New report from COASTAL Biogas Project reveals that anaerobic digestion of cast seaweed leads to multiple socioeconomic benefits

By | 2021-12-16

The objective of the COASTAL Biogas project is to provide solutions based on anaerobic digestion of cast seaweed to coastal regions to tackle eutrophication, contribute to the transition to a circular bioeconomy and improve prosperity. The project revolves around anaerobic digestion and utilisation of the digestate as an organic fertiliser which brings several socio-economic benefits to the region where the solution is implemented.


Eutrophication has both ecological and social consequences and is a major environmental threat to the ecosystem in the Baltic Sea. The excessive inflow of nutrients to the Baltic Sea creates a viscous circle where the rapid growth of microalgae results in algae blooming. At the end of the blooming, the algae sink to the seafloor and consume oxygen during their decay. The anoxic condition in turn results in the release of phosphorous that has been bound in the seafloor sediment, providing more nutrients to the algae. The result is oxygen completion and dead zones. A good summary of the effects of eutrophication can be found on the HELCOM (Baltic Marine Environment Protection Commission – Helsinki Commission)

Despite different measures to reduce the inflow of nutrients, 97% of the Baltic Sea is affected by eutrophication according to HELCOM. The COASTAL Biogas concept attacks the problem of eutrophication from the other end, by physical removal of nutrients from the sea. Cast seaweed is collected and transported to an anaerobic digester where the nutrients are recovered, and biogas produced. The digestate, with all the nutrients, is used as an organic fertiliser, offsetting the use of synthetic fertiliser. In this way, the nutrients loop is closed, and nitrogen and phosphorus are continuously removed from the sea.

Solrød Biogas plant in Denmark has implemented the COASTAL Biogas concept on an industrial scale and takes advantage of the socio-economic benefits associated with anaerobic co-digestion of cast seaweed and utilisation of the digestate as an organic fertiliser. In 2019, the Solrød Biogas plant processed 1,522 tonnes of cast seaweed.

Socioeconomic benefits


Reduced eutrophication

Even if external loads of nutrients that enter aquatic ecosystems may be limited, nutrients have accumulated over many years in the sediment at the seafloor. A self-perpetuating process continues as internal loads of stored nutrients are repeatedly released into the water, where they feed the renewed growth of plants. Removal of cast seaweed from the Baltic Sea beaches implies that nitrogen and phosphorus are physically removed from the Baltic Sea. It is difficult to give an exact number how much nitrogen and phosphorus are removed, because it depends on the type of seaweed and how long it has been in the water and on the beach before being collected.

By processing the cast seaweed in an anaerobic digester, the nitrogen becomes more available for the crops and less nutrients are returned to the sea through leaching and run-off. The anaerobic digestion also makes it possible to store the nutrients as digestate until it is time to put the fertiliser on the field.

Reduced smell

The decay of cast seaweed creates problems for inhabitants and tourists due to the smell. It hampers recreational opportunities in the area and lower the value of residential properties along the coast. The characteristic smell of rotten eggs comes from the hydrogen sulfide (H2S) released during the decay. Hydrogen sulfide is a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. The toxicity of H2S is comparable with that of carbon monoxide. It binds with iron in the mitochondrial cytochrome enzymes, thus preventing cellular respiration.

Reduction of spontaneous methane emissions

When the seaweed decays on the beach it releases methane. Methane is a strong greenhouse gas with a Global Warming Potential of 86 in a 20-year perspective and 34 in a 100-year perspective. That is, methane is 86 times stronger greenhouse gas than carbon dioxide calculated over 20 years. When the seaweed is collected, these emissions are reduced significantly. Findings from Roskilde University indicate that 4 tonnes of spontaneous methane emissions are avoided when 1,000 tonnes of wet and sand free seaweed are collected and treated in an anaerobic digester. In total 355 tonnes of CO2-equivalents are avoided calculated in a 100-year perspective. This figure includes displacement of fossil fuel by the produced biogas, displacement of synthetic fertiliser by the digestate and avoided methane and nitrous oxide losses from decaying seaweed.

Organic fertiliser

The removal of 1,000 tonnes of wet and sand free seaweed implies that approx. 8 tonnes of nitrogen and 200 kg of phosphorus are removed from the sea. The nitrogen and the phosphorus remain in the digestate, which can be used as an organic fertiliser, offsetting the use of synthetic fertilisers. The digestate improves the possibility for organic farming where synthetic fertilisers are not allowed. The anaerobic process also contributes to make the nitrogen more accessible for plants compared to the direct use of seaweed as a fertiliser.


The biogas production from the collected seaweed depends on how fresh the seaweed is and which type of seaweed it is. The fresher, the higher biogas yield. Findings from Roskilde University indicate that anaerobic digestion of 1,000 tonnes of wet and sand free seaweed gives rise to approx. 36,000 Nm3 of biogas.

Local value chains, regional development and new job opportunities

The Corona pandemic has made all of us aware of the need for local and resilient energy and food production systems. Turning local low-quality biomass and waste into a high-quality fuel and organic fertiliser creates value for the local society and the region. Some jobs are directly related to the anaerobic digestion of cast seaweed, such as collection and transport of the seaweed, operation and administration of the anaerobic digestion facility and transport of the organic fertiliser, while others are of a more indirect character such as increased tourism, increased value of residential properties, a more attractive place to live and hence the need for more services.

Transition to a circular bioeconomy

The COASTAL Biogas solution is fully aligned with the concept of the circular bioeconomy. The anaerobic digester is the centre piece, which recycles the nutrients and the solar energy stored in the organic material without creating new waste streams.

Fulfilment of local climate plans

The EU Covenant of Mayors for Climate & Energy brings together thousands of local governments voluntarily committed to implementing EU climate and energy objectives. The Covenant of Mayors was launched in 2008 in Europe with the ambition to gather local governments voluntarily committed to achieving and exceeding the EU climate and energy targets. Solrød Municipality, which is part of the Covenant of Mayors initiative, has worked strategically with climate action plans since 2010. The last 10 years, the reduction of greenhouse gases amounts to 59,190 tonnes of CO2 equivalents. The largest impact is attributed to the Solrød Biogas plant, where cast seaweed is co-digested with industrial residues and manure. The biogas plant was implemented as means to handle the cast seaweed collected at Solrød beach according to Preben Larsen, board member of the Solrød beach cleaning team.

For more information on the COASTAL Biogas Project, please visit: 

You can also download the full report COASTAL Biogas – a Swiss army knife of socioeconomic benefits below.