Roadmap for valorization of the digestate from a biogas plant

Abstract

Digestate is the major product coming from the anaerobic digestion process, therefore, valorization of this stream is crucial for any biogas plant to be viable. Depending on digestate characteristics and local conditions, digestate might be a valuable by-product for agriculture, energy production, and other industrial applications. How to improve the quality and/or the value of the digestate in the market can be difficult to define since many variables are involved. In this thesis, a methodology (roadmap) was developed for selecting the valorization route for the digestate from a case study. The roadmap included: 1) Pre-treatment of raw digestate, 2) Decision tree evaluation of available uses and treatments, 3) Local conditions and demand of products in the market, 4) Definition of potential valorization routes, and 5) Calculations. The methodology was applied to full extent from steps 1 to 4. In step 5, the cost evaluation was not included.

For the case study, it was assumed a biogas plant processing only sewage sludge, producing 100,000 t/a of raw digestate, and located in the municipality of Hanko, Finland. In the pre-treatment, a centrifuge separated the raw digestate into solid (29,000 t/a) and liquid fraction (71,000 t/a). For the valorization of the products, alternatives with a final use in agriculture, a high maturity of technology TRL>6, and performed on-site were preferred over the others. Due to its high phosphorus content, the solid fraction was not valorized on site. It would be sent to an external composting facility or transported to other parts of the country with higher phosphorus requirements. For the valorization of the liquid fraction, it can be used to produce nitrogen–rich fertilizer (15,000 t/a), ammonium sulphate (2,572 t/a), or ammonium water (1,360 t/a). The areas and distances required for spreading these fertilizers were calculated for four scenarios depending on the demand of the products in the market (25, 50, 75, and 100%). A cost evaluation is needed to define the best valorization route for each fraction.

So far, the valorization of digestate has been mainly focused on overcoming technical and legislative challenges. There has been less discussion about the importance of the local market (including public acceptance). Not clarifying this aspect since the early phases of a project can result in unrealistic conclusions and unnecessary work. No biogas plant can predict and control all the factors affecting the valorization of digestate during all the operating years, but having a good understanding of the parameters considered in the roadmap might increase the chances to find a solution that is viable in the long-term.