Anaerobic Digestion Sector Forming a Clearer Picture

An association in Canada has issued anaerobic digestion guidelines for the growing sector, while a technology provider sees pasteurization as critical.

May 8, 2020

Anaerobic Digestion 

 

Anaerobic digestion (AD) as a conversion technology for discarded organic materials appears poised to retain its momentum in attracting investments in the waste and recycling sector.

The Ottawa-based Canadian Biogas Association (CBA) is tracking AD and other biogas installations north of the border and also has produced an AD Guideline document for existing and prospective AD plant operators.

Atlanta-based HRS Heat Exchangers, meanwhile, foresees downstream pasteurization technology as a likely pocket of AD-related investment in the agribusiness sector in particular.

A north star

An industry snapshot compiled by the CBA shows Canadian companies and municipalities have invested significantly in biogas projects during the 21st century.

The organization has identified more than 60 operating food and agricultural residue AD facilities and 86 wastewater treatment plants that produce biogas. Canada also has some 53 sites converting landfill gas into a usable energy product.

“AD has seen incremental growth in Canada over the last 10 years in the agricultural and commercial space,” says Jennifer Green of the CBA. She says municipalities have been using AD in the wastewater space “for decades,” and this sector is “now contemplating opportunities for co-digestion.”

An announcement about a new AD investment in Canada was made in February, when Montreal-based Xebec Adsorption Inc. and Bähler Biogas Inc., also based in Canada, completed an agreement to develop a facility to process “various organic wastes for the production of renewable natural gas (RNG) and biofertilizer.”

The Québec, Canada, facility will be designed to process more than 45,000 metric tons of organic waste per year through an AD process. The two-stage process will produce biogas that can be upgraded into renewable natural gas (RNG), according to the two companies.

Equipped to produce

An anaerobic digestion (AD) solids pasteurization system installed by Allen’s Farm in the United Kingdom consisted of equipment now being marketed in North America by Atlanta-based HRS Heat Exchangers, which is part of the U.K.-based HRS Group.

The HRS twin-tank pasteurization system chosen by Allen’s Farm is a batch-fed system, which can be an alternative to HRS Heat Exchanger’s more commonly supplied three-tank system. The twin-tank system “is an ideal choice for farms with space constraints,” says HRS, because “it uses the same technology as the popular HRS three-tank technique, but its compact design reduces the physical footprint of the system by a third.”

The system at Allen’s Farm includes a primary storage tank placed before the pasteurizer and a post-treatment storage tank designed to feed the treated digestate into a lagoon for longer term storage. When first commissioned in December 2015, the unit treated around 1,060 cubic feet of digestate sludge each day, but it was designed to cope with twice that amount if the plant increases in size.

Capture 1 
 Provided by HRS Group

 

The digestate fed into the HRS pasteurizer contains up to 14 percent dry solids. A double tube heat exchanger exposes the material to temperatures of up to 167 degrees Fahrenheit. According to HRS, this allows for variation in the sludge consistency and its incoming temperature, ensuring the digestate is always properly pasteurized.

In order to maintain a constant throughput, while one tank is being filled, the other is heated, pasteurized and emptied. The tanks can also be used individually, which can provide time to schedule routine maintenance.

“This twin-tank version of our widely used HRS three-tank pasteurizer brings all the benefits of the established system, but in a more compact package,” says Matt Hale, international sales manager for HRS.

We can see it being of particular interest to smaller plants, farmers and those who want to increase the efficiency of an existing plant where there is limited room to expand,” he adds.

Hale also touts the diversity of the twin-tank configuration.

“This particular twin-tank pasteurizer has been carefully designed to allow Allen’s Farm to expand its biogas operations, not only by providing sufficient treatment capacity for the enlarged facility in the future but also by allowing a wider range of waste feedstock to be used in order to fuel the planned expansion.”

Xebec says it will supply the “turnkey biogas upgrading equipment package” for the facility that will be able to produce more than 150,000 gigajoules of RNG and 7,500 metric tons of biofertilizer annually. The plant is expected to be commissioned in early 2021.

The $19.3 million plant will sell its RNG under a 20-year off-take agreement and the biofertilizer produced will be sold and distributed to farmers through an undisclosed biosolids management partner.

“Renewable natural gas presents a unique opportunity to both divert organic waste from landfills and produce a valuable source of clean energy to displace fossil natural gas and create a biofertilizer for farmers,” says Kurt Sorschak, president and CEO of Xebec. “Xebec plans to be a valuable technology provider and financial partner in the waste-to-RNG space in Canada,” he adds.

Beyond tracking AD and biogas installations, the CBA issued its AD Guideline document in 2019, available directly from the organization.

Welcomed guidance

“The AD Guideline was developed by industry for industry to assist stakeholders in the deployment of AD facilities in a manner that improves outcomes and ensures environmental sustainability,” says Green.

She says the CBA’s more than 130 members “span the entire value chain of the sector and are made up of: biogas/RNG owners and operators—comprised of farmers, municipalities and private sector [operators]; biogas/RNG technology developers and product suppliers; utilities; waste management companies, consultants and regional representatives.”

Input was received from all these sectors, says Green, to produce a document “intended to be used by developers, operators, government entities and any organization or individual with an interest in—or a role to play in—developing, siting or operating AD facilities.”

Green says the CBA had several objectives as it set out to produce the AD Guideline document, including to:

  • create a clear outline of best practices for biogas projects;
  • assist developers and stakeholders with the regulatory process and remove barriers to support growth in the green economy;
  • inform proponents to minimize or prevent the exposure of any person, property, plant or animal life to adverse effects associated with the operation of food and organic waste AD facilities, and
  • provide a reference document for the design, approval and operation of safe and efficient AD facilities.

In order to address those ambitious goals, Green says the CBA had to focus on several aspects of AD planning and operations, including: legislation, permit approvals and standards; planning and siting considerations; site design and operation considerations; commissioning and starting up an AD facility; operating an AD facility; odor prevention, control and treatment; contingency planning, safety and “spill” response; communication; and monitoring, sampling and documentation.

The CBA’s AD Guideline “does not provide any technology recommendations,” says Green, but around the world, technology providers continue to develop equipment and techniques they think will move AD forward as a waste conversion option.

A European import with potential

The production of both biogas and fertilizers via the AD process entails the need for plant operators to meet considerable industry quality standards and government health and safety regulations.

Atlanta-based HRS Heat Exchangers, part of the United Kingdom-based HRS Group, says a downstream pasteurization process can help make sure AD plant operators stay well within such quality and safety parameters for the solid byproducts created.

Capture 2 
 © uliab | stock.adobe.com

 

“In Europe, pasteurization is commonly used as part of the AD process to allow digestate produced from foodstuffs or animal byproducts to be used or sold as a fertilizer and soil improver,” HRS writes in a white paper issued in December 2019.

Pasteurization also can be used up front in some AD situations, according to HRS, which writes, “Some farm plant owners also pasteurize their feedstock or digestate in order to reduce the risk of plant pathogens and weed seeds carrying through in the digestate.”

While pasteurization has not been embraced in North America to the same extent it has in Europe, HRS says it can cite a number of factors to suggest that, going forward, “the pasteurization of digestate produced by on-farm AD plants in the United States will become more attractive to plant operators and, therefore, more widespread.”

Factors on the HRS list include:

  • The increasing use of AD overall in the U.S.: There are currently around 250 on-farm AD plants in the U.S., according to HRS, “but there is believed to be the potential for as many as 8,000 on-farm plants nationally.” An increase in the use of AD technology will also boost the amount of digestate produced, meaning more treated land will lead to greater levels of attention to safety.
  • The increasing use of food waste for anaerobic digestion: As AD becomes more widespread as a method of food waste treatment, the amount of digestate produced from non-crop sources will increase accordingly. Such material will need suitable treatment if it is to be applied as a fertilizer or used for other purposes such as landscaping, soil improvement or manufacturing.
  • The need for thorough pathogen control: While research suggests that certain plant pathogens and weed seeds, such as tomato or black-grass (Alopecurus myosuroides), are not always destroyed by mesophilic anaerobic digestion at 99 degrees Fahrenheit, pasteurization has been shown to effectively control most plant pathogens and seeds, according to HRS.

HRS appears poised to help introduce the pasteurization step to North America, and it cites an example from the U.K. when stating its case.

Down on the farm

In Europe, some farmers choose to go beyond what is required by legislation and use pasteurization as part of their anaerobic digestion process even where crops from their own holdings are used as the feedstock. One such plant, says HRS, is Allen’s Farm near Colchester, England. That company grows around 300 acres of onions, 300 acres of potatoes, 1,000 acres of cereals and 500 acres of corn.

Three years ago, Allen’s Farm contacted Fullcircle Energy Ltd. to upgrade its existing 499-kilowatt AD plant by installing an HRS twin-tank continuous batch pasteurization system. The AD plant, supplied by Austria-based Thoeni, is fueled with what HRS calls “energy crops” and byproducts such as brewer’s grains and 8,820 tons of on-farm waste.

The biogas produced is burnt in a combined heat and power (CHP) engine to produce electricity for the distribution grid, creating an amount of power equivalent to supplying more than 600 homes.

The heat produced by the engine was previously wasted, but is now used to pasteurize the solid digestate. HRS says this improves the quality of the digestate biofertilizer by removing potential crop diseases and weeds. Adding the pasteurization step also allowed the farm to expand its renewable energy capacity.

Says David Hunter of Allen’s Farm, “The idea of pasteurizing is that we can feed more byproducts and waste from our potato and onion crops into the digester. We can then put the digestate onto our land without worrying about potential issues with weed seeds or spreading crop diseases. It will also remove the need to obtain a permit to spread the digestate, which can be very onerous.”

Hunter says the additional step also “allowed us to double the capacity of our plant and run two digesters rather than one, and we will then be able to export the electricity directly to the nearby University of Essex.”

Matt Hale, international sales manager for HRS, sees a bright future for pasteurization systems to take hold beyond Europe. “As the demand for pasteurization at on-farm AD plants increases in the U.S., having access to a variety of [system] options means that farmers and plant operators will be able to select not only the most appropriate technology for their situation, but also the most cost-effective solution,” he remarks.

This article originally appeared in the April issue of Waste Today. The author is a senior editor with Waste Today and can be contacted at btaylor@gie.net. Information on pasteurization was provided by HRS Group.

 

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