Pig and abattoir slurry for biogas

Faced with increasing pressures on their handling of pig and abattoir slurries, a farming family found the solution in a modern biogas unit producing electricity for the National Grid and a high nutrient value liquid for applying to land.

“It is really our farm diversification and also providing a service to the local community,” says Andrew Rennie, who farms with his father John, and his mother Monica.

“For the farm it is closing the circle in that we grow cereals to feed our pigs that produce the slurry that goes through the biogas unit to give us ‘fertiliser’ to grow the cereals. In the process we are also producing and selling pigs and electricity.”

Their Gask Farm on the outskirts of Turriff, Aberdeenshire, is a 700-acre arable unit also carrying a pig unit based on 260-270 hybrid breeding sows with offspring finished for pork.

Cropping includes 120 acres of Bravour oilseed rape, 120 acres of Riband and Robigus winter wheats, with the balance down to winter and spring barleys.

The winter barley is a mix of varieties, including some blends and split equally between two- and six row crops. Spring barley is Braemar. Apart from the oilseed rape, all crops are grown for home feeding using a mobile mill-mix service.

The farm also has an extensive agricultural contracting business, while John ‘keeps out of Andrew’s way’ by running an agrochemical distribution service for a local agent.

“We have used our own pig slurry on the land and taken slurry from local abattoirs for some years. The abattoir slurry was always applied using deep injection techniques.

“We were finding ourselves coming up against increasing legislation. This was particularly the case for the abattoir slurry since BSE, and it was clear that we would have to find a way of processing it if we were to continue to apply abattoir waste to our land.

“To add to this there seemed to be amendments to the various regulations coming through every few months so we were continuously having to reassess the situation.’’

The idea of a digester and biogas plant started about four years ago. Apart from providing a means of treating the slurries, says Andrew, it also meant being able to make the best use of the plant nutrients in the slurries.

“We had seen a small advertisement for biogas units in a pig magazine and decided to find out more. One question was the siting of a new unit. Ideally, we would have liked to build it alongside the pig unit so that it could take slurry direct from the existing lagoon, but because of handling abattoir wastes it had to be placed some distance away.

“Another consideration was that it had to be near National Grid power lines in order to feed electricity into the system.

“Although pig production is going through a difficult period, we have no plans to close our unit. However, the viability of the plant is not dependent on the slurry from our pig unit. All that is needed is a small amount of pig or cattle slurry to ‘feed’ the digester, otherwise it would operate perfectly well on the abattoir waster alone,” said Andrew.

Andrew and John went to Germany to see biogas plants under construction and fully operational before deciding to go ahead with their own.

In September 2006, the concrete apron for the plant was laid, a team from Germany then installed the unit and the farm started up the digester in December that year.

Andrew admits it does take some time to bring the digestion process up to temperature and ready to accept slurries but the unit came into production in March last year.

“We deliberately chose to buy a complete package from a single supplier as we wanted to minimise any problems and to make sure that we had good support if, and when there were any problems,” says John.

The unit has three 100cu.m capacity reception pits continuously feeding the 2,245cu.m capacity digester. The digester is fed 10cu.m of various slurries five times each day. The insulated digester unit runs at 38.5degC with new material coming in at about 18 per cent dry matter.

Newly introduced material rises to the top of the digester, gradually migrating to the bottom during the digestion process, which takes about 45 days. The resulting fluid containing about 3 per cent dry matter is then drawn off from the bottom of the digester.

During the digestion process gasses are produced and rise to the top of the digester. These are 67 per cent methane, 32 per cent carbon dioxide, and a small amount of other gasses. The methane is used to power two 230kW engines, which in turn powers an electric generator. A smaller engine acts as a backup unit.

About 85 per cent of the electricity generated goes into the National Grid, the rest being used to meet the needs of the biogas unit itself.

The engine is water-cooled with hot water from the engine being passed through a heat exchanger in a pasteurisation plant treating fluids drawn from the digester plant. This is a legal requirement because of the use of abattoir waste.

“The whole operation is fully computerised and needs only about half an hour each day checking everything is working well. If there is a problem a signal is sent to a mobile phone and the problem can be identified remotely using the house computer or a laptop. Usually it can also be solved remotely as well,” said John.

“The liquid from the unit is high in nitrogen so suits winter crops though we will use some as a top dressing on spring crops.

“The liquid is analysed for plant nutrients and this is matched to soil analyses and crop requirements to make best possible use of the nutrients available.

“A 5,000cu.m capacity covered store for the liquid from the digester gives flexibility in timings of applications, again enabling optimum benefit to be gained from nutrients. This gives us major savings in bought-in fertiliser costs.

“We use a variety of methods for spreading the liquid, though it is well suited to umbilical systems,” says Andrew.

“Overall, the biogas unit works extremely well. There is an art to getting the best out of the system, including the right mix of pig slurry and a little dry matter to feed the digester.

“The unit has been designed to allow for future expansion, but there are no plans to increase the capacity of the plant in the foreseeable future.

“The main limiting factor is the amount of ground we have to spread the liquid from the plant and we estimate that any worthwhile expansion would need 2,000 acres rather than the 700 acres we have now.’’

The biogas system earned Andrew the 2007 NFU Scotland innovation award and the unit also won a ‘highly commended’ in the Renewable Energy Association awards.

Apart from a small Scottish Government diversification grant, just over 1 per cent of the capital cost, the unit was funded through the farm business. The unit has a SEPA PPC part A licence.

“Pig production is going through a difficult period. We are losing about £20 per pig sold at the present time, but we like our pigs and they fit in well with our overall farm system so there are no plans to drop them,” says John.