A review on generation, composition, and valorization of dairy processing sludge: A circular economy-based sustainable approach

This dairy industry has grown considerably in the last decade to meet the increasing requirements of the ever-rising human population. Large volumes of solid waste are generated during the processing of dairy wastewater, known as dairy processing sludge (DPS). Organic and inorganic chemicals, carbon, nitrogen, phosphorus, chlorides, sulfides, fats, oils, and grease comprise most of the DPS. Discharging untreated DPS into surrounding water sources harms the environment because of its elevated organic contents. Alternatively, the DPS can potentially be transformed into struvite, char, and ash with some fertilizer equivalence values. DPS contains heavy metals, pathogens, and synthetic organic compounds (hormones and pesticides) and requires pretreatment before its direct application as a fertilizer equivalent. DPS can be effectively treated by non-conventional methods, such as earthworms converting it into nutrient-rich fertilizer. Moreover, circular economy principles can be fulfilled by recycling DPS into value-added products, such as bacterial growth media for rhizobia, stabilizing contaminated soil for growing biofuel plants (e.g., Jatropha), and further used as a potential substrate for biodiesel production. This review elucidates the generation, composition, treatments, and opportunities for reusing DPS in a sustainable and eco-friendly manner that minimizes environmental pollution and identifies important future research prospects.