Author Archives: Elliot Volpe

Anaerobic Digestion

How to Maintain Biogas Quality

Posted on by Elliot Volpe
21
Apr

The biogas quality is one of the most important aspects of the process. Biogas is made up of Methane and CO2. However, the part we are most interested in is the Methane. This is what powers our CHP unit or can be injected back into the grid. This means in order for the process to be profitable we need the biogas quality to be sufficient.

What Can Affect Biogas Quality?

The biggest factor that affects biogas quality is the stress on the digester biology. There are many different ways the digester becomes stressed and can stop performing as it should. These include:

The most common stress on digester biology is the feeding of fresh feedstock into the digester. Fresh feedstock coming into the digester often causes spikes in the process. As the biology seeks to break down the feedstock it is working even harder to do so. This often means the biogas production drops as a result of this.

Another common reason for poor digester output is co-digestion of incorrect feedstocks. Some feedstocks complement each other well in aspects such as retention time and biologically stability. Others can work against one another and will produce a volatile mixture inside the digester.

How to Control Biogas Quality

Workout the correct HRT for the particular feedstock coming into the digester. HRT is the time needed for the feedstock to go through its biological breakdown to releasing methanogens. This value varies for each type of feedstock and also the volumetric size of the digester.

Another way to keep the issue under control is through the use of a multi tank system. This is where a receiving tank can be used to allow the feedstock to be fed in there, in the correct amounts. This can then be held in the reception tank and fed into the digester at a continuous rate and with stable biology.

The dry matter content can also be reduced in the feedstock. It is well known that the higher the value of dry matter content the less stable the process is. This is because of the requirements for efficient breakdown are much higher than that of manure. If liquid is added to the feedstock or co-digestion with manure takes place then the feedstock will breakdown in a more stable way. This means that the biogas quality will remain consistent and the process itself will not see as many peaks and troughs.

 

Reference – Roots Organics Ltd.

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Anaerobic Digestion

Anaerobic Digester Design Considerations

Posted on by Elliot Volpe
13
Apr

There are vast amounts of variables in the Anaerobic Digestion process. Many variables can depend on site location and capacity, as well as the feedstock and its flow. However, it is essential to get the digester design correct for each individual site to operate correctly and be profitable.

More on Process Design

Anaerobic Digester Process Design

Temperature range

There are two main temperature ranges in AD. These are Mesophilic and Thermophilic. Dependent on the feedstock and what the process owner is actually trying to get out of the process will determine the temperature range the digester is operated at.

More on Digester Temperature

Desired Operating Range

If the process requires a higher temperature to break the feedstock down more efficiently and meet the requirements to enable the process to operate as it should, then this will be stated in the design process and worked around.

Digester Size & Design

The specification of a digester is done by assessing many different aspects of the process. One key consideration is the actual size of the land the process will be built upon. If the space isn’t physically big enough for the process that has been designed then it will not work. Another consideration is the amount of feedstock that is going to be processed by the digester and the retention time needed for the feedstock to be broken down properly.

Feedstock Types & Source

When designing the process and more specifically the digester, the feedstock type is a primary consideration. This is because it simply determines many other variables in the process. If the feedstock is a certain type and has characteristics when reacting in the digester then the levels of additives and other process optimisation techniques need to be determined based on the feedstock itself.

The source of feedstock relates to where it is coming from and how much of it is going to be in the digester. For example, if the feedstock is manure coming from cows off of a farm then we know exactly what the feedstock type is, which means we can get a good understanding of how it is going to act inside the digester. We also know roughly how much feedstock is going to be processed per day.

Feedstock Receiving Strategy & Feeding Plan

In addition to the actual feedstock being processed and where it is coming from we also have to take into consideration what we are going to do with that feedstock once we have obtained it. Is the feedstock going to be fed into the digester continuously for 24 hours of the day or will it be done in batches? We then have to determine if the feedstock coming in is going to be held in a storage vessel until it is ready for processing and ensuring the time between receiving the feedstock and actually processing it is monitored so we do not end up with a poor performing digester.

Pre or Post Pasteurisation

When the digester is being designed the pasteurisation aspect will need to be considered. Ensuring this stage is correct is essential. The digester or one of the digesters will need to be heated up to a certain temperature to kill off any pathogens in the feedstock. This is to ensure that the biological process is as stable as possible.

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Anaerobic Digestion

Anaerobic Digestion: Process Design

Posted on by Elliot Volpe
01
Apr

In Anaerobic Digestion the process design is critical to how the plant operates. Incorrect design can lead to the flow of material being improperly treated or the digester can start to inhibit biogas production. This is why every step of the process is essential to get right by the system designer.

Commonly a plant will be designed in two main parts. These are the biogas production part and the energy generation element. Within these two main categories come different sub sections that make the process tick. For example, pre- and post-treatment and the digester itself.

Process Design Considerations

There are many different things to take into consideration when design an Anaerobic Digestion plant.

Site Location – Considering where the site is located is the first step. The process designer needs to ensure that there is sufficient access and egress. This is dependent on the type of license the site holds and whether it is a central location with resources being brought in often during the day. Potentially creating traffic and other hazards. This would obviously need permission from the local council.

Normal Weather Conditions – Weather conditions can have an effect on how the system gets designed. It can also affect the materials used. An AD plant that sits in direct sunlight all day will be different to that which faces cold weather, rain and winds constantly.

Layout – Site layout is probably the most important aspect of design. From deciding the layout of process equipment and instrumentation to assessing the flow of medium throughout the process. If the site layout is incorrect or not as good as it could be then the end user will either find the process difficult to manage or their outputs from the process will be significantly reduced.

Capacity – The need for identifying capacity is essential in process design. There is no point designing and building a plant and holding tanks that can handle 10 tonnes, yet the feedstock being delivered to site is well in excess of that. A plant must be designed per population or the amount of feedstock coming in. In addition to this, the same is also true for post-treatment. It would be inefficient if there wasn’t the capacity to handle the digestate after it has been in the digester. This also applies to energy generation too.

Access & Lifting – Having the required access around process equipment for maintenance purposes is essential. If a plant is designed with insufficient space and no lifting davits or even enough space to set up a portable lifting frame then any maintenance will become extremely difficult.

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Anaerobic Digestion

Measures to Mitigate Biogas Losses

Posted on by Elliot Volpe
09
Mar

As mentioned in a previous article (How to Reduce Biogas Losses) biogas can cause havoc if it is exhausted into the atmosphere. It not only means there is something fundamentally wrong with the process, but it also means that it can cause damage to the environment and humans. This can lead to large fines and complications.

Technical Measures to Mitigate Losses

  • Ensuring seals are achieved where necessary. This can include valves, joints, fittings and any other hardware that has the potential to cause leaks.
  • Covering process tanks with gas-tight covers. This can include process, storing or mixing tanks.
  • The process designer will take into consideration plant emissions during the design phase of the plant. Through this, they can incorporate emission-reducing technology and also look at relevant pipework.
  • The need for regular maintenance is essential and the replacement of any dome membranes is key to ensuring there is no biogas leak from any tank in the process. The membrane should be checked for permeability and also replaced as per the manufacturer’s recommendation.

Operational Measures to Prevent Biogas Losses

  • Regular maintenance and inspection procedures to check for leaks
  • If new components are installed then a check for leaks after this has taken place is essential
  • Regular maintenance of the CHP unit needs to be carried out to ensure there is no chance of gas leaks
  • Ensure the feedstock isn’t fed in too close to the planned maintenance period otherwise it heightens the chances of biogas getting into the atmosphere
  • Regular maintenance of hardware e.g. vales and gas equipment to ensure that all seals are still in check and prevent biogas leaks
  • Ambient temperature effects due to different weather e.g. weathering of membranes
  • Aeration post-treatment
  • Emissions testing and recording measurements with an action plan if needed

Biogas leaks are important to mitigate. Through routine checks and planned maintenance, these potential risks can be stopped. When maintaining the plant and equipment it must be done by a competent person. Regular routines along with system monitoring means operators and plant owners can significantly reduce the risks of biogas losses.

 

Reference – Roots Organics Ltd

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Anaerobic Digestion

Anaerobic Digestion: 4 Common Gases

Posted on by Elliot Volpe
02
Mar

The Anaerobic Digestion process can potentially be dangerous due to a buildup of gases over time that can make the atmosphere become explosive. This being said it is important to be able to recognise the gases and know the damage they can cause if humans come into contact with them.

4 Common Gases

 Methane – CH4

Methane is the desired output from the system so will be present in every biogas application. This doesn’t mean that it is not harmful.

  • Odourless
  • Lighter than air
  • Highly flammable (Explosion limits 5-15% CH4 in the air)
  • Displaces air (O2)

Carbon Dioxide – CO2

  • Smells sour
  • Heavier than air
  • 6-10% CO2 means increased respiration, high blood pressure, heart palpitations and headaches
  • 10%+ CO2 means paralysis of breathing, narcosis and death

Hydrogen Sulphide – H2S

  • Smells like rotten eggs
  • Humans can smell up to 250-300ppm and then after this our receptors become numb, and you cannot smell the gas
  • Heavier than air
  • Highly inflammable (explosive at 4.3-4.5% vol in the air)
  • Effects of exposure
    • <100ppm – life-threatening over several hours
    • >100ppm – life-threatening <1 hour
    • 500ppm – life-threatening in 30 min
    • 1000pm – life-threatening in a few minutes
    • 5000ppm – deadly within seconds

Ammonia – NH3

  • Commonly causes eyes to water and visual problems
  • Smell threshold 5-30ppm
  • Lighter than air
  • Highly inflammable
  • Caustic on moist skin and mucus membranes
  • Deadly at a volume of 0.5% (5000ppm) after 30-60 minutes

 

 

Reference – Roots Organics Ltd

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Anaerobic Digestion

How to Reduce Biogas Losses

Posted on by Elliot Volpe
22
Feb

Biogas losses can be a costly expense in the Anaerobic Digestion process. The first reason is when the biogas escapes the system it cannot then be turned into electricity or injected back into the grid. The second reason is the cost of environmental damage. If the extent of damage to the environment can be proven then the process owner is liable to incur large fines.

Losses can occur under normal operating conditions from particular points in the process or they can occur when certain variables or parts of the process start to change. This being said there are common circumstances where biogas is lost. These are:

  • Emissions from tanks downstream e.g. holding tanks
  • Leaks from the gas system e.g. fixings, joints and valves
  • Emissions and venting from safety devices e.g. Pressure relief valve
  • Emissions from open or semi-open digestate storage tanks or lagoons
  • Emissions from Combined heat and power units, engines and boilers

 

How Can We Reduce Biogas Losses?

There are a few methods used to prevent biogas losses depending on the circumstances. If for example, the biogas losses are from the gas system such as joints and valves then we can introduce a regular inspection and maintenance procedure on these components to ensure they are fit for use and if not, then any work is carried out correctly.

If the losses are being caused by open or semi-open tanks and lagoons, then we can look to introduce a fixed solution to ensure all gas is retained in the tank. This coincides with tanks that may be used for digestate.

Regular inspection and maintenance should be carried out on CHP units as per the manufacturer’s guidelines. This will help prevent unnecessary biogas losses when the CHP unit is in operation.

We can also use system monitoring to spot gas leaks in the process. Given the benchmark data that we could expect to see at any given time, we can compare this to the actual data. If we know that we are putting the same amount of feedstock into the process and getting less biogas out, then we know there is a problem somewhere. This could be a biogas leak somewhere in the system or biological inhibition within the digester.

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Anaerobic Digestion

Digester Souring: How Does It Happen?

Posted on by Elliot Volpe
14
Feb

Digester souring can be one of the most detrimental outcomes in the process of Anaerobic Digestion. Every operator or process owner knows that this is the worst thing that can happen to the process and is one of the hardest to recover from. It often results in having to empty the digester and go through the start-up process all over again.

What Is Digester Souring?

Digester souring is when one of the many variables starts to play havoc with the biological makeup of the contents inside the digester. The biological contents then start the acidification process, which in turn starts to inhibit methane production. Because of the acidification, the process is relying on the buffer (FOS/TAC & Buffers) to prevent the digester from souring. Unfortunately, when the buffer is then used, and the methanogens continue to be inhibited then methane production drops. This then drops the pH of the digester, causing it to sour. Any biogas production is stopped below a pH of 6.5

                         

What Are The Main Causes of Digester Souring?

As always in Anaerobic Digestion there are so many variables that can affect how the process operates and the amount of biogas that is produced by the system. However, the most common culprits can be broken down into 4 steps:

  • High OLR – Overfeeding the digester and stressing the biology
  • Low retention time – Not calculating the correct retention time for the medium to be properly broken down
  • Incorrect commissioning – Not setting up the plant to be effective
  • Irregular feeding – Feeding the plant too often causes peaks and troughs in variables

How Can We Avoid Digester Souring?

Digester souring can be avoided by implementing a few changes and creating routines within the process.

To solve the OLR problem we can identify the optimum feeding loads into the digester. This can be done through the calculation to a certain extent and then trial and error to get the best results. The best way to overcome digester overloading is routinely feed the digester with the correct amount of feedstock that it has been designed to deal with.

This coincides with the feeding regime. It is vital to feed the digester periodically to allow it to operate as efficiently as possible. If we introduce irregular feeds we then start to have the variables of the digester rapidly changing, which then causes instability in the digester.

Another prevention method is establishing benchmark data from when the digester is performing at its best and then comparing it to the results on a daily or weekly basis. Being able to compare this data is invaluable for optimising performance and preventing digester souring. It allows the operator to spot any anomalies and deal with them before they become a problem.

 

Reference – Roots Organics Ltd

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Anaerobic Digestion

Anaerobic Digestion: Indicators of Process Instability

Posted on by Elliot Volpe
01
Feb

There can be many warning signs to anaerobic digestion process instability. Your digester will often start to show typical symptoms of poor performance in many different ways. It is the job of the operator to spot these signs through physical checks and also biological tests. Once the problem has been identified, an appropriate solution can be made.

What is Anaerobic Digestion Process Instability?

Instability can occur at any moment in the process. There will be more trigger points when loading the digester or changing one of the variables slightly. On the whole, process instability is when the digester becomes unstable in such a way that it starts to have an impact on the performance of the system as a whole.

What Are the Main Indicators?

There can be many indicators as to why a digester is becoming unstable, but there are a few common signs:

  • Low methane concentration (poor biogas quality)
  • High Hydrogen Sulphide (H2S) concentration in biogas
  • Reduced biogas/methane production
  • Foaming (More on foaming)
  • Digester acidification
  • Highly sensitive against acid production

The Effects of Anaerobic Digestion Process Instability

Process instability can cause havoc in your digester. In some cases it can mean the digester has to be emptied and the whole process has to start from the beginning. This can be damaging to the environment and also your profits.

If the digester is performing sub-optimally then we can get poor electricity generation from the CHP unit, and low-quality biogas/methane to inject back into the grid, depending on the type  of the system being used.

The worst effects of instability typically come from an overloading of the digester. This can either be the OLR being too high for how the digester has been designed. This causes a shock to the digester and makes the biology become unstable. Another example is irregular feeds into the digester. If the feeds are not regulated then it can mean the process variables are spiking and dropping quite quickly in short spaces of time. Whereas, if regular feeds into the digester are introduced then it can allow the digester to avoid spiking.

 

Reference – Roots Organics Ltd

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Anaerobic Digestion

How to Avoid Anaerobic Digestion Process Issues

Posted on 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:

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

A Guide to Ensuring Suitable Feedstock

Posted on by Elliot Volpe
19
Jan

Whether your AD plant is brand new or has been constructed for years, the feedstock still has to be suitable for the process. If we don’t get the right feedstock in the process it can lead to damaging results to the digester microbes and the biogas yield.

In this article, we cover what to look for in your feedstock and how suitable it will be for your digester. After all, if you get the wrong medium it will start causing havoc in the digester and ultimately ruin your profit margins.

How do we Ensure Suitable Feedstock?

Biodegradability

The biodegradability is essential to know as you need to know the optimal breakdown periods of the contents. If a digester was designed to suit manure, but there was heavy fats or fibrous materials then the set-up would be wrong.

Feedstock Mixing, HRT & OLR

Equally, the feedstock has to be suitable for the process as a whole. If the medium has an extremely high organic loading rate, then the digester won’t be stable if it has been designed for manure like applications. The method of mixing has to be suitable. There are many ways a digester can be mixed to ensure the continuous breakdown of the feedstock and prevent the digester from becoming unstable.

Efficient & Robust Microbial Population

Microbial populations play an important role in the digester. Having the right balance can lead to a biochemical process that is efficient enough to allow the breakdown of the medium as needed but is stable enough to withstand slight changes in the process variables without then becoming volatile.

Feedstock Temperature, Alkalinity, pH & VFA’s

As mentioned in previous articles, there are many process variables in the world of AD. The main process indicators are commonly taken from the temperature of the system (Mesophilic or Thermophilic), the pH of the digester and the VFA profiles within the digester. Different feedstocks will have different requirements from a system. Some may need to be at a higher temperature or given their biological makeup they may need a slightly altered pH to allow optimal degradation. These are all considered when the system is designed.

More on process temperature

More on pH in the process

Toxic & Inhibitory Substances

Co-digestion can lead to two different types of feedstock helping each other to break down more efficiently. As one feedstock may have higher acidity or levels of metals that the other feedstock needs more of to break down efficiently. However, sometimes the feedstocks can inhibit the gas production process due to their nature. Heavy metals, biocides and solvents can cause havoc inside the digester.

 

Reference – Roots Organics Ltd.

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