Liquid extraction of substrate from manure for efficient biogas production

Abstract

In search for both new renewable energy and an effective way to reduce emission of
greenhouse gases from farming, anaerobic digestion that produces methane is now considered
the most efficient process.
The main challenge in implementing this process in typical small- to medium-scale farming is
the relatively large investments necessary compared to the economic benefits achieved.
Transport of manure through pipes, tanks, reactors and settling tanks all add up to a cost
higher than the economic benefits for the farm. A way of reducing these costs by simplifying
the process is analysed here. Drainage of liquids from existing storage facilities both reduces
the necessary amount of construction and of the reactor volume for biogas production. The
drained substrate liquid from the storage is fed a high efficiency biogas reactor. The reactor
outlet is returned to the manure storage tank.
The test necessary to quantify the achievable amounts of drained liquid was done using a
3.4 m3 vessel, containing slightly diluted cow manure (14% diluted). A 1.4 m long common
corrugated drainpipe (80/100 mm) was used for drainage. This was wrapped in common
mosquito net in order to avoid clogging and increase the area of filtration. For observation of
scaling effects, a small lab scale feeder, (I: 1000 regarding the vessel, the scaling of the filter
area is I: 17.5) with a subsequent reactor was used in addition. In both cases peristaltic pumps
were used to assure the necessary flow accuracy and provide a vacuum suction of 0.1 bar.
Maximum flux of drained liquid achieved though the drainpipe was 341 L/m2
.d in the pilot
vessel and 48 L/m2
.d in lab scale, implying that the proposed biogas process based on liquid
feed extraction from manure is feasible.