Biogas production also produces domestic fertilizers

The use of recycled fertilizers is a pro-environmental act, as the already recycled nutrients are utilized again in agriculture. By using recycled fertilizers, farmers are able to lower their fertilizer expenses by 30–40% in addition to improving the condition of the soil. Produced during the production of biogas, Gasum is already providing solid and liquefied digestate fractions to about 700 farmers as recycled fertilizers.

When recycled fertilizers are the topic of discussion, it usually brings to mind the recycling of the nutrients that are in fodder back into the soil as manure. The waste sludge that is produced as a by-product of biogas production can be used as recycled fertilizers, instead of traditional fertilizers.

During the production of biogas, the raw material's nutrients are transformed to better suit the requirements of organic plants. At the biogas plants, the refined and hygienized digestate is centrifuged into liquefied or solid sludge.

"The liquefied digestate that is born during the process, is an excellent fertilizer that has less noticeable odor, no hazardous microbes nor, for example, weed seeds. The quality of recycled fertilizers is also monitored carefully," says Ari Suomilammi, circular economy, Finland, Biogas, Gasum.

All approved fertilizers receive a certificate from the Finnish Food Safety Authority (Evira) based on analysed samples that have been collected from the plants. The certificate states what raw materials were used in the production, their pH-value, the proportion of organic matter, as well as the proportions of primary and micro nutrients. The certificate also shows the hygiene-level of the fertilizer and toxic metal concentrations that must be less than very strictly defined maximum levels.

Gasum already has about 700 farmers as customers

Gasum is Finland's largest company that refines societal waste sludge through biogasification. Currently, Gasum has about 700 farmers as customers, who receive fertilizers produced by the company.

Gasum's annual production is some 400,000 tonnes of liquefied fractions, an average of approximately 570 tonnes per customer. The modern technology used in slurry tankers ensures that the hectare dosage remains steady even if there are changes in the driving speed. As the amount of distribution changes according to farmers' wishes, from 15 to 28 cubic meters per hectare, one field hectare receives 85–165 kilos of nitrogen (39–73 kg soluble) and 18–26 kilos of phosphor (12–22 kg soluble).

Gasum delivers digestate fractions free of cost to farmers that live within a 70-kilometer radius of a biogas plant. On occasion, fractions have been delivered even further away.

"Finding suitable storage containers all over Finland has been crucial for arranging transportation and costs. When possible, we have utilised existing sludge tanks, shallow manure pits and above ground silos that are no longer in use. In addition, we have also participated in building new sludge ponds. It is also important to develop storing. By building remote storage facilities, we have simplified the freighting and distributing process. However, there are still many points of improvement in the storing system," states Juhani Viljakainen, Gasum's product manager of recycled fertilizers.

Many farmers consider using recycled fertilizers to be an environmental act and argue that they should be utilized even more in agriculture. The use of recycled fertilizers is also a concrete way of developing a circular economy. According to estimates, the annual cost of purchasing artificial fertilizers has decreased by 30–40% at farms as the result of the long-term use of solid and liquefied digestate fractions. In addition, the amount of the complex organic substance, humus, on the fields, as well as the granular structure and the state of the soil, have improved.