Opportunities and prospects of biorefinery-based valorisation of pulp and paper sludge

•Paper sludge varies widely in chemical composition with feedstock and pulping.•Paper sludge valorisation reduces industry waste and has a negative feedstock cost.•Paper sludge valorisation in a biorefinery context can improve carbon conversion.•Integration of bioprocesses with thermal process to improve the energy yields.•Techno-economic assessments to determine the profitability of process options. The paper and pulp industry is one of the major industries that generate large amount of solid waste with high moisture content. Numerous opportunities exist for valorisation of waste paper sludge, although this review focuses on primary sludge with high cellulose content. The most mature options for paper sludge valorisation are fermentation, anaerobic digestion and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as integrated biorefinery. The objective of integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation and value addition. Assessment of selected processes for biorefinery varies from a detailed analysis of a single process to high level optimisation and integration of the processes, which allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a roadmap of technologies that can generate economic benefits, and reduce carbon wastage and pollution load.