a “product” that can be put to some kind of use, reducing the mass of MSW, improving the qualities of the MSW for subsequent disposal or processing – for example as a pre-treatment for landfill.

The principal effects of the composting process are biodegradation, drying, increasing bulk density and physical attrition.  The waste components that are most changed are those that are biodegradable.  Composting is of relatively short duration – weeks to months depending on the processing route, hence rapidly biodegradable materials are those most affected.  More slowly biodegradable components may persist through the composting process, and even as the compost is matured.  This persistence can be a particular problem for various types of “biodegradable” plastic (Colyer 2004), but also for paper, card and wood – including woody components of garden waste, and notoriously for MSW composts: cigarette ends (filters).

The possible effects that composting has on non-biodegradable components such as glass or many plastics is that of physical attrition, drying and the removal of adhering organic matter.  These components are often referred to as “inerts” since they are not affected by biodegradation.  Mechanical pre-processing and compost refining processes (discussed elsewhere in this review) seek to remove these inert components as concentrates, to leave a more organic rich “compost” product.  Inert components are detrimental to compost quality either as visible contaminating components, or as sources of potentially toxic substances in compost, or both.  They may also pose physical risks to grazing animals, or to people using MSW compost simply by virtue of being sharp.

Hence the quality of any compost produced from MSW is constrained by the proportion of so called “inerts” in the feedstock, and the effectiveness of processes to remove them before and after composting.  The “inert components” are not necessarily chemically inert, for example metal ions may leach from batteries.

In many cases MSW fractions are one component of a compost feedstock, and other compostable materials may be added, most commonly sewage sludge.  In the UK, sewage effluent, from both domestic and industrial premises, is treated at wastewater treatment plants.  There are three standard stages of treatment:

The composition of MSW feedstocks can be considered in three ways, its physical characteristics, its biological characteristics and its chemical characteristics.  The composition of MSW is very variable.  Some of this variation is related to seasonal trends, the approach to waste collection, and the locales waste is collected from.  However, even within a given locale and time of year composition is variable.  This makes extrapolations of conclusions from one area to another highly problematic.  Regional comparisons are further complicated by differences in analytical approach, and a standardised methodology for solid waste analysis could enable greater comparability and accuracy of waste data within the European Union (Dobson 2003).

MSW can contain hazardous components, and its degradation can cause hazards.  Health and safety issues for composting plants are outlined in the Critical Review Section, Health and Safety, Emissions and Emissions Control.  However, this is not a comprehensive treatise on the subject and plant managers should seek professional advise on risk assessment and compliance with health and safety regulations.