Composting Techniques

Composting techniques and their context have been widely reviewed, e.g. Anon 1989 and 1991, Bidllingmaier and L'Hermite 1989, Border 1999, Brunt et al. 1985, CIWM 2002, Composting Association 2001, de Bertoldi et al. 1986, Diaz et al. 1993 and 1996, DTI 2000, EC 2002, Efstathios and Stentiford 2004, EC 2002, Epstein 1997, Environment Agency 2001, Finstein et al. 1987, Golueke 1972 and 1974, Hansen 1996, IWM 1994, Jackson et al. 1992, Kitto 1988, Lindeberg 1997, Newport 1990, Obeng and Wright 1987, Pescod 1991, Wiley 1963, Wix 1961. A useful general web link is the Environment Agency’s web page: the Waste Technology Data Centre at: http://www.environment-agency.gov.uk/wtd/

The main aims of the composting techniques reported in the literature include one of more of the following:

· to provide a location for composting to take place which is convenient for both inputting and removing materials and holding materials while they compost

· to optimise the composting process - typically to achieve the fastest possible throughput, which needs to be balanced with achieving the greatest stability and maturity in the product

· to contain the composting process – for example to prevent access by animals and birds, to prevent the escape of odour, to protect the process from the extremes of the weather

· to further mix materials, ensuring even and thorough distribution of the moisture, nutrients and substrates.

From a processing point of view composting is often considered in two phases:

· a rapid “active” phase that includes composting to the end of the thermophilic processes, and

· a longer period of “maturation” occurring at mesophilic temperatures – see the Critical Review Section, Biology of Composting - Process Optimisation. Maturation tends to be a slow process taking 3 to 6 months (Bagstam 1979). Maturation benefits from moisture management and occasional turning.

The “active” phase may be further broken into two steps, for example an “in-vessel” treatment followed by an open aeration treatment.

Many composting systems incorporate artificial aeration, others incorporate mixing or turning, and some both. Biological processes in systems that rely on turning alone may be limited by oxygen supply. Systems that do not include some form of turning, agitation or mixing may suffer from problems in moisture control and poor processing at surfaces or interfaces with containment systems (edge effects) and a lack of process homogeneity - see the Critical Review Section, Biology of Composting -Process Optimisation. From the point of view of an optimal composting process some combination of aeration and mixing / agitation seems best.

Composting approaches can be divided across four basic categories:

The traditional compost heap as used by farmers and gardeners, where the only aeration is provided by diffusion, assisted by convection currents as the waste self-heats. The main problems are that the system can quickly become anaerobic and therefore odorous and inefficient and results in a poor quality product and is not considered further in this review.

Using turned windrows, where elongated piles (windrows) are formed and turned according to a regime which aims to maximise the rate of degradation. Turning has the advantage of exposing fresh surfaces to degradation processes.

Aerated piles, where air is forced through heaps of compostable materials. Aeration is controlled according to temperature or time or both, to maximise degradation. Aeration may be positive (blowing) or negative (sucking). No mixing of the waste is carried out once the pile has been constructed (hence >static=). The compost piles may be “static piles”, or may be turned intermittently, for example in bay systems.

“In-vessel” systems, most of which utilise turning or forced aeration or both. These are generally variations and combinations of the basic control methods of mechanical turning and forced aeration, although the fact that the composting material is enclosed means that the ability for control of the process may be enhanced.

These categories should not be regarded as absolute. For example mechanically agitated systems typically have one surface open, to allow access by mixing screws for instance, however they tend to be described as “contained systems”. However, this categorisation is a convenient way of describing composting approaches in generic terms.

The requirements of the Animal By-Products Order and the EC Regulation that it is derived from have an important bearing on how composting techniques can be applied to mechanically segregated fractions of MSW, and is discussed in detail by the Composting Association (2004). An important aspect, in terms of processing, is a requirement for two phases of thermophilic composting, for example an in-vessel treatment followed by a turned windrow treatment. In practice this is not far removed from what has always been the case for MSW composting: a high throughput reactor based treatment (most frequently in rotating drum reactors) followed by a second phase of treatment including composting and aeration. The reasons for this have been largely economic, in that the cost of containing the entire thermophilic processing in-vessel would be astronomical, and in part accidental. The proponents of many in-vessel systems believed they could achieve very rapid completion of composting, whereas in practice the processes continued to occur at the rate biology dictated.

Composting is often taken to include “vermicomposting”, which is the digestion of waste materials by worms (CIWM 2002). This has been applied to mechanically segregated fractions of MSW, but is not reviewed here.

Composting may be combined with anaerobic digestion in “hybrid systems” (CIWM 2002, Kayhanian et al. 1991, Rogers et al. 1992, Von Felde and Doedens 1999), which allow both for some methane recovery and the benefits of composting in producing a dry friable product.

The remainder of this section covers:

· turned windrow approaches

· open aerated systems (e.g. static piles, bay systems)

· contained systems (including: vertical units, horizontal units, reactors with compost agitation)