| Energy
Agri-Tech East farmers optimising use of Anaerobic
Digestate
Farmers, NIAB & Cranfield University to improve nutrient use efficiency.
e want to get back to the soils
of 20 years ago,” says Patrick
Allpress, Farm Director at
Allpress Farms Ltd in Chatteris,
Cambridge. As part of a
consortium of six farmers led
by Agri-Tech East, Patrick is undertaking field
trials to optimise the use of liquid and fibre
‘digestate’ (organic fertiliser) produced as a by-
product from Anaerobic Digestion (AD).
With funding and support from the Innovative
Farmers’ network, Agri-Tech East has set up a
field lab, using high tech tools to measure the
impact of digestate. The farmers will be
investigating how to ensure the maximum
amount of nitrogen is available to the crop and
the best way to prevent it escaping as ammonia
gas.
AD has become an attractive technology for
many farmers to generate energy from waste
products. It is a controlled microbial process
where organic materials are broken down into
organic compounds in the absence of oxygen.
Digestate is the material remaining after AD,
alongside biogas. Digestate has three forms:
whole (similar to livestock slurry), liquor (the
whole digestate with most or all solid matter
separated) and fibre (similar to compost – the
separated solid material).
Allpress Farms invested in a 500kW AD plant
in 2014, feeding it 50% onion waste and 50%
leek waste. The plant is fed 12,000 tonnes of
this feedstock per year, producing around
11,500 tonnes of digestate. In one year, the
plant can produce a potential 4.4 million
kilowatt hours, which is roughly enough to
supply electricity to 900 houses.
“The first point is getting the value out of
digestate – we know what it costs, but
measuring the value of it as a farm input is
difficult,” explains Allpress. “By doing the trials
we [Allpress Farms] hope to use digestate like
a regular fertiliser; an alternative to inorganic
fertilisers.”
W
www.farmingmonthly.co.uk
The project originated from discussions at
the Royal Norfolk Show, as many farmers with
Anaerobic Digestion (AD) plants wish to
maximise the value of the by-products of AD.
“The farmers in this field lab want to find
ways to ensure that the maximum proportion of
nitrogen is available to crops,” says Liz Bowles,
Head of Farming at the Soil Association – a
partner in the Innovative Farmers network. “The
desired outcome is to find techniques which
allow this to happen – for example, acidification
might be helpful in reducing the pH of the
digestate.”
Nitrogen, in the form of nitrates can be lost
through the soil profile or through volatilisation
(as ammonia) and lead to diffuse pollution if
preventative measures are not employed.
Cranfield University will be evaluating
laboratory scale trials through various
acidification options that can be used to
mitigate pH levels and control loss of nitrogen.
Dr Ruben Sakrabani, Senior Lecturer in Soil
Chemistry at Cranfield University says:
“Optimisation of digestate NUE (nutrient use
efficiency) in crops will not only provide a
solution for soil health but also food production
and minimise environmental pollution.”
Andrew Blenkiron, Estate Director at Euston
Estate in Suffolk is also taking part in the trial.
He is set to speak at Agri-Tech East’s annual
REAP conference this November.
He says: “It is a big challenge to work out
the economic benefits of spreading 30,000
tonnes of organic material from the anaerobic
digester, compared to the cost of artificial
applications - but I know that the organic
material will have a longer-term benefit on soil
health.”
In addition to cutting-edge scientific input
from Cranfield University, NIAB are also lending
their expertise.
“Farmers in the East of England are among
some of the most innovative in the country and
NIAB is working with a group to maximise use
of digestate on farm,” says Dr Lydia Smith,
Head of NIAB Innovation Farm.
“In this field lab, NIAB aims to help them take
this a step further and to use this resource in
combination with other strategies to improve
soil quality and hence function. We will draw on
the results of several long term studies at NIAB
including use of cover crops and interactions
within the vital soil microbial community.”
Cover crops are of great interest to the
group, as they help reduce soil erosion,
improve structure and composition, retain key
nutrients and water in the soil and