Contamination is a challenge for AD

Written by: Ricardo Energy & Environment | Published:
Image courtesy of Biogen
I am trying to eliminate reasons why our AD is not working properly (we do livestock farming in the ...

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Dr Andrew Godley, Stefania Trivellato, Phil White and Dr Adam Read at Ricardo Energy & Environment's resource efficiency & waste management practice explain some of the problems faced by the operators of anaerobic digestion plants – including contamination – now that the range of feedstocks has been expanded beyond traditional sewage sludge and industrial wastewaters

Anaerobic digestion (AD) is a long-established microbial treatment process. It produces renewable energy and digestate that can, if of suitable quality, be a valuable route to recycle plant nutrients as fertiliser. It is widely used in sewage treatment where it has for a long time offered a waste treatment function with biogas and digestate benefits. It has also historically been widely used for industrial waste water treatment, again a waste treatment function with benefits.

In both these cases the rationale for the use of AD was principally cost-effective waste treatment. There was no specific motivation to generate revenue from the products, as in most circumstances any energy generated was used on-site.

Certainly in the past in the UK, treatment of other wastes by AD was rarely practised – principally, it can be said, because of the cost being uncompetitive against alternatives such as landfill. The Landfill Directive, Landfill Tax, and fiscal incentives to encourage the development of renewable energy, have changed this position.

Joining the AD bandwagon

Now, many wastes and putrescible materials such as food waste, manures, farm waste, catering waste and the organic fraction of mixed waste are being anaerobically digested.

Unlike sewage sludge and industrial wastewaters, many of these ‘solid’ AD substrates have presented technology challenges.

Sewage sludge and industrial wastewaters are composed of small macerated solids or soluble organic matter and are generally free of contaminating material such as plastics, metals, stones and grits.

These contaminants are not welcome in an AD system as they cause damage to equipment, block pumps and pipe work and accumulate in the digesters, affecting performance.

Where such contaminants are present in these feedstocks it means that significant effort has to be made to protect the AD process by removing the contaminants at the front end. This represents one of the main technology challenges that AD processes with these feedstock types have had to face.

Once the material is fed to the digesters, another technology challenge is ensuring that the mixing system is adequate for the process. Some mixing systems might perform adequately in sewage treatment plants, but less so when it comes to food waste or agricultural feedstocks.

Probably the most challenging technology in this regard is when the AD plant forms part of a mechanical biological treatment (MBT) system for mixed residual waste treatment. The preparation of a suitable-quality AD feedstock from such inputs has known problems that have plagued the successful deployment of MBT-AD.

The compromise that emerges is that the level of purification required for trouble-free AD operation is substantial and results in a low yield of potential biodegradable substrate actually being presented to the AD.

On the other hand, it would seem that separately collected household kitchen waste would be amenable as a substrate for AD without significant contamination issues.

However, this is not often the case.

Householders understandably like to contain food waste in bags and wrappings especially if they have to retain the waste themselves for several days before collection. These bags then are undesirable contaminants in the feedstock and need to be removed. Even the use of biodegradable bags are problematic as, while they may degrade sufficiently during several weeks of composting, they are not so readily degraded in AD systems and may also cause blockages to pumps and pipe work.

Pesky packaging

Commercial food waste from supermarkets is also often used as AD feedstock. Here the issue is the large amount of packaging associated with the waste that can include cardboard and potentially glass. To enable such materials to be used, the food waste packaging has to be opened and processed to separate the undesirable packaging from the desirable food waste. Implementation of such measures adds significant capital costs to pre-treatment and can lead to losses of substrate and consequently potential biogas yield.

Overall, expanding the range of feedstocks outside of traditional sewage sludge and industrial wastewaters for AD have meant challenges for feedstock preparation. For separately collected kitchen and commercial waste, solutions have been developed but represent additional cost and complexity. For residual mixed waste systems, the challenge remains, making these AD systems problematic to operate robustly.

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I am trying to eliminate reasons why our AD is not working properly (we do livestock farming in the UK). The manufacturers of our AD unit say that it is due to contaminants within the unit such as stones or other such materials. Out of the last 90 weeks we have had one week of effective use from our AD unit. Does this sound plausible and if so what would be the remedy. Roger Everington

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