What type of pumps are used in an AD/Biogas plant?
Anaerobic digestion (AD) is a complex process which breaks down organic matter using microorganisms. This process creates a methane-rich gas called biogas, which can be used as fuel for heating, or electricity generation. Due to the wide variety of bio-organic feedstocks used and the changing properties of the material as it progresses through this process, pumps for anaerobic digestion are varied and need to be carefully chosen depending on the material being pumped and its stage of digestion.
Pumps play a critical role in the anaerobic digestion process as they enable the movement, mixing and handling of sludge, cake, liquor, and digestate and other challenging materials. The right pump, in the right place of the biogas plant can help reduce the risks of failure, inefficiencies or inconsistencies in the final outcomes.
This article will help you understand the different types of pumps used in biogas plants as well as how to choose the right type throughout each step of the process, and their specific characteristics (for example, viscosity levels or solids content they are able to handle).
Intake “Tip Hall”:
For AD plants that receive mixed or dry waste, typically from municipal household and business waste, there is an additional sorting stage. At waste receiving facilities, input material is often tipped from bin collection lorries into a receiving hopper, then transferred into the cleaning and sorting system using screw, or moving belt, conveyors.
Peveril Machinery is an establish manufacturer of bulk and abrasive dry-material handling equipment, and part of the Atlantic Pumps group.
Feedstock Pumping
AD Feedstock is bio-matter which can come from sewage (treatment works), food waste and food by-products, animal manure, or specially grown crops (“energy crops”).
Depending on the source, the material may need depacking, sorting and crushing. The aim is to remove as much non-biological matter as possible; items such as plastic, metal, and stones - diverting them to their own recycling streams.
Washplants and depacking machines require high-pressure clean water, which is usually supplied by vertical centrifugal pumps, such as SlurryPro, Audex submersibles, or Ebara multi-stage pumps.
Process water, including a percentage of recycled water (returned sludge) is mixed with the material to aid the AD process. The now slurrified liquor contains a high amount of entrained bio-solids, and it is inevitable that some abrasive or sharp items will get through the initial separation stage. Foodwaste, farm slurries and municipal waste typically contain grit, stones, glass and ceramic chips, broken eggshells, and plastic packaging.
Due to the challenging nature of this sludge, high-volume anaerobic digestion (AD) plant pumps need to process large solids and have high abrasive resistance. A good example is SlurryPro, a centrifugal slurry pump commonly used for its robust and wear-resistant design. LSM pumps are another contender, especially where the sludge has a particularly high viscosity.
From washplant to digester tank
Anaerobic digestion provides better results when a consistent feed rate is in place. This is why positive displacement pumps tend to be the recommended option, as they move a set volume of fluid per rotation, allowing convenient control of the flowrate via a variable speed drive (VSD).
An important detail to remember is that many types of positive displacement pumps, such as lobe and ram pumps, are susceptible to rapid-wear and blockages with abrasive feedstocks, and those with high fibrous content. In those instances, a large peristaltic pump, such as LSM, tends to be the better option. Heavy-duty progressive cavity pumps, such as Toro-Kronoa PC pumps, are also another potential option as they are optimised for challenging sludges. They can also be fitted with an intake hopper and mixing or crushing attachments, which can aid the AD and biogas process.
Digester tank mixing
Once the digestate has been placed in the digester tank, it needs to be continually mixed to avoid the formation of layers (for example, scum foam, different temperatures and mass) which can degrade the process.
Whilst mechanical arm stirrers and paddle mixers are popular options, operators have found that pumps are often a better alternative. Since those stirrers and mixers are difficult to inspect and replace any worn or corroded parts, Audex submersible pumps tend to be the recommended option as they can be suspended in the tank, making it very easy to retrieve them during maintenance, without having to empty the entire tank.
If internal pumps are not the ideal option for your anaerobic process, external mixing pumps for digesters are also an option. Whilst they are even easier to maintain, it is important for the pipe and pump to be sealed in order to stop the introduction of oxygen and avoid any odours or gases from being released. We would then recommend using centrifugal pumps with double mechanical seals, or self-sealing pumps such as diaphragms and peristaltics.
Returned Activated Sludge (RAS)
Returned Activated Sludge (RAS) is used to transport useful bacteria and microbes back to the beginning of the AD and biogas process, which can help stabilise and make the treatment more sustainable. Centrifugal pumps (for example, SlurryPro), and progressive cavity pumps (for example, Toro Kronoa) are excellent options during this step of the process.
Digestate treatment pumps:
Since not all the solid waste gets broken down, the undigested residue will usually go through further treatment to recover useful product. This digestate contains minerals and fibrous matter that are beneficial in horticulture, agriculture, and farming.
Cleaning, grading and dewatering this digestate can involve filterpresses and washplants, which rely on high-pressure pump such as Ram, Progressive Cavity or SlurryPro pumps. In a washplant, digestate moves along a vibrating screen while high-pressure water is sprayed through it. This produces graded fines (soil conditioner), clean grit, and ligin fibres that can be made into animal bedding, compostable plant pots - completing the circularity loop.
The process water is recovered for return to the intake mix, and recirculated through the AD system.
Heavy-duty slurry pumps, such as SlurryPro, submersible pumps like Audex, and specialist pumps able to handle high-solid contents tend to ensure the highest efficiency and productivity, especially when it comes to grit-heavy sludges.
Digestate Cake Transfer
Digestate cake transfer is an important output of the AD and biogas process, which is a near-dry, high-solids sludge that is dewatered, dried and used as a soil conditioner. Pumps that can handle high-viscosity, gritty material, such as LSM peristaltic pumps are ideal for transporting the semi-dry digestate, while Peveril Machinery offer solutions for dry-conveying, storing and bagging.
Digester emptying and cleaning
It is important to keep in mind that, no matter what type of AD and biogas process you run, silt and debris tend to accumulate at the bottom of the tanks, which can impact the digester’s capacity and potentially hide corrosion. Regular maintenance is crucial to ensure this doesn’t become a critical issue, and it is important to conduct the digester emptying, cleaning and maintenance process efficiently to minimise downtime.
The flammable methane gas needs to be purged out before the digester tank is opened or worked on. This is usually done by pumping an inert gas into the tank.
The normal digestater pumps are usually well capable for drawing off the upper layers of digestate ‘soup’, until the lower sedimentation is reached. This bottom layer consists of highly dense, settled sludge mixed with undigestable solids that have accumulated over time, which is why we would recommend using LSM pumps or vacuum trucks to get this done.
Vac trucks have good suction power, but tend to be slower and blockage prone when compared to LSM pumps. LSM pumps operate continuously and at a controllable flow rate, making them particularly beneficial for moving material across site, for example to holding tanks or sludge treatment units.
Finally, high-pressure pumps like SlurryPro and Audex take care of clean down duties, allowing inspection of the internal walls and ‘wet parts’ of the digester.
At Atlantic Pumps, we offer pump solutions for optimised AD and Biogas processing, from high-pressure clean water pumps to models proven for longer lifetime in abrasive applications and high-solids sludge. For more information, download our guide or contact us.
