The literature on monodigestion of U. lactuca is dominated by
lab scale work and even more so dominated by BMP analyses.
The Tokyo Gas Company (Matsui et al., 2007) however commissioned
the development of a large scale biogas plant which ran
for 150 days. It ran two trials digesting 2 different macro-algae,
brown algae (Laminaria) and green algae (U. lactuca). Trials for U.
lactuca operated for 70 days with a biomethane yield of between
15 and 17 m3/ton of seaweed added. The seaweed sampled had a
DS of approximately 10% (lower than in this experiment). There
was no volatile solids percentage given in these trials. The results
are similar to sample 1 (Fresh: 20 m3/t: Table 6) and sample 2
(Wilted & unwashed: 18 m3/t: Table 6) in this experiment.
However the conditions which are described fit more closely to
sample 4 (washed and wilted) which in this experiment yielded
35 m3 CH4 per tonne wet weight at a dry solids content of 31%.
At 10% dry solids content this would equate to approximately
12 m3/t. We can say that our results are not dissimilar to those
of the Tokyo Gas Company.
However it is well documented that there tends to be a reduction
in the biomethane yield when operating a continuous system
when compared to batch BMPs. A recent paper which followed on
from the work completed in the Brittany region on anaerobic
digestion of Ulva, co-digested collected Ulva with pig slurry (Peuet al., 2011). A total yield was not given between the combined
substrates but a specific yield of 128 L CH4/kg VS was given for
the Ulva by subtracting the yield from a reactor with pig slurry
only from the total biomethane yield produced from a mix of Ulva
and pig slurry (48% and 52% respectively). H2S as predicted was
present in large amounts (up to 35,000 ppm) but did not inhibit
the process. This successful digestion of Ulva provides a future avenue
of research where a co-digestion potential feedstock is dairy
slurry with Ulva. The Argideen estuary is surrounded by an intensive
dairy farming industry, which is partly to cause for the development
of the macro-algae bloom. With over 38 million tonnes of
cattle slurry produced in Ireland each year, this provides a possible
potential co-substrate to digest with Ulva (CSO, 2011).