Algae as a Feedstock

Algae and Seaweed: The prevalent species are the green seaweed common eelgrass – Zostera marina, the brown seaweed bladder wrack – Fucus versiculosus and the brown algae – Pylaiella littoralis. They are marine species, which grow on available substrates and provide many services for the ecosystem. Macroalgal roots serve as natural erosion protection for the sediment and have high importance as food and breeding habitat. Mats of macrophyta are able to float over great distances and accumulate at the water-line on beaches.

Algae can absorb, therewith purify and remove nutrients from the surrounding water volume. They can take up excess nutrients and unbound elements and, in this regard, multiply their biomass quickly. Worldwide the excessive growth of macroalgae is occurring as a sign of the eutrophication phenomenon. In the process high amounts of nitrogen (N) and phosphorous (P) are consumed from environment in which the algae are located. This characteristic is excellently applicable for the selected uptake of element surplus from the environment.

Based on currently applicable techniques seaweed (macroalgae) is collected along the coast line on the beach (onshore) and in the shallow water nearby the beach (offshore). First, the algal biomass is pre-treated via sieving and washing. The pre-treated material can potentially be fermented (co-digested) with other residual material in an oxygen free (anaerobic) decomposition (digestion) procedure at a biogas plant or wastewater treatment plant. Biogas can be used to fuel a combined heat and power unit or upgraded to biomethane. Depending on the season and region, contaminant thresholds in algal biomass may be too high. In such cases other applications can be found for the organic residues.

Collection and Transport

Collection: A comparative analysis has been made of seven different collection techniques. Reports and experience suggest that a tractor with a grating bucket should generally be preferred. This technique can be used to collect seaweed from shallow waters (up to 1 meter) and from the near shore sandy beach. This technique is estimated to be applicable to 70% of the coastal areas where seaweed can be collected. The other techniques can only be used to a lesser extent. Pontoon machines can be used to collect seaweed in port areas, which are only estimated to cover 5% of the total collection areas in the five partner countries.

Sand content of around 50% is common. Some municipalities have a system of visual control for determining if the algae can be sent to the biogas plant. They allow the collected piles to dry for a day or two and then check the colour. Lighter piles have a higher sand content and darker piles are better for the biogas plant. Piles with high sand content can be re-washed in the water and/or pre-treated in order to remove as much sand as possible.

Transport is typically managed by dump trucks or similar vehicles. It is, therefore, better if the biogas plant is located closer to the coast.

Pre-treament

Pre-treatment at Solrød biogas plant

Solrod has been processing algae at its plant since 2014. The plant has equipment to pre-treat the algae and remove as much sand as possible.

If you are interested in further background information on the Solrod biogas plant you can find it here.

Other options for pre-treatment (not yet realised at industrial scale)

- Sand separation is more effective in an acidic solution (pH 2)

- Mechanical grinding of marine biomass reduces the efficiency of sand separation in both acidic and neutral solutions

- Hydrothermal pre-treatment shows the highest biomethane yield increase compared to untreated seaweed and pure cattle slurry (12-27%).

- Biomethane yield achieved for acid pre-treatment did not far differ from pure cattle slurry. The change is in the range from -3.7% to -0.2%.