By Rashi Gupta
Posted November 13, 2015 to the WEF Blog
As our industry advances towards resource recovery, enhancing energy production through the anaerobic digestion process remains a topic of much interest. Depending on digester feedstock and available co-generation systems, the process can produce enough biogas to power the entire plant…and sometimes, even enough power to export back to the grid! While a variety of feedstocks are available for co-digestion, this post deals with fats, oils, and grease (FOG) collected primarily from restaurant grease traps.
Facilities began implementing FOG receiving stations to reduce sewer clogs and overflows, reduce power purchases from utilities, increase the production of renewable energy, and meet the demands of FOG haulers seeking to reduce hauling distances. Many of these plants have seen significant increases in digester gas production. Those facilities that successfully charge tipping fees for FOG acceptance have received additional revenue. The reduction of clogs in collections systems and the provision of local disposal options for FOG haulers also provide benefits for the utility and community. However, as the number of such facilities has grown, so have reports of operational difficulties and higher than expected maintenance requirements.
The quantity and quality of FOG can vary greatly. In some regions, FOG haulers have numerous outlets, so consistently large FOG quantities cannot be counted on – especially in terms of assumed tipping fee revenue. Restaurant grease trap waste can contain grit, pebbles, utensils, and other contaminants that are abrasive, resulting in equipment wear. Often, delivered FOG has low pH and tends to be odorous. Excessive loading of FOG to digesters can cause the digesters to become upset, and cold weather can result in pipe clogging. Even in light of these O&M issues, many facilities across the country have been operating FOG systems for years, reaping the benefits in terms of digester gas and energy production, fewer sewer system clogs, and sometimes, tipping fees charged for FOG acceptance.
To mitigate the O&M impacts as much as possible while retaining the benefits from co-digesting FOG, receiving system designs should build on lessons learned at currently operating facilities – in terms of reliable FOG quantities and realistic tipping fees, equipment type, system components and control, materials of construction, and actual labor necessary to operate and maintain the systems. The associated costs in time and material should be studied and weighed against the potential benefits. And the O&M staff that will eventually be responsible for the system should be brought into the conversation early in the feasibility study phase.
Digestion of FOG can benefit the community and increase a plant’s energy production, but the potential challenges should also be acknowledged upfront. Proper staffing, stakeholder support, and realistic expectations for dealing with this variable product are critical to successful and sustainable system operations.
WERF research and the LIFT program are working to further knowledge and advance technology on this topic. WERF has several research projects related to co-digestion including FOG such as ongoing projects Developing Solutions to Operational Side Effects Associated with Co-Digestion of High Strength Organic Wastes and Co-Digestion of Organic Waste – Addressing Operational Side-effects, The WEF/WERF LIFT program also has a Focus Group on Digestion Enhancements which is working with end-users to move new pre-digestion and co-digestion innovations into practice.