David Ellis presented a methodology developed specifically to optimize anaerobic digestion systems at the Value of Biogas Webinar Series hosted by the Canadian Biogas Association (CBA) in June 2020.
The 2020 Value of Biogas Webinar Series delivered a variety of biogas and renewable natural gas (RNG) research and industry information in a three-part webinar series. This event followed the postponement of the in-person Value of Biogas East events due to the COVID-19 pandemic. More than 200 attendees participated over the three two-hour sessions featuring subject matter experts who shared their expertise, research, and industry knowledge. The Optimizing Digester Performance webinar session on June 18, 2020 included a presentation by Azura’s David Ellis titled “Beyond FOS/TAC – Digester Performance and Stability Measurements to Help Manage Changing Feedstocks in a Black Swan Event.”
In this presentation, David shared an evidence-based methodology, designed to optimize biogas production in food and organic waste anaerobic digestion systems. This information is based on Azura’s original work, research, and experience in the anaerobic digestion field for almost 30 years. This methodology was used to successfully recognize an imbalance and proactively tune the digester’s nutrition for optimal performance.
To demonstrate this methodology, a case study from an anaerobic digestion failure was used. This particular digestion facility was only fed with food and organic waste and used a two-stage digestion process to produce biogas, a methane-rich renewable energy source.
Prior to the failure, the anaerobic digester was struggling with poor and inconsistent biogas production for about 10 months. During this time, the operators conducted routine testing. They made some adjustments in the feedstock and it looked like the digester was recovering for a time. Then, the digester failed again, and the operators contacted Azura for assistance.
As a core part of their service, Azura uses a two-tiered process of optimizing anaerobic digester performance. Building on the work done by the plant’s experienced operators, Azura was able to bypass the first tier of testing and conducted the more-detailed second tier of process testing. The presentation included a list of both the Tier 1 and Tier 2 tests and conventional bench-mark values for most parameters.
One of the most used digester performance parameters monitored is the VA/BA or FOS/TAC ratio. These parameters use a simple and low-cost titration test to estimate the concentrations of organic acids and the buffering capacity of the digestate. The concentrations of organic acids (also called volatile acids, VAs, or volatile fatty acids, VFAs) and alkalinity, a measure of buffering capacity, can be estimated using several slightly different titration procedures. The alkalinity measurements can be in the form of the total alkalinity (TA) or the bicarbonate alkalinity (BA). These measurements result in calculated ratio values such as the Ripley Ratio, the VA/BA, VFA/TA, and FOS/TAC.
FOS/TAC, sometimes written as “FOSTAC”, is from the German Fluchtigen Organischen Sauren (FOS), a measure of the volatile organic acids and the Totales Anorganisches Carbonat (TAC), the total inorganic carbon.
Whichever terminology the facility uses, this VA/BA or FOS/TAC ratio suggests the amount of food a digester can accept without negatively affecting the ecosystem within the digester. Some use terms like “full” or “hungry” to describe the digester’s VA/BA ratio. Many operators consider this ratio to be a sufficient indicator of digester health and productivity. However, there are limits to the insight that the VA/BA ratio can convey.
The VA/BA ratio does not indicate potential inhibitory parameters. There may be a buildup of nitrogen, fats, sanitizers, or exotic toxins. During the current COVID-19 pandemic, increased sanitation chemicals in food processing plants are a common concern. Additionally, this VA/BA ratio does not indicate ammonia toxicity. A high ammonia content can appear as excess alkalinity, resulting in an inaccurate VA/BA reading due to chemical imbalances. These limitations of the simple titration VA/BA test mean that other monitoring methods are needed to ensure stable digester operation–hence the need for Tier 2 testing.
In this specific case, the Tier 2 testing revealed that the concentrations of long-chain fatty acids (LCFAs) and VFAs were beyond the productive ranges. Due to the inhibitory impact of these compounds, the anaerobic digester ceased biogas production. After an aggressive intervention campaign, supported by frequent laboratory testing, the biogas production was restored within 2 months.
David rounded out the presentation by sharing interesting developments with the audience that included tools for continuous real-time measurement of the biological activity within the digester. This technique has the potential to identify overloading situations or pin-point exactly when a ‘bad load’ of feedstock containing some inhibitory chemical is fed into the digester.
Azura’s presentation shared the different tools digester facility operators can use to monitor their process and ensure stable production of reliable, high-quality biogas. These tools ranged from online to on-site to off-site laboratory testing and included both well-established and leading-edge monitoring methods. For more information on the Canadian Biogas Association’s 2020 Value of Biogas Webinar Series, please visit the website here: