Although composting and anaerobic digestion between them account for by far the bulk of biological waste treatment globally and each has a well-established track record, as is so often the case with practical applications of environmental biotechnology, neither is a clear winner. Dependent on the specifics of the situ-ation, the particular waste composition, local factors and so on, either can have clear benefits to offer and, as we have seen, both can form adequate responses to the growing demand for biowaste diversion. At the same time, each has its limitations also. For AD, the airtight nature of the reactor, the gas-handling arrangements needed to guarantee that the potentially explosive methane pro-duced can be safely managed and the demands of internal environmental control contribute heavily to the overall capital cost of the plant. In addition, on a practi-cal note, there are certain inherent limits on the levels of contamination by other waste fractions that can be tolerated. Clearly, for some applications, these factors may prove major barriers to use.
On the other hand, composting is essentially less of an engineered solution and in many of the versions often seen at local authority sites, it is a very simple process. The major practical limitation, at least as a sole method of bulk treatment, lies in the physical amount of material, since the typical retention period for composting is longer than AD and the final volume of product derived is greater. Consequently, a relatively large area of land is required for processing and a sizeable market capacity or disposal arrangement is necessary for the compost. In certain circumstances, these issues may be significant blocks to its adoption.
Applications for which neither of the major technologies is an immediately easy fit have, at times, led to interest in the potential of other methods of biowaste treatment. It is beyond the scope of this book to discuss the wider political and economic issues surrounding biowaste initiatives, though it should be clear that such factors commonly play an indivisible part in their implementation. Suffice it to say that these local modalities can often form the most critical deciding factors in determining the suitability of a given approach. This is something of a mixed blessing, since, though it can make direct comparisons between individual methods exceedingly difficult to do in a meaningful way, it does leave space for novel or less well-known technologies to play a role.