Power from Poo

It’s really amazing when you think about it. Electricity from cow poo! But it’s so much more than that.

We’d heard about farm anaerobic digesters in Germany 10 years ago. The idea seemed great and when they started building them in the UK we went along to look at a few. These were all very large, expensive, had an army of engineers running around monitoring pressures, temperatures, feed rates, gas flows, inputs and outputs of all kinds of things and required huge amounts of crops and/or industrial food wastes to keep them going. Wilma had a fit when she discovered that the monitoring equipment extended to the bedroom so that alarms were heard 24/7.

This was not what we wanted.

If we were to build one on the farm it had to be cheap, simple to operate (farmer friendly – me!) and just use the farm wastes, so that we’d have control of everything that went in and came out.

All the AD industry people said that to be financially viable – that is, pay for itself in less than 10 years – the minimum size was 150kW. But these cost over a million and required as much silage as the total we needed for our cows. Cows or AD? Food or energy? Then there was the 3-phase connection to the grid which could cost as much as the AD.

That was until we met Marches Biogas from Shropshire. Things must have been quiet at that time for the AD industry because they spent quite a lot of time with us developing an AD system to suit our requirements and then gave us the technical support to build it.

That was 3 years ago and I’ll not bore you with the details but it took us until this winter to get the thing to work ‘properly’ (not helped by the fact the AD company has re-focussed on much bigger ADs and are flat out installing them, and the one man band we bought the engine from has gone bust - so no technical support for us!)

But we’re just about there. The green-topped tank (that Jim and Abe clad with insulation and timber) is gradually filled with the mix of slurry and silage waste – now known as ‘feedstock’ – and the naturally occurring bugs in the slurry feed on this mix in the absence of air (anaerobic) to produce, mostly, methane gas which is piped to the engine that has been converted to run on this biogas (you can see the steam from the engine exhaust pipe in the photo). This engine drives a small (25kW) electric generator and the hot water from the engine heats the wash water for the dairy as well as warming the digester store to help the bugs feed, multiply and produce gas. Great!

But that is not all.

The liquid we draw out after digestion is a much better fertiliser and is far less polluting than the raw slurry we put in. And it doesn’t smell!! Not only that but an independent consultant looked at the figures and reckons it will have paid for itself in less than 7 years. So it’s good for us, good for the farm, good for nature and good for neighbours and visitors.