Table 7 outlines the results of the kinetic modelling of the Ulva
for year 1 and 2 and for co-digestion of Ulva with dairy slurry.
Higher K values, (indicating faster degradation rates) were recorded
for Ulva collected in year 1 than year 2. The highest K value
(0.23/d) was associated with dried Ulva in year 1. With respect to
Figs. 4 and 5 the K values are indicative of a steeper curve and as
such a faster degradation rate, a shorter retention time and a smaller
cheaper digester. This K value may be compared with values of
0.433/d for food waste and 0.239/d for grass silage from trials carried
out by the authors (data not included in this paper).
R2 values indicate the fit of the model to the result, with an R2
value of 100 suggesting a perfect fit. D is the lag phase which represents
the time taken for the process to produce significant
amounts of methane. T50 is the half life (when half of the methane
has been produced). It may be noted that these values would suggest
relatively small digesters with retention times typically less
than 30 days. Typically substrates with large lag phases have longer
half lives and those with small lag phases have short half lives.
The ideal substrate would have a high K value, a short lag phase
and a short half life. Dried Ulva year 1 would appear to be the best
substrate.