Factors Affecting The Process Archives - Haigh Pipeliner Macerators

Anaerobic Digestion

How to Avoid Anaerobic Digestion Process Issues

Posted on January 27, 2022February 1, 2022 by Elliot Volpe

27
Jan

Process issues in AD can be detrimental. Problems in the digester can be the difference between profit and loss. This means that keeping your process out of trouble and running as effectively as possible is essential.

What Are AD Process Issues?

Process issues are when the variables that make up the anaerobic digestion process become out of balance with how the system was designed. This imbalance can then start to cause the digester to react in certain ways. These include scenarios such as foaming and souring. In some cases, the digester can become explosive, although this is extremely rare.

What Are the Main Causes of AD Process Issues?

Given the number of variables in the AD process, there are so many factors that can play a part in process issues. Some of the main variables include:

Temperature – (More on process temperatures)
pH – (More on pH in the process)
VFA
OLR
Irregular Digester Feeding
Plant Commissioned Incorrectly

The Effects of AD Process Issues

Process issues can be extremely expensive. Not only can it damage profits when poor amounts of biogas are produced, but it can also be expensive when you have to stop the process and empty the digester if it has soured. Every minute of downtime is costly to a process that commonly relies on continuous operation.

The process can become more odorous once the digester has started to perform poorly. With the imbalance in VFA’s and an increase in Hydrogen Sulphide, the smell can rapidly become unpleasant.

Process issues can also have damaging consequences on the other equipment in the process. Hardware such as pumps, macerators, valves even pipework can become damaged due to the process being unstable. This is because the content can erode and wear parts much quicker. In some cases, pressure increases can cause valves to blow.

Process issues can also cause damage to the environment. This is partly because of the incomplete digestion in the digester and therefore the digestate may still contain harmful elements that it wouldn’t have if the digester had gone through its complete cycle. The other part of this is that process issues can cause pollutants to be given off into the atmosphere due to digester emptying or leaking from a specific part of the process.

How Can We Avoid AD Process Issues?

As detrimental as process issues can be, many can be avoided by adopting good operational practices and being consistent with how you operate and monitor the system. This coupled with sufficient operator training can remove many problems with the process.

Feeding the digester at regular intervals can greatly improve the stabilization of the process. If the process is designed for a continuous flow then this will aid with consistency. It is then down to the process owner to ensure the feedstock is a consistent mix and the organic loading rate is kept consistent.

If the feedstock is to be altered at all in any way then these changes should be done gradually to allow the digester to not become stressed by the sudden change in biology. The same can also be said for the temperature in the digester. If the temperature has dropped then it is important to not increase it rapidly. This increases the chance of shocking the digester. It is better to increase the temperature as slowly as possible.

Ensuring the digester is being adequately mixed is another important part of the prevention of process issues. When the process is designed and commissioned this will be a main focal point. Considering the flow of feedstock, its contents and how often it is running will be some of the main factors when considering a mixing system.

Continuously monitoring the process and taking data samples to compare against the benchmark data is one of the most essential steps in avoiding process issues. The process needs to be monitored and measured daily. This ensures that any part of the process that may be operating inefficiently can be spotted early, and a solution can be created. If the process is not being measured frequently then it can often be too late to reverse the problem.

Reference – Roots Organics Ltd.

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Anaerobic Digestion

6 Important Factors Affecting Your Digester

Posted on January 4, 2022February 1, 2022 by Elliot Volpe

04
Jan

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 100m³digester 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