Tag Archives: System Health

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 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

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

Anaerobic Digestion Process Metrics

Posted on by Elliot Volpe
10
Jan

Anaerobic digestion process metrics can be extremely important overall performance of the system. The efficiency can be measured in two different ways. Both of these focus on two different aspects of the process. These are:

  1. Biochemical process
  2. Physical process

Biochemical Process

Conversion Efficiency of The Inputs

Within the biochemical reaction that takes place throughout the process, we can use certain data to measure how our digester is performing.

Firstly, we can use the organic matter that we put into the system to measure the outputs from the process. If the biogas yield was required then we could use (m3/tOM). With m3 representing the biogas yield from the output of the digester and tOM being the tonnes of organic matter loaded into the digester.

If we wanted to find the electricity yield from the process we could use (kWhe/tOM). Where kWh is the electricity produced by the CHP unit in the system and tOM is the tonnes of organic matter being loaded into the digester.

Digester Productivity

Secondly, we can look at the productivity of the digester itself and how it is performing. To do this we would use (m3CH4/m3D day). Where m3CH4 would be the biogas that the digester is outputting and m3D day is the volume of digestate being produced per day from the process.

Focusing on both of these outputs from the process will give a clear understanding of how the digester is operating. From this, we can then compare this against benchmark data to see if it is underperforming.

 

Physical Process

Conversion Efficiency of The Outputs

When assessing the performance of the direct outputs we have to look at the physical processes that are taking place. In Anaerobic digestion process metrics, this is typically done by looking at the performance of the engine, typically known as the combined heat and power (CHP) unit.

This can be done through two methods. The first is the yield of electricity from the CHP unit. We can derive this from using the formula (kWe/m3CH4). Where kWe is the number of kilowatts of electricity that is being produced by the unit and m3CH4 is the amount of methane being produced by the digester per day.

The second method is total engine efficiency. This is the sum of electric output added to the sum of thermal output and divided by total fuel energy input (see below). From this equation, we would expect to see a value of around 60-80% efficiency from our CHP unit.

 ηo =    We + ∑ QTH  /  QFUEL

 

References –

Roots Organics Ltd

Methods for Calculating CHP Efficiency | US EPA

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Anaerobic Digestion

9 Ways to Know Your System’s Health

Posted on by Elliot Volpe
29
Nov

There are many indicators within a biogas system that can determine how well the system is operating. It can be an immediate response from the system or over a long period. Due to the process being made up of many variables it can often be difficult to locate the specific problem. However, from the response in the digester, we can then assess the actions needed.

Short Term

Biogas Quality

The immediate indicator from the digester will be the methane and hydrogen sulphide levels. In a normal digester, running efficiently, the methane levels will be expected to be around 75%, with carbon dioxide being a large proportion of the rest. If this percentage has dropped it may be cause for concern. Foaming and other process issues could be interfering with the production of quality biogas.

Biogas Volumes & Energy Yield

Much like the biogas quality, the volume will also be affected by inefficiency or a problem in the digester. This coincides with having benchmark data for your process. If there is a set standard of biogas you expect to see from the process per day and this is not being reached, then it requires further analysis. This then translates into the amount of energy yield. With the digester being inefficient and producing an inadequate amount of biogas, then the energy output from the system will be considerably less.

Medium Term

VFA’s, FOS/TAC & pH

Medium-term effects on the digester occur when particular levels in the process start to become abnormal. Telltale signs are the VFA (volatile fatty acid) profiles, the pH within the digester and the FOS/TAC levels.

(More on FOS/TAC)

Floating Layers & Foaming

Once the imbalance begins in the digester, it is followed by layers beginning to form on the surface of the digester. These can either be in the form of a ‘crust’ or foaming, depending on the imbalances. To counteract the layers forming, a good method of mixing is essential to keep the liquid moving. Also, clearing or breaking the layer can be useful to ensure the biogas can escape. Following this, it is essential to measure and monitor the levels in the system and compare them to the benchmark data.

(More on foaming)

Long Term

Digestate quality

The long-term indicator of a healthy system is the quality of the digestate. Once the digestate has been pumped out of the digester and de-watered, we can then assess its quality. These assessments are done primarily on the smell of the digestate, texture and also a visual inspection.

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