Mid Sweden University

The current economic system is not sustainable, as it is based on a linear flow of resources from extraction, production, use and disposal. The linear flow of resources must be replaced with a circular flow where waste is seen as a resource. The Circular economy has been suggested as a vision for how to organise production and consumption by reducing waste and supplying sustainable resources. In a circular economy materials should be kept at their highest utility and value at all times. As an example, wet, non-solid waste such as sludge from wastewater treatment facilities for household and industry sewage contains valuable resources. However, it is a challenge to valorize because of its low energy value, content of potential pathogens, pesticides and heavy metals. Pulp and paper bio-sludge (PPBS) is an example of this kind of difficult waste for which the current management methods only recover low values such as energy and compost. Thus, the discovery of sustainable ways to maximise the value of PPBS is needed.Ranking of valorization methods for PPBS was done based on assessment of technology readiness level, degree of circularity, PPBS characteristics and economic drivers (a top-down perspective).

The ranking procedure aimed at finding circular valorization methods with high potential in the mid-range of technology readiness level. Producing protein from PPBS was the highest ranked valorization method. A novel recycling method to produce protein from waste material is conversion of the waste by insects.

Valorization of PPBS into protein by Black soldier fly larvae (BSFL. Hermetia illucens) was assessed by lab scale rearing. However, the nutrients in PPBS are not readily available to the larvae. Simplexmanipulations such as adding reference diet leachate (nutrient solution) mixed into the PPBS or as free liquid surface did not significantly increase larvae weight gain or bioconversion rate. It is clear that low nutrient availability, the occurrence of toxic substances such as pesticides and other organic toxins, toxic elements such as heavy metals and pathogenic and/or competing microbes inhibit the growth of BSFL. PPBS therefore needs pre-treatment to improve its feasibility as feed for BSFL.

Pre-treatment of PPBS by fermentation dissolves a part of the PPBS and dissolved substances in the fermentation liquid are readily converted into larvae biomass. However, the bulk of the lignocellulose is not dissolved, thus most of PPBS nutrients remain unavailable for growth of the larvae. The BSFL did not grow well and pre-treatment of PPBS by fermentation did not improve its feasibility as feed for BSFL.

Hydrothermal pre-treatment of PPBS reduces microbial occurrence and increases nutrient availability. The growth of the larvae improves which leads to increased weight of BSFL thus the PPBS’s feasibility as feed for BSFL improves.