Digestate Transfer Pump for AD Plant Reduces Fertiliser Use

A potential new client contacted us recently about improving the transfer rates from their digestate pond to a nursing tank at their anaerobic digestion plant.

What is anaerobic digestion?

This is a natural biological process whereby micro-organisms break down biodegradable material to create two valuable by-products: biogas and digestate (Anaerobic simply means without oxygen being present.)

What is digestate?

Digestate, the byproduct of anaerobic digestion, is a complex mixture with varying properties, and the conditions under which it must be pumped can be challenging. Anaerobic digestate doesn’t need oxygen to be produced and consists of dead micro-organisms and left-over indigestible material which can be for use as a fertiliser. 

The amount of digestate produced will be around 90% to 95% of what was fed into the digester.  Therefore, it has enormous potential within the agricultural industry as it offers a viable, long-term alternative to widely-used mineral fertilisers, as well as closing the nutrient cycle to reuse essential minerals, and create less waste matter.

The challenges of digestate transfer

Selecting the right digestate transfer pump requires careful consideration due to several factors that influence the pump's performance, efficiency, and longevity.. Here are key reasons why careful selection is essential:

• Viscosity and Solids Content

Digestate can have a wide range of viscosities and solids content, depending on the type of feedstock used and the digestion process. Some digestates are relatively fluid, while others are thick and contain large solid particles. Pumps must be capable of handling these varying consistencies without clogging or losing efficiency. A pump that can't manage high solids content or thick slurry may experience blockages, leading to downtime and maintenance issues.

• Corrosive and Abrasive Properties

Digestate often contains corrosive substances like ammonia or acids, and it may have abrasive materials, such as sand or small stones, depending on the feedstock. This requires selecting a pump made from materials that can resist corrosion and abrasion. Using a pump made from inappropriate materials can lead to rapid wear and tear, increasing maintenance costs and reducing the pump's lifespan.

• Flow Rate and Pressure Requirements

The required flow rate and pressure for transferring digestate can vary depending on the size of the operation, the distance the digestate needs to be moved, and the height (head) it must be pumped. Choosing a pump that doesn't match these requirements can result in insufficient flow, leading to inefficiencies in the transfer process or excessive energy consumption.

• Pump Design and Compatibility

Different types of pumps (e.g., centrifugal, positive displacement, or progressive cavity pumps) are suitable for different applications. The design of the pump must be compatible with the characteristics of the digestate and the specific application needs, such as whether the pump needs to handle continuous or intermittent operation. Incorrect pump selection can lead to operational issues, such as cavitation in centrifugal pumps or excessive wear in positive displacement pumps.

• Energy Efficiency

Pumping digestate can be energy-intensive, especially in large-scale operations. Selecting an energy-efficient pump that matches the application requirements can significantly reduce operational costs. Pumps that are not energy-efficient will increase energy consumption and, consequently, the cost of running the equipment.

• Maintenance and Reliability

Digestate pumping systems often operate continuously, and any downtime can disrupt the entire anaerobic digestion process. Therefore, selecting a pump known for reliability and ease of maintenance is crucial. Pumps that are difficult to maintain or prone to frequent breakdowns can lead to costly repairs and operational delays.

Digestate management and use

Digestate can be used in liquid or solid form and as such requires considerations to the types of pumps used in the processing and transfer stages of it’s production.

• Liquid Digestate

The liquid fraction of digestate can be applied to fields using irrigation systems. Pumps transfer this nutrient-rich liquid from storage tanks to irrigation equipment, ensuring even distribution across crops. This enriches the soil with essential nutrients like nitrogen, phosphorus, and potassium, promoting healthy plant growth.

It can also be processed into biofertilisers which requires pumps to move the digestate into mixing units where it is combined with other organic materials to create customised fertilizers. These biofertilisers improve crop yield and reduce reliance on chemical fertilizers.

Additionally, digestate can be used in hydroponic systems, where pumps circulate the nutrient-rich liquid to support plant growth without soil. This method maximises water efficiency and allows crops to grow in areas with poor soil quality.

•  Solid digestate

The solid fraction of digestate can be used as a soil conditioner. Pumps transfer solid digestate to composting units or directly to fields where it is spread to enhance soil structure, increase microbial

activity, and reduce erosion.

In precision farming, advanced pumping systems, integrated with sensors and automated controls, deliver precise amounts of digestate based on real-time soil and crop needs. This optimises nutrient use, reduces waste, and minimises environmental impact.

What’s the difference between slurry and digestate?

Slurry is a semi-liquid mixture, typically consisting of water combined with solid particles, which can vary in composition depending on its use. In the context of agriculture, slurry often refers to a mixture of manure (from livestock such as cows, pigs, or chickens) and water. This mixture is commonly stored in slurry tanks and is used as a natural fertilizer for fields.

Digestate, on the other hand comes from anaerobic digestion and is a better fertiliser for heavier soils with high clay and carbon levels. The nutrients in digestates are more accessible than in raw slurry, making it easier for plants to make use of the nutrients.

What our client wanted

A potential new client contacted us recently about improving the transfer rates from their digestate pond to a nursing tank at their anaerobic digestion plant.

The liquid being transferred was mechanically-separated anaerobic digestate which contains screed at less than 1mm and only 5% solids. The mixture also contains a small amount of sand.

They were struggling to achieve a flow rate of 100m³h, and wanted to actually double this up to 200m³h. The digestate transfer pipeline was 300m long and fed into a nursing tank, as well as into digestate tanks as a feedstock or seed for future batches.

They draw the digestate liquid from a lagoon 4m in height and 18m away. To double their existing flow rate and hydraulically mix their lagoon, they were looking for an industrial-strength, robust self-priming digestate transfer pump.

What was our specification?

Here at North Ridge Pumps, our wide-ranging expertise means we can specify from a wide range of pumps for a variety of industrial and commercial applications.

We are not limited to a single pump technology and our pump manufacturing expertise enables us to offer a bespoke solution. This means we can select the most appropriate pump for any application at the lowest lifetime cost.

For this demanding application, we specified the XR625, and built and supplied it in just over 6 weeks. This is a 6” self-priming pump manufactured from ductile iron and a hard faced seal which is unsusceptible to the solids present in this application.

Our pump is designed to deliver a digestate transfer flow rate of 200m³h with only an 18.5Kw motor. To maximise output flow, we advised the client to run the pump at 60Hz via a frequency inverter to achieve the required flow and pressure.

Our XR pump range

These are heavy-duty self-priming pumps which are solid handling, efficient and extremely durable. This is thanks to the armoured volute which surrounds the impeller, improving pump efficiency and protecting the pump casing from wear.

The XR’s double curved impeller helps improve pump efficiency. In this particular application, the existing pump achieved a flow rate of just 100m³h with a 25kw motor. Our new pump delivered 200m³h with a motor power of only 18.5Kw.

Protected against corrosion

Another important consideration when selecting the pump was the properties of the digestate itself. It can vary in quality and be very corrosive. So we had to make sure that all internal parts were easily accessible and are not held in place by screws which can corrode over time.

Our XR pump series has design features including oversized inspection ports, and no internal screws in the pump unit.

Another clear advantage of our XR range is that traditionally self priming pumps have a wear plate covering the impeller which is replaced as it wears. The issue with this design is that when the wear plate is replaced pump performance is not restored to that of the original design as the casing has worn. Our range features an armoured volute covering the impeller which when replaced ensures the pump works to the original performances.

For more information on Digestate Transfer Pump for AD Plant Reduces Fertiliser Use talk to North Ridge Pumps Ltd