Focus on digester health for efficient biogas operations

Turning organic waste, such as manure, food scraps or expired food products into biogas is a modern type of alchemy that uses process-based technologies including an anaerobic digester, the organic waste feedstock, bacteria and by maintaining the proper temperature and pressure levels.

Biogas systems already provide enough renewable energy to power the equivalent of almost 70,000 average American homes, according to EPA Connect, an Environmental Protection Agency blog. The federal government has taken notice of the industry’s potential and is hoping to encourage further development. If its full potential was realized, the biogas industry could produce enough energy from the livestock sector alone to power 1 million average American homes, notes EPA Connect.

The biogas industry today, however, is still relatively small. There are currently more than 191 anaerobic digesters on farms and about 1,500 more operating at wastewater treatment plants, according to the American Biogas Council. With the proper support, however, more than 11,000 additional biogas systems could be deployed, according to the federal government’s Biogas Opportunities Roadmap released in August 2014.

Failure to maintain the proper conditions, however, could compromise biogas production. The process of producing biogas sounds easy enough: The digester takes the feedstock in and, after about 30 to 90 days, transforms it into biogas, a combination of methane and carbon dioxide, and an organic fertilizer. However to maintain optimum output, owners and operators should keep in mind that this process is one that is truly living and that can easily be disrupted. Care must be taken from the very beginning with the planning, design and construction of the digester. The design specifications, for instance, will vary depending on the type of feedstock and amount of solids that will be used. Once the feedstock is identified, owner/operators should consider how they will obtain the feedstock to avoid costly business interruptions associated with an inconsistent supply. A predictive and preventative maintenance program also should be implemented for all equipment, especially those systems critical to the plant operation.

Design and Construction of the Digester

Before starting a biogas facility, the owner/operator should consider the type of feedstock that will be used. Each biogas facility is designed to work with specific types of feedstock and excursions outside these parameters make it more difficult to efficiently digest the material and produce biogas. Some digesters, for instance, may be designed to use livestock manure, while others may use solids derived from wastewater, mortalities (dead animals) or other organic materials such as food waste. The digester should be designed with the intended feedstock in mind.

Once the preferred feedstock has been identified, owners and operators should seek out experienced designers, engineers and construction teams to build the digester to ensure it will operate as intended. After the digester is constructed, it should be properly commissioned to ensure that it has been designed and built properly. This should be done by an independent third party and not the general contractor, who could have a potential conflict of interest with the owner.  Full production levels may take several months to achieve.

Testing the Feedstock

With the digester built and ready for operation, the digester will need a steady supply of feedstock. The source and the purity of the feedstock should be taken into consideration. During the digester process, the feedstock breaks down into biogas gas as well as organic fertilizer. Feedstock, however, may be contaminated or diseased. Mortalities, for instance, may carry prion diseases, such as the bovine spongiform encephalopathy, also known as mad cow disease.

The wrong feedstock could contaminate the biogas digester, potentially incurring clean-up costs and an interruption of the gas production process. Feedstock that includes a diseased animal could contaminate the resulting fertilizer, which could contribute to the spread of the disease. If mortalities are utilized, the feedstock should be pasteurized. 

Running the Digester

To operate properly, the digester needs a constant supply of feedstock, seeded bacteria for anaerobic digestion, as well as the proper temperature and pressure levels. Temperature levels may vary depending on the type of digester. There are two main types of digesters: mesophilic and thermophilic. Operators should know the required temperature levels of both types, with thermophilic digesters operating at a higher temperature than the mesophilic digester.  Operation of a digester outside of the bacteria’s range can result in a decrease in biogas output and can also cause the bacteria to die off and stop the digestion process.

With the proper amount of heat and the correct bacteria, the feedstock will begin to decompose, creating the biogas and the byproduct. Once the digester operations begin, Biogas begins to be produced.  It can take anywhere from about 30 to 90 days to produce the biogas at maximum capacity. Digesters typically run under fairly low pressure. The key is to be able to release the pressure to prevent either an explosion or implosion of the tank. Pressure release devices should be manufactured and certified to a recognized national standard. Digesters also should be equipped with a flare system that will allow for the release of gas when needed. The flare system will allow excess gas to be released and then burnt off. 

Maintenance

 

To preserve the health, wellbeing and operational longevity of the biogas facility, the facility must be effectively maintained. Once expected baseline operational parameters are established during the commissioning process, a formal maintenance program must be developed and followed to make sure that the equipment and related components are in proper working order. Critical, hard-to-replace spare parts should be kept on hand so that any problems can be resolved quickly without significant operational downtime in the event of an unplanned disruption or equipment failure.

Owner/operators should keep in mind that while the facility’s staff may be qualified to conduct some of the maintenance, a third party service repair expert may be required for other more sophisticated tasks. Proper maintenance helps to ensure that the digester will continue to operate with minimal disruption. In many cases, the digester will continue to produce gas during maintenance outages, necessitating the flare off of gas that could otherwise be used to produce electrical generation revenue.

Keep Digesters Running Smoothly 

As new biogas facilities are built in the coming years, owners and operators should take care to make sure their digesters run smoothly. They should give careful consideration to the type of feedstock that will be used and the design of the digester. The feedstock should be tested and appropriate measures taken to ensure its quality and proper temperature and pressure levels should be maintained. Finally, a formal maintenance plan should be put in place ensure digester health and to avoid possible disruptions and downtime.

Craig Bierl is an assistant vice president, senior energy and risk specialist, loss control, for the Chubb Group of Insurance Companies in Chicago, Illinois. 

http://www.renewableenergyworld.com/rea/news/article/2015/03/focus-on-digester-health-for-efficient-biogas-operations