Challenges and opportunities in tackling paper mill sludge waste

The foundation industries including chemicals, paper, metals, ceramics, glass, and cement are among the largest contributors to global emissions and waste generation. Among these, paper production generates a by-product known as paper mill sludge (PMS), with an estimated 27.5 million tonnes expected annually by 2050. This review critically evaluates current PMS management practices and explores emerging opportunities for its valorisation. Drawing on a systematic analysis of over 275 research articles, the study identifies key valorisation pathways, including energy recovery (e.g., anaerobic digestion yielding up to up to 3 PJ/year), material reuse (e.g., bricks with 10–20 % PMS content showing 30 MPa compressive strength), and biofuel production (e.g., bioethanol yields of 0.25–0.35 g/g dry PMS). The review also highlights the environmental benefits of these approaches, such as a over 50 % reduction in global warming potential when PMS is used in cement production. The paper advocates for a biorefinery model in which paper mills co-produce paper alongside biomass, biofuels, and biogas, thereby enhancing sustainability and supporting circular economy principles.