Closed composting system for local compost production

05 September 2022 - Published by Hanne Lakkenborg Kristensen
Written by Esther Hessel. In Denmark the Startup ComFerm ApS is developing a closed and automatic composting facility; the ComCrop-komposttromle. Or in English a compost drum.

The ambition of the easy to control system is that growers or grower groups can make their own compost on local or company scale. And at the same time save otherwise leached nitrogen as a concentrated fertilizer. The ComCrop facility is a 100% closed system where local waste streams are upgraded to soil improvers and high quality by-products as CO2 and heat for glass house cultivation, clean water and ammonium fertilizer. ComFerm ApS, Aarhus University (SOILCOM-ComCrop project collaboration) and Aartsiderne A/S are working together to deliver the starting material, to measure the characteristics of the different input and output streams, and gain experience and knowledge about this closed composting system.


Controlled system with zero emissions

The starting material for the composting process is brought into a drum. This material for composting in the ComCrop facility can be from different origins. At the moment the vegetable waste of the organic vegetable company Aarstiderne is used. Straw pellets are added to the vegetable waste to gain a C/N ratio of the starting material of 30 and a proper moisture content. Lower C/N ratios will be tested later as part of the science undertaken in collaboration with the university. During the year the composition of the vegetable stream is changing, depending on the type of vegetables grown in that season. For example, in winter more cabbage leaves are in the input material. In the future also horse manure or other animal manures could be part of the input. Special focus will be on green manure crops which with proper use in the system can yield all the nitrogen needed for organic crop rotations.

When the material is in the drum, the composting process starts. The drum rotates constantly at very low speed and the material is composted in 4-7 days. Material is added and discharged continuously. The process is controlled by measuring among others the levels of CO2, O2 and temperature.

After this ‘high speed’ composting process a fresh compost is the end result. A compost which can be used directly, depending on the needs of the field and crops. Or an after-composting step can be added, were the fresh compost is further composted to obtain a mature product. The targeted applications will be further investigated in soil and vegetable experiments.

What makes the system very interesting is that all the by-products of the composting process are collected. Where in outdoor composting facilities heat, CO2, ammonia, methane and nitrous oxide is lost to the air, in this case CO2 and heat can be used in glass houses.  Another interesting product is the water with high concentrated ammonium. Which can be used as an ammonium fertilizer. And no methane or nitrous oxide emits.


Measuring compost quality

At the moment the ComCrop facility is a prototype, which is still developed and optimized. Crucial is to control the process. Therefore ComFerm and Aarhus University are working together to measure both the quality of input streams as well as the quality and characteristics of the output products. Knowledge about the characteristics of input and output will lead to an optimal closed composting process. Aarhus University did already some germination tests using different levels of the ComCrop compost application.

ComCrop is a very interesting concept, were all the ambitions on regional and EU scale are coming together; the re-using of local waste streams in a circular economy, soil improvement and the use of organic fertilizers. And a closed system where by-products are used and zero emissions. The ComCrop project is funded by the national programme Organic RDD5 under GUDP and the SoilCom project is funded by EU through Interreg North Sea Region.

More information about science in composting? Contact Hanne Lakkenborg Kristensen,

Information about the ComCrop facility? Contact ComFerm ApS, Pia Strunge Folkmann,