Tag Archives: Process Stability

Anaerobic Digestion

Haigh Pipeliner: Stopping Costly Downtime

Posted on by Elliot Volpe
16
Nov

An Anglian Water waste water treatment works was suffering from system downtime due to a number of problems. The system originally consisted of a progressive cavity pump and integral macerator. These units were not fit for purpose and frequently caused blockages in the system.

The System

The system itself contains a raw sewage wet well at the inlet with the medium then drawn through the Pipeliner for treatment to 10mm in particle size. The sewage is then pumped out for further treatment using an Energy Maxi diaphragm pump at 1l/s. The medium is pumped into a rotating biological contactor (RBC) to be treated.

The Problem

Rag and debris would often find themselves getting into the wet well at the start of the system which meant it was being drawn through the pumping system and making its way into the RBC. This often led to expensive repairs and downtime of the equipment. Occasionally, the buildup of rag and debris would be so large it would get stuck in the pumping system itself, due to the macerator not being fit for purpose.

The Outcome

With the Haigh Pipeliner successfully protecting the diaphragm pump, and the system as a whole, it means there will be no expensive unplanned maintenance or system downtime that operators and maintenance teams will have to deal with.

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

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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Anaerobic Digestion

How to Keep Your Anaerobic Digestion Process Stable

Posted on by Elliot Volpe
22
Nov

The digester goes through multiple stages from start-up through to being stable. Each one of these is critical. If at any point the digester becomes too unstable, it can have serious consequences for the whole process. But, getting the Anaerobic digestion process to eventually become stable is a difficult and lengthy task.

Start-Up

At this stage, the process is extremely fragile. Because of the initial feed into the digester, there will be millions of different variations of biology present. If the feedstock is lignocellulosic, found in fibrous material, then it will be biologically different in comparison to acidic feedstock such as food waste.

Stable Operation

From the start-up to being a stable digester, this is called primary succession. Here the microbes inside the digester will have matured and become robust. Meaning they don’t become unbalanced as easily as in the startup phase. As the digester moves through the four stages of anaerobic digestion some species begin to dominate, becoming ever more acclimatised and balanced.

Unstable Operation

If the digester is overloaded or one variable in the process changes suddenly, then this can have a massive effect on the digester. Even a small change in pH can stress the digester and make the process unstable. See more on pH levels here – https://pipe-liner.com/2021/08/31/ph-its-all-about-balance/

Mature biology then becomes fragile, leading to further Anaerobic digestion process instability. If action is not taken on balancing the digester then it is at risk of souring. Meaning the digester would have to be emptied and the start-up phase would have to take place again. This would be costly and time-consuming.

If the digester becomes unstable but action is taken to stabilise the process, then the digester will slowly start to become healthy again. However, biology will be poor in variety and will be somewhat limited.

How to Avoid Process Instability

Many variables within the digester can affect the process such as feedstock contents, FOS/TAC, pH, temperature and many more. It’s a balancing act to keep the digester running at its best.

The first step to process stability is trying to keep the feedstock relatively similar in characteristics. Too many variations in the feedstock can allow the digester to become stressed and start to decline in performance.

The next precaution is that the digester is not being overloaded. Each digester will be able to handle a certain number of volatile solids per unit digester volume per day. If this is exceeded then it can lead to process issues such as foaming and potentially souring.

Process monitoring is an integral part of any AD process. To keep the process running as smoothly as it should be it is essential that all variables are being measured and analysed. From this adjustments to the process should be made to ensure it is running efficiently.

 

Reference – Roots Organics Ltd

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