Current trends and future perspectives in dairy by-product valorisation

The increase in milk production and demand for dairy products worldwide has led to the emergence of a wide variety of products in the sector, whilst a high volume of solid (sludge) and liquid waste is also generated. Dairy industry wastes, due to their high organic load (biological oxygen demand, chemical oxygen demand), high moisture content and variable composition, can cause significant environmental and economic problems when disposed of directly. The aim of this research is to determine current methods and possible future trends for the valorisation of dairy industry by-products (whey, casein and lactose-containing wastes, wastewater and sewage sludge). Literature from 2010–2025 was systematically screened in databases such as Web of Science, ResearchGate, ScienceDirect, PubMed and Google Scholar in line with the PRISMA guide, and a total of 34 documents were examined. According to the results, various technologies aimed at conversion into valuable products (valorisation) and integration into the circular economy have come to the fore. Thermochemical methods such as hydrothermal carbonisation (HTC), hydrothermal liquefaction (HTL), pyrolysis and gasification increase energy and chemical substance recovery from wastes with high moisture content. Biological methods (anaerobic and aerobic digestion, composting, microbial fermentation) yield value-added products such as biogas, compost, bioplastic and single cell protein (SCP). The necessity of integrated systems combining both thermochemical and biological approaches is also established in the literature.