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EffiSludge for LIFE – a demonstration project to reduce carbon emissions from the treatment of pulp and paper mill effluent

Dr Francesco Ometto completed his ATBEST fellowship in March 2016. We catch up with him to find out how his career has progressed. 

After two years at Scandinavian Biogas as Experienced Research working on ATBEST, the company offered me the possibility to stay and take the lead of a new EU project submitted by Professor Jörgen Ejlertsson back in 2014. I could not refuse such a great opportunity.

EffiSludge for LIFE is a project that aims to demonstrate the advantage for operating conventional Activated Sludge Plants (ASP) at low sludge age. This is contrary to the standard operation, where high sludge ages are preferred to maintain low growth rates and therefore minimising the cost of sludge disposal.

In the project, an existing ASP located at the Norske Skog Skogn mill  - the largest producer of newsprint in Norway will be modified to achived the flexible operation required by EffiSludge conditions. Treating approximately 20 000 m3 of wastewater per day, the ASP currently operates with a sludge retention time close to 18 days and external nitrogen and phosphorus is added into the system to secure biomass growth.

The EffiSludge concept

By lowering the sludge retention time below 10 days, higher sludge production occurs and lower aeration is required per unit of treated wastewater. Lower the aeration needed, lower the energy demand and the related carbon emissions. Furthermore, to justify the higher amount of sludge produced, this will be processed onsite for biogas production capable, in principle, to satisfy part of the heat and power required by the mill.

A new integrated AD plant

In the specific context of the Skogn site, the produced excess activated sludge will be co-digested with fish waste adding to the stream of rejected water post anaerobic digestion a high load of nitrogen and phosphorus. Recirculated in the WAS system, such loading of nutrients could be able to offset current external dosing of nitrogen and phosphorus. With a capacity of 25 million (12.5 in the first year) cubic meters of liquefied biogas (LBG), the plant is currently under construction and it is expected to enter in operation by the end of 2017.

Construction site at Skogn. Francesco Ometto (left - Scandinavian Biogas Fuels) with Pål Nygård, (Biokraft)

Construction site at Skogn. Francesco Ometto (left – Scandinavian Biogas Fuels) with Pål Nygård, (Biokraft)

Linked to the work on seaweed digestion completed as ATBEST fellow, I am also involved in a parallel project, receiving financial contribution from the Research Council of Norway. The COMPLETE project investigates the possibility to integrate the new AD facility at Skogn with algae production – both seaweed and microalgae – to enhance an energy efficient biogas production by recirculation of nutrients and complete utilisation of resources (COMPLETE).

Existing Norske Skog facilities with an artistic representation of the under construction anaerobic digestion plant including possible future cultivation of seaweed.

Existing Norske Skog facilities with an artistic representation of the under construction anaerobic digestion plant including possible future cultivation of seaweed.

25 years of LIFE

The project is entitled of 1.8 million Euro as financial contribution from the European programme LIFE celebrating this year its 25th anniversary. Launched in 1992 and investing so far more than 3.4 billion Euro, LIFE is the main instrument for the European Commission to support the development of the action plan for Climate Change Mitigation and Environment. Coordinated by Scandinavian Biogas Fuels AB, EffiSludge for LIFE (LIFE14 CCM/SE/000221) is implemented in cooperation with Biokraft AS (Associated Partner) and Norske Skog Skogn. Started in September 2015, the project will last until December 2019.

Project website:


Twitter: @EffiSludge

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UK- China workshop and Biogas developments in China

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Early Stage Researcher Jingxiao Liang from Queen’s University Belfast recently attended a workshop in her native China. Here, she shares her experiences from her visit….

A three-day UK-China workshop funded by The Newton Fund and the National Natural Science Foundation of China for early career researchers on advanced Technologies for energizing sustainable urban transport was held in Beijing, China, from 16th to 18th May 2016. The event was co-sponsored by Beijing Institute of Technology (BIT), a prestigious Chinese university and Queen’s University Belfast (QUB), one of the UK’s leading research-based universities. Professor Patrick Johnston, President and Vice Chancellor of QUB and Professor Hu Haiyan, President of BIT both give speech at the opening ceremony on 16th May, encouraging collaboration between QUB and BIT. The workshop served as a networking event between researchers in China and the UK, laying a solid foundation for further cooperation in sustainable energy(1).

Figure 1: Participants at the Uk-China Workshop

Figure 1: Participants at the Uk-China Workshop

Around 60 lecturers and scientists attended this workshop; 21 of them were from the UK, including representatives from Queen’s University Belfast, Newcastle University, Aston University, Cardiff University, University of Aberdeen and Ulster University. The remainder were researchers from China, including those from Beijing Institute of Technology, Harbin Institute of Technology, XI’an Jiaotong University, Shanghai Jiaotong University, Chongqing University, Qingdao Institute of Bioenergy and Biomass Technology, Chinese Academy of Sciences among others.

On the third day about writing high quality proposals, given by Professor David Rooney from QUB, who was also one of the organisers. Nearly one third of researchers who attended are working in the area of biomass and biofuels.

Figure 2: Prof. Rooney leads a discussion group

Figure 2: Prof. Rooney leads a discussion group


China is an agricultural country, with half of its population living in the rural area. As a country, they are facing serious environmental pollutions and more energy needs than ever before. Developing a biogas industry is a perfect way to help overcome these problems.


From statistics produced by the Chinese Ministry of Agriculture, in 2015 the annual biogas production was 2.258 billion m3, with potential biogas production over 200 billion m3. The electricity production from biogas was 467 GWh, enough to power 1.92 million households. Chinese biogas plants are divided into four types, dependent on production:

a) super large-scale, daily production over 5000 m3

b) large-scale, daily production between 500 to 5000 m

c) medium-scale, daily production between 150 to 500 m3  

d) small-scale, daily production below 150 m3.

There are 6713 super large-scale and large-scale biogas plants, 10087 medium-scale biogas plants and 86346 small-scale biogas plants, at the same time, more large-scale biogas plants are under construction.

It was a great pleasure for me to get an invitation from HongChuan Xin, who works for Qingdao Institute of Bioenergy and Bioprocess Technolog (QIBEET), part of the Chinese Academy of Science. He introduced me to a group of experts with many years’ experience in biogas separation and compression.

Figure 3: Presenting to QIBEET fellows

Figure 3: Presenting to QIBEET fellows

At the meeting, I presented the ATBEST project which the group was very interested in.  The QIBEET fellows also presented their research. Shengjun Luo, Senior Engineer, is working on methane hydrate – a mixture of methane and water under high pressure and low temperature. Three of his PhD students are working on using surfactants, graphene and carbon nanotubes as activators and they have achieved significant progress in this area which will hopefully lead to new technologies for methane compression into a more convenient for long-distance transport.

Figure 4: Getting a tour of the lab from Senior Engineer Luo

Figure 4: Getting a tour of the lab from Senior Engineer Luo

Engineer Gang Guo’s focus is on engineering design and modeling. He has made a lab-scale biogas upgrading unit (Figure 4); consisting of two parts, an absorber on the right and a scrubber on the left, it can deal with a throughput of 1 m3 biogas per day.

When they showed me their lab, I am surprised and impressed by their creativity – they designed most of their reactors themselves. Below are two of their reactors, Generation 2 and Generation 3 (Figure 5).

Figure 5: Reactors - Generation 2 and 3

Figure 5: Reactors – Generation 2 and 3

Compared to natural gas imported from aboard, the price of biogas as energy is not yet competitive in China. The Chinese government have provided financial support to sustainable energy for years; however, as opposed to the European situation, the government subsidies to biogas tend to favour large-scale plants instead of small ones. This was not always the case; for instance, back in 2005, farmers obtained ¥2000 from local government when they built a household biogas plant, with total expense of ¥3000. This policy was abandoned as it turned out these household biogas plants did not last long, as they were typically poorly maintained and have low efficiency.

By 2015, the National Development and Reform Commission had issued a biogas transformation and upgrading program, investing ¥2 billion to support 386 large-scale biogas projects (collectively have a daily biogas production of over 500 m3), and 28 super large-scale biogas projects (with a total daily biogas production of over 10 000 m3).

Table 1: List of 28 super large-scale biogas projects supported by government in 2015



Project title

Total investment


Government investment


Daily biogas production

(10,000 m3 )

1 Hebei Gucheng biomass 8,922 3,934 1.500
2 Hebei Sanhe biomass 14,100 4,500 1.800
3 Neimenggu Balinyouqite biomass 18,656 5,000 3.000
4 Neimenggu Wengniuteqi biomass 18,151 5,000 3.000
5 Neimenggu Wuyuan biogas 10,125 4,050 2.700
6 Liaoning Liaoning biogas 17,233 5,000 1.800
7 Jilin Huadian biogas 8,930 5,000 2.190
8 Jilin Jilin biogas 20,814 5,000 5.500
9 Heilongjiang Baoquanling agricultural  organic waste utilization 15,277 4,000 1.600
10 Jiangsu Dafeng biogas 4,834 3,000 1.200
11 Anhui Maanshan biogas 7,505 3,000 1.200
12 Shandong Leling biogas 9,800 5,000 2.470
13 Shandong Penglai biogas upgrading and compression 9,506 5,000 4.200
14 Shandong Yinan biogas 6,127 3,000 2.000
15 Henan Biogas utilization 10,047 3,630 1.620
16 Hubei Biogas and organic fertilizer 10,375 3,750 1.500
17 Hubei Zhongkai biogas 11,763 4,500 1.730
18 Hunan Xiangcun biogas 10,868 4,950 1.980
19 Hunan Yueyang biogas 6,845 2,700 1.020
20 Guangxi Muti-feedstock biogas 12,677 5,000 2.000
21 Hainan Chengmaishenzhou biogas 5,386 3,375 1.350
22 Sichuan Rongxianwangjia biogas 7,202 3,000 1.200
23 Sichuan Organic waste utillization and biogas 10,416 5,000 2.880
24 Guizhou Maotai biogas 19,259 5,000 3.230
25 Yunnan Erhailiuyu biogas 10,513 5,000 3.000
26 Gansu Gaotai biogas 12,045 5,000 2.000
27 Ningxia Zhongwei 6,250 2,500 0.954
28 Xinjiang Hutubi biogas 10,850 4,970 2.300
Total: 314,501 118,859 60.920

 Luo’s group have been involved with the design of Leling biogas plant (Number 12 in Table 1). Their role was with the biogas upgrading unit, by designing it and building it themselves, they were able to save approximately 40% on the cost they were quoted by a Swedish manufacturer. Chinese engineers have good skills in the production of infrastructure such as digestion tanks and scrubber columns; but for process items such as compressors and CHP engines they cannot yet match the efficiency of those produced by more developed nations.

Hopefully the links made by events such as this and through the ATBEST project, the Chinese biogas industry will make strategic links with Western supplier; allowing biogas to make a significant contribution to China’s energy provision in the future.



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And then there were 13 :(


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As she reaches the end of her ATBEST fellowship, Experienced Researcher Dr Laura Gil Carrera from Gas Networks Ireland reflects on her experiences in the project.

On the 3rd of November 2015 another ATBEST meeting took place in Belfast, however this one was different for me;  it was my last meeting with a great group of people. I still remember the first meeting two years ago when I walked into the Council Chamber in Queen’s University, which was filled with incredibly qualified individuals from around the world. We all presented our projects in that room crowded with strangers at that time, who became friends along the way. From the very first day with ATBEST, everyone welcomed all fellows as members of the team and genuinely expected us to make a contribution.


6Over the last couple of years, I’ve been working on the project “Developing strategies to facilitate the integration of biogas into the existing gas network”, in another words investigating the optimal model for rolling out a biomethane industry in Ireland. I hit the ground running, working on data collection and evaluation of concepts and literature to get a good flavour of the biogas industry in Ireland.


My first year was full of work on Irish ground, from literature reviews, several workshops all over the country, meetings with biogas producers, potential producers, academics, and politicians in order to develop strategies that suit the Irish context for biogas utilization. Besides biogas, I got an insight into the natural gas business and energy markets, which is essential for the future integration of renewable gas into the energy system in Ireland.

3This year has been extremely challenging and exciting, trying to manage numerous field trips and getting my analysis and models done before November. During 2015 I had the opportunity to attend a few conferences across Europe and present my work at them. Feedbacks and engagement at Green Gas Research Outlook, REGATEC and Progress in Biomethane Mobility were very fruitful to achieve successful outcomes in my project.  I also got the great chance to collaborate with Scandinavian Biogas, QUB and UDE through secondments which broad my knowledge and gave me hands on experience, not only within my field of expertise but also in the anaerobic digestion itself, operation of biogas plants, upgrading plants,  biomethane logistics and application of biogas in liquid fuel production.

4Both the tight collaboration with our ATBEST partners and the intensive exchange with peer experts from all over Europe helped me to get a better technical and economic understanding and develop models incorporating novel innovative technologies and novel biogas substrates to grow the Irish biogas industry.


5But the work—meaningful as it was—was only a small part of what made my experience so special. Everyone I met, from ATBEST fellows, project coordinators, GNI colleagues, associate partners… helped me grow both as an employee and a person.


I just want to thank all ATBEST community!!Thanks for two great years!! See you soon! :)


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And so we are just over half way there…….

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With the ATBEST project having reached the halfway point, Project Coordinator Sam McCloskey reflects on the progress and achievements to date.

What lessons have we learned along the ATBEST project journey and what have we got to look forward to in the future?  We have just received our formal feedback from the EU Project Officer from the project mid-term review that took place in Essen, Germany in May 2015. It is a credit to the Project Manager, all of the ATBEST fellows and their Academic Supervisors that feedback was very positive. Let’s summarise the achievements to date:

  • 12 Early Stage Researchers in post – all studying for PhD’s
  • 2 Late Stage Researchers – linked to industry
  • 8 Project Partners in 4 countries (5 Academic Institutions and 3 Industry partners)
    • UK, Ireland, Sweden, Germany
    • Three regional meetings – Belfast, Cork and Essen
    • Two summer schools – Germany & Sweden
    • Dissemination activity including articles, events and posters
    • A wide range of training activity for the Fellows
    • Secondments to industry are now taking place



The research projects follow the life cycle of biogas production from optimum feedstock in to the biogas plant through to maximising the output and use of the biogas product. Projects cover a range of solutions to issues that industry is facing including the use of novel probes, biogas for transportation, the gas grid, fuel cells, storage and logistics. So far there have been a number of notable scientific highlights including:

  • That the addition of hydrogen from surplus renewable energy production increases the methane yield from mono-digestion of grass silage (Markus Voelklein & Professor Jerry Murphy)
  • Methane production from co-digestion of grass silage and cattle slurry compares well against mono-digestion with grass silage (Himanshu & Padraig O’Kiely)
  • Miniature probes can provide online monitoring solutions to AD plant and the ability to detect unstable conditions within AD plant early (Professor Dr Michael Bongards. Dr Christian Wolff & Rob Eccleston)



Going back to basics then, the original aim of the project was “to develop new and innovative technologies for the biogas sector, to enable Europe to implement its Energy 2020 strategy and to address the challenges of increasing energy demand and energy generation costs.” Is the project on target to achieving those aims?


Well, the ATBEST research puts biogas production right at the heart of the three pillars of sustainability, where traditionally it has been viewed as environmentally acceptable there has been scepticism over the social and commercial viability of large scale biogas production. The project aligns with EU biogas policy in the SET Plan and the EU 20/20/20 vision and specifically focuses on maximising the sustainability of biogas production, enhancing the commercial value of the biogas product and at the same time, providing jobs and opportunities not just for the 14 researchers but for the industry as a whole.


However, there is plenty of work still to be done in the remaining 18 months of the ATBEST project and the team has much to look forward to. Plans are being put in place to road map knowledge / technology transfer from each of the research projects with potential pathways to product and service commercialisation now being developed. There is also the November 2015 meeting in Belfast followed up by the summer school in Northern Ireland and our final conference in Linkӧping in Autumn 2016.

Well done to all the ATBEST team for the successes so far and keep up the great work!

Sam McCloskey (ATBEST Coordinator)

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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]

2. [Online]

3. [Online]

4. [Online]

5. [Online]

6. [Online],465992,en.pdf.

7. S. Murray, E. Groom, C. Wolf,. WRAP – feasibility reports. [Online] October 2012.

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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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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.

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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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