Azura’s David Ellis presented two different case studies based on troubleshooting the failure of food waste anaerobic digesters as part of Dr. Daniel Zitomer’s short course titled: “Anaerobic Treatment of High-Strength Industrial and Agricultural Wastes.” This course has been offered annually by Marquette University for more than 19 years.
For the 2021 edition of the course, Azura’s David Ellis was invited back as a presenter having previously presented during the 2020 edition. David’s first presentation covered a case study from a market-digester that had received food waste feedstock shortly before experiencing a complete failure. Following a detailed investigation and intervention, the digester recovered 80% of the total potential biogas production within 2 months and achieved full production a few weeks afterwards. The second case study looked at a well mixed wet digester which had abruptly stopped production. After a detailed investigation and subsequent intervention, the digester was back up to 93% production.
The focus points of the case studies were on digester monitoring and testing that could help operators spot potential issues and proactively act before getting into trouble. Dave spoke about:
- Achieving a detailed understanding of the feedstock blend. It is essential to understand the totality of your feedstock using a Total Mixed Ration profile;
- Practical steps to combine different feeds to ensure a balanced recipe; and
- How to regularly monitor the digester to ensure production optimization and reduce risks.
Tiered Testing
During the presentation, David introduced the idea of establishing different tiers of tests for digester operating parameters. Tier 1 tests are simple tests that can be performed daily at the digester site by the plant operators. These tests include temperature, pH, biogas composition, and a titration to estimate the concentrations of organic acids and the buffering capacity of the digestate. The concentrations of organic acids, also called volatile fatty acids (VFAs), and alkalinity (a measure of the buffering capacity) can be measured by a titration method using several slightly different procedures which result in calculated ratio values such as the Ripley Ratio, VA/BA, VFA/TA, and FOS/TAC.
The FOS/TAC ratio includes the German Fluchtigen Organischen Sauren (FOS), a measure of the volatile fatty acids, and the Totales Anorganisches Carbonat (TAC), the total inorganic carbon.
Tier 1 tests can give a good indication of how the digester is performing and stable tier 1 test results usually indicate stable digester operation. However, one major limitation of these tests is that they cannot identify more complicated issues that might disrupt the future operation of the digester.
Tier 2 tests are more sophisticated and are usually done at an off-site facility such as a specialized environmental laboratory. Some of the tier 2 tests include speciated volatile fatty acids (VFAs), long-chain fatty acids (LCFAs), different forms of nitrogen and phosphorus, and trace metal concentrations within the digester. Nitrogen, phosphorus, and trace metals are well known in the scientific literature to be important for digester operation. To achieve effective digestion, the right balance of micro and macronutrients needs to be met. Too much or too little, however, can cause digestion problems, inhibition, or toxicity-related failure. VFAs are known to be important to anaerobic digestion, since acetic acid is one basic input for the anaerobic biomass to make methane. However, VFAs are also known to be inhibitory in high concentrations and require regular monitoring.
The LCFA concentration is a parameter that is gaining attention in the academic world due to the increased interest in anaerobically digesting high-fat feedstocks for biogas production. Several studies have sought to understand the inhibitory nature of LCFAs. However, most of these studies were done in controlled, laboratory settings. Little practical data from operating digesters are publicly available. Over several years, Azura has built a proprietary database of LCFA data from a wide range of full-scale operating anaerobic digester systems.
The digester involved in this case study originally contacted Azura because the plant’s biogas production was not able to achieve the facilities nameplate capacity. Days after the first round of samples were collected, the digester crashed completely resulting in zero biogas production. The case study presented detailed the testing procedure, discussed the results, and shared with course participants benchmark values for the various tests.
Dr. Daniel Zitomer’s short course: “Anaerobic Treatment of High-Strength Industrial and Agricultural Wastes.”
The course serves those who deal with high-strength industrial wastes or who are involved in operating and designing anaerobic treatment processes. The short course is designed to walk people through the anaerobic treatment processes, from fundamental principles to case histories. The case history presentations demonstrate anaerobic technologies in real-world industrial settings and allow for questions regarding specific problems encountered and solutions developed.
For more information on Dr. Zitomer’s short course, please visit the website here: https://zitomerlab.wixsite.com/group
Dr. Daniel Zitomer
Professor Daniel Zitomer is the chair of Civil, Construction and Environmental Engineering at Marquette University in Milwaukee, Wisconsin. Dr. Zitomer completed his graduate degrees at Vanderbilt University where he worked with industrial and anaerobic wastewater pioneers Drs. Richard Speece and Wesley Eckenfelder. Dr. Zitomer’s research group hosts an anaerobic digestion short course every year for the purpose of helping wastewater professionals. Dr. Zitomer also invites experts from the wastewater industry to present in his short course.