Anaerobic Co-digestion of Cast Seaweed and Organic

Collection and quantification of cast seaweed
In framework of this project historical data related to annual collection rate of cast seaweed
was performed at Solrød municipality. The cleaning of the beach is performed three times a
year during the period from 1st of May to 1st September. The first cleaning begins about 1st of
June and ends just after Sankt Hans (23th June). The second one is performed during a week
in early August, and final cleaning at the end of August.
Two collections were performed at Solrød beach and Skodsborg beach during the time frame
of this project. Both took place during the months of June in 2014 and 2016. During 2015 was
not possible to perform a collection at Solrød beach as we were informed by the representative
of the Solrød beach homeowners that a shortage of seaweed took place. To overcome this
problem, cast seaweed from other locations in Denmark (already in stock at our lab facilities),
namely, Hanstholm and Skive were used. Unlike the samples collected at Solrød beach, these
samples were collected manually a mainly consisting of Laminaria digitata and Ulva lactuca,
respectively.

Based on cast seaweed volumes treated in Solrød Municipality in 2009, where the amounts
collected on a 3.7 km stretch corresponded to approximately 4,000 tonnes, an available quantity
of 1081 tonnes yr-1 km-1 beach has been estimated4. However, more precise data on annual
collection of cast seaweed were obtained directly from Solrød Municipality. This annual average
collection of cast seaweed corresponds to 1,500 tonnes yr-15. However, considering the entire
Køge Bay (38.6 km of coast line) an annual average collection of cast seaweed has been
estimated to be 42,000 tonnes yr-1 which corresponds to 1 141 tonnes yr-1 km-1 of coast line.

Technologies for cast seaweed collection
Currently, cast seaweed at the Solrød beach is collected and cleaned by means of a back-hoe
with big shovel in front, with a loader tractor, and with a special beach cleaning machine. Algae
waste is removed from the beach with a dump truck and put in a temporary place where they
stay for a few days and get drained there. Afterwards, the algae waste is taken away from the
beach by trucks. Data on this technology respect to fuel consumption, working hours, type of
fuel were retrieved so that can be included in the LCA.

Physicochemical characterization of cast seaweed
Samples collected at Solrød and Skodsborg beaches mainly consisted of debris, a considerable
amount of sand, Zostera marina (eelgrass), Pilayella littoralis and Ectocarpus (filamentous
brown algae). After sorting and cleaning the cast seaweed, a complete characterization was
performed including the samples previously collected at Hanstholm and Skive.
Results from the physicochemical characterization showed that C:N ratio of cast seaweed were
out of the range (C:N 20:1-30:1) for optimal anaerobic digestion process; particularly the cast
seaweed collected at Solrød beach which exhibited the lowest C:N ratio (17.8:1). This potentially
can lead to problematic digestion due to excess levels of ammonia nitrogen which may
inhibit the methanogenic community. Therefore, co-digestion with dissimilar substrates is
highly recommendable to shift the C:N ratio.
Another important ratio that needs to be taken into account, particularly when seaweed is
considered as potential substrate for biogas production, is the C:S ratio. A substrate with a
C:S below 40 will tend to have larger accumulations of H2S gas as experienced by seaweed
digestion trails in other studies. Results from characterization showed a C:S ratio of cast seaweed
collected at Solrød below 40; therefore, problems due to inhibition caused by H2S could
be expected.

Another important result is the quantification of metals and nutrients (K, N, P) in the cast
seaweed samples. Cadmium content has been relatively constant in previous years (in the
range 0.50-0.80 mg kgTS-1)6, however, it has been shown that the content in recent years
were somewhat higher than 1 mg kgTS-1. This fact coincides with the cadmium content report
in this study (1.47 mg/kgTS). As also observed, the sample taken in January 2009 also exceeded
the maximum level for cadmium. On the other hand, the cadmium content for samples
collected at Hanstholm was lower of the limit of detection.