Biological Waste Treatment
The aims of biological treatment are relatively straightforward and can be summed up in the following three points:
1. Reducing the potential for adverse effects to the environment or human health.
2. Reclaiming valuable minerals for reuse.
3. Generating a useful final product.
Broadly speaking, this effectively means the decomposition of the biowaste by microbes to produce a stable, bulk-reduced material, during which process the complex organic molecules originally present are converted into simpler chemi-cals. This makes them available for literal recycling in a wider biological context.
To some extent these three aims can be seen as forming a natural hierarchy, since removing environmental or health risks, and deriving a stable product, forms the bottom rung of the ladder for all biological waste treatment technolo-gies. Clearly, whatever the final use of the material is to be, it must be safe in both human and ecological terms. The recovery of substances, like nitrogen, potassium and phosphorus, which can be beneficially reused, forms the next level up, and is, in any case, closely linked to stabilisation, because these chemicals, if left untreated within the material, would provide the potential for unwanted microbial activity at a later date. The final stage, the generation of a useful end-product, is obviously dependent on the previous two objectives having been met with some degree of efficiency. The possible uses of the final material, and just as importantly, its acceptability to the market, will largely be governed by the certainty and effectiveness of the preceding processes of stabilisation and recla-mation. Thus, while the hierarchical view may, in some ways, be both a natural and a convenient one, these issues are not always as clear-cut, particularly in respect of the implications for commercial biowaste treatment, as this approach might lead one to believe.
In practical terms, the application of this leads to two major environmental benefits. Firstly, and most obviously, the volume of biowaste consigned to landfill is decreased. This in turn brings about the reduction of landfill gas emissions to the atmosphere and thus a lessening of the overall greenhouse gas contribution, while also freeing up space for materials for which landfill genuinely is the most appropriate disposal option. Secondly, good biological treatment results in the generation of a soil amendment product, which potentially can help lessen the demand for peat, reduce the use of artificial fertilisers, improve soil fertility and mitigate the effects of erosion.
As has been mentioned previously, stabilisation is central to the whole of bio-logical waste treatment. This is the key factor in producing a final marketable commodity, since only a consistent and quality product, with guaranteed freedom from weeds and pathogens, will encourage sufficient customer confidence to give it the necessary commercial edge. As a good working definition, stabilisation is biodegradation to the point that the material produced can be stored normally, in piles, heaps or bags, even under wet conditions, without problems being encountered. In similar circumstances, an incompletely stabilised mate-rial might well begin to smell, begin renewed microbial activity or attract flies. Defined in this way, stability is somewhat difficult to measure objectively and, as a result, direct respirometry of the specific oxygen uptake rate (SOUR) has steadily gained support as a potential means to quantify it directly. Certainly, it offers a very effective window on microbial activity within the matter being processed, but until the method becomes more widespread and uniform in its application, the true practical value of the approach remains to be seen.
The early successes of biowaste treatment have typically been achieved with the plant matter from domestic, commercial and municipal gardens, often called ‘green’ or ‘yard’ wastes. There are many reasons for this. The material is readily biodegradable, and often there is a legal obligation on the householder to dispose of it separately from the general domestic waste. In the UK alone, the production of this type of biowaste is estimated at around 5 million tonnes per annum (DETR 1999b), making this one area in which biological waste treatment can make very swift advances. Nowhere is the point better illustrated than in the USA, where the upsurge in yard waste processing throughout the 1990s, led to a biowaste recovery rate of more than 40%, which made an effective contribution of nearly 25% to overall US recycling figures. In many respects, however, discussions of waste types and their suitability for treatment are irrelevancies. Legislation tends to be focused on excluding putrescible material from landfills and, thus, gener-ally seeks to make no distinction as to point of origin and applies equally to all forms. The reasons for this are obvious, since to do otherwise would make practical enforcement a nightmare of impossibility. In any case, the way in which waste is collected and its resultant condition on arrival at the treatment plant is of considerably greater influence on its ease of processing and the quality of the derived final product.
There are three general ways in which waste is collected: as mixed MSW, as part of a separate collection scheme, or via civic amenity sites and recycling banks. From a purely biowaste standpoint, mixed waste is far from ideal and requires considerable additional effort to produce a biodegradable fraction suitable for any kind of bioprocessing, not least because the risk of cross-contamination is so high. By contrast, suitably designed separate collection schemes can yield a very good biowaste feedstock, as a number of countries around the world have successfully shown. However, not all separate collections are the same, and they may vary greatly as a result of the demands of local waste initiatives and specific targets for recycling. As with all attempts to maximise the rational use of waste, the delivered benefits of any scheme inevitably reflect the overall emphasis of the project itself. Where the major desire is to optimise the recovery of traditional dry recyclables, biowaste may fare poorly. Systems deliberately put in place to divert biodegradable material from landfill or incineration routes, however, generally achieve extremely satisfactory results. In many respects, the same largely holds true for recycling banks and amenity sites. Dependent on local emphasis, the operation can recover very specific, narrow waste types, or larger, more loosely defined, general groups. Where ‘garden’ waste is kept separate, and not simply consigned to the overloaded skip labelled ‘other wastes’, the biowaste fraction produced can, again, be of a very high quality and readily acceptable for biological treatment. Indeed, it is generally accepted that this material is the cleanest source available for processing and it constitutes something in the region of three-quarters of the biowaste treated yearly in the UK (DETR 1999b).
For those approaches to collection which do not involve separation of the putrescible fraction at source, obviously some form of sorting will be required before the material can be taken on to any kind of biological processing. It lies beyond the remit of this work to attempt to describe the methods by which this can be achieved, or their relative merits. Suffice it to say that whatever onsite sorting is used must be matched adequately to the demands of the incoming waste stream, the intended treatment biotechnology and the available local resources. However the biowaste-rich fraction is obtained, the major consideration for processing is its physical form, which is of more fundamental significance to biowaste than any other refuse-reclaimed material. For traditional dry recyclables, chipping, crushing or baling are mere matters of convenience; for biotreatment, particle size, purity and consistency are indivisible from the process itself, since they are defined by the requirements of the microbes responsible. In general terms this means that the biowaste is shredded to break it down into small and relatively uniform pieces, the exact requirements being dictated by the particular treatment technology to be used. This not only makes mixing and homogenisation easier to achieve but also, by increasing the surface area to volume ratio, makes the material more available to microbial action.
There are a number of processes currently available in varying degrees of commercial readiness, and others under development, to deal with biowaste. While the underlying aims and basic requirements of all these biotechnologies are essentially the same, there is some variance of detail between individual methods. Two general approaches in particular, composting and anaerobic digestion, are so well established and between them account for such a large a proportion of the biowaste treated worldwide, that the discussion of specific technologies must begin with them.
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