The benefits of Multi Stage Anaerobic Digestion                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

Multi Stage Anaerobic Digestion is becoming increasingly popular in the renewable energy space – this process provides optimal process stability, increased energy efficacy and providing better control over performance and energy recovery. But, what is Multi-Stage Anaerobic Digestion and why, if it comes with so many benefits, is Single Stage A.D. Still the leading process? Here, Ross Henderson explains...

What is anaerobic digestion?

A.D. (anaerobic digestion) is the biological process of producing biogas from organic feed materials, where biogas is a mixture of carbon dioxide and methane with small amounts of other gases.

Organic feed materials – also known as feedstock - can be anything from garden waste, food waste, manure, grass silage etc; indeed, any material made up of protein, carbohydrate, and fats/oils.

The biogas produced during the A.D. process can be used directly to fuel a CHP (combined heat and power plant) or it can be upgraded to remove most of the carbon dioxide and other contaminants, which allows it to be injected into the natural gas grid.

The basic biochemistry of an A.D. plant involves four main processes.

Hydrolysis -> Acidogenesis -> Acetogenesis -> Methanogenesis

The main limiting process in any A.D. plant is the hydrolysis process.

There are two main types of bacteria that are involved in A.D. The first is hydrolytic and acidogenic bacteria and the second is the methanogenic archaea (acetogenesis can be carried out by both acidogenic and methanogenic bacteria).

Single stage Anaerobic Digestion

Single stage anaerobic digestion is where all four biochemical processes are carried out in the same tank.

This type of digester is currently the most used as they are perceived as simple to operate.

However, they are not as productive as multistage digesters and they often have significant amounts of undigested material in their output as they often operate below the residence time for full digestion of the feed material.

The main limiting process in any A.D. plant is the hydrolysis process, especially where the feedstock has a high ligno/cellulose content, such as grass.

The time it can take for a single stage A.D. plant to digest this type of feedstock can be up to 45 days.

This is the main reason why very few single stage A.D. plants feed grass only feedstock.

The residence time makes it uneconomical to do so. To overcome this, all single stage A.D. plants feed only easily digestible feedstock, such as maize silage or food waste “soup”.

The main reason a single stage A.D. plant needs a high residence time to digest high lignin/cellulose feedstock is that it is trying to keep 2 very different types of organism “happy” and therefore is not able to operate ideally for either.

A single stage A.D. plant must be very careful regarding feed rate and heat control. Quick changes in temperature will upset the methanogenic archaea which will affect biogas production.

Also, quick changes in feed rate will negatively impact the methanogens. Too much feed will cause the pH to drop which if left unchecked will start to inhibit methanogenesis, which will further drop the pH and possibly lead to acidic runaway.

Too little feed will cause the population of methanogens to decrease, which will limit the amount of feed that can be fed to the A.D. plant until the population is built back up again.

What is Multistage Anaerobic Digestion?

Multistage anaerobic digestion involves separating the two main biochemical processes of hydrolysis/acidogenesis and acetogenesis/methanogenesis, which allows for optimizing the conditions for each of these processes. This allows the A.D. plant the flexibility to operate with different feedstocks or with feedstocks that other A.D. plants wouldn’t be able to use economically.

The multistage digester allows the operator to feed the hydrolysis/acidogenic bacteria at the optimal rate for the feedstock while feeding the methanogenesis tank at the optimal rate for biogas production.

The optimal conditions for hydrolysis is allowing the pH to drop below 5.5 and bringing the temperature up to between 50 and 55 degrees Celsius. This significantly speeds up the rate of hydrolysis which is the rate limiting step for all anaerobic digestion. When the pH is between 5.5 and 5 acidogenesis can occur uninhibited, which allows the product of the hydrolysis/acidogenic phase to be a mixture of short chain fatty acids and acetic acid. This is the perfect feedstock for methanogenesis as acetogenesis can be carried out by both hydrolytic and methanogenic organisms.

A hydrolysis/acidogenesis tank operating in this manner will not produce biogas, the gas will be a mixture of carbon dioxide and hydrogen, which can be sparged into the methanogenesis tank to further increase biogas production.

Hydrogen is not currently able to be exported into the national gas grid so reducing the amount of this gas before exporting is necessary.

The methanogenic phase will perform at its optimal rate when the temperature is kept between 38 and 42 degrees Celsius, and the pH is kept between 7 and 8.

To keep the feed material from the hydrolysis/acidogenesis phase from reducing the pH locally in the tank, some refeeding of digestate mixed with the incoming feed will increase the pH and reduce the temperature to the conditions the methanogens prefer.

The cost of a multistage digester versus a single stage digester isn’t significant, especially when compared to the increased efficiency a multi stage digester can deliver.

The benefits of Multistage Anaerobic Digestion

Increased yield of biogas is considered to one of the key benefits of Multistage Anaerobic Digestion as Multistage AD enhances the breakdown of complex substrates and increases the availability of easily digestible compounds.  

Alongside this, other benefits include enhanced nutrient recovery, increased process efficiency & stability, and improved overall feedstock conversion.

Why are single stage digesters still being built?

There is a perception that single stage digesters are less complicated to operate and less expensive to build than multistage digesters.

This is most probably a sign that the biochemical process understanding of anaerobic digestion within the anaerobic digestion industry is poor and that more needs to be done within the industry to help transfer the wealth of research knowledge that has been developed over the past 15 years into the operational side of the industry.

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In short, there are multiple benefits to Multistage Anaerobic Digestion which make it an increasingly popular solution in the renewable energy space; favored particularly for it’s efficiency and increased biogas production.

And, although this doesn’t entirely take away from Single Stage Anaerobic Digestion, increasing research and understanding of Multistage Anaerobic Digestion is leading to a shift in processes – and we’ll likely see more Multistage Anaerobic Digestion plants emerge.