First HRS Twin-Tank Pasteurization System Helps Farm Expand AD Plant | HRS Heat Exchangers

First HRS Twin-Tank Pasteurization System Helps Farm Expand AD Plant


For many on-farm anaerobic digester (AD) plants, space can often be the biggest limitation when considering an expansion. So, when Fullcircle Energy Ltd at Allen’s Farm near Colchester in Essex was considering increasing its renewable energy production by pasteurizing digestate using feedstock from outside sources, such as brewer’s grains, they needed a compact solution. With insufficient room on site for a standard system, HRS’s Twin-Tank Pasteurization System ticked the box.

Features of the new HRS Twin-Tank pasteurisation system

The unit, which is Renewable Heat Incentive (RHI) compliant, includes a primary storage tank, where digestate is held before the pasteurizing treatment takes place. There’s also a post-treatment storage tank, which feeds the treated digestate into a lagoon for longer-term storage. It has the capacity to treat around 60 cubic meters of digestate sludge daily.

The HRS pasteurizer uses a double tube heat exchanger to heat the digestate, which contains up to 14 per cent dry solids, to 75°C above the required pasteurization temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurized.


Reducing the physical footprint by a third, the HRS Twin-Tank continuous batch pasteurizer provides all the benefits of the standard HRS 3-tank unit, including the treatment of crop pathogens. It works by:

  • Feeding energy crops into the digester where natural bacteria turn them into an energy-rich gas (biogas) and a remaining biological material, known as digestate, which the farm uses as an organic biofertilizer.
  • Producing a high methane biogas, which is burnt in a combined heat and power (CHP) engine to produce electricity for the grid.
  • Reusing heat produced by the engine that would otherwise have been wasted to pasteurize the digestate.
  • Improving the quality of the biofertilizer through pasteurization, by removing potential crop diseases and weeds, before it is spread on the farmland.
  • Maintaining a constant throughput – while one tank is being filled, the other is heated pasteurized and emptied.

The tanks can also be used individually, allowing for routine maintenance.


By capturing surplus heat from the existing CHP engine, the energy requirement of the new pasteurizer has been reduced while the flexibility of the Twin-Tank pasteurizer allows the AD plant to continue to function in the same way as before, without restricting throughput.

Now capable of processing energy crops, such as maize, and some waste and by-products, the AD plant is:

  • Running at double the capacity of its 499 kW AD plant, by operating two digesters rather than one.
  • Able to treat approx. 60 cubic metres of digestate sludge daily.
  • Helping to produce enough electricity annually to power 600 Essex homes and the University of Essex.
  • Saving more than 1,600 tonnes of CO2 emissions.


“By pasteurizing, we can then feed more by-products and waste from our potato and onion crops into the digester and can then put the digestate onto our land without worrying about potential issues regarding weed seeds or spreading crop diseases. It also removes the need to obtain a permit to spread the digestate, which can be very onerous.”

David Hunter of Allen’s Farm

“This Twin-Tank version of our widely used HRS Three-Tank pasteurizer brings all the benefits of the established system, but in a more compact package. It is particularly beneficial to smaller plants, farmers and those who want to increase the efficiency of an existing plant where there is limited room to expand, and in this instance also allows for a wider range of waste feedstock to be used in order to expand.”

Matt Hale, International Sales Manager for HRS