Digester Archives - Haigh Pipeliner Macerators

How Digestate Can Be Used

Posted on June 30, 2022July 6, 2022 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.

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Anaerobic Digestion

Digester Mixing: How it’s Done

Posted on June 20, 2022June 20, 2022 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

Anaerobic Digester Design Considerations

Posted on April 13, 2022April 13, 2022 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.

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.

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

What Is Anaerobic Digestion?

Posted on August 13, 2021February 14, 2022 by Elliot Volpe

13
Aug

What Is Anaerobic Digestion?

The process of anaerobic digestion dates as far back as the 19^th century. The process itself has evolved over time from open lagoon systems, through to creation of closed tanks to get optimum yield from the process.

So, what is it? Anaerobic digestion is the breakdown of material in the absence of air. Feedstock is fed into the digester tanks and left to sit there at a set temperature. Over time the material goes through different stages and gets broken down by microorganisms, with the result being a methane rich biogas.

Once the reaction has taken place, the methane can then be used to power a combined heat and power (CHP) unit, which often is used to generate electricity to allow sites to be self-sufficient. Any unused electricity is sold back to the grid. Some sites take the methane and export it straight back into the grid as soon as it is extracted from the process.

As a bonus, the digestate that is left in the system once the reaction has occurred is a nutrient rich fertiliser, which farmers can then spread on their land.

How Does AD Cut Emissions?

Anaerobic digestion is a renewable energy system, as the feedstock can be replenished at no harm to the environment. Often, cow manure is used for the feedstock, which is the first stage of the process. Then, at the end of the process the nutrient rich digestate is then spread onto the fields in which the cows eat the grass from. This means it is seen as almost net zero carbon emissions, making it a popular choice in the drive for renewable and clean energy.

In some cases, food waste is used as a feedstock. This means that the food waste that would usually decompose and emit harmful gases into the atmosphere is now being used as the input to a system that can then be turned into energy.