Saturday, September 13, 2014

Biogas Market Grows Despite Uncertain Policies & Biomethane Inconsistencies

biogas



Paul Thompson, who is head of policy at the Renewable Energy Association (REA) has it right in his Guardian article "Renewables battle on in face of uncertain policies". Certainly, here in the UK the anaerobic digestion sector is seeing advances in AD plant scale and the sophistication of the technologies being installed which could only have been dreamed of even less than 5 years ago. Our readers might like to look at the Guardian article below:

Renewables battle on in face of uncertain policies - The Guardian

"Renewables battle on in face of uncertain policies: The Guardian
Paul Thompson, head of policy at the Renewable Energy Association (REA), says there's been increased entrepreneurial activity in biomass, bio-methane, anaerobic digestion and ground-source heat pumps. Solar PV is also starting to imagine life without ...and more »"
To find out more about "Renewables battle on in face of uncertain policies - The Guardian" and anaerobic digestion, click here

Nobody is suggesting that it isn't still a tough climate for renewable energy companies, suppliers, and green entrepreneurs. All credit to those companies who have sustained their businesses through tough times since the 2007/8 finacial crisis and can now begin to grow from a sound financial and product base. The race will now be on to achieve compliance with the targets for increased use of renewable energy sources, which in many cases were in place before the current UK political regime, have weathered it intact, and must continue to be complied with, if the UK as a nation expects others to follow with them, and after them.

One area in the Anaerobic Digestion Sector, which was largely a theoretical one but which is now growing rapidly in importance, is biogas upgarding and what is referred to as the production of biomethane, or natural gas/ CNG replacement. Success brings it own problems, and in this area a lack of consistent requirements from the operators of the existing national and regional gas networks is making life more difficult than it need be for the producers of biomethane.

That why it is cheering to read that International Energy Association members may get some help from an initiative by the IEA to improve consistency in the gas quality requirements and associated technical issues between the many gas distribution grid companies. The following article explains what is being done:

The biomethane market needs clear frame conditions for further growth - Phys.Org
"The biomethane market needs clear frame conditions for further growth. Biomethane as a substitute for the fossil energy carrier natural gas offers a variety of options and applications for a sustainable energy supply. Nevertheless, a consequent market penetration is still pending because of a lack of standardized and ..." http://news.google.com/news/url?sa=t&fd=R&ct2=us&usg=AFQjCNF36nsas7i8RJUkfw_HG5N5aak8gQ&clid=c3a7d30bb8a4878e06b80cf16b898331&ei=_U4UVOiBAo2YjAbq44H4DA&url=http://phys.org/news/2014-09-biomethane-conditions-growth.html
Finally, to end of this News Round-Up with a ground-breaking first commitment of its kind, we would like to highlight that still on the subject of biomethane, a major UK retail store has decided to back anaerobic digestion by buying the energy for its own use:

M&S buys into Biomethane Certification Scheme - Energy and Environmental Management (EAEM) Magazine

"Energy and Environmental Management (EAEM) Magazine: M&S buys into Biomethane Certification Scheme
The leading UK supermarket, Marks and Spencer plc, has bought the majority of Biomethane Certificates (BMCs) from the green gas produced at Future Biogas's new gas-to-grid anaerobic digestion plant near Doncaster. M&S is the first FTSE100 buyer to join ... M&S strikes landmark deal to buy biomethane certificates. Biomethane certification scheme wins M&S's backing as buyer - EPR Retail News (press release) news articles »" http://news.google.com/news/url?sa=t&fd=R&ct2=us&usg=AFQjCNFxLR-oe4WNpDIvmklCbBXgfz8vUg&clid=c3a7d30bb8a4878e06b80cf16b898331&cid=52778605188089&ei=_U4UVOiBAo2YjAbq44H4DA&url=http://www.eaem.co.uk/news/ms-buys-biomethane-certification-scheme
This should help yet more potential biogas producers, who are still on the fence about investing in new biogas plants, to feel confident in the technology and the future market demand, which in our opinion will certainly continue to grow and should accelerate.

Wednesday, September 03, 2014

Biogas Success - Weltec Biopower To Build Piddlehinton AD Plant 1.1 MW Extension

Weltec Biopower Awarded 1.1 MW Piddlehinton AD Plant Extension

A highly successful food waste biogas plant constructed by Weltec, after only 2 years of operation is being enlarged. Now there's confidence in the profitability of food waste anaerobic digestion!


Watch our video on this project below:


Press release · Communiqué de presse
Vechta, September 2014: 

WELTEC BIOPOWER receives order for 1.1 MW extension in Dorset

Award-Winning AD plant processes food waste to energy

Weltec Biopower (UK) Ltd, based in Stoneleigh, Warwickshire, UK, has been granted the contract to extend the award-winning food waste AD plant in Piddlehinton, Dorset. Plant owners Eco Sustainable Solutions Ltd. are expanding with a further 1.1MW of food waste processing capacity. 

This follows on from the Weltec successes of commissioning two other plants in early 2014. The original Eco-Dorset AD plant was also built by WELTEC and was commissioned in 2012. After two years of successful operation, the operator assigned WELTEC with the extension. The extension will be completed in autumn 2014. 

Another advantage core to WELTEC’s continued success in industrial and agricultural applications is the fact that the digesters and digestate storage tanks are constructed from stainless steel. Aside from being a higher-quality product, stainless steel and is corrosion resistant to aggressive materials and offers a faster construction time, which is increasingly important to all developers with the FIT framework deadlines providing tight construction windows. 

Watch our new video "How Does a Biogas plant Work?"

After two years of successful operation, the operator assigned WELTEC with the extension. The extension will be completed in autumn 2014. The plant is fed by local authority food waste as well as out of date food products which prior to digestion are unpackaged, sorted and pasteurised at the site. After the extension, approximately 37,000 tpa of food waste will generate an electrical output of 1.6 MW. 

The Eco-Dorset approach is an excellent example of the benefits of intelligent design, co-location and mutually beneficial plant integration. Electricity generated at the plant as well as excess gas is fed to an adjacent feed mill. When the mill is not operational, the power is fed to the National Grid. The digestate produced by the plant, which is in the process of gaining PAS 110 accreditation, is collected and used by local farmers. 

Despite the latent renewable energy in commercial and municipal food wastes, much of this valuable resource still ends in landfill. Much of it also ends in composting plants, which use expensive imported energy to bio-stabilise (or destroy) what, when processed in an AD plant, is a valuable renewable energy resource. After all, one tonne of organic waste processed through AD, could be converted into 600 to 800 kilowatt hours of energy – enough to power five refrigerators for one year. 

The equivalent tonne of waste through a composting plant costs energy and money to process. As energy demand and prices continue to rise and security of supply concerns worsen, more and more Local Authorities and private waste companies look set to follow the excellent example set by Eco Sustainable Solutions Ltd. and capitalize on the excellent income generation opportunity that organic waste presents. 

WELTEC understands the needs of every client and tailors the most cost-effective solution on a case by case basis. The partnership between the plant manufacturer and his clients also maintain after the design and build phase. “We recognize that an AD plant is a 20 year partnership and that maintaining solid trust-based working relationships with our clients is crucial to our ongoing success. 

Therefore we now have a dedicated UK-based Service and Maintenance capability with locally available spare parts”, sales manager Kevin Monson explains. 

Company Portrait

In 2014, WELTEC BIOPOWER GmbH, one of the first manufacturers of biogas plants, celebrated its 13th anniversary. WELTEC is one of the pioneers of the constructors of biogas plants. 

Based on the modern approach of experienced engineers, the company from Vechta, Germany with its expertise of a staff of close to 80, offers complete biogas plants from one source and has developed to a leading constructor of biogas plants in the world. As the hydrogen sulphide and ammonia compounds contained in biogas corrode unprotected parts, WELTEC builds the digesters from stainless steel. This ensures a long useful life of the plant. The manufacturing depth also guarantees a consistently high standard regardless of the location and ensures an export rate that is far above the industry average. 

The plants have a modular structure. WELTEC uses only proven system components and develops most of the technologies along the entire value chain internally: digester technology, mixing technology, control technology, hygienisation systems, and digestate processing solutions originate from the company. 

By means of its comprehensive services, WELTEC ensures the technical and economic stability of the biogas plants. The CHP service guarantees stable output, the biological supervision ensures continuous monitoring of the relevant parameters, and systematic repowering makes sure that the biogas plant is always up to date. 

Operators can choose from a range of service packages of different types and scope. One of the main strengths of WELTEC is the ability to deliver individual and flexible solutions worldwide – from compact plants to large computer-controlled plants in the megawatt range, waste recycling plants, and biogas parks with gas processing technology.

Ann Börries
Marketing
Phone: +49 4441/99978-220
Email: presse@weltec-biopower.de

Wednesday, August 20, 2014

New Low Temperature Anaerobic Digestion Process Reviewed

Low Temperature Anaerobic Digestion - It Is The Third Type Of AD Process

I was fascinated to discover a new company with their own stand at the AD & Biogas Association (AD&B) Exhibition and Conference 2014, which was held last month at the NEC Birmingham, UK.  Irish start-up NVP Energy was the company in question, and they were there to present their unique and innovative low temperature (< 20°C) Anaerobic Digestion (Lt-AD) wastewater (WW) treatment technology for medium to low-strength industrial effluents.

And, yes! You did just read “less than 20 degrees Centigrade”, so this is a truly ambient temperature process for use in our cold northern European climate, without costly insulation and heating.

If you have ever wondered why so many digesters in the UK (of the mesophilic and thermophilic variety), are a lot less productive than those un-insulated anaerobic digestion systems which seem miraculously easy to build and operate in hot climates, it has got an awful lot to do with our climate being too cold, and the cost of heating!

Watch our sponsored Lt_AD explainer video below:

To view this video on the YouTube website click here and watch Low Temperature Anaerobic Digestion being explained.

So, how remarkable to find that a spin-off technology, which has taken 15 years microbiological lab and pilot scale testing at the National University of Ireland, Galway is now available for the Food and Drink Wastewater (WW) industry. Not only that there are many other industries which produce similar strength organic effluents such as the Municipal WW industry, Pharmaceutical WW industry, and also the ethanol processing WW industry. All thse can now use this technology to treat their wastewater without the ever-rising energy (running) cost implications of current aerobic treatment systems.

But like all other anaerobic digestion based processes LT-AD has, is also by nature biogas plant, with all the implications that brings for net energy export (and revenue), reduced carbon footprint for the business, and even possible sales for other products such as CO2, and the digestate as a fertilizer.

Traditionally all these industries have used aerobic wastewater treatment systems, the Lt-AD technology does what those high energy consuming systems can do. It recovers the carbon in the WW and transforms it into biogas. This means that the NVP Energy technology is a carbon neutral and energy-positive process technology which is great news for the global environment. But, lets not forget the benefits to the bottom line for those that use this new process, due to those reduced carbon costs.

If you have been watching how quickly anaerobic digestion taking hold as the preferred treatment method of choice, then just take a look at this blog! Due to its clean green energy credentials Ad is leaping ahead, and this development should be very attractive to the Food and Drinks industry, plus all other businesses producing similar effluents.
AD-Lt Anaerobic digestion 3D Image cut-away
The NVP Energy process has been given the name of “Lt-AD technology” and they say that it can work equally well as a retrofit, an add-on technology to their client’s business expansion, or of course can be the core technology treatment method in any new build scenario.
For example, if you operate a Food & Drink plant that has an existing inefficient WWT system such as the activated sludge system, then their technology can:
  • Lessen the load on your current system and greatly reduce your aeration costs due to our high COD attenuation
  • Significantly decrease your sludge volumes. Their technology has been shown to generate up to 90% less organic sludge compared to the activated sludge process.
On the other hand, if you operate a Food & Drink plant that elutes all your wastewater to sewer, then their technology can:
  • Significantly reduce your trade effluent charge/Mogden charge due to high COD & TSS attenuation.
In both cases the biogas that is produced is 100% available for reuse on-site to generate heat and/or electricity thus reducing your operational expenditure.

They say that the system will prove very attractive to Food and Drink plant operators that currently discharge their wastewater to sewer.

Where this is the current disposal method they say that NVP Energy can significantly reduce the high trade effluent discharge costs that arise from the application of the “Mogden” calculation of TE Charges due to high chemical oxygen demand (COD), and total suspended solids (TSS) attenuation.

I spoke to Tracey Giles (NVP Energy, Design Engineer) who said:
“For Food and Drink plants that treat their wastewater on-site, NVP Energy can considerably decrease their aeration costs. In addition, the Lt-AD technology can reduce organic sludge volumes by up to 90% when compared to the activated sludge process thus reducing sludge management costs.”
“The technology is very attractive as it has produced effluent which adheres to effluent release standards, e.g. Urban Wastewater Directive (UWWD) COD release standards, without the need for post-treatment. This is proven from our pilot-scale trials using our “low-strength” effluent definition, a type of effluent that will apply to very many businesses”.
Benefits of Lt-AD are listed as:
  • Significantly reduced OPEX due to lower discharge costs through a high COD attenuation (approx. 80% removal rate) capability & reduced TSS levels (approx. 50% removal rate).
  • Reduced OPEX - up to 90% less organic sludge produced than the activated sludge system.
  • Energy Efficient - No WW heating or aeration required. Lt-AD has been successfully trialled at temperatures as low as 40C.
  • Potential savings from biogas production – can be used to generate electricity/heat that will offset use of fossil fuels.
  • Potential revenue from biogas production - FITs/RHIs for on-site energy generation.
  • Modular design allows flexibility in WW volumes.
  • Integrates easily with existing WW treatment systems.
  • Aids in adhering to specific discharge requirements e.g. UWWD.

Summary:

NVP Energy offers an exciting new and innovative energy-positive, low temperature anaerobic digestion technology (Lt-AD).

Lt-AD treats wastewater (WW) highly efficiently at temperatures as low as 4°C and will significantly reduce low strength wastewater treatment costs and TE charges. The Lt-AD technology target markets are Food and Drink WW including Dairy, Brewing, Malting and Distilling applications; Municipal WW; certain pharmaceutical WW streams and also ethanol processing WW.

For more information visit: www.nvpenergy.comhttp://nvpenergy.com

Tuesday, August 12, 2014

New Schmack Carbotech Plant to Shows Extent of Swedish Commitment to Biogas Biomethane Production

Shmack carbotech biomethane upgrading
Schmack Carbotech Stockholm: Sweden’s role in bioenergy production, with the world's largest installed biogas upgrading capacity, is set to continue its march toward transport fuel sustainability ahead of other nations. That is because of the announcement that Schmack Carbotech has been signed-up as the "design and build" contractor for a 2000 Nm³/h biogas upgrading facility to be located to the south of Stockholm.

Schmack Carbotech’s innovative Pressure Swing Adsorption (PSA) technology will be used in a development that will enable the Swedish capital to produce pure methane (biomethane) from more biogas.


(Image: (c) Schmack)
This will increase the proportion of biomethane, it uses by 50%, this will provide biofuel for vehicles currently powered by natural gas. The biomethane produced will be a high purity gas leaving the PSA with a methane content in excess of 97 per cent.
Fed by organic waste and waste kitchen oil, the plant, which will be located in Sofielund, Huddinge, will produce biogas with an annual energy yield of nearly 100 million kilowatt hours. That's enough to supply approximately 5,000 vehicles clocking up an average of 20,000 km per year each.

Part of the Viessmann Group, Schmack Carbotech’s Pressure Swing Adsorption (PSA) principle distinguishes itself from other technologies because of its favourably low energy consumption, efficient heat extraction and high methane yield, as the company’s International Business Manager, Eberhart Wusterhaus Gomez, explains:
“This important new contract at Sofielund brings to bear all of our 30 years ‘experience in the development, engineering and manufacturing of turnkey gas upgrading plants.
“As industry pioneers and market leaders in biogas purification, we are very keen to play our part in delivering a low energy yet high yield technology that in this case fits in perfectly with the Swedish government’s long-term thinking to become the world’s first completely oil-free economy”.

(Image: (c) Schmack)

Setting new standards in biogas process efficiency, Sofielund’s new plant will be equipped with a CNG station, a Carbotech PSA plant, off-gas cleaning with high heat recovery system combined with a peak load boiler. The plant will also benefit from ultra-reliable VOC and Hydrogen Sulphide removal, making Carbotech’s robust technology ideal to produce biomethane from biogas, sewage gas and landfill gas for grid injection and CNG vehicle fuel. As well as being a specialist in upgrading biogas plants, the Viessmann Group is actively implementing complete biogas turnkey solutions worldwide. Installations include award-winning biological technology, with special wet fermentation (from Schmack Biogas) and batch-dry fermentation (BioFerm technology) now integral parts of more than 300 cogeneration and biomethane plants in Europe and beyond. This includes Europe’s largest dry-AD-from-organic-waste plant, located in Fife, Scotland.

The Viessmann Group is a leading international manufacturer of heating technology systems. Founded in 1917, the family company has around 11,400 employees with group sales of EUR 2.1 billion. Viessmann has an international presence with 27 production companies in 11 countries, sales companies and representations in 74 countries and 120 sales branches worldwide. 55 percent of sales are generated abroad.

For more information, contact:

Press:
Petra Krayl
Schmack Biogas GmbH
+49 (0) 9431 / 751-285
petra.krayl@schmack-biogas,com
www.schmack-biogas.com






Thursday, July 31, 2014

Lukeneder Offers Biogas Plant Owners a Leap in Biogas Plant Efficiency

http://images.anaerobic-digestion.com/meme/view/Biogas Plant Efficiency - Iron Additive Dosing/53d2d3b33111e
The German Company Lukeneder recently added the UK to its list of countries to which it supplies its Deuto-Clear® Sulfo product, which is an iron dosing chemical. That means that UK anaerobic digestion plant operators can now obtain substantially improved biogas plant efficiencies, and raise the profitability of their operations.


View this description of the Lukender Deuto-Clear® Sulfo product on YouTube.

The Anaerobic Digestion industry tends to spend too much time scrubbing the biogas, when in most cases, a far better solution to producing a better quality of gas, is to tackle such problems at the source of the problem, in the digester. By doing it in the digester there are so many spin-off benefits. In fact, operators will end-up wondering why they did not previously pursue all avenues for optimizing the conditions in the reactor to suit the micro-organisms they want to encourage!

A policy of H2S prevention, combined with the avoidance of high (and biochemical process inhibiting) ammonia concentrations, not only helps to stabilise the whole methane production process, it also allows the healthier biomass that results, to produce more gas of a better quality. This is achieved, it is understood, without changing the feedstock, and occurs over a period of time.

This looks like a true “win-win” situation, and involves the daily dosing of the digester with Deuto-Clear® Sulfo. This according to the Lukeneder brochure:

"Is a ready-to-use solution containing metal salts and trace elements. It improves the performance of your biogas plant and its dosage will be adjusted to fit the specific needs of your digester. Deuto-Clear® Sulfo features a broad range of trace elements, minerals, and other agents to improve the quality and stability of the digestate."
Deuto-Clear® Sulfo removes hydrogen sulphide and ammonia gas from the digestate and adds nutritious trace elements and minerals to produce a healthy and productive environment for anaerobic bacteria.
 Added benefits are listed as:
  • HOMOGENISATION OF FERMENTATION SUBSTRATE
  • QUALITY OF FERTILISER IS IMPROVED AS AMMONIUM
  • AND SULPHUR COMPOUNDS ARE BOUND IN THE DIGESTATE 
  • NO LONGER NECESSARY TO DOSE WITH OXYGEN TO CONVERT H2S.
To read our more comprehensive article go here.

For more information and copies of supporting documents contact:

Dr. Naoimh McMahon, UK Representative, Lukeneder GmbH (BIOGAS & ANIMAL HYGIENE)
Telephone: +44 (0)20.3151 0883
Email: naoimh.mcmahon@lukeneder.com
Internet: www.lukeneder.com

Wednesday, June 25, 2014

How to Stop Water Pollution Risks Leading to Water Pollution from Biogas and AD Plants

There was recently quite a serious UK biogas plant digestate tank failure at Harper Adams University recently, which we reported on in our last post. It led to digestate leaking into a bunded area, and because the right precautions were taken with containment it is extremely unlikely that any pollution has occurred.

That led us to consider what we could do to help those people who are responsible for Anaerobic Digestion and Biogas Plants to understand the UK legislation on watercourse and groundwater protection. Most will be familiar through their site environmental permitting arrangements anyway, but we wanted to provide an accessible reminder which would also be useful to our international readers who might view it as "good practice" worth following, even if similar local regulations don't exist in their location. 

The idea is that a better appreciation of the law in this area, and what causes most spillages, should help to ensure that those responsible, such as site managers and plant operators carry out what is in fact their legal duty (in the UK). That is to assess risks from their liquid storage installations and reduce all such risks to their minimum which in most cases (as for anaerobic digestion plants) means providing suitable containment.

The answer, we thought, would be to post the infographic image below which is based upon a UK Chartered Institute of Waste Management Fact File.

It provides the legal basis for compliance by owners/ operators of factories and other potentially polluting sites, making sure that they appreciate their legal liability should they fail to follow the guidance. It then provides a summary of some of the most important requirements for minimizing pollution risks from processes like anaerobic digestion, where collapse or even a leak can harm large areas of groundwater, or long stretches of rivers if either the mixed-liquor, stored digestate, or even uncontrolled maturation slab run-off reaches rivers, or soaks into the ground and into the groundwater.


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Here, in text, is what the CIWM said in their Fact File:

OPERATORS OF permitted facilities are responsible for complying with their environmental permit and for preventing pollution of air, land and water.
Waste management facilities have the potential to cause significant environmental harm, which could threaten water supplies, public health and wild life in the event of an environmental incident such as fire, explosion or spillage.
 A facility found guilty of causing a pollution incident could face a fine of up to £50000 in the Magistrates Court.
In order to prevent environmental harm you should be aware of the following:
The source of the contaminant;
  • the most common pollutant in the UK is oil 

The pathway 
  • this could be the site's surface water drainage system or via off-site surface drainage, direct run-off, foul drainage system or into the atmosphere 
The receptor
  • ie a river, groundwater, the local population. 

Potential causes of environmental incidents include: 
  • delivery and use of materials 
  • plant or equipment failure 
  • containment failure 
  • fire, explosion or failure to contain fire fighting water 
  • wrong connections of sewers and pipes 
  • discharge of partially-treated or raw effluent 
  • vandalism 
  • flooding of part or all of your site.

Those operating waste facilities need to assess the risk from every one of the above listed potential causes, in the context of the source (degree and nature of pollution), the pathway to the permeable ground, river, ditch or stream which would become the receptor

Tuesday, June 03, 2014

Harper Adams University Biogas Plant Explosion and Tank Collapse Produces 2nd Spill of Liquor at Troubled Green Power Facility

This is appalling news for a flagship UK anaerobic digestion plant which was put into operation less than 3 years ago.

As far as can be seen from the Shropshire Star report referenced below, there was an explosion which resulted in a digester tank collapse and that led to the contents of the tank being dropped on the ground around the plant.

We found the short video below on YouTube, briefly showing the clean-up:



There was also previous spill at the plant last year, which appears to be beset by incident.

Thankfully, we don't think there were any injuries on either occasion, but there may have been some of the biomass which entered the local stream, again the articles referenced below don't make it clear just what the effect was on the local water quality. Hopefully, there would not be expected have been any significant loss of liquor outside of the bunded area for the plant, and therefore the clean up will adequately remove any future risk of pollution.

Sludge power plant collapses in Shropshire

"A power plant using farm waste today exploded at Harper Adams University, spilling tonnes of slurry. A 200-metre exclusion zone was today put in place by police, who described it as a “chemical incident”."
The incident took place on Friday 30 May, and the following further article was published on Monday 2 June:
"The clean-up operation continues at the HarperAdams University anaerobic digestion plant today. The access road to the AD plant was fully cleared over the weekend. Digestate is now [contained]."


 Credits: Clean-up continues at Harper Adams anaerobic digestion plant - shropshirelive.com (press release)

Today the Farmers Guardian had more to say, as the staff worked on at the clean-up:
THE clean-up at the Harper Adams anaerobic digestion (AD) plant is continuing today (Tuesday) with specialist contractors working on limiting the impact of the spill on the environment. The university has been working to tackle the major leak since one ...

 Credits: Harper AD plant clean-up continues after major spill - Farmers Guardian


It is clear that this was a major explosion to cause this degree of damage, and from the articles the presence of watercourse pollution remains unknown while tests are carried out:
"A Harper Adams spokesman said: “The Environment Agency is also conducting routine monitoring of the local watercourse. No release of pollution has been detected so far.”
I would predict that the University will carry out an inquiry into the cause, and that in due course there will be lessons to be learnt for the UK biogas industry.
We have previously highlighted the potential for explosions at Anaerobic Digestion Plants on this blog, and the need for this to be heeded at all times both during design and construction, and throughout biogas plant operation.
In this case there does not appear to have been anyone present at the automatically operated plant when the incident took place simply because there is no mention of a staff, or maintenance contractor's presence at the time, so we would guess that an equipment failure of some sort is the most likely cause.
In the video below we see the biogas plant in happier times, on opening day in 2012: