Anaerobic digestion: is it right for you?

Anaerobic digestion is an attractive option in the current waste climate, but how best can it fit into local authority organic recovery schemes? Matt Pumfrey imparts some advice

With the publication of Waste Strategy 2007 last year, ever closing LATS targets and the Government's vocal support of anaerobic digestion (AD), there has never been a higher emphasis on organic recovery through AD. However, AD is not a new organic waste processing technique. It is well established in the water and agricultural sectors and has been used to process municipal-derived organics in Europe for a number of decades.

AD is a naturally occurring biochemical process where microbes break down organic materials in the absence of oxygen. Through this process biogas is produced containing roughly 60% methane and 40% carbon dioxide. The biogas is collected, cleaned and can then be used as a renewable energy source. This is traditionally through use in a combined heat & power plant producing electricity and heat, or in some cases it is converted for use as a vehicle fuel.

As a general rule, the more putrescible the waste stream, the greater the quantity of biogas produced per tonne of input material. The digestion process normally takes between 14 to 21 days and also produces a solid residue, digestate, similar to compost, and a liquid fraction that can often be used as a liquid fertiliser. The digestate is normally composted after digestion for the production of a compost product.

To mix with green waste, or not?
Most local authorities are looking to develop source-separation collection schemes to access the organic fraction of the domestic waste stream. The primary question being asked when looking at source-separation is whether or not food waste should be collected on its own or with green waste. This impacts on many areas, including the type of AD system suitable for the processing of the collected waste.

Pure food waste collections produce a dense, high moisture content material that is more suitable for processing in a wet AD system. Wet AD tends to process a feedstock with a moisture content >85%, while dry AD will process a more woody, drier material with a moisture content <85% - in other words, food waste mixed with green waste. Wet AD plants can be aligned with traditional green waste windrow composting where process digestate that has been processed to the ABP Regulations can be blended with green waste and composted to produce a quality compost product.

Where combined food and green waste collections are in place, dry AD will be the most suitable processing option. The drier material is 'handled' rather than 'pumped' through the processing stages and the final solid digestate can often be cured with a minimal addition of woody matrix. It is important to note that dry AD plants produce a lower amount of biogas per input tonne than wet AD.

Some authorities are looking to employ AD at the back end of a mechanical biological treatment operation to produce biogas from the organic fines. While this process will allow for the production of renewable energy, it will also produce contaminated digestate that will have limited and restricted end uses.

After further treatment, the residue may be suitable for use as an RDF material - however the cost of this infrastructure could prove prohibitively expensive. This route may also come under threat from changes in European legislation that may force the separate collection of food waste for recovery. Something that seems more than likely in the coming decade.

Make sure it's fit for purpose
When it comes to choosing a technology, there are a number of AD systems available in the European market along with a small number of proven home grown technologies. From a risk point of view, the usual rules apply and it is vital that procurement personnel fully understand the system and implications of the process guarantees being offered to ensure that the final plant chosen is fit for purpose and performs as expected.

As with traditional in-vessel composting, it is important to look at the collection and digestate end markets when assessing technology for your operation. However an added issue to be addressed are the wider infrastructural requirements of utilising the energy the AD plant will produce. Grid connections can have huge financial implications and care needs to be taken when determining how and where excess heat can be utilised to fully assess its revenue generating potential.

Finally, while there are many benefits attached to AD, the ultimate decision on developing an AD facility will come down to the financial viability of the project. The proposed doubling of renewable obligation certificates will undoubtedly have a positive financial impact on AD, but ultimately developing commercially-sized AD plants is capitally intensive. Ensuring that all potential revenue streams are maximised will be vital in achieving the right financial outcome of any AD operation.

In the current waste climate, AD is an attractive and viable option, however AD might not be the right technology for your situation and a detailed analysis should be made before ruling out other more traditional composting technologies.

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