Analysis of the intensive phase of biowaste composting process with the addition of sour buttermilk

Abstract The sustainable management of dairy industry waste remains a major environmental challenge, particularly for sour buttermilk, which is characterized by low pH and high concentrations of slowly degradable biogenic compounds. Its discharge to wastewater treatment systems contradicts circular economy principles and highlights the need for alternative valorization pathways. In this research, the feasibility of incorporating waste sour buttermilk into green biomass during composting was evaluated, and its effects on the intensive phase, selected physicochemical and microbiological properties, and the phytotoxicity of the resulting material were assessed. Sour buttermilk was applied at proportions ranging from 0 to 30% (w/w). Its addition reduced ammonia concentrations in emitted gases from maximum values exceeding 1000 ppm in the control to approximately 600–700 ppm in treatments with 9% and higher additions. At levels above 6%, the addition of sour buttermilk limited the extent of drying during the thermophilic phase. However, additions exceeding 12% adversely affected process performance, as thermophilic conditions (> 45 °C) were achieved only briefly and at relatively low temperatures. This inhibition was associated with decreased pH, increased moisture content, and reduced air-filled porosity. Higher application rates also increased the phytotoxicity of the obtained materials. No potentially pathogenic bacteria were detected in any treatment. Overall, the results indicate that controlled incorporation of sour buttermilk may influence composting dynamics, with 9% (w/w) showing the most balanced process performance under the tested conditions.