Closing the loop in bioproduction: Spent microbial biomass as a resource within circular bioeconomy

Successful transition to a circular bioeconomy relies on the availability and efficient use of organic feedstocks such as agricultural and food waste. Advances in industrial biotechnology provide novel tools to valorize these feedstocks differently. Less attention, however, has been directed towards assessment of the organic side-residues arising from industrial biotechnology, such as spent microbial biomass (SMB). This study aims to reflect the current state of SMB within bioeconomy and create awareness of this growing industrial resource. Data from a range of published fermentation processes is used to estimate the amount of SMB formed per product (weight per weight, wt/wt) across different types of bioproducts, namely organic acids, alcohols, polymers, amino acids, antibiotics, protein and vitamins. Varying amounts of SMB are generated depending on the bioproducts and bioprocess, where bulk bioproducts, e.g. alcohols, generate less SMB than bioproduction of high-value low-volume specialty products, e.g. vitamins. It is estimated that more than 50 million tons of nutrient-rich SMB was generated in 2013, with SMB from bulk and specialty bioproduction accounting for roughly equal amounts. Furthermore, the composition of six industrially relevant organisms is summarized and compared, highlighting the general features of SMB as a carbon-rich substrate mainly consisting of protein. The results indicate that SMB is a growing resource with a reliable supply and predictable composition. The predictable nature of SMB could make it a favorable substrate for further innovation in industrial applications and nutrient circulation within the bioeconomy, for example, by using it as a co-substrate for valorization of other biomasses.