Feedstock suitability in anaerobic digesters

Sylke Chesterfield Published on 19 November 2012

Anaerobic digestion to turn waste into energy is a topic that continues to garner growing interest from the agricultural community – and rightfully so. The benefits of biogas systems are many and they span the environmental, societal and economic arenas.

Economics drive the pursuit of co-digestion, which is the addition of organic waste materials to manure. Manure, an already digested material, yields relatively little energy in comparison to some organic waste materials. By adding higher-energy substrates, much greater quantities of gas can be produced, resulting in greater potentials for calorific, electric and thermal energy, as well as revenues realized from these sources.

But not all waste is created equal and every feedstock must be evaluated on its own merits.

When considering the impact on overall gas production, food processing waste with concentrated levels of certain nutrients can result in a greater quantity but a lower quality of gas produced. Other feedstocks may be too low in energy. And substrates that have high concentrations of salt or ammonia may actually inhibit gas production altogether.

For the complete-mix process flow, substrates must be pumpable and the entire slurry must remain in a state of fluidity that allows for appropriate agitation of the tank contents – between 8 and 12 percent total solids.

To ensure an optimized anaerobic digestion process, attention must be paid to overall slurry fluidity, temperature, nutrient load, pH level, agitation rate and retention time. Of course, the process will function with any one of these factors not necessarily ideal. At least for a while … when it doesn’t, the consequences can be costly, to say the least.

So how do you know what might complement an existing feedstock mix or what materials you should be looking for or avoiding? At what quantity will a certain feedstock compromise the integrity of a digester?

At which rate should the new feedstock be introduced to the digester? Does it have to be pretreated? Does it need to be mixed with manure? All are important considerations for optimized performance of the digester.

To get some answers to these questions, there are a few laboratory tests that can be carried out. The most common is biomethane potential (BMP) testing. This test determines the suitability of a feedstock for anaerobic digestion and the potential biogas (methane) yield of the feedstock or combination of feedstocks.

This test blends the feedstocks with an anaerobic inoculum in a vessel at mesophilic or thermophilic temperatures, simulating anaerobic digestion on a batch scale. The gas yield is compared to a control test to determine the methane production. Additional testing might be pursued if more information is required.

Depending on the biodegradability of the feedstock, this test takes between 10 to 30 days (or more) to complete. Good news: The technology to model feedstock scenarios in a much shorter timeframe and the expertise to optimize the process exists.

A computer-based simulation model has been developed that replicates the anaerobic digestion process in nature, laboratory settings and scale-size biogas systems. The proprietary program simulates feedstock performance in the anaerobic digestion process, closely matching laboratory result predictions for biogas quality and quantity, pH levels, acids and ammonia inhibitions.

Pairing this technology with the expert knowledge of biochemical engineering staff allows for cost-effective means to analyze feedstock suitability; you can have an answer in as little as a few hours! When considering the use of additional feedstocks, operators of existing digesters should obtain an analysis to assess the feasibility of a waste product for their system.

Maybe even more important, accurate predictions of feedstock suitability and the gas production potential of specific feedstock recipes under consideration play a vital role during the planning stages of an anaerobic digestion system. Realistic expectations for the quality and quantity of the biogas that will be produced will undoubtedly reassure financing entities.

Knowing the impact of feedstocks on system stability, together with the proper guidance for optimizing process controls and any adjustments in operational and loading schedules, ensure an overall reliable anaerobic digestion process and the long-term viability of a well-designed anaerobic digestion project. PD

Sylke “Silka” Chesterfield has been active in the biogas industry since 2001 in various roles of educating and assisting interested parties, as well as project development. She can be reached at (518) 632-4530 or Click here to email her.

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