Anaerobic Digestate: Disposal, or Recovery?
As we plan our MSW Anaerobic Digestion Plants the question arises for each site as to whether we should dispose of our digestate as we always did before, to a landfill as a waste material, or go one step further and produce quality controlled end products from the digestate which in principle would be a liquid fertiliser and a fibrous material.
The solid component may, it has been suggested, be used as a soil conditioner, or possibly a fibrous material which could be further processed (pressed) into blocks etc, and used in buildings.
The primary advantage seen by many Waste Management Officers for Anaerobic Digestion in Europe, where strict targets exist for the diversion of waste away from landfills, will simply be whether the end product of the AD process will be classified as an organic waste (specifically Biological Municipal Waste or BMW) or not.
To the European Regulators implementing waste diversion rules the important requirement is to reduce organic waste or more precisely from their point of view the landfilling of BMW.
To the Waste Officer seeking to achieve compliance with waste diversion targets as long as the end result of Anaerobic Digestion has been to obtain an agreement from the regulator that the output is no longer BMW he can send it to landfill.
In a sense this is the regulations working as intended, and the impact of the landfill will be reduced as this BMW will be much lower in the organic content which could be rapidly released to cause environmental nuisance in the landfill.
So Anaerobic Digestion offers an expensive but effective method of protecting existing landfill operations, by eliminating the worst of their environmental consequences, but for most of the public the most tangible benefit is the ability to produce an attractive and desirable product such as a soil conditioner or soil substitute for agriculture, forestry or land reclamation.
To produce a soil substitute has been tried before, with composting, and the lessons are clear enough. Anaerobic digestion provides good protection against pathogens and subsequent composting of the digestate provides a further barrier to infection. There is sufficient waste heat available from the gas for drying and pasteurising of the product, if required. The problem areas are the presence of metals, plastics and glass.
Source separation schemes are beginning to help to diminish metal contents in our domestic refuse. Plastics are unsightly and can discourage the use of refuse-derived materials on land where animals graze. Broken glass is a major hazard and greatly diminishes the possible outlets for the material. But, with the co-operation of the public, who will probably need to be incentivised in some way as well, source separation can avoid the presence of troublesome quantities of these materials.
The experience with compost and the Compost Association promoted quality standards has again shown that with good source separation and fine screening before use, a thoroughly acceptable compost can be provided, and the same will no doubt apply to the best AD compost products.
Considerable downstream processing of the digestate, and strenuous efforts in marketing, will be required to provide the best possible product and to obtain assured outlets for the processed digestate and this will tend to be expensive and thus offset any profits.
So this returns us to original question and the subject of this article. Should we send anaerobic digestate to disposal to landfill, or to recovery?
So to summarise we would say that it is going to depend on local circumstances to a very large extent. The classification of the output from AD as either a BMW or not will be the first point in question. If the material has been processed to reduce its organic content sufficiently to no longer be BMW, and landfill capacity is available locally, there will be an economic and risk aversion pressure to simply continue to landfill this material.
Hopefully, nevertheless as time progresses source segregation becomes established practice, and better uses are found for the digestate, coupled with a greater acceptance of the crop benefit value of good anaerobic digestion derived soil conditioners, will be likely to result in a much greater uptake of the AD output as a product rather than a waste.
Nevertheless, let’s not forget that using Anaerobic Digestion even if only to render the output sufficiently low in organic content to classify it as no longer BMW will be both protecting the environment around our landfills, and also very substantially reducing the tonnages placed in our dwindling stock of landfill void space.
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