Promise for biogas

The development of the biogas sector of the waste conversion industry is marked by three main features:

• U.S. anaerobic digestion/biogas market takes root;
• expanded options for maximizing organics capture from municipal solid waste (MSW); and
• development of new projects remains challenging, but hopeful.
   

Market growth

Although industrial and municipal wastewater treatment plants (WWTPs) in the U.S. have converted dilute effluent streams to biogas and biosolids for years, the U.S. market for anaerobic digestion (AD) of organic waste from MSW is now just beginning to take root. Since 2011, 18 dedicated facilities have come into operation to process waste streams that include—at least in part—organics that were predominantly part of the waste stream going to landfills or composting.

These include food waste, yard or green waste, soiled nonrecyclable papers and a variety of organic residuals from food and agricultural processing facilities. These are generally classified as either source separated organics (SSO), where organics are collected separately, or the organic fraction of mixed solid waste (OFMSW), where the organics are separated from mixed solid waste in a mixed waste materials recovery facility (MRF), sometimes referred to as “dirty MRF” and are normally less than 2 inches. The MRF also can combine separately collected yard or green waste while producing the OFMSW.

Another 12 facilities are reported to be under construction, and 10 more have been announced as contracted and currently in the permitting phase. During the same period, eight announced facilities have been cancelled or indefinitely delayed.

The amount of waste to be processed is a bit tougher to determine, since some facilities report the fresh tons of waste to be received, while others report the working capacity of the AD facility, which may include dilution water for wet digestion systems. Many facilities codigest SSO or OFMSW with biosolids and do not specify what portion of the waste is SSO or OFMSW.

According to published reports, the 18 operating facilities have a combined capacity of 660,000 tons per year, and those under construction and those in permitting each bring another 500,000 tons per year capacity by the end of 2017, bringing the total to nearly 1.7 million tons, less than 3 percent of the estimated organic fraction of MSW.

This initial growth is actually faster than the initial growth of AD plants in Europe processing only SSO or OFMSW, where the published capacity from 1990 to 1995 went from 132,000 tons at 3 facilities to 365,000 tons at 17 plants, and then to 1.3 million tons by 2000 across 60 plants. This year, total European SSO and OFMSW digestion capacity is estimated at 9.7 million tons spread across 290 facilities. This does not include any quantities being codigested with manure at on-farm facilities, nor with sewage at WWTP facilities.
 

Maximizing recycling

Maximizing organics recycling, which consists of both the conversion to useful energy and the reuse of the organics, requires sufficient separation and preprocessing to enable the AD process to function reliably.

The SSO originating from preconsumer sources, such as food processing or preparation, can be accommodated in nearly any AD technology with typically only a requirement for size reduction. To address postconsumer SSO or OFMSW containing as much as 5 percent to 45 percent contamination respectively, four different options have emerged.

Some newer wet AD systems—either completely stirred tank reactors (CSTR) or horizontal plug flow (HPF)—are preprocessing SSO waste using a press to extract the organics. The waste is initially passed over a 6-inch screen to remove large contaminants that will cause issues for the press, and then the remainder is extruded in the press, sometimes with the addition of water depending on the moisture content.

The wetter organic fraction and small inert material permeates the press and is diluted and sent to the digester, while the rejected fraction—metals, conventional and biodegradable plastics, nearly all of the paper and some organics—is disposed of.

The digestate is consequently quite clean, containing only the smaller inerts (provided they have not settled in the tank). Because of the high dry matter content and the high amount of inerts in OFMSW (and the propensity for these to settle), OFMSW is not typically processed in wet AD systems.

In batch tunnel AD systems, SSO preprocessing may only consist of a bag breaker to allow the fresh waste to be premixed with a front-end loader and to expose the waste to the biological activity in the tunnel, or it may also include a mechanical sieve for culling out large contamination.

As a result, the digestate contains nearly all of the incoming contamination and requires mechanical separation steps after digestion (proper odor control and personal protective equipment is essential due to offgassing) in order to obtain a saleable organic product. The separated contaminants are then landfilled. OFMSW is not typically processed in batch tunnel systems because of the high amount of small inerts in OFMSW that occlude or clog the percolation system.

In continuous high solids AD systems (HSAD, where the waste is processed at up to 45 percent total solids without water addition), SSO is normally minimally pretreated to remove gross contamination such as metal or glass and then is reduced in size to less than 2 inches. If biodegradable collection bags are used, these are shredded with the waste and also enter the digester. The resulting digestate is quite clean and can easily be dewatered. In addition, remaining inorganics are easily removed during final compost screening.

It is also now possible to capture and recycle the most difficult organics—those combined with mixed solid waste. The HSAD plant can process an OFMSW stream that was screened on 2 inches without further pretreatment. The digestate will contain 100 percent of the nondigested materials since no sedimentation occurs.

To obtain a clean, saleable compost, the digestate can then be further processed in a postdigestion wet processing step. This approach obtains the maximum landfill diversion and organics recycling. See the flow chart above of a new facility in France incorporating postdigestion processes.
 

Addressing challenges

Several factors have combined to make it difficult to develop profitable biogas projects using SSO or OFMSW. Landfill fees have not risen as quickly as expected in the last five years, and organics diversion mandates have been only been implemented in limited areas while some green waste landfill bans have been overturned.

Biogas plant construction costs have increased as the general economy has improved, while the market value of the energy produced has gone down along with conventional fuels, wind and solar. At the federal level, financial incentives are also currently limited for nonfarm systems.

To address these challenges, the American Biogas Council (ABC), Washington, continues to push at the federal level for renewal of the investment tax credits and inclusion of all uses for biogas; expansion of the Renewable Fuels Standard (RFS) which includes biomethane for vehicle use; and maintaining the funding levels established in the 2014 Farm Bill.

The ABC has prioritized its efforts on passage of organics diversion legislation in an additional 15 states, and ensuring that reasonable permitting procedures for AD facilities are established.

Encouraging for future growth of the biogas market was the 2014 issuance of the “Biogas Opportunities Roadmap” by the White House, which recognizes the multiple environmental and economic benefits of increasing the implementation of biogas plants for waste conversion and organics recycling. The Roadmap encompasses the responsibilities of the U.S. Department of Agriculture (USDA), Department of Energy, Department of Defense and the Environmental Protection Agency to:

• remove policy or regulatory barriers to new project or biogas use;
• quantify the size, output and economic impact of the biogas industry on an annual basis;
• ensure biogas is included in all renewable energy programs and that biogas projects receive parity or better incentives in recognition of their multiple benefits; and
• increase participation in the biogas industry through presence at conferences, sponsorship of research initiatives and workshops focused on industry growth.

The ABC was instrumental in its creation and continues to work with policy makers and regulators each week to maintain momentum. The USDA has set an ambitious goal of 500 new AD systems in just five years, and while predominantly for manure, these could accommodate some amount of clean, wetter organics. In July, the first Biogas Industry Stakeholder Forum was held in Washington with both government and industry participants literally packed into the room. The first annual Roadmap progress report is due for publication soon.

Norma McDonald is North American sales manager for Organic Waste Systems (OWS), Cincinnati, and also a board member of the American Biogas Council, Washington.