28 Oct Municipal Solid Waste: What to do with the biodegradables?
National Academy of Science and Engineering (acatech) 2010
Based on the disciplines of biology, molecular biology, chemistry, biotechnology as well as information technology and engineering sciences, a new field of research is evolving, called synthetic biology. It has attracted special attention recently, also on an international basis.
Synthetic biology can make an essential contribution to the gaining of knowledge in basic research. Beyond that, it opens up options for biotechnological applications in the medium term, i.e. in the area of new and improved diagnostic agents, vaccines and drugs as well as the development of new biosensors and biomaterials or even biofuels.
Concurrently, the research area opens up new questions, e.g. about legal aspects in the context of biological safety or the protection from abuse; likewise, questions about the economic use and ethical aspects.
Against this backdrop, the German Research Foundation (DFG), acatech, which is the German Academy for Science and Engineering, and the German Academy of Sciences Leopoldina have pooled their strengths and crafted a joint statement on possible opportunities and risks of synthetic biology.
From a legal standpoint, the European Union adopts directives which must be transposed by Member States in their own legislation within a given period of time. This report includes a short analysis of the main directives of interest for the subject treated. The new Directive 2008/98/EC is examined in detail; it introduces a waste hierarchy in 5 points: prevention; preparing for re-use; recycling; other recovery, e.g. energy recovery; disposal. The present status of legislation in Belgium is also described briefly.
The best available techniques for the treatment of biodegradables contained in MSW are examined, restricting the scope to techniques that “have been developed and tried with success on an industrial scale allowing implementation in the relevant industrial sector, under economically and technically viable conditions” as defined in Directive 2008/1/EC. Accordingly, R&D processes and pilot plants are not included. However, some processes which still depend on subsidies for survival are discussed in the report.
Among the high temperature processes, incineration may be considered as pertaining to the best available techniques for the treatment of MSW, because it complies with all the conditions imposed by the relevant EU directives, including environment protection. Biodegradables contained in MSW are easily processed in mass-burn, modular or fluidized-bed incinerators. There is no need for separate collection or pre – liminary sorting out. They can be burnt as such, even in cardboard or plastics packaging. A preparation step is however required before introduction in a fluidized bed incinerator. If the incinerator plant generates enough energy to comply with the requirements defined in Annex II/R1 of the Directive 2008/98/EC, it can be called “Energy recovery plant”.
Other high temperature processes (pyrolysis, gasification and plasma processes) are not considered today as “best available techniques” in the European Union for various reasons mentioned in the report. They should be monitored for their technical and economical progresses. The low temperature processes are not able to treat MSW as such. They can only cope with the biodegradables fraction contained in MSW, and necessitate either separate collection of the biodegradables at the source or adequate sorting of MSW before loading in the process. There are two possibilities: aerobic treatment (composting) and anaerobic digestion (biomethanisation). The two processes depend on microorganisms for their correct functioning. Only part of the carbon is converted to CO2.
Composting can be operated over a large range of scale from very small (home and backyard) to large centralized composting plants. At the small scale, there is the advantage that biodegradables are re – moved from the waste stream. At the large scale, the process becomes more difficult to operate because of the need to feed correctly air and water to the load and also because the liquids must be recovered andtreated. There is no energy recovery. Good quality compost can be considered as a fertilizer and a soil amendment. However, a true market for this compost does not seem to exist.
Biomethanisation received a lot of attention during the past decades, because it generates methane that can be used to recover energy. There are various ways to operate the process, and potential feedstocks from different origins may be envisaged (agricultural origin; industrial origin; MSW and sewage sludge). There are difficulties to control properly the process especially if the characteristics of the load change with time. Energy recovery is much lower than with incineration. Good quality digestate can be considered as a fertilizer. Again, a true market for this digestate does not seem to exist. The process is economically viable only when subsidies are available. A detailed discussion is included in the report. In preliminary remarks it is stated that: any chemical element present in the incoming stream will anyhow be present in the outputs in the same quantity (this holds especially for heavy metals); no process may claim any “greenhouse effect” advantage (after decomposition, compost and digestate end up with CO2 and H2O); for energy recovery, when the global process is split into two partial processes, with the first of the partial exothermic, the net calorific effect of the second partial is reduced (this is the case for the combustion of methane from anaerobic digestion); most flawed waste policies forget and leave out thermodynamics; “not in my backyard” emotional reactions are ruled out if the technology does not justify them. The discussion is split in two parts: the first one is limited to scientific, technological, economical and environmental considerations; the second to legal considerations.
Finally, the conclusions present the necessary elements for the authorities to make correct decisions. After reducing by all possible means the amount of biodegradables contained in MSW, the main decision deals with proceeding or not to the separate collection of the biodegradables remaining in MSW at the source , taking all elements in consideration. There are also recommendations. Among them appears the need for new European directives and BREF documents for composting and anaerobic digestion: this could help in generating markets for compost and digestate.