Centralised anaerobic digestion plants, are a great idea, because they allow the pooling of resources to achieve multiple aims. These include not just for the biogas plant operator to make a profit from a successful energy (biogas production) business, but also to sanitise waste for the health of a community, but also reduce water pollution, control odours, and more. The concept originated in Denmark, where they possibly still have highest number of plants.
This type of biogas plant is referred to a CAD plant.
CAD plants characterised for design purposes as very flexible plants capable of processing a wide variety of feed stocks. However, they are invariably at their core a manure treatment plant based design, and agricultural.
In the most common application of CAD, several farms co-operate to treat their phone animal wastes in the single facility. It is not uncommon for local industrial and municipal wastes, also to be accepted and as a rule of thumb these may be taken as feed materials providing no more than 10 per cent of the total input material. This 10 percent can be viewed as an additional revenue because a gate fee will be charged. As long as the imported materials remain below 10%, any contamination will be unlikely to affect significantly the quality of the output as digestate, in reality.
However, unfortunately this commonsense approach to allowing the output to be used as fertilizer by the farms within a co-operative, may be impeded by difficult to comprehend and apparently unnecessary rules applicable in many European countries at the current time. Notwithstanding this difficulty, which will hopefully be resolved in the not too distant future, by a reclassification of many so-called waste materials.
Given, that as we said earlier, CAD plants are at their core based around agricultural waste treatment, it is not surprising that characteristically they are almost all based upon tried and tested manure digestion technology. By this, it means that they are low solids, mesophilic temperature range, type plants. They are designed around the water contents of a typical slurry from farms. This technology also happens to be very similar to the sewage sludge treatment biogas plant design. These designs are some of the earliest and most thoroughly proven which are available within the AD industry. Many contractors offer these types of plants, so this market is highly competitive and good value should be achievable from the best contractors who are active in supplying this type of biogas plant.
Four CAD plants to be run successfully, really proactive management is necessary to ensure that the right next of different feedstocks are continually being delivered to the plant, not only by the other members of the co-operative, but also by those organisations also contributing organic MSW, and industrial or commercial organic waste. Providing that this balancing act, is achievable given seasonal fluctuations, the nature of holidays in tourist areas, and the need for continuation of feedstock supply in times when there may be growing competition for feedstocks, this is a good business plan.
The biogas generation rates from CAD facilities in reality varies as much as the feedstock does. Estimates are hard to come by, but the publication titled; “Biological techniques in solid Waste management and land remediation”, published by the Chartered Institution of Wastes Management, 2009, CIWM Business Services Limited, Northampton, UK, WWW.CIWM.CO.UK, suggests typical rates.
Using data from Denmark an average generation rate of 37 m3/m3 has been calculated. That sounds quite definitive, but in reality spread of biogas yields is large and varies from 23 to 90 cubic metres of biogas per cubic metre of feedstock. If these plants are operating on manure alone it is suggested that the yield in biogas would be something like 20 cubic metres of biogas cubic metre of feedstock. From that it can be assumed that the highest yields will be those from the CAD plants which are excepting highly calorific feedstocks in the form of the wastes such as household food waste.
In the examples used here, and based on experience from Denmark, the viability of these anaerobic digestion plants is heavily dependent upon them using their own farm and as the output destination for their digestate. These plants would not generally be economically viable if they were not able to use their own co-operative’s digestate and fertiliser, and instead had the cost of digestate treatment to finance.
Of course, this should not be allowed to detract from the fact that they produce biogas which is every bit as good as that produced by other types of processes other than anaerobic digestion plants. It can be used for heating the farms, schools and other facilities in the locality. It can raise steam for food processing/ bottling plants, generate electricity for off-grid locations, and provide vehicle fuel for farm vehicles and local transport.
Very few people would not agree that there is a bright future for centralised anaerobic digestion plants and energy costs rise, as they surely will.