The Magic of Combined Anaerobic Digestion with Composting of Municipal Solid Waste

The portion of the municipal solid waste (MSW) inamount of renewable energy freed satisfies the
developed economies, which is organic in nature, andenergy need of the whole treatment plant, i.e. for
derived from household, industry, gardens, public parksdigestion, pre- and post-treatment as well as for
and the treatment of waste wood, has beenautomatic turning and aeration of the compost. A very
commonly determined to comprise between 40% andlarge amount of heat and - depending on the
60% of the total by weight. The proportion is alsotechnologies applied - a surplus of electricity may be
expected to rise over the next few years, assold to other users. (In the UK this is considered a
increased recycling rates are achieved for man madegreen energy and attracts a higher unit rate from the
re-useable materials.electricity company to which it is sold, under the ROCS
Therefore, as countries put in place sophisticatedScheme, than for non-renewable or fossil fuel based
waste processing techniques to reduce the amount ofelectricity.)
waste generated, and then still wish to reduce the use* The waste heat from generation can be used in
of landfill still further, the degree of mass reduction, thebuilt-up areas in "combined heat and power schemes",
manner in which this organic fraction is processed andin which instead of the low grade heat after generation
the amount of readily useable and non-renewablegoing to a cooling tower where it is wasted to the
energy (replacement fuel) produced by theseatmosphere, this water (which is actually still very hot in
processes is becoming of greater importance.terms of house heating use), can be pumped to
Two processes, which have always existed in nature,provide heating for homes, or to warm factories etc.
are popular for the processing of organic MSW, and* Instead of using all the energy to generate power,
these will remain favourites with the public becausemuch of the biogas may be used as a fuel.
their use avoids burning (incineration). These two are* The liquid generated during digestion of separately
composting, and digestion.collected MSW and non-hazardous organic industrial
The defining difference between composting andwaste is rich in nutrients. If used for irrigation on
digestion, which are both natural microbiologicalcompost, it improves and eventually accelerates the
processes is that in composting decomposition takescomposting process.
place in the presence of sufficient free oxygen (air) to* This liquid is sterilised by the heat of the digestion
maintain the system aerated (aerobic), whereas inprocess and provides an excellent liquid crop fertiliser in
digestion there is no free oxygen, and the only oxygenthe place of chemical fertilisers. * Anaerobic digestion
present is in chemical compounds, and in thereadily complies with the precautions necessary to
microorganisms such as the bacteria which areensure that infections like the Foot and Mouth disease
present.cannot be spread by waste processes. For most
All MSW contains both woody materials and "wet"organic waste composting with good practise hygiene
organic material such as grass cuttings. The lignifiedtechniques is sufficient, but for some "animal
("more woody") wastes have to be composted, wetby-product" (high infection risk wastes) anaerobic
and easily degradable wastes, and sludges, aredigestion provides easy and dependable sanitisation as
suitable for anaerobic digestion. In addition these highan inherent component of the process.
moisture content wastes will cause odour problems in* If digestion plants are located near oil pipelines carbon
composting facilities, unless a very high degree ofshift technology can be incorporated for optimum
attention is paid to blending and mixing to ensure that acarbon capture, by removing carbon gas (carbon
fibrous freely ventilating and completely aerobicmonoxide and carbon dioxide) from the biogas and
composting (fibrous) material structure is retainedpacing it underground in long-term storage, thus
throughout. In fact, although expert opinions vary, forproviding the ultimate in the preservation of our planet
more than one third of the total potential organicas we know it.
material digestion is a better solution than composting.* Furthermore, detailed investment and operating costs
In many countries previously heavily dependent onfor aerobic and anaerobic combined process
landfill, and embarking on organic waste processing fortreatment plants are likely to be cheaper than
the first time, it makes good environmental andcomposting alone, when all lifetime benefits are
economic sense to start by building compostingconsidered. These benefits include carbon emissions
facilities. Composting can be by simple windrowing, andreductions (even without carbon capture), and a
the first waste streams treated are usually just thosepremium biogas which can be used (after scrubbing
derived from kerbside collection of garden waste,and compression) for fuelling motor vehicles.
which is readily mixed and composted in most climates.Combined plants cost less over their lifetime than
Nevertheless, not only are the markets limited for theplants which treat the wastes exclusively by way of
compost products produced, the undoubted value ofcomposting, especially if the increasing willingness of
this material is very difficult to realise as a financialgovernments to provide operators with financial and
return.tax reduction incentives toward carbon emissions
In addition, there is an increasing realisation that thisreduction continues, and there is good reason to
readily compostible kerbside (source segregated)expect that such incentives will rise dramatically over
waste once catered for by the provision of suitablethe next five to ten years.
composting facilities, is only a first step on the road toAs described earlier, when all things are considered,
achieving reasonable overall waste minimisationcombined anaerobic digestion and composting can not
targets.only be very significantly cheaper than composting
At a point, and probably not too far into the future foralone, but the mass of the residue sent to landfill in the
many developed nations, the waste industry, politicians,end will also be reduced, and global warming emissions
and the public will seek more useable products, andare also minimised. Therefore, the treatment of organic
look for benefits in other ways from their waste.MSW in combined plants is highly recommended, and
These will such as in the production of useable formsall those involved in the planning and design of waste
of fuels, and the avoidance of greenhouse gastreatment facilities should consider these benefits.
emissions by using organic waste derived fuel as aWhile it must always be recognised that although
fossil fuel replacement.combined anaerobic digestion will not always be the
This is where the magic of combined composting andbest option for all district locations, residents and
digestion plants will be realised.ratepayers would do well to press their local politicians
Combined plants, where the digestion is directlyto ensure that combined anaerobic digestion is
integrated with composting, show many advantages,considered carefully by their municipal engineers,
such as the appropriate treatment for differentbefore alternative waste treatment technologies are
substrate fractions, use of the same machineries forpromoted.
the pre- and the post-treatment, self-sufficiency inFor more information on Anaerobic Digestion of
energy and net energy production, as well as utilisationWaste, including Municipal Solid Waste click here.
of the waste water derived from liquid-solid separationFor more information on Anaerobic Digestion of
at the end of the digestion for the irrigation of theWaste, including Municipal Solid Waste click here.
windrows.For more information on UK Waste Technology,
Important advantages of digestion, and the synergiesincluding Anaerobic Digestion, composting, and MBT,
available fom combing the two processes are:-etc click here.
* During anaerobic digestion, biogas is produced. The