There can be many different factors affecting your digester. It is the job of the operator to spot when things are not performing as they should and to react quickly to solve problems. These factors determine the rate and efficiency of the anaerobic digestion process.
6 Factors Affecting Your Digester
Type of Feedstock
The feedstock is critical in the process. Without it, there would be nothing to process. However, the feedstock can be one of the biggest causes of process inefficiencies. This is due to the nature of the feedstock, its biological make-up and how it acts. Feedstocks, such as manure, have high water contents with low biogas yields. This also means the organic loading rates are not as high as energy crops. These energy crops have a high yield, but their biology means it is easy to overload the digester.
Process Temperature
The temperature of the process can be equally as important for the process. As we know there are two main temperature ranges for the AD process: mesophilic and thermophilic. Each system will be designed with the temperature range at the forefront and constructed around this. Mesophilic temperature processes are commonly steady but stable. However, thermophilic temperature ranges allow the process to take place quicker, but it is more unstable.
Presence of Toxic Materials
Toxic metals in the AD process can be a nuisance for any process operator. Heavy metals can cause damage to equipment and the system itself, which could mean costly downtime and maintenance. Once the heavy metals make it into the digester they can then start to cause havoc on the biological process. The metals react with the process differently from the normal feedstock.
The same is also true for other contaminants that enter the system. If the system ends up with high levels of ammonium, sulphates, sodium, calcium and potassium then the process can become just as unstable. This is where continuous system monitoring is essential to spot the problem before it becomes catastrophic.
pH & Alkalinity
As mentioned in previous articles, the pH of the process is an extremely important factor. This combined with the temperature can be a major factor in digester biology. It can good indicator for process health. However, it is not always the best way to use pH alone as it can sometimes not give us the full picture. That’s where FOS/TAC levels are best for measuring digester health. From this, we learn the alkalinity buffer of our system and what we need to add to the system to make it stable.
Hydraulic Retention Time (HRT)
HRT is the average amount of time the medium stays inside the reactor for. It is usually expressed in hours or days. To work this out you simply divide the volume of the tank by the influent flow rate. E.g 100m3 digester tank with an influent flowrate of 50m3/hr = 100/50 = 2 days.
Rate of Digester Loading (OLR)
The organic loading rate refers to the feedstock contents and its VFA profile. When it comes to energy crops, fats oils and grease (FOG), or glycerol, these feedstocks have extremely high OLR rates, which means they have a high yield percentage, but due to their biological contents, they are volatile, meaning the process can become unstable much quicker. Feedstock materials such as cow manure have much lower OLR rates but because of their biological make-up, the process is much more stable. This means the biogas yield is less than a higher OLR feedstock.
How Are These Factors Affecting Your Digester?
These six factors come together in the process of anaerobic digestion to create robust biology within the digester. This robust biology means that when there is a slight imbalance with one aspect, then it does not harm the digester as a whole. Process efficiency is down to these fundamental aspects, which as we know, can be the difference between making money and losing it.
Reference – Roots Organics Ltd
