Tag Archives: Process Optimisation

Anaerobic Digestion

What Is Sludge Thickening?

Posted on by Elliot Volpe
11
Nov

What Is Sludge Thickening?

Sludge thickening occurs when the liquids are removed from the sludge. This reduces the total sludge volume but the number of total solids that are suspended in the sludge is increased, often by 2-5x the amount. Depending on how much of the water is removed, the process is referred to as thickening or dewatering.

How Is Sludge Thickening Done?

There are 5 main methods of sludge thickening:

  • Gravity thickening
  • Centrifugal thickening
  • Flotation thickening
  • Gravity belt
  • Rotary drum

Gravity Thickening

Gravity thickening is done by using a tank or vessel that contains the sludge. Once the sludge enters the tank the heavier particles then sink to the bottom and the liquid stays at the top. There is a scraper that rotates at the bottom of the tank to allow the solids to stay in suspension. The sludge is then pumped out from the bottom of the tank with the liquid or effluent being pumped from the top of the tank to be further processed.

Credit: eddypump.com

Centrifugal Thickening

Centrifugal thickening works by introducing a centrifugal force upon the sludge. This actively encourages the heavy solids to move outwards, and the liquid effluent will be removed conveniently.

Credit: Sludge thickener & Centrifuge) | Download Scientific Diagram (researchgate.net)

Centrifugal techniques can be used in thickening and dewatering applications. However, there are some differences in the way they operate. These include:

  • Rotational speed
  • Throughput
  • Solids contents percentage of the product generated

When dewatering is wanted to be achieved the outcome will be a dry cake that can’t be moved by the use of pumping systems. Whereas thickened sludge will still be able to be pumped where needed.

Flotation Thickening

Flotation thickening is commonly referred to as dissolved air flotation (DAF). DAF works by injecting air into the water to reach saturation. This is then pumped at high pressure into waste-activated sludge. As the sludge then enters the vessel the solids float to the top and are removed and the effluent is removed from the bottom of the tank to be further treated.

Credit: Judd Water & Wastewater Consultants

Gravity Belt

Gravity belt thickening uses the same principle as gravity thickening, hence the name. However, a rotating belt is introduced into this method. The belt itself is porous which allows the excess water to pass through leaving the sludge sitting on the belt, which is then moved along by the felt and eventually drops off into the next part of the process. The excess water that has fallen through the belt drops into a sump and is then transferred for further treatment.

Credit: Judd Water & Wastewater Consultants

Rotary Drum

Rotary drum thickening works similarly to gravity belt thickening. A rotary drum uses a drum typically of 0.5-1.5m in length that is porous. Sludge is passed through the drum and the excess water falls through the holes in the drum into a sump to be further treated. The sludge continues to stay in the drum moving along continually until it reaches the end where it is then moved for further processing.

Credit: VIRIDIS Engineering 

What are the Benefits?

There are multiple benefits to the sludge thickening process. One of the main benefits is that sludge volume reduction brings down capital costs to treat high volumes of sludge and the further treatment steps that are needed. Sludge thickening is also beneficial from a process stability standpoint because of the reduction in mass of the sludge which means there are fewer heating requirements, smaller tank sizes needed, and less external input required.

 

Reference – Sludgeprocessing.com

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Anaerobic Digestion

What Are Biogas Blowers?

Posted on by Elliot Volpe
15
Sep

In the Anaerobic Digestion process biogas is produced which needs to be transferred from one place to another. The transfer of gas can be to the flare if it is excess gas, directly into a CHP unit or the national gird. This article explains how biogas can be moved from one part of the process to another.

What Are Biogas Blowers?

Gas blowers are often rotary, positive displacement machines that are used for the transportation of gases. Commonly, they are two or three lobe rotors. However, there are also centrifugal type blowers on the market. Biogas blowers can be used to supply, drive or extract biogas wherever needed in the process.

How Do They Work?

Positive displacement blowers have lobes inside that do not touch, meaning the gas is oil free. They rotate within the housing causing a pressure flow scenario. This means that the pressure varies to meet the systems requirements.

Centrifugal single stage blowers are constructed with spark proof aluminum casing and impellers. They do not require lubrication. Multi-stage blowers suck the gas into the inlet volute, from there every stage is linked to the first stage through a return channel. The main advantages of centrifugal type blowers are they are easy to install, low noise, minimal maintenance and no contamination.

References –

What is a Positive Displacement Blower? | Discover the Benefits of PD Blowers & Other Rotary Air Blower Solutions – Blocker and Wallace

Centrifugal Type Gas Blowers (ggepower.com)

Process Gas Rotary Blowers in Process Industry | HIBON

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Anaerobic Digestion

Biogas: Now More Than Ever

Posted on by Elliot Volpe
07
Jul

The future of biogas is extremely bright. This is largely due to business opportunities being exploited, developing infrastructure and the knowledge base to support rapid development. As a collective, the world has realised it needs to reduce its greenhouse gas emissions and find cleaner sources of energy.

Economic Impacts Fueling Biogas

Whilst many countries are putting in place plans to achieve net-zero by 2030, there are still many challenges that need to be addressed. There have been many dictating factors as to why biogas is becoming ever more important.

Whilst the COVID-19 pandemic seems somewhat behind us, we are still feeling the effects. None more so than the rate of inflation, with many people struggling due to the cost of living rising so sharply. Part of this is the cost of electricity and gas. Prices are continuing to rise with no sign of slowing down. In many places energy prices have more than doubled which makes for completely unsustainable living for people and families who were already struggling.

Another major factor is the Russian invasion of Ukraine. Whilst Russia supplied many countries with their gas, these countries have now cut ties with Russia and now they are forced to find other suppliers which are more expensive. This price increase is then being passed on to the consumer.

Knowledge Development

Many biogas project developers now have a vast knowledge base on carrying out successful projects with maximum profitability. This makes the proposition far more attractive for potential investors as they know their capital won’t be at as great a risk. This means there are more investors and growing knowledge bases, which means growth for the sector as a whole.

Biogas Market Growth

Th Biogas market is expected to see a 9.3% CAGR between now and 2028. The market was hit extremely hard during the COVID-19 pandemic and because of this we saw declines of -39.1% in 2020. However, the industry seems to be very much out of this phase with continuous growth predicted for many years to come.

Biogas Plant Market Size, Growth, Share | Global Report [2028] (fortunebusinessinsights.com)

Local Government Actions

With the demand for renewable energy increasing and the need for reduction in food waste, local governments in the UK have started to introduce schemes for households to separate food waste from all other waste by providing them with a dedicated food waste bin. This food waste will then be collected and taken to a centralised processing plant where it will be turned into energy. This new environmental bill is expected to lead to a reduction of 1.25 million tonnes of greenhouse gas per year.

Household food waste to be collected separately by 2023 and 50,000 city trees to be planted in Urban Tree Challenge Fund – Defra in the media (blog.gov.uk)

Improved Technology

Technology within the Biogas world is constantly improving. A key area this is true is the upgrading methods that have been developed. We are now seeing far more advanced techniques when it comes to extracting methane from the biogas. Because of this we are getting a better-quality biogas over a shorter period of time.

Also, other parts of the biogas process are improving, such as the CHP engines, feedstock processing equipment and the biological management of the process as a whole.

Conclusion

It is almost certain that AD will continue to grow at a significant rate due to the rising demand for cleaner energy. This aligns with government commitments and incentives and the technology suppliers making processes more efficient and providing education to the industry as a whole. For all aspects of the AD process, there has never been a better time to get involved in a movement towards a cleaner future.

Anaerobic Digestion

How Digestate Can Be Used

Posted on by Elliot Volpe
30
Jun

Once the feedstock has been digested in the digester we then get a residual. This residual is called digestate. The end product can be extremely good quality and useful for farmers to either pack and sell or spread it on their land. Spreading it on their land then means that it helps the crops grow. These crops are then either harvested and potentially put into the digester or are eaten by livestock whose waste feeds the digester. This makes it a circular economy.

What is Digestate?

Digestate is a residual outcome from the process of Anaerobic Digestion. It is extremely rich in nutrients and therefore a great fertiliser. The main nutrients in digestate are Nitrogen, Phosphorous and Potassium.

Different Types of Digestate

There are 3 common types of digestate:

Whole digestate – looks similar to livestock slurry and typically has less than 5% percent solids content

Liquid digestate – this is similar to whole digestate; however, the solids contents have been removed.

Fibre digestate – this is a compost like substance. Fibre digestate is the solids that were removed from the whole digestate.

Where Can It Be Used?

There can be many different uses for all types of digestate. Because the digestate has had the majority of toxins removed it makes for extremely good fertiliser. Liquid fertiliser is commonly spread on land to help crops grow. Fibre digestate can also be used as a compost/soil for personal use or it can be packaged and sold.

Regulations for Digestate

If the digestate is not to be considered as waste then it must meet the standards of the quality protocol and PAS110. These two standard ensure that the digestate that is being sold is fit for purpose and is non-toxic. These are two of the pillars that make up the biofertiliser scheme.

More on the benefits of Digestate

 

GP Biotec – What is Digestate?

BSI PAS 110: Producing Quality Anaerobic Digestate | WRAP

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Anaerobic Digestion

Digester Mixing: How it’s Done

Posted on by Elliot Volpe
20
Jun

When feedstock is pumped into the digester it is important to ensure the solids remain in suspension. If this isn’t done it can lead to many process issues, with the result being poor biogas production. The most popular ways to do this is through digester mixing.

What is Digester Mixing?

In a digester, there are no moving parts. Therefore, solids will naturally settle and inhibit biogas production. Because of this digester mixing is commonly used. Digester mixing is when the medium inside the digester is mixed or stirred to ensure solids stay in suspension. This can be done through the use of mechanical mixing using propellers, or through the use of pumps or gas injection systems that keep recirculating the fluid.

Types of Digester Systems

Continuously Stirred (CSTR)

A continuously stirred digester is the most common in the AD world. As the name suggests the digester is continuously mixed. The liquid coming in displaces the liquid going out and this ensures the process stays balanced.

Plug Flow

A plug flow digester means that the contents are that much thicker that the solids continue to stay in suspension with little to no use of additional mixers. However, the liquid is displaced by incoming feedstock.

Different Methods of Digester Mixing?

Gas Injection

Gas injection systems can often be grouped into four common types:

  • Lances
  • Floor mounted diffusers
  • Draft tubes
  • Bubble guns

These systems are designed to introduce gas into the tank from different directions to allow the sludge to effectively move around and avoid causing the floating layer on the top of the fluid, as this is what inhibits the production of biogas.

Mechanical – Propeller

Propeller mixers are one of the most common types of mixers in an AD system. They’re efficient, easy to maintain and cost effective. They come in many variations. Such as wall/side mounted or even floor mounted. Due to their nature they are not removing or adding anything to the process but simply stirring everything round in the digester.

Mechanical – Pumping

Another way of digester mixing is to recirculate the fluid by means of pumping. This involves using a pump to draw the liquid out of the digester at a certain point and then pressurising it and pumping it back into the digester. The pressure that the fluid is being pumped back into is high enough to move the solids in the tank.

 

References

Different Types of Anaerobic Digester Mixing Systems (phsenesacinc.com)

Digester Mixing Fundamentals | BioCycle

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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: 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: 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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