Large amount of untapped energy in aquatic plants

How do we make better use of surplus aquatic plants in the form of unwanted seaweed on our beaches and  weed cuttings from streams and lakes? Aikan A/S has received support from the Danish Government’s EUDP program to examine the possible use of surplus aquatic plants as feedstock for the production of methane and certified compost, which can be used as fertilizer.

Most of the time, yard waste and seaweed from beaches and stream weed cuttings are composted or just disposed of on agricultural land. This results in unwanted emissions of CO2 as well as the more potent greenhouse gasses methane (CH4) and nitrous oxide (N20).

Minimizing greenhouse gas emissions

In this pilot project we demonstrate a full scale implementation of the Aikan technology to show how it can restrict unwanted greenhouse gas emissions while also transforming surplus aquatic plants into a valuable resource.

We estimate the currently available feedstock resources in the form of yard waste, easily accessible seaweed (from beach cleaning) and stream weed cuttings allow for the production af at least 15-30 million cubic metres of pure methane per year. These types of feedstock amount to 300,000 tonnes of organic dry matter, comparable to the total amount of biodegradable municipal solid waste currently available in Denmark each year.

Estimated production of 30 million cubic metres of pure methane per year corresponds roughly to the energy consumption of 18,000 households. This includes electricity, heating and hot water, based on an estimated yearly consumption of 16,500 kWh for a household with four persons.

Overcoming known obstacles

In conventional wet AD systems there are obstacles to using surplus aquatic plants and yard waste. Often sand will build up as unwanted sediment in the reactor tanks, and twigs and other large materials tend to clog the pumping systems. Using the Aikan system, in which anaerobic digestion and composting takes place in an integrated two-phase process without moving any of the solid fractions, we foresee that we will be able to overcome these obstacles.

Globally, the potential for making gas and compost out of surplus aquatic plants is huge. In the developed countries, with collection in place and demand for renewable energy and fertilizers, large-scale exploitation seems straight forward, provided that the pilot project is successful.

Projects updates

You can read our project updates below, starting with the most recent:

September 2016

We have completed the last initial methane potential analyses. We are preparing for full-scale testing. The first batch for a full-scale experiment will be a mixture of seaweed from beach cleaning in Odsherred municipality to be mixed with locally collected yard waste.

January – August 2016

We have completed the following preparatory steps:

Gathering information on beach cleaning machinery and procedures.

Workshop with participants from three municipalities (Kalundborg, Odsherred and Holbæk) about the collection, handling and processing of seaweeds (mainly from public beaches).

Methane potential analyses of seaweed, stream cuttings and yard waste (ongoing).

Step 1-11

photo documentation

of the process

1 - Unwanted surplus seaweed on one of the best beaches in Denmark.
2 - Once gathered, the seaweed is left in a pile to dry in order to minimize weight.
3 - Even when seaweed is collected using specialized equipment, it contains substantial amounts of sand.
4 - Often surplus weed accumulates in fresh water lakes and in canals as shown here.
5 - Semi-automated gathering of surplus weed in a canal using a coarse filter.
6 - Once gathered, the surplus weed is left in piles to dry.
7 - Gutweed (ulva iintestinalis], a green alga, gathered in shallow salt water.
8 - Preparation of sea lettuce (ulva letuca) for analysis of its biomethane potential.
9 - Blended sea lettuce ready for analysis.
10 - A sample of bladder wrack (fucus vesiculosus) for analysis.
11 - Blended bladder wrack ready for analysis.

Methane potentials

Prior to the full-scale experiments, we determined the biomethane potentials (BMP) of different seaweeds (Ulva sp., Fucus sp.), seagrass (Zostera marina) and yard waste. For the laboratory analyses of BMP we used an automatic methane potential test system (AMPTS II)  from BioProcess Control.

The results obtained are as follows:


BMP (m3 CH4/tonne VS)

Fucus vesiculosus (bladder wrack, brown algae)


Ulva lactuca (sea lettuce, green algae)


Zostera marina (seagrass)


Fine fraction of yard waste




Biodegradable municipal solid waste (BMSW)


VS=volatile solids = organic dry matter

The measured BMP values are comparable with values found by others. As the BMP values of seaweed and yard waste are substantially lower than the BMP values of biodegradable municipal solid waste (BMSW), we expect the gas output per tonne of feedstock will also be lower than for BMSW.


The project is scheduled to run throughout 2016 and 2017.

By the end of the project period, a report will be made available

on this website and on this EUDP webpage