From waste to resource: A review on advancing whey permeate valorization through anaerobic digestion

The increasing demand for whey protein has led to a significant production of whey permeate as the byproduct. It is characterized by its high organic load, mostly in the form of lactose, but low in other nutrients. This represents both environmental risk and resource with high valorization potential. Despite of this, valorization of whey permeate is still less common when compared with other forms of wastewaters from the dairy industry. Among several methods to valorize the byproduct, anaerobic digestion (AD) has emerged as an attractive solution by offering simultaneous bioresource recovery and organic load removal. This review synthesizes current knowledge on whey permeate valorization through AD. First, whey permeate production, composition and current findings on direct and indirect utilizations of whey permeate are outlined. It is highlighted that while current utilization methods offer add value, many of whey permeate utilizations remain constrained by limitations (e.g., consumer acceptance, lactose crystallization, limited processing capacity and complexities for large volumes, and generation of secondary waste) that can be tackled through AD. The theoretical foundation of AD is then presented, with focus on process stages and key factors influencing AD performance. Published studies on whey permeate AD are critically reviewed, highlighting experimental designs, AD performance, and methodological limitations. This review identifies strengths in current approaches while underscoring persisting challenges such in effective optimization strategies. Finally, future research perspectives are discussed, pointing towards standardization of terminology to enhance reproducibility, process optimization, and viable route for advancing whey permeate AD.