Facebook LinkedIn

Tuesday May 30 2023

AD (Anaerobic Digestion) plant map – how to make the most of each stage of the anaerobic digestion process

Anaerobic digestion is renewable energy source that is growing in popularity as it deals with waste and creates and reliable source of heat and power for our businesses and homes. Plus, the leftover digestate from an AD plant can then be used as a nutrient-rich fertiliser for our fields, helping to preserve food security for a hungry world. 

The efficient running and maintenance of the anaerobic digestion process is critical, as a loss in efficiency, a breakdown in parts and any downtime at all, can be costly to the operator. For example, each percentage lost in efficiency will cost thousands of pounds a year and, if you have to shut down operations because of a broken part, you will never recoup the lost money because the feed-in tariff is for a fixed period of time, and you won’t be granted any extra days to benefit from the financial incentive. 

For plant operators to maximise the efficiency and uptime of their AD systems, they need to understand each stage of the process, carefully balancing the complex biological processes, ensuring each key process is efficient as possible and, critically, working to prevent any failures in the key components at every step. To truly fulfil the potential of your plant, you will need expert support to get expert analysis of the biology, provide routine servicing and maintenance and, most importantly, make sure parts can be quickly delivered and installed when something breaks down, minimising any downtime. 

The increase in efficiency you will see from optimising each stage of the process will easily outweigh the investment and help to deliver substantial returns from your AD plant. 

In this blog, we share a simple AD plant map to help explain each key process of anaerobic digestion and go into detail about each of these stages below. Crucially, we look at how you can maximise the potential of your AD plant by increasing efficiency and preventing any downtime. 

Biogas plant labelled diagram. 

In this image we break down each stage of the anaerobic digestion process, showing how feedstock is fed into the digester, how the resulting biogas is used and processed, and completes the renewable energy circle, as digestate and energy is used to fuel the start of the cycle once again. 

Biogas plant equipment and each stage of the AD plant process 

Anaerobic digestion is, on the face of it, a relatively simple process where bacteria breaks down organic matter in the absence of oxygen and produces biogas and a nutrient-rich digestate. This process involves several stages, and each stage is important to the efficiency of the AD plant. Below we explore each step in the process and how AD plant operators can maximise efficiency. 

Feedstock for anaerobic digestion 

The anaerobic digestion process begins with the feedstock, and this can range from specific energy crops through to food waste, wastewater sludge and livestock manure. This blog explores feedstock types in more detail, but a vast range of organic materials can be used. 

Before the feedstock can be fed into the digester, it will need to be processed to create a rich source of food for the bacteria or to remove any materials that have no calorific value or may be harmful. This could involve grinding or chopping the material to reduce particle sizes and increase the surface area for the bacteria. 

To help improve efficiency, the feedstock should also be analysed to understand it’s value and predicted biogas output, helping you to build a detailed insight of your AD plant’s processes. 

Biogas plant biology 

Once the feedstock is added to the digester, a series of complex biological processes begin and these can be broken down into hydrolysis, acidogenesis, acetogenesis, methanogenesis which generate biogas. 

Hydrolysis in an anaerobic digester 

The first stage in the anaerobic digestion biological process is hydrolysis and this is where the organic matter is broken down into simpler molecules, such as sugars and amino acids. This process is driven by hydrolytic bacteria that release enzymes to break down digest the initial organic material. 

Acidogenesis in an anaerobic digester 

In the next phase, the simpler organic molecules are broken down further by acidogenic bacteria into volatile fatty acids, with alcohols also being produced as a by-product. 

Acetogenesis in an anaerobic digester 

The volatile fatty acids and alcohols are then broken down again by a new group of acetogenic bacteria and these produce acetate, hydrogen, and carbon dioxide. 

Methanogenesis in an anaerobic digester 

In the final, and the most important stage, of the anaerobic digestion process, the acetate, hydrogen, and carbon dioxide are then consumed by a group of bacteria called methanogens. In consuming these elements, the bacteria produce biogas as a by-product and this biogas can then be extracted from the digester. 

Optimising anaerobic digestion biology 

The biology of the anaerobic digestion process is complex and needs to be closely monitored and analysed to ensure it can reach its full potential. Creating the perfect environment for the bacteria is crucial as it can be easy to get it wrong and see a significant drop in efficiency or, worse still, see the bacterial reactions completely breakdown. 

Managing the biology of an anaerobic digester takes careful analysis and you can read more about the importance of this analysis and what you need to know in this blog from Birch Solution’s expert AD plant biologist. For AD plant operators, it means not only having support to test the biology of the plant, but to also get expert interpretations of those results and what you need to do to improve the efficiency of your plant. 

The digester is essentially a digester of nutrient-rich soup filled with bacteria but still has several critical components and will need regular maintenance and support. Again, it is vital to have a reliable AD plant parts and service supplier to reduce the risk of parts failure and costly downtime. 

Using AD plant digestate for agriculture 

Once the biological reactions are complete, the remaining organic matter in the anaerobic digester is separated from the biogas and removed. This nutrient-rich leftover material is known as digestate and is then treated to remove any remaining solids or harmful materials.  

Despite being leftover material from the digester, this digestate is a valuable fertiliser and can be used to grow crops, starting the AD energy cycle again. Food waste plants must adhere to strict regulations when dealing with Cat 3 Animal Protein waste, so this doesn’t re- enter the food chain or cause a potential biohazard or pandemic such as BSE. This waste is pasteurised to ensure any nasty bugs are killed, deeming it safe to spread to land under PAS110, APHA and EA regulations.  

Biogas output from an AD plant 

While the digestate is useful, the real prize in the process is the biogas. The biogas produced in the digester is a mix of around 55% methane and 45% carbon dioxide with lesser amounts of other gases such as ammonia, and hydrogen sulphide. 

The gas can be used in this form to generate heat or power using a CHP (Combined Heat and Power) engine, creating green electricity or it can be upgraded further and be fed into the national grid or as a fuel for vehicles. 

Using biogas for heat and power 

The biogas created in the anaerobic digester can be fed into a combined heat and power unit (CHP) to generate electricity or fuel heating systems. This power and heat can be used for homes and businesses, but some can be used to regulate the digester itself as well as drive the processes that are generating feedstock, from heating livestock sheds through to powering sewage treatment plants. 

The CHP process must also be carefully maintained and looked after as, it too, is key to the overall efficiency of the AD plant. Biogas contains impurities that can be corrosive, and you need to regularly service CHP units to ensure they can provide the electricity and heat you need to drive your other key functions. Again, it is worth having a parts and service provider who can respond rapidly to tackle any issues. 

Upgrading biogas for biomethane 

To put biogas into the gas grid or use it as a fuel, it must be upgraded. This upgrading process cleans the biogas and removes impurities such as moisture, the harmful gases, and the carbon dioxide. This process creates biomethane and you can learn more about upgrading biogas in this blog

The carbon dioxide that is removed can be sent on to industry that needs CO2 for their products and processes but the biomethane can now be utilised in a variety of ways. The first is to put it directly into the national grid so the gas can be used for cooking, heating and at electricity generation plants. 

The second option is to use the biomethane as a fuel source for vehicles and this is rapidly rising in popularity with a variety of vehicles now offering methane-powered engines as an option. Several businesses with their own AD plants are powering fleets of trucks while, on farms, farmers are even opting for a range of new methane-powered tractors from New Holland

This is another critical stage of the AD plant map and is critical for your revenue. As always, it is worth having expert engineers on hand who can source the parts you need quickly and efficiently. 

Maintaining an AD plant 

Despite saying anaerobic digestion is a relatively simple process, each stage is dependent on a series of complex biological reactions or equally as complex mechanical processes and expert support is critical to ensuring that you get the maximum efficiency from your AD plant and, as a result, the maximum revenue. 

Birch Solutions offers a range of parts and service support packages that give you access to expert analysis, a team of skilled engineers and, most importantly, the spare parts you need to maintain your plant and increase your efficiency. 

For more information about how increasing efficiency could increase the profitability of your anaerobic digestion plant or to learn more about how Birch Solutions can support you, contact enquiries@birchsolutions.co.uk or call 01652 686060.