Anaerobic digestion is a natural process where solids are broken down into a slurry and gases in an oxygen-free environment. In nature, this continually happens on the floors of bodies of water. Similarly, a cow’s digestive system turns feedstocks into manure and biogas, a methane-containing gas that is expelled into the atmosphere.

Thus, a biogas system is basically a technical reproduction of the processes that occur in a cow, but since methane can be used as a fuel – often in engines specially designed for it – the gas is harvested for its energy.

Any dairy producer can tell you that a cow’s health and comfort directly impact its production, and each producer has an approach, feedstock recipe and feeding regimen that he knows works best to maximize production of his herd.

The same is true for a biogas system: The characteristics of the feedstock mix, the addition of off-farm substrates, the feeding schedule, the quantities fed and the processes within the system itself determine the health of the system and the composition of the digestate, as well as the quantity and quality of the gas it produces.

However, some things matter more than others … Let us explain.

The digestion process itself has four stages: Hydrolysis, acidogenesis, acetogenesis and methanogenesis. Each step prepares the feedstock going through the system for the next step in the digestion process.

However, as it turns out, the initial stage has a much greater impact on the subsequent processes than one might suspect.

Some of the co-digestion systems that we were monitoring received similar feedstocks and operated in similar conditions, and thus were ideal candidates for comparison monitoring.

The biochemical experts noted that those systems that had optimized their feedstock handling with hydrolyzers – vessels equipped to optimize the feedstock mix before the latter stages of digestion – produced a significantly greater amount of gas than those without this crucial step.

It was concluded that this is related to the facts that hydrolysis occurs in a different pH range than do the remaining steps of the digestion process, and it typically takes longer than the subsequent steps, which also occur in quicker succession.

Additionally, hydrolysis is also the part of the digestion process in which solids are broken down into liquids.

What does all this mean for biogas systems?

Based on research and field experience, we can state the following for anaerobic digester systems where the focus is to maximize the production of biogas:

1. Material handling for off-farm substrates in the receiving phase of the system is best divided into solids and liquids receiving. This allows for an optimization of the feedmix and schedule.

2. The hydrolysis phase must be separated from the remaining digestion processes, as it renders the feedstock mix more easily digestible for the methanogenic bacteria in the following steps.

3. When the feedmix proceeds from the hydrolyzer to the next phase, it is a homogeneous slurry that presents minimal disruption to the next phases of the digestion process, keeping the ideal environment for maximized methane production intact.

4. Adding a hydrolyzer to the digestion process provides system operators with greater flexibility for variable feedstock delivery schedules.

5. Since hydrolysis breaks down organic solids, operators can more easily receive packaged organics, as indigestible material can be removed after the hydrolysis phase.

Whether a biogas system is designed to maximize gas production or to more fully break down solids (while also eliminating pathogens and weed seed), any co-digestion system may benefit from the addition of a hydrolyzer to facilitate either or both objectives.

Of course, it makes sense to include a hydrolyzer as part of a new digester system design, but existing co-digestion systems can also be retrofitted. If you want to get more energy, a hydrolyzer, in combination with the right feedstock, can increase gas production by up to 30 percent. PD

Sylke Chesterfield
Business Consultant & Coach
Chesterfield Associates