From dairy waste to butyric acid: Scaling the valorization process.

To move towards a circular dairy sector and reduce its environmental impact, sustainable management of its by-products is essential. This study presents a novel approach to valorize milk permeate through the production of butyric acid. Key operating parameters, including hydraulic retention time (HRT) and pH, were tested at a laboratory scale and the best conditions were validated at a pre-industrial scale in a dairy factory. A relatively short retention time (2 d) and pH 5.5 was demonstrated to be appropriate for the conversion of milk permeate into volatile fatty acids (58% yield), likely due to the permeate carbohydrate composition, which is primarily lactose. Furthermore, the acidic pH (5.5) promoted a shift in the product spectrum, increasing the molar fraction of butyric acid from 0.2 to 0.6. These operational conditions resulted in butyric acid productivities of 9.0 g HBut/L·d and 3.1 g HBut/L·d at lab- and pilot scale, respectively. This study represents a significant step towards a promising valorization route for milk permeate at a representative scale, while highlighting the need for process optimization to overcome scale-up challenges to ensure industrial applicability.