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.

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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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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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Gas flaring is a common occurrence at many Anaerobic Digestion facilities. Being able to flare off gas is important for the process and also from a safety perspective. Failure to flare off gas correctly can lead to catastrophic consequences to the process and its hardware.

What is Gas Flaring?

Gas flaring is the process of burning off excess gas within a process. The gas is often not needed due to surplus or there may be a buildup of gas that could potentially become dangerous to the plant and people.

Why is Flaring Needed?

When the Anaerobic Digestion process has taken place and the biogas has been produced, it is then used in two ways. This is either through using the energy to power a CHP unit or through direct injection back into the grid. Often at times there is more energy produced by the process than is needed for the output. This then presents problems not only from a safety perspective but also an environmental point of view.

The two main options are:

• Storage
• Flaring

It is possible for biogas to be stored, usually in lagoons, whilst it is waiting to be used up. Commonly, it is only stored for a few hours before moving through to the desired end use. Biogas can be compressed but this is a more costly process and is usually used if there is an upgrading phase.

Alternatively, a flare can be used to allow the excess gas to be safely burned off in the event of excess gas or if the plant has failed and become potentially dangerous.

Different Types of Flare

Open Flares

When a process uses an open flare it often comprises of a burner with a windshield around it to protect the flame. Often controlled manually by means of a valve. The use of an open flare can be inefficient and difficult to achieve the correct mixture of gases for complete combustion. Many do not meet the efficiency demands and regulations that countries are putting in place and therefore are not a popular choice.

Enclosed Flares

Due to the design of the enclosed flare it promotes a uniform burn with the emissions being much lower than that of the open flare. There is much closer monitoring of emissions within an enclosed system and there is also better control over the combustion ratio too. Due to these reasons the enclosed flare is a popular choice for system designers.

Typical Flare Design

There a many common features that a flare must have to make it operational but also to ensure the safety of the plant and people. These include:

• Knock out pot – the role of a knockout pot is to remove any condensed liquids or droplets from the flare gases
• Flow control valve – a flow control valve is designed to ensure there is the correct amount of gas flow into the flare so that the burn takes place as designed
• Gas booster/blower – a booster is used to pressurise the gas entering the flare in order to create better combustion
• Failsafe valve – cuts off the flow of gas to the flare in the case of an emergency
• Flame arrest – allows gases to pass through but prevents the transmission of a flame which reduces the risk of fire or explosion
• Flame shroud – a shroud is used as a safety barrier round the flare and burner to avoid anyone coming into direct contact

Reference – Flaring_4-4.PDF (iea-biogas.net)

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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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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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