Cambridge University’s Building Energy Manager Adam Fjaerem will be joining us at the #FarmingCleanLeanGreen event on 11th May to talk about the AD plant built at Park Farm, which was fully operational in August 2021

The University Farm milks 220 cows with approx. 80 calves and heifers. The slurry from the milking unit feeds the AD plant to generate power to fuel general farm operations as well the many electric vehicles on site. There is a desire to move all farm machinery to electric as technology advances.

Adam has lots of advice, support and lessons learnt from managing such a project and we took the time to chat to him to find out a little bit more about the AD plant.

As part of the demonstrations during the #FarmingCleanLeanGreen event, Adam will be demonstrating how electric is produced from the AD plant and can power anything within reason. As a visual aid, we’ll have some cars ready to charge as well as some battery packs – proving how cow slurry is providing power to run a sustainable and environmentally mindful farming system. (poo into power!)

What was your main driver for the project, energy/ carbon saving or helping with the slurry problem?

All of the above to be honest. I wanted to build the AD plant as it is an all-electric farm which is only going to increase its electrical consumption as we electrify vehicles and machinery. To generate our own power will reduce the carbon from using grid based electricity and to use the slurry twice helps the farm with its circular economy agenda. Plus, to reduce the methane emissions into carbon emissions is a big win and gives the farm valuable lessons ahead of the need to cover all slurry tanks as part of the clean air act.

How much slurry (m3) is produced on a typical day and is it this fairly constant through the year?

The AD plant takes 24,000 kgs of slurry from the dairy barn reception pit each day (4 x 6,000 feeds) and this pretty much balances the slurry production. Any more than this is automatically pumped into the Heifer slurry tower. We are in the process of diverting the rain drainage so that dirty water i.e. nothing but rainwater from the yard is taken to a third tower to spread all year round.

Does the water content of the slurry have to be reduced and if so how is this achieved?

No, the initial tests suggested that our slurry content contained too much water and that we might have to process it but this hasn’t proven to be the case. This is thought to be down to increasing the size of the scraper slots at the end of the barn so they don’t need to be washed down as much and also (I think) that the length of the slurry supply pipe (approx. 120m) means that the slurry gets dried out between the reception pit and the reactor. Once we have the dirty water separated this might create a problem as rain water is currently getting into the slurry and it is noticeable that there is a reduction in generation following heavy rain.

How much biogas does the AD produce on a typical day (m3) and do you have any figures available for it’s calorific value?

The calorific value was tested at the start but my gut feel is that it changes a lot throughout the days/month/weeks. Volume wise the buffer (the dome) of the AD plant needs to contain biogas between 350cm and 500cm for the engines to work – this varies throughout the day but is usually >420cm which means both engines are running. This drops at night with the colder temperature of the last few months so that one engine usually stops and restarts in the morning.

How many hours do the electrical generators run on a typical day, or what is the typical equivalent output (eg 44kW x 12hrs)?

One will be running 24/7, the second usually about 14 hours a day. This does depend on the amount of rain, the internal temperature of the reactor, the feed the cows are having (four days of them not getting maize reduced output for weeks).

A Youtube video was produced during the build and can be viewed here: