Monthly Archives: September 2014

Operating a Digester

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In this post, ATBEST researcher at Cologne University of Applied Science, Robin Eccleston shares his experiences of a training course on operation of anaerobic digesters.

I recently attended a three day intensive course, ran by the IBBK (International Biogas und Bioenergie Kompetenzzentrum) held in St Ives, Cambridgeshire. The course covered a wide range of areas, and furthermore, alongside all of the technical details each of speakers added in information about what they have seen in reality, and how it differs from literature. I wanted to share some of the information that I found particularly interesting and which may not be so clearly documented.

Whilst the majority of literature suggests that mesophillic fermenters operate at 35°C and that a digester temperature of 37°C to 38°C is optimal, in reality plant operators frequently find better results at 40°C. For example, in Germany it is common for many plants to run at 42°C to 43°C, particularly in the south of Germany.

During digestion, self-heating occurs during the breakdown process inside the digester.  In summer, some plants may even require cooling to keep the temperature at the optimal level. This can be a problem for energy crops, however is not a problem for other substrates.

The course included a visit to a local AD-plant, with a tour given by the plant operator. The plant was owned by a farmer who was growing his own maize for use in the digester. Part of the plant can be seen in the following photo. The digester extends approximately 2 m under the ground.



To the very left is the hopper where the silage is fed in to the digester. The left most section of the digester has a green removable cover over the top, with the flat concrete section on the right being the main digester. In the background, the green dome is for gas storage. On the digester, the cover on the left can be opened without exposing the main digester to air. Most of the contraries remain in this section, which helps with the maintenance of the digester as the majority of the contraries can be removed at regular intervals without interrupting the operation of the remainder of the digester.

With energy crops, it is common to only have a harvest once per year, and so the crop must be stored correctly over the remainder of the year and slowly fed in to the digester as required. The plant visit also included seeing the storage. As the plant was new, there were still some issues that were being learnt about. To store the feedstock, it is clamped – it is compressed very tightly and covered with an airtight plastic cover to prevent aerobic degradation. However this plant had encountered the problem that the exposed face of the clamped maize was too wide. Consequently, as maize would be removed from the clamp, the layer of fresh maize behind would start to degrade too rapidly, so that when it was added it would have already started to compost and was not producing as much biogas as had been anticipated.

Rather than disposing of the maize and not running the plant until the next harvest, glycerine was added to the silage to aid in the digestion process. Additionally, some imported maize was also mixed in. As the problem was due to the exposed face being too large, in this case fitting a diving wall in the centre would have substantially helped with the problem, as then it would have been possible to utilise maize silage from one half, whilst the other side remained covered. This would halve the area of maize that was exposed to air, and to add the same volume to the digester, twice the depth would be removed each day. A 1MW plant would typically require €800k – €1M in silage per year. If the silage is not stored correctly, it is easy to lose 30% of the silage, which can obviously have a huge impact on operating profits.

silage clamp

The biogas produced is directly dependant on the organic dry matter which is added to the digester. This can vary significantly based on location. The example was given of a company that had built digesters in Germany which were fed local grass matter and were ran successfully. A similar setup was attempted in Ireland however there were problems with the digester which were discovered to be due to the substrate. The dry matter in the grass was only approximately 20%. By comparison, in Germany it is typical for the grass to have a dry matter of between 28% and 45%. Initially as grass was being used in both digesters, it was not considered that the local variation would have been the cause of the problem. Additionally a higher DM content would also require stronger mixing. For this reason, clarification of the local material is important.

Clarification of local legislation is also important. In Germany since 2014, all plants are required to be fitted with an emergency flare. The plant shown above in the visit did not have a flare, but rather all gas could be burnt in the CHP unit, with the surplus electricity turned in to heat by a resistive load, and the heat radiated in to the atmosphere.

When looking at CHP units, it is important to check what methane concentration the quoted efficiency is given for. The manufacturer may provide an efficiency figure for 60% methane concentration, but if the biogas only contains 50% methane then this can impact the efficiency. The percentage of methane in the biogas is highly dependent on the substrate. In addition to this, the efficiency of the CHP unit will change over the life of the unit. For larger generators, over 250kW, a gas analyser should always be fitted so that in the event of a problem with the generator, it is possible to prove to the manufacturer that the gas composition is not the cause of the problem. Of course this can also be useful to help analyse the condition of the plant.

It is typical that digesters will have to be opened and cleaned due to sedimentation. It can vary depending on the feedstock and level on contaminants, but this can typically vary between 2 to 8 years. When sedimentation is not addressed this can lead to the mixer shafts or mounting bolts shearing and can result in huge amounts of work to fix.

In the event of a problem in the digester where draining the tank for maintenance would be extremely costly, it is possible to pay for a “slurry diver”, who will be lowered in to the tank by his team and remove or fix the problem. This can cost around £600 per hour, but when the alternative is draining a tank, which might takes months until it is back at the previous output levels, then this can be a cost effective option.

Robin Eccleston, ATBEST Researcher, Cologne University of Applied Sciences

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Co-digestion – A sustainable solution for renewable energy production and organic waste management.

Ireland, being an agricultural country produces significant amounts of organic wastes. For example, it is estimated that 3.2 million m3/annum of pig manure is produced in Ireland. The 1.07 million dairy cows of Ireland can produce 18.4 million m3/annum of slurry. The traditional waste management of this slurry/manure is to use as a fertiliser/soil conditioner because it is rich in nutrients such as nitrogen, phosphorus and potassium. However, environmental legislations, such as EU Nitrates Directive, impose restrictions on their direct land use. An addition to the above mentioned, traditional waste management is to use the slurry as an energy resource in anaerobic digester. The anaerobic digestion of slurry has a number of advantages over traditional organic waste management, such as:

  • Production of renewable fuel e.g. methane
  • Increased fertiliser value of the digestate due to higher homogeneity, enhanced nutrient availability, better C/N ratio and improved flow characteristics
  • Reduction of pathogens, flies and odours
  • Reduction of the organic fraction from the overall waste stream

Ireland’s climate is suitable for grass production and 81% of its agricultural land is used as grasslands. Ireland produces 1.7 million tonnes of dry matter (DM) from grasslands in excess of the livestock requirements. This yield can be increased up to 12.2 million tonnes of dry matter per annum by more intensive grassland management. In European countries, grass is often conserved by ensiling. Ensiled grass or grass silage is more homogenous, has a highly digestible organic matter and volatile solids content and is an excellent feed stock for anaerobic digestion.


Co-digestion of grass silage with animal slurry can have a number of advantages over the mono-digestion or either grass silage or slurry:

  • Supply of additional essential nutrients e.g. cobalt by slurry which may lack in long term mono-digestion of grass silage.
  • Maintenance of optimal pH for methanogens due to buffering capacity provided by slurry/manure.
  • Reduction of ammonia/ammonium in the AD and thus reduction of associated inhibition during anaerobic digestion.
  • Better C/N ratio in the feedstock.
  • Higher biogas yield compared to mono-digestion of silage or slurry due to possible synergistic effects.

small BMP

The synergistic effect is the associative effect where the response is greater for the mixture than the arithmetic calculation using the responses for the sole constituents. Synergy in co-digestion tends to be associated with one substrate with a low C:N ratio coupled with a substrate of high C:N ratio. However, there is limited research being done relating to synergies in grass silage and slurry combinations. The synergistic effect in the combined feedstock of grass silage slurry and slurry is being further explored at Teagasc – Animal & Grassland Research and Innovation Centre, Grange Ireland using batch digestion of various grass silages and various slurries.


Suggested literature:

Lehtomäki, A., S. Huttunen and J. Rintala (2007). “Laboratory investigations on co-digestion of energy crops and crop residues with cow manure for methane production: effect of crop to manure ratio.” Resources, Conservation and Recycling 51(3): 591-609. DOI: 10.1016/j.resconrec.2006.11.004

Murphy, J., D. M. Wall and P. O’Kiely (2013). Second generation biofuel: biomethane from co-digestion of grass and slurry. Grassland Science in Europe.

Seadi, T. A., D. Rutz, H. Prassl, M. Köttner, T. Finsterwalder, S. Volk and R. Janssen (2008). Biogas Handbook. Esbjerg, Denmar, University of Southern Denmark.

Wall, D. M., P. O’Kiely and J. D. Murphy (2013). “The potential for biomethane from grass and slurry to satisfy renewable energy targets.” Bioresource Technology 149: 425-431.  DOI: 10.1016/j.biortech.2013.09.094

Himanshu, ATBEST researcher, Teagasc

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