Comparative evaluation of pre-treatment methods to enhance phosphorus release from digestate

This paper aims to evaluate various pre-treatment methods to improve the release of phosphorus (P) in the liquid fraction after solid–liquid separation of digestate (input: (dairy) manure and organic biological by-products provided by the food industry). As such, the P recovery potential from the liquid fraction as concentrated fertilizer product can be increased, whereas soil organic carbon can be maintained through local recycling of the resulting thickened fraction. Four different mechanical pre-treatments (microwave heating, conventional heating, ultrasonic treatment, and orbital shaking) were tested in combination with the chemical dosing of three different acids (citric acid, formic acid, and HCl) down to pH 4, 5, and 6. The pre-treatments released 13–28 % soluble ortho-P relative to total P. When balancing the recovery efficiency with the acid costs, the addition of HCl down to pH 4 under microwave heating was most feasible. Nevertheless, if no pasteurization is required for fertilizer marketing, then orbital shaking (mixing in practice) with HCl addition may be more attractive