[Dave]

What’s the frequency you suggest folks sample & test for the speciated VFA profiles to generate this information?

[Vanita]

Fortnightly is a good start to do VFA Profiling. For stable digesters fortnightly may work, but during process disturbances or optimization, weekly or even daily monitoring is recommended.

High propionate concentration occurs due to thermodynamic challenge because of high concentration of hydrogen in the reactor. This indicates that hydrogenotrophic methanogens are inhibited.

We have to release hydrogen stress for propionate degradation to happen.

High butyrate means acidogenesis is faster than methanogenesis in anaerobic digestor. That means the loading rate is high. This leads to lowering of pH in the digestors and creates non-favorable conditions for methanogens. Hence there will be a shifting microbial community of the digestor. This can be corrected by regularly monitoring of pH of the digestors.

High iso-valerate concentration leads to ammonia stress due to protein degradation. This can be monitored by weekly monitoring ammonia concentration in the reactor and C/N ratio in the feed. This can be mitigated either by reducing the OLR or increasing HRT and most important is reducing or maintaining C/N ratio.

High acetate concentration generally means there is a high concentration of acidogens as compared to methanogens in the reactor. If the digestors have sufficient quantity of Acetoclastic methanogens then methane yield can be increased by balancing feeding rate. This can be monitored daily basis by monitoring pH and VFA and Alkanity in the reactor

Two stage digestion can be helpful in recovering more methane.

[Tejas]

Thank you so much for brief explanation. It is quite interesting.

[Hatem]

The inoculum is important in a such rebalancing. Selecting an appropriate inoculum is a critical factor for the stability and performance of an anaerobic digestion (AD) process. In particular, H₂-adapted microbial consortia can prevent thermodynamic inhibition associated with hydrogen accumulation during biomethanation. Studies show that when the microbial community adapts to hydrogen-rich conditions, its structure changes significantly: the abundance of Methanocorpusculum can increase markedly (from roughly 5% to about 59%), while the overall proportion of Archaea rises from around 9% to 23% of the total community, together with the enrichment of syntrophic bacterial families. Fast-growing Methanocorpusculum species rapidly consume the initial H₂ load, while slower-growing Methanoculleus species—characterized by higher H₂ affinity—maintain very low dissolved hydrogen concentrations. This microbial synergy stabilizes the thermodynamics of the process and supports sustained methane production, highlighting the importance of selecting a well-adapted inoculum when designing or operating AD systems.

Ref.: “Hydrogen addition can accelerate propionate degradation kinetics during in situ biomethanation,” INRAE, Univ. Montpellier, LBE, Narbonne, France.

[Dave]

Excellent point about the inoculum affecting the gas quality and quantity–that is one reason why we see full-scale food waste and SSO AD plants with own-growth biomass rarely match the results of a BMP study that uses a different inoculum, sometimes from a local domestic sewage digester and under ideal laboratory conditions.

I’ll go further to state that batch BMPs that are started with healthy biomass inoculums grown on different feedstock are NOT sufficient for designing AD plants except for the most simple feed streams where other factors such as inhibition or toxicity and nutrient issues have been resolved at numerous reference sites with similar feedstocks.

[Hatem]

Dave,

The microbial community must be functionally adapted, not just active. I agree that using the same inoculum from a BMP test to start a plant is not practical and this is why:

1- The microbial community may not be adapted to the plant feedstock or process conditions.

2- Industrial start-up usually requires large volumes of digestate from an operating digester.

I think that the BMP tests should instead be viewed as:

1- Feedstock characterization tools

2- Inputs for kinetic calibration and reactor modeling (e.g., determining degradation rates for simulations)

This approach is consistent with advanced modeling workflows such as those based on Anaerobic Digestion Model No. 1 (ADM1), where BMP data are used to calibrate kinetic parameters rather than to define the operational inoculum.