“Where ideas come to life” – visit to Linkӧping

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“Där idéer blir verklighet” – “Where ideas come to life”. This motto of city Linkӧping has been particularly appropriate since the formation (1975) of the University of Linkӧping (LIU) that is one of the ATBEST collaborators (from 2013). Linkӧping itself is a calm, average-sized city (ca. 150,000 inhabitants and 42 km2) in the south of Sweden. It has many interesting places to see – such as the locks of Berg on the Gӧta Kanal or Gamla Linkӧping (Old Linkӧping; fig. 1 and 2).

Fig 1: Old Linkӧping

Fig 1: Old Linkӧping

Fig 2: Old Linkӧping

Fig 2: Old Linkӧping

Linkӧping, like the rest of the country, tries to be environmental-friendly and close to the nature. This is reflected in the fact that Swedish establishment to produce 50% of the whole energy from renewables by 2020 has already been reached. Of course, some of this green-energy is produced from the biogas. Sweden has 264 (year 2013) full-scale biogas plants, amongst which the most abundant (more than 50% of all digesters) are reactors operating with sewage sludge as a feedstock. All reactors produce 1,686 GWh/year (data from 2013). Biogas in Sweden is mostly used as a vehicle fuel (>50%; ca. 200 filling stations – year 2012) and for heating purposes (>30%; year 2012 and 2013). Due to very beneficial financial policy (i.e. no energy tax or financial support for manure-operating anaerobic digesters), there is a huge possibility for the development of energy production from biogas in Sweden. It was proposed that the energy production development will have been 1-3 TWh (in the worst scenario), 58 TWh, or 5-10 TWh (in the best scenario) by 2030. This is very promising forecast.

Fig. 3. Full-scale anaerobic digesters in Sweden (divided by type of the feedstock) (2013; data from: Statens energimyndighet and Energigas Sverige (2014))

Fig. 3. Full-scale anaerobic digesters in Sweden (divided by type of the feedstock) (2013; data from: Statens energimyndighet and Energigas Sverige (2014)) 

Recently, myself (Joanna Grebosz, QUB) and my supervisor – Professor Michael Larkin (QUB) have had a pleasure to visit Linkӧping city for a two days. We loved it from the first sight, especially because the city welcomed us with a very warm and sunny weather as well as very friendly people (most speaking fluent English everywhere – impressive!). During our first day of the visit, we met researchers (fig. 4 and 7) connected with Linkӧping University (Annika Bjorn, Luka Safaric, Bo Svensson, Carina Sundberg, Sepehr Shakeri Yekta, Magali Genero Marti, Eva-Maria Ekstrand and Anna Karlsson) and Scandinavian Biogas Fuels (Francesco Ometto) in order to discuss our projects and any possibilities of the collaboration in our research that brought the positive results and conclusions.

Fig. 4. LIU, QUB and SBF researchers

Fig. 4. LIU, QUB and SBF researchers

Fig. 5. LIU campus

Fig. 5. LIU campus


Fig. 6. Linkӧping river

Fig. 6. Linkӧping river

Fig. 7. LIU, QUB and SBF researchers at the main square in Linkӧping

Fig. 7. LIU, QUB and SBF researchers at the main square in Linkӧping

Linkӧping University (Tema M laboratory) is focused mostly on the research of the trace elements in terms of biogas process optimization. Researchers are also interested in the optimal use of Swedish paper mill wastes, hydrolysis optimization (i.e. the use of enzymes) and the rheology (crucial role of trace elements – see Luka’s blog piece) in the anaerobic digestion process. They believe that the optimized trace elements addition might result in higher methane production even by 15-20% (ca. 100 GWh). LIU has an impressive research facilities that we were able to see on the second day of our stay in Linkӧping during the walking lab-tour guided by Annika Bjӧrn (fig. 8, 9 and 10). She explained the lab-work they’re doing and showed some of the lab-scale digesters they’re running at the moment. Also, LIU works closely with Scandinavian Biogas Fuels (SBF) which focuses on the best proportion of algae in the reactors in the co-digestion of the other feedstock at different temperatures. SBF is testing algae as a feedstock (in the co-digestion) in their 5 L mesophilic and thermophilic CSTR reactors (fig. 10).

Fig. 8. Oxygen-replacing machine at LIU

Fig. 8. Oxygen-replacing machine at LIU

Fig. 9. Anaerobic chamber at LIU

Fig. 9. Anaerobic chamber at LIU


Fig. 10. Annika Bjӧrn (LIU). In the background - CSTR thermophilic and mesophilic lab-scale anaerobic digesters (property of SBF)

Fig. 10. Annika Bjӧrn (LIU). In the background – CSTR thermophilic and mesophilic lab-scale anaerobic digesters (property of SBF)

The QUB co-operation with the researchers from LIU and SBF would be very beneficial for broadening the knowledge about the microbial community of anaerobic digesters – because that is the issue I’m dealing with. My project is connected with the genomic optimization of the hydrolysis step of anaerobic digestion process. Thus, I’m using metagenomic (454-pyrosequencing) and molecular biology (PCR, qPCR, RT-qPCR) techniques in order to analyze microbial content (and microbial population shifts as a result of changes in different AD parameters) in the reactors.

The visit in Linkӧping was very productive in terms of networking and planning the future co-operation between researchers from QUB, LIU and SBF. I’m really looking forward for the secondment in Linkӧping!



I’d like to thank all of the researchers we’ve met in Linkӧping for very valuable discussions and kind help.


1.  Statens energimyndighet, Energigas Sverige (2014) Produktion och användning av biogas och rötrester år 2013. Statens energimyndighet, Energigas Sverige [Online] http://www.energimyndigheten.se/Global/Statistik/officiell%20statistik/Produktion%20och%20anv%C3%A4ndning%20biogas%202013.pdf

2. Persson T, Baxter D (ed.) (2014) Task 37. Biogas Country Overview (country reports). IEA Bioenergy [Online] http://www.biogasportalen.se/BliProducentAvBiogas/MerLitteratur/~/media/Files/www_biogasportalen_se/BliProducent/Rapporter/Countryreportsummary2013.ashx

3. Dahlgren S (2013) Realiserbar biogaspotential I Sverige år 2030 genom rӧtning och fӧrgasning. WSP [Online] http://www.biogasportalen.se/BiogasISverigeOchVarlden/~/media/Files/www_energigas_se/Publikationer/Rapporter/BiogaspotentialSverige2030.ashx

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Cranfield University’s Anaerobic Digestion Research Pilot Plant Facility

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In our first guest blog, Dr Cecilia Fenech from Cranfield University gives an overview their AD Research pilot plant. Dr Fenech was previously a fellow in the Marie Curie ITN ATWARM, which was also coordinated by the QUESTOR Centre at Queen’s University Belfast. 

Cranfield University’s AD research pilot plant facility provides a unique integrated facility for companies to use as a ‘plug-and-play’ opportunity for research and development. The commissioning and opening of the plant is one of the Bio-Thermal RED (Biological and Thermal Renewable Energy Demonstrator) project milestones. The Bio-Thermal RED project is partly (40%) funded by the European Regional Development Fund (ERDF), with match funding (60%) by Cranfield University. Additionally, the AD reactor vessels and part of the commissioning cost were donated by Shanks Waste Management.

The AD pilot plant at Cranfield University will treat food-waste arising from the Cranfield University campus and be available for large-scale R&D projects. Companies can use the demonstration facility as an open access “plug and play” facility. Thus companies can robustly and objectively demonstrate, de-risk and develop their technology as required to commercialise their products with subsequent promotion through the knowledge hub events and services. This will create and develop regional expertise in AD design and deployment ensuring regional businesses and the knowledge base are at the centre of developments within these sectors and thus best positioned to capture the emerging opportunities.


The Cranfield University AD’s units are at the m3 size to ensure initial manufacturing and development costs for company equipment are kept as low as possible. The demonstrator is of a modular construction and mounted on skid-type frame assemblies. This facilitates integration of skid mounted equipment into the AD plant. In addition state of the art laboratories for independent testing of materials are available and qualified staff can help trouble shoot and optimise equipment. In addition to its function as a research facility, the plant will divert more than 10 tonnes of food waste from landfill, save around 5 tonnes of carbon dioxide emissions and produce 8 tonnes of fertiliser each year.

In addition to the creation of the demonstrator facility, the Bio-Thermal RED project was also responsible for setting-up a knowledge and networking hub for SMEs in the East of England region involved in bioenergy.  Over the past  two years the Bio-Thermal RED project has been involved with a number of SMEs based in the East of England that are part of the renewable energy chain, by providing free project-based support and a number of topical workshops relevant to the AD and thermal renewable energy SME sector, combining Cranfield University’s world class expertise in biological and thermal engineering.

Projects carried out to date include work on the utilisation of new feedstocks for AD, engineering optimisation, feasibility studies and technology analyses. The workshops delivered so far have also covered a wide variety of topical subject, including finance and planning for AD, nutrient recovery and digestate management, biogas treatment and upgrading and thermal technologies for energy from waste. In addition to this various technology show case demonstrations, business-to-business networks and on-line support have also been delivered.


The outputs of this project so far resulted in:

  • The creation of 2 new jobs
  • Support to over 25 East of England SMEs involved in renewable energy
  • Delivery of over 65 innovation initiatives
  • Delivery of over 30 environmental initiatives

For more information contact:

Dr Raffaella Villa: r.villa@cranfield.ac.uk or Dr Cecilia Fenech: c.fenech@cranfield.ac.uk

Developing an AD plant in Northern Ireland

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The QUB ATBEST fellows were recently lucky enough to visit an AD plant under construction by ATBEST associate partners AgriAD. In this blog, researcher Fabio De Rosa describes the visit and what he learned from it. 

Most of the jokes in my country start with “there were once a German, a French, a British and a guy from Naples…”. Things were slightly different during this trip, not only because of the company, but because when it comes to biogas we are not talking about jokes but about a great business instead.

Rawan (Lebanon), Liang (China), Joanna (Poland), Simon (Ireland) and I (the Italian guy) went to visit a 500 KW AD plant under construction located in Banbridge, County Down, Northern Ireland, owned by the directors of agriAD Ltd Thomas Cromie (Figure 1) (Figure 2).


Figure 1 - QUB fellows with Mr Thomas Cromie

Figure 1 – QUB fellows with Mr Thomas Cromie


Figure 2 - Aerial view and map of the AD plant in Banbridge, County Down, Northern Ireland from AgriAD

Figure 2 – Aerial view and map of the AD plant in Banbridge, County Down, Northern Ireland from AgriAD

Thomas Cromie has 20 years’ experience in small and medium-sized enterprises active in the agricultural and energy industries. He has worked in collaboration with local government like the Department of Enterprise, Trade and Investment of Northern Ireland (DETI NI), for the development of regional policies in these sectors and on matters related to the exploitation and commercialisation of science, technology and R&D.

He is founder and partner in agriAD Ltd, which deals with the development and operation of joint-venture biogas projects, providing funding, technology and expertise in anaerobic systems. Thomas comes from a farming family, and graduated in geology at Queen’s University Belfast and after a working experience for an oil company in Saudi Arabia (back when the oil price was still 25$/barrel) he came back to the Emerald Island in the late 80s, recognizing what was raising in Germany during those years as a possible bargain for his family agricultural business: anaerobic digestion.

It took several years of travelling to the USA, Germany, Italy, France and Netherlands to understand the developing market, build contacts and meet politicians.

AgriAD’s purpose is to develop a network of standalone farm-based AD plants across Northern Ireland. It is proposed that the AD plants will be strategically located on large farms and operate in partnership with the farmer. Under the partnership, the farmer will provide a suitable site with planning permission for the AD facility, guaranteed feedstock (typically grass silage and animal manure) and the manpower to operate the facility. AgriAD will in turn provide the majority of finance, engineer, procure and construct (EPC) contractor, project delivery experience, project development and process management, biological support and plant maintenance.

The advantages from this network and the partnership between farmers and landowners include project experience and credibility, purchasing power, operational economies of scale and operational resilience (e.g. feedstock pooling, mitigating the key supply risk). The plant is currently under construction and will be operational in June 2015.

As Thomas says, “anaerobic digestion is a matter of opportunity”. Anaerobic digestion in Northern Ireland is a well-established technology and a great opportunity thanks to the perfect grassland conditions for AD and the most attractive financial regime in Europe. Indeed, Renewables Obligation Certificates (ROCs) represent the driving force of this economy.

ROCs are green certificates issued in UK to operators of renewable generating stations for the renewable electricity they generate and are basically used by suppliers to demonstrate that they have met their obligation [1]. Every year the RO requires UK electricity suppliers to source a specified proportion of the electricity they provide to customers from renewable sources. ROCs are tradeable commodities that have no fixed price, i.e. the amount an electricity supplier pays for a ROC is a matter for negotiation between the supplier and generator [2] and changes according to the technology priority. Currently AD ( 500kW), Hydro ( 20kW), Onshore wind ( 250kW) and Solar Photovoltaic ( 50kW) technologies have all been assigned the highest values for 2014/2015 [3], which is equal to 4 ROCs. There is significant on the ground interest, with 86 AD project permissions granted in 2014 [4] (Figure 2).

According to Mr. Cromie, AD brings more benefits to the rural communities. For example wind turbines have virtually no operating costs, whereas AD generates 200-300k£/year that can be spent to further develop the renewable energy sector. Moreover electricity generation through biogas has a constant load, whereas wind energy is more fluctuating.

It might seem odd, but also giant oil companies like Shell and its subsidiaries are very interested in renewable energies, photovoltaic and biogas on top [5]. This is what looking ahead means.

The electricity grid in Northern Ireland was constructed to push the electricity from three large fossil fuel generating stations (Maydown, Kilroot and Ballylumford) around the coast into the interior of the country. Reconfiguring the grid to allow generation in other locations requires major investment.

Figure 3 - Projects with planning approval in Northern Ireland

Figure 3 – Projects with planning approval in Northern Ireland

The challenges for AD in NI are the funding scheme, small scale and individual projects as a barrier, the limited experience of project financing within the NI farming sector, the infrastructures and also the number of farmers in the UK that has been decreasing of the 4-5% per year in the last decade.

Nowadays only the 7% of the AD projects in NI is operational, 10% are under construction, 54% have been approved but seeking for funding while 29% is waiting for planning. Only 16% of the total fundings are secured. Regarding the source, 50% are provided by banks, 23% by contracts, 14% by leasing and 9% are unsecured [6].

The total project cost of the plant in Banbridge is around £2.5m and two thirds of the heavy concrete construction is already done. Usually it takes 9 months to build the plant, under and EPC contractor: fixed price and time and penalty in case of late delivery of the plant.

At the beginning the whole feedstock will be grass silage secured within Cromie family landholdings.

Later industrial wastes from a near milk factory will be integrated, representing the 25% of the feedstock. The advantage is that the feedstock composition is known (there is no need for pre-processing) and in any case these wastes have to be pasteurized before disposal, so that using them in an AD plant makes sense.

The biogas yield will be equal to around 2Mm3/year. The combined heat and power system creates the opportunity to transport heat to a number of major heat users nearby (Figure 4). It will work for 8000 h/year in order to produce 4MKWh of electricity with a 91.3% operational efficiency. There are operation responsibilities and should the electricity generation be less than planned a penalty has to be paid. Knowing the outputs is paramount with respect to the funding.

Statistically 55-60% of the problems in an AD plant arise from CHP inefficiencies and only the 10-15% is due to human operators.


Figure 4 - Combined heat and power (CHP) unit

Figure 4 – Combined heat and power (CHP) unit

Figure 5 - CHP engine

Figure 5 – CHP engine 

The CHP engine is basically a modified Diesel engine, factory made and tested, operating with a biogas consisting of 55% of methane (Figure 5).

There are radiators for the excess heat developed in the CHP unit, activated carbon filters and a gas boiler to keep the biological temperature during the start up of the plant and in case of problems with the CHP unit (Figure 6). It takes 4-5 weeks to heat the system up to the biological temperature of the process.

Figure 6 – Gas boiler, radiators for excess heat and activated carbon filter.

Figure 6 – Gas boiler, radiators for excess heat and activated carbon filter.

There is a primary digester, which receives the feedstock by gravity, and a secondary digester (Figure 7). The tanks are factory-made, which means that the concrete wasn’t poured on site, but each slab was shaped singularly elsewhere and put together in loco. This allows for a faster construction.

Figure 7 - Primary and secondary digester and control room (on the left)

Figure 7 – Primary and secondary digester and control room (on the left)

The control building is on the left in Figure 7, with the pumps in the cellar.

Usually the retention time depends on the feedstock and it is around 40 days. It will be around 100 for this plant, because of the grass silage.

The pumps and the pipes going to the second digester from the control room cellar offer a high operational flexibility to this plant (Figure 8).

Figure 8 - Pump room

Figure 8 – Pump room

There are 3 points in the tank at three different heights where the temperature is monitored (Figure 9). The optimal value is around 35-40°C and when the variation between the three points is no more than 0.1°C then it means that there is a good mixing inside the tank.

The sampling point is at 3-4 feet, while the other 2 holes below are the drain points.

In both tanks there is a propeller mixing system. When you have to fix the mixing system in case of faulty, large tanks have to be emptied first. These two tanks have a particular sleeve which allows for fixing the propellers without emptying the tank and venting the gas.


Figure 9 - Inside of the digester

Figure 9 – Inside of the digester

Figure 10 - Primary and secondary digester from the inside

Figure 10 – Primary and secondary digester from the inside

The concrete tank lifetime is equal to 25 years, also because of the black coating in the top part. It protects the concrete from the biogas, which is corrosive (Figure 9 and Figure 10).

Small amount of oxygen can be fed in the overhead in order to oxidize hydrogen sulphide, producing elemental sulphur. This is a way to get a grass fertilizer which is even better than the commercial ones (11% more yield).

The digestate will be first collected in a storage tank and then spread in the surrounding fields after September (Figure 11). This is because NI is a nitrates and phosphorous vulnerable zone and spreading is not allowed through the winter.

Figure 11 – Digestate storage tank

Figure 11 – Digestate storage tank

Operation should start in 2 months, with slurry at the beginning and a little of digestate from another AD as inoculum. Before that the two domes have to be put in place and a pressure test, using just water, has to be carried out.

As a conclusion, this trip was extremely interesting and stimulating. We had the chance to see first-hand how an anaerobic digester plant is constructed and moreover to discuss with a professional in the sector.



1. [Online] https://www.ofgem.gov.uk/environmental-programmes/renewables-obligation-ro.

2. [Online] https://www.gov.uk/government/policies/increasing-the-use-of-low-carbon-technologies/supporting-pages/the-renewables-obligation-ro.

3. [Online] http://www.detini.gov.uk/existing_and_confirmed_roc_per_mwh_levels_from_1_april_2013.pdf.

4. [Online] http://www.planningni.gov.uk/downloads/renewable_energy_apps_decided_by_fy_by_type_renewable_energy-2.pdf.

5. [Online] http://www.ipwatchdog.com/2015/03/06/shell-oil-provides-surprising-developments-in-renewable-energy-along-with-oil-and-gas-tech/id=55211/.

6. [Online] http://questor.qub.ac.uk/GeneralFileStorenew/DO-Bioenergy/Filetoupload,465992,en.pdf.

7. S. Murray, E. Groom, C. Wolf,. WRAP – feasibility reports. http://www.wrap.org.uk/. [Online] October 2012. http://www.wrap.org.uk/sites/files/wrap/DIAD%20I%20Queens%20University%20feasibility%20report.pdf.

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Introduction to innovative technology of biogas production

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Biogas production facilities use a variety of reactor configurations.  In this blog piece, Markus Voelklein, ATBEST researcher at University College Cork, discusses and compares single stage and two stage reactors. 

The anaerobic microbial conversion of organic carbon takes place in a four phase degradation process. It consists of hydrolysis, acidogenesis, acetogenesis and methanogenesis, which theoretically can be identified via the presence of characteristic bacteria in each phase.

One-stage reactor system

In waste water treatment and agricultural plants the one‑stage digestion is dominating due to its decreased capital costs. This is probably the most important reason why most facilities have previously been designed for one-stage operation. Nevertheless, non optimum growth conditions prevail in this reactor especially for the bacteria involved in the first two phases (hydrolysis, acidogenesis) of the degradation process. Furthermore, since all four steps of fermentation take place simultaneously in one reactor the generated biogas during fermentation results in a mixture of all gas compounds of each phase. The digester of a one‑stage biogas plant is typically implemented as a cylindrical vessel, whose principle is shown in figure 1.

Figure 1: One stage biogas plant

Figure 1: One stage biogas plant

Two-stage reactor system

The two-stage biogas production has not commonly found its way into the biogas technology. The main reason lies in considerably higher capital costs regarding the construction of reactors and stirring technology. The requirements caused by decreased pH values for vessels, fittings and equipment in terms of corrosion have to be considered during the construction. Nevertheless, this technology is sometimes used in large scale industrial applications for example in waste water treatment or in the processing of easy-acidifying waste products in the food industry.

A two-stage biogas plant excels itself trough the spatial separation of process phases, which arise as a consequence of the different microbiological requirements of the four phase anaerobic process. In an upstream reactor acidification takes place and in a second reactor methanogenic bacteria produce methane. The two-stage system possesses the advantage of adjusting the environmental settings to the optimum requirements of the microorganisms. First of all, the pH value and loading rate can be adjusted individually. The spatial separation allows a directed acidification of the input substrate in a first vessel. The following Figure 2 illustrates a schematic layout of a two-stage biogas plant.

Figure 2: Two-stage biogas plant

Figure 2: Two-stage biogas plant

The substrate is fed into the first reactor where acidification takes place leading to hydrogen production and accumulation of acids and alcohol, which are precisely the precursor for the acetogenic microorganisms in the second stage. The hydrolysis phase is either carried out batch wise or continuously and connected to the second methanogenic reactor. The ratio of both reactor volumes is defined by the different retention time and generation time of the bacteria. This requires smaller volumes in the hydrolysis and bigger in the methane reactor, because the retention time in hydrolysis is significantly lower. At fibrous and high solid content enriched input substrate, mixing problems could occur in the hydrolysis phase and form a swimming layer. This can result in incomplete hydrolysis of the substrate. In order to prevent this condition, proper stirring technology has to be chosen in the design phase. Due to conversion of solid carbon into the liquid phase a liquefaction of the substrate occurs.

After a certain retention time of 1 to 5 days the acidified substrate is pumped into the methane reactor. Because of the already pre-liquefied substrate and the existing digested substrate in the methane reactor, the solid content is relatively low and requirements concerning the stirring technology decline. In contrast to the one‑stage process and as a result of the already largely liquefied carbon, a rapid degradation to biogas takes place, which leads to shorter overall retention times. An increase of methane yield or an extension of the input substrate range at higher loading rates is consequently feasible. Also poorly degradable lignocellulosic materials can be partly broken down trough the prevailing conditions in hydrolysis phase. Thus the bioavailability of these materials increases significantly and can be digested to biogas.

Biogas composition of two-stage digestion

The most fundamental difference and substantial advantage compared to the one‑stage system is represented by the separate gas collection of each vessel. This allows an active influence on the concentration of individual gas compounds or even a separated utilisation of the produced biogas. Figure 3 shows a comparison of a one and two-stage biogas plant system.

Figure 3: Comparison of one and two-stage biogas plant

Figure 3: Comparison of one and two-stage biogas plant

The gas compounds of an acidification reactor consists mainly of carbon dioxide, hydrogen sulphate and hydrogen. The composition can be influenced by parameters like retention time, loading rate, pH value and temperature in order to gain high carbon dioxide stripping. Due to this removal during acidification, a biogas with enhanced methane content is received in the methane reactor. For substrates like food waste or renewable raw materials a methane content of 60 to 70 % can be achieved. Therefore, the previous carbon dioxide stripping in a two-stage digestion system, enhances the efficiency and reduces the costs of a following biogas purification facility.

However, the loss of hydrogen and carbon dioxide in the first phase can contribute to reduced overall energy yields. On the other hand, the degradation rate rises due to the partial break down and a more complete utilisation of the substrate.

Comparison of both technologies

The one‑stage digestion constrains the biochemical settings of a digester. Inevitably non optimum growth conditions for hydrolysis bacteria prevail in this reactor. As a consequence an optimization should take this limiting step into consideration. The two-stage operation allows an individual optimization due to a separation into two-phases. Two-stages anaerobic digestion has been developed to minimize the inhibition of the hydrolysis and acidogenic microorganism by creating acidic conditions at a pH of 5.5. Slow growing methanogenic bacteria, which require a more neutral pH, are cultured in the second phase at longer retention times of 15 to 20 days. The investment costs for the two-stage fermentation are higher than for a one‑stage fermentation. The exceeding costs have to be refunded by advantages regarding the process stability and higher rate of substrate degradation, leading to higher biogas yields from the same amount of substrate.



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Biogas Upgrading – What are the real challenges?

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Biogas upgrading is vital part of the biogas supply chain to ensure compatibility with existing consumer technologies. Here, Keren Rajavelu, ATBEST researcher at University Duisburg-Essen, discusses the issue. 

The Bioenergy supply chain has become a thriving research field with the feasibility of technologies, cost and logistics. Specifically, issues in transporting the gas to grid or using it as a fuel for vehicles has gained more recognition in terms of both research and marketing. Biogas Upgrading is defined as the removal of carbon dioxide and other impurities from biogas to produce a gas with high methane content (greater than 96%). This gas is called Biomethane and is suitable for use in heat and power production or as a vehicle fuel.


Biogas plant with upgrading unit commissioned in Dec 2006 Stadtwerke Aachen AG, Germany

Biogas plant with upgrading unit commissioned in Dec 2006 Stadtwerke Aachen AG, Germany

The decision of an AD plant operator to utilise biogas directly or to first upgrade to Biomethane depends mainly on technical and legislative factors. A decade ago, there were only 20 biogas upgrading plants operating globally. This scenario has improved drastically to 220 operating plants at the end of 2012. Most of them are in Germany (96) and Sweden (55) according to  figures published by the IEA in 2013.

Although upgrading technologies such as Pressure Swing Adsorption (PSA), Amine scrubbing, Water Scrubbing, Organic solvent scrubbing, Membrane separation and Cryogenic Separation are all established; it is PSA, water scrubbing and amine scrubbing that dominate. PSA has a market share of 40 %, water scrubbing 23% and amine scrubbing 22%.


Fig 2: Amine Scrubbing Unit in Lunen, Germany

Fig 2: Amine Scrubbing Unit in Lunen, Germany

When choosing a biogas upgrading technology, we must keep in mind that our primary goal is to remove maximum CO2 to increase the methane yield (97%) and produce a cleaner gas that is free of sulphur. All technologies mentioned above have the ability to fulfill this requirement.

What then are the main challenges of upgrading biogas? Whilst technically achievable at all scales, gas upgrading at small scale biogas plants can be too expensive.

This is the challenge that is being met by the ATBEST research project – producing new or improved technologies for the biogas supply chain that close the gap between existing technologies and what is economically sustainable. With respect to biogas upgrading, the techniques under investigation  include in-situ methane enrichment, coupled with an integrated biological sulphur removal process; the use of new absorbents like CaO ash and ionic liquids.

Fig 3:  Biogas Desulphurization Unit in Small scale Biogas plant Lunen, Germany

Fig 3: Biogas Desulphurization Unit in Small scale Biogas plant Lunen, Germany

In conclusion, there is an increase in biogas production in many countries and therefore in the efficient use of upgraded biogas. The uptake of new technologies in this area will depend on both the technology costs and the ability to meet the demands of the end user.

Suggested literature:

1. Dr. Wolfgang Urban. “Experiences and future perspectives of biomethane in Germany from a regulatory perspective”. Nature Conservation and Nuclear Safety

2. Bauer F, Hulteberg C, Persson T and Tamm D, Biogas upgrading (2013). “ Review of commercial technologies”. SGC Rapport 270. Swedish Gas Technology Centre.

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AD Sludge Rheology – What is it? What does it tell us? Why bother?

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In this post, Luka Safaric, ATBEST researcher at Linköping University discusses the importance of an understanding of the rheological properties of AD sludge. 

When operating a biogas reactor, we generally follow several parameters to make sure it is running as efficiently as possible. Rheology is however often not one of them. But should it be? Surely everything seems to be running just fine without worrying about this aspect of anaerobic digestion, so why should we spend our time, effort, and money on it?

And what exactly is rheology anyway?

Well, the “official” definition is that rheology describes the deformation of a body under the influence of stresses. Bodies in this context can be either solids, liquids, or gasses (Schramm, 2000).

So let us focus on fluids (as that is mainly what our reactors contain). When describing their behaviour, we use terms such as viscosity, shear rate, and shear stress. Viscosity is used to describe the fluid’s resistance against irreversible positional change – therefore the higher it is, the more energy is needed to make the fluid flow. If we now imagine an example when the fluid is positioned between two parallel plates, then the shear stress would be the force applied tangentially to the liquid through one of the plates as it moves in a parallel direction in relation to the other one, divided by the area this force is acting upon (i.e. the area of the plate-liquid interface) (Schramm, 2000). In more practical terms, shear stress is the force exerted on the liquid when we stir it. Shear rate on the other hand, describes the drop in flow speed across the gap size (between the both plates in our example) as the liquid will develop thin layers across this gap, flowing at different speeds (decreasing from the moving, towards the stationary plate) (Schramm, 2000). This can therefore be considered as analogous to the speed with which we mix the fluid.

Generally we can separate liquids into two distinct categories – Newtonian and non-Newtonian. The former, also known as ideal fluids, are characterized by the fact that their viscosity does not change with changes in shear rate. In other words – their viscosity is unaffected by how intensively we mix them. We are quite used to this type of liquid behaviour because we deal with water on a daily basis. In reality Newtonian fluids are rare in comparison with non-Newtonian ones (Schramm, 2000). Some examples (besides water) are methanol, olive oil, and glycerol (Björn et al., 2012).

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Non-Newtonian fluids on the other hand, can behave in many interesting ways and are thus further classified into several different categories. For example, we have the pseudoplastic fluids, which are characterized by drastic viscosity decreases at increasing shear rates. The more intensively we mix them, the less viscous they are (up to a certain point). Examples of such fluids are corn syrup and ketchup (Björn et al., 2012).

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Then there are liquids, showing a dilatant-flow behaviour, which is opposite from the previous type, as their viscosity increases significantly at increasing shear rates. Therefore the more intensively we mix them, the more viscous they become. This can be seen with honey, cement, and ceramic suspensions (Björn et al., 2012).

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Fluids may also exhibit plasticity. In this case they behave like pseudoplastic fluids, only that they additionally feature a so-called yield point. Such a liquid will initially behave as a solid, until a certain threshold in shear stress is reached. After that, the internal forces of the liquid will no longer be strong enough to resist the outside forces and it will start to flow. Blood and some sewage sludges can behave in this way (Björn et al., 2012).

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There are even some liquids whose behaviour is affected by their shear history. In other words, they are capable of “remembering” if and how much they were mixed recently. They are called thixotropic and rheopectic liquids and they are in a way opposites of one another. Thixotropic liquids solidify if left alone for a sufficient amount of time due to interactions between particles and/or molecules developing in the liquid. These are often hydrogen or ionic bonds, which are relatively weak, so they rupture easily if the dispersion is subjected to shear over a sufficient period of time, transitioning back into their fluid state. The solid structure is then reformed if the liquid is allowed to rest again for a while. What this means is that the shear history of the liquid affects its rheological properties, i.e. the longer you mix it, the lower the viscosity (to a certain level). This can be observed with some paints, soap, wastewater, and sewage sludge (Björn et al., 2012). On the contrary, the rheopective liquids are characterized by a viscosity increase related to the duration of shear stress. When they are allowed to rest, they will recover their original low viscosity. Such liquids are very rare (Schramm, 2000).


But why should we care about any of this while running biogas reactors? In most common reactor designs, the anaerobic sludge inside them is a complex dispersion of solid particles and soluble molecules in water. Most often it can be classified as one of the non-Newtonian fluid types. Which one and what viscosity it has is very much dependent on many different parameters in the process, and can change due to significant shifts in said parameters. It has even been observed that two reactors of the same type (e.g. CSTR), digesting the same type of substrate, can exhibit different fluid behaviour (Björn et al., 2012). This has important implications as it might happen that a reactor was designed with a different fluid type in mind than the one it actually contains, thus leading to possible operational problems.

For example imagine mixing a dilatant-flow type of sludge very intensively. It would resist your attempts by increasing its viscosity and causing you to spend extra energy while losing mixing efficiency. Another example would be using an intermittent mixing regime on a sludge that exhibits thixotropic behaviour. By stopping the stirring, you would allow the viscosity to increase, again increasing your energy consumption and decreasing mixing efficiency when the stirrers are turned back on. Different sludges therefore need to be treated differently.

Currently, when biogas reactors are being designed, liquid viscosity is often only estimated based on reference data that is based on the relationship between the total solids concentration of sewage sludge digester fluids and their viscosity. This can be problematic when used for reactors that will be digesting other substrates, which may impose different rheological characteristics on the sludge despite having similar total solids contents. The stirring equipment and the stirring intensity might therefore not be optimal for the type of sludge in question. This can be problematic because stirring is an important part of the anaerobic digestion process as it brings the microorganisms into contact with new feedstock, facilitates the release of biogas from the sludge, and helps with temperature distribution. Inadequate stirring therefore has detrimental effects on the general efficiency of the reactor, and can potentially lead to severe operational problems such as foaming. Because of this, more attention should be directed to actual rheological characteristics of the sludge than it currently is. (Björn et al., 2012)

We can accurately determine these characteristics through rheological analyses. We take samples of the sludge and analyse them with a rheometer. These come in many different shapes and sizes, but the most suitable for anaerobic sludge characterization, are the rotational rheometers, which spin a cylinder in a cup with the sample to determine its rheological properties. The result is a graphical representation of the relationship between shear stress and shear rates, called a rheogram. Alternatively, viscosity may also be plotted against shear rates, thus creating a viscosity curve (similar to the example images for different fluid types above). Based on the shape of these curves, we can determine which type of fluid we are dealing with. We can also use values attained by our measurements and compare different sludges to one another.

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A more advanced use of rheological results can be done through computational fluid dynamics, which allows us to model and predict mixing behaviour of the sludge in relation to our specific reactor design. We can then optimise mixing time, power consumption, flow patterns and velocity profiles (Latha et al., 2009).

We can therefore conclude that rheology is an important, yet often overlooked parameter of anaerobic digestion that can have big implications for reactor performance.


Björn A., Segura de La Monja P., Karlsson A., Ejlertsson J., Svensson B.H. 2012. Rheological characterization. In: Sunil Kumar (Ed.), Biogas. Chapter 3 (63-76). Tech publisher, Rijeka, Croatia. ISBN 979-953-307-221-9.

Latha, S, Borman, DJ and Sleigh, PA (2009) CFD multiphase modelling for evaluation of gas mixing in an anaerobic digester. In: Aqua-enviro, TT, (ed.) UNSPECIFIED 14th European Biosolids and Organic Resources Conference and Exhibition, 9‐11th November 2009, The Royal Armouries, Leeds, UK.

Schramm G. 2000. A practical approach to rheology and rheometry. 2nd Ed., Thermo Haake Rheology, Germany.

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BioGaC Study Tour November 2014

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In this blog, ATBEST researchers at Christian Jenne (University Duisburg-Essen), and Laura Gil Carrera (Gas Networks Ireland), tell us about their experiences of attending a study tour (BioGaC) throughout Sweden and Finland.

BioGaC (Biomethane and LNG in the North for Growth and Competitiveness in the EU) is a €4.4 M European Union research project in Sweden which started in March 2014. The research study covers the pilot deployment of two new CNG (Compressed Natural Gas) filling stations in Härnösand and Umeå, as well as improvements to existing stations at Sundsvall and Skellefteå in northern Sweden. The aim is to increase the number and density of the CNG filling stations, encourage the use of CNG and create a market opportunity for CNG/LNG (liquefied natural gas) investors.

Proposed CNG/LNG filling stations in Sweden

Biofuels region was the project co-ordinator for this study tour, which opened a call for 12 participants in taking part on a study tour. This two day study tour started on 21st November 2014 in Skellefteå (Sweden) and finished in Vaasa (Finland) the following day.

Laura Gil Carrera from Gas Networks Ireland and Christian Jenne from University Duisburg-Essen have been nominated for this field trip. Demand for this research study trip was huge and was oversubscribed with submissions from eight different countries.

On our first day we met all participants at the central bus station in Skellefteå. From there we drove to a local biogas plant which was led by Mr Ola Burström. He was the main driver behind this biogas project since 2003. The biogas plant was fully commissioned and officially opened on 26th February 2007. All the food waste in this region is treated in this biogas plant and a “brown bin” was introduced to collect all local organic waste products. The owners of this biogas plant are mainly ordinary people living near the city and five company investors.

Plant field


This biogas plant has two 100% CBG (Compressed Biomethane Gas) filling stations: One onsite for trucks and commercial vehicles and one located in the city centre with the aim to increase the customer base. The biogas filling service is used by some haulage companies, local bus services, taxi drivers, pizza service and ordinary car drivers. Each customer has an individual fuel card and all biogas fuel purchases are deducted from their registered accounts by the end of each month. Due to a huge popularity (18 buses and trucks and 7 taxies and cars) the filling station needs an upgrade.

Filling 1-3

Filling process in comparison to a regular diesel fill for a bus can take up to 25 minutes which causes long waiting times for refueling. The future plan is to install a few more fast charging refueling points. This also will be housed in an open building with a night time refueling option for commercial vehicle. This would reduce waiting time on the filling station during the day and will translate into more efficiency for drivers due to filling process during the night time. This would mean the vehicle can be refueled over night without any supervision and therefore less man hours required.

On the second day after a short bus trip we met Leif Åkers, CEO at Stormossen Oy, and Johan Saarela, process engineer. Leif gave us a nice presentation on their biogas plant and how they are working to upgrade the biogas to biomethane. To handle the ”Chicken and egg” – problem, Leif firstly approached the municipality and he has been working very close with them. The municipality will have their own biomethane buses ready to run once the biogas is upgraded to biomethane.

The anaerobic digesters are fed with waste from household and sewage sludge. Such feedstock generates the equivalent to about 1.6 million litres of diesel. It is currently being used for electricity and heat production through CHP, however electricity price is very low in Finland so they are getting very low revenue, hence they are willing to upgrade the biogas and use it for transport. Leif showed us around the biogas plant, the waste processor, AD, gas holder and the CHP engines.

Afterwards we met Kurt Stenvall, CEO at Jeppo Biogas. Kurt talked about their biomethane plant, which has three digesters and produces 20- 25 GWh of biogas/year out of manure, offal and green masses from non-food agriculture. The biogas is sold to two nearby industries, Merkki and Snellmans and transported by pipeline to their premises. At the moment only a small amount of biogas is upgraded to biomethane, however they expect it to grow when a filling station will be built next year in Stormossen. It was very interesting to hear the challenges that they faced to make such a huge project real and successful.

The next talk was given by Mauri Blomberg, CEO at Vaskiluodon Voima Oy. He told us about the largest biomass gasification plant in the world, built in Vaasa. The power plant is mainly using coal for electricity (1.2-2.5 TWh/y) and district heat production (0.8 TWh/y) and integrating the biomass gasification plant to the existing coal-fired boiler  is a way of contributing to a more sustainable society and also prolong the life span of the plant. The syngas is used directly and co-fired with coal. Since the plant is air-blowned, it is not possible to use the syngas in a methanation process. The total investment was €40M, the gasifier has a fuel input of 140 MW and reduces the use of coal by 25-40% as well as CO2 emissions by 230,000 tons/year. It was great to hear Mauri’s talk but we needed to have a look of the technology in the field, so the bus took us to the gasifier and we could see the biomass dryer (called Titanic) as well as the gasifier itself.

gasifier 1-3

The visit was coming to an end, but a very interesting session was waiting for us in the conference centre, where we discussed how to solve biomethane and CNG market issues. We discussed on the different challenges and concerns in our countries and the mechanisms to face them. It was very rewarding session since we could hear interesting experiences, from the other participants, that could help us to face and solve our problems.

Besides lectures and visits to biogas plants, there was also time for entertainment and fun. After the long but very fruitful day we enjoyed some Swedish beers and a tasty Mexican dinner in a van J.

Dinner 1&2

Overall, it was very interesting to see how biogas business is constructed in Sweden and Finland. It was very impressive to see all the biogas plants successfully up and running and how they are working together from production to fuel and the market. It was great to share our experiences with such knowledgeable and interesting group and I believe we all learn something.

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Destination: Cambridge

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In this blog, ATBEST researcher at QUB, Rawan Hakawati tells us about her experience of attending an IChemE conference with some of her colleagues.

On the 17th and 18th of September, 2014, The Institute of Chemical Engineers, IChemE, organized a conference titled as “Applied Catalysis and Reaction Engineering”. It took place in one of the most prestigious universities ranking second on the QS World University Ranking for the year of 2014, University of Cambridge, United Kingdom. The program included various presentations covering topics from catalyst development to reactor design. The presenters were PhD students and Post-docs from different universities in the UK such as University College London , Bath University, Newcastle University, Manchester University, Cardiff University, Heriot-Watt University, Liverpool University and of course our beloved Queen’s University Belfast. The two-day program also included three invited presenters who gave their time to share some industrial experiences; Professor Freek Kapteijn from TU Delft talked about “Large Scale Reactors” and their ongoing research in optimizing their activity . Dr. Adeana Bishop from ExxonMobil talked about the “Role of Basic Science in Industrial Experience” and Professor David Cole Hamilton from the University of St. Andrews took the time to introduce the importance of producing “Chemicals from Waste Bio-oils.”

Personally, I was very interested in one specific piece of work presented by Mr. Paulo Brunengo, Vice President- Reaction Technologies at Johnson Matthey Davy Technologies Ltd, entitled as “Interdependence Between Reactor Design and Catalyst Development: An Innovative Reactor Design Opens Opportunities to Enhance Catalyst’s Activity”. Mr. Paulo talks about the industrial heterogeneous catalytic reactors’ limitations in terms of heat transfer and pressure drop. The latter imposes using “non-optimum” catalysts when a reaction that is limited by heat/mass transfer is scaled up to the industrial scale.1 He also talks about the factors that affect the efficiency and selectivity of the catalyst and the product yield. Johnson Matthey Davy Technologies Ltd, a Johnson Matthey Plc. company was able to develop a new design for a catalyst carrier which is configured to fit in the reactor tube in a tubular reactor. This innovation optimizes heat and mass transfer with acceptable pressure drop due to its small pore size which means that the reactor can be operated at high productivity. The catalyst carrier allows the use of catalysts with less than 1 mm particle size irrespective of their strength meaning that they can be fragile or powder state. The carrier allows much better control of temperature and can be readily removed and transported for disposal or regeneration. The carrier may be used in a wide range of processes such as methanol and ammonia production, methanation reactions, endothermic reaction such as dehydrogenation and also in Fischer-Tropsch reactions.

Figure 1: Catalyst Carrier developed by JM

Figure 1: Catalyst Carrier developed by JM


Aside from the technical side of the trip, Fabio, Leanne and I were happy to support our colleague from QUB, Colin McManus, who presented his work on TAP systems, “Expansion of Temporal Analysis of Products (TAP) Pulse Responses for More Accurate Data Analysis.”  The two-day trip drew us closer to one another and allowed us to meet students from different places like Mexico, United States of America and Germany etc… and hear their random stories about living abroad. We roamed the streets of Cambridge, dined in a must-try Baked Potato place called “Tatties”, probably one of the best stuffed baked potatoes we have ever had (mine was stuffed with goat cheese and marinated chicken breasts on a bed of spinach leaves, YUM.)

On the second day during one of the coffee/tea breaks, I couldn’t but notice a tall-aged grey-haired man sitting in the far corner of the room. As I approached him, he greets me with a simple “hello”, yet I can sense he is very shy. I notice a few books (on the table beside him) all aligned perfectly and replicas of each other, so I assumed there was a mini book-signing event. However, the man did not say a word. Then I asked him what these books were for; this was the start of a very interesting conversation. He explained that he is trying to make people read so I tell him that this is a wonderful thing as I point towards the books and ask him whether these books were for borrowing. He says that he is trying to sell them and starts talking about the biography of this man in the book, and then to my surprise it appears that he is the author of the book so my interest escalated, and I asked more about the book and about his source of inspiration. The author’s  name was Peter Varey;  deeper into the conversation, I learn  that he graduated from Chemistry at Cambridge University, he taught industrial processes and was the editor of “The Chemical Engineer” magazine. He published books and organized events for the Institution of Chemical Engineers. He now decided to become a freelance writer and write a book about a man he knew personally, a very inspirational character in the name of Peter Danckwerts. “Peter Danckwerts was a leading protagonist of a more scientific approach to chemical engineering during the 1950s. He became Executive Editor of the new journal of “Chemical Engineering Science”, and when he was appointed to a new chair at Imperial College, he became the first Professor of Chemical Engineering Science.”  (Danckwerts Memorial Lecture, Engineering science—or scientific engineering?, Chemical Engineering Science 57 (2002) 1075 – 1077, R. W. H. Sargent )


Figure 2: Peter Varey's Book about Danckwerts

Figure 2: Peter Varey’s Book about Danckwerts

I bought the book and promised to email him with my feedback about the book which I’m very excited to start reading! Overall, I had a very fruitful trip to Cambridge and I enjoyed every bit of it.

Figure 3: QUB ATBEST fellows

Figure 3: QUB ATBEST fellows

Figure 4: Attendees at the IChemE conference

Figure 4: Attendees at the IChemE conference


Figure 5: Dinner at Zizzi Restaurant

Figure 5: Dinner at Zizzi Restaurant

Figure 6: Farewell Picture

Figure 6: Farewell Picture

Rawan Hakawati, ATBEST Researcher, Queen’s University Belfast

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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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