Hydrothermal carbonization of dairy processing waste: Optimizing phosphorus recovery scenarios

Ever since phosphorus (P) was declared a critical raw material by the European Union (EU) in 2016, the ongoing quest for securing sustainable resources of P has been growing continuously. Industrial wastes have been identified as potential secondary resources for the recovery of nutrients, including P. In parallel, the growth of the global dairy industry induced a significant increase in dairy processing waste (DPW), which can pose an environmental threat in the case of traditional treatment. Therefore, the need for proper management of dairy processing waste coincides with the quest for securing secondary P-resources to define P-recovery from treated dairy processing waste as a step towards achieving sustainable resource recovery route.

In this study, hydrothermal carbonization (HTC) was applied as a thermochemical treatment of two types of dairy processing waste (chemically treated and biologically treated) to investigate P-recovery from HTC solid and liquid products. Experimental HTC runs were performed on dairy processing sludge at different operating conditions to assess the effect of parameters such as temperature, initial pH, and pre-treatment. Finally, a kinetic study was performed, and its outcomes were utilized as input for a simulation model of an industrial scale HTC of DPW.

The results showed that HTC enhanced the concentration of P in solid hydrochars of DPW, yet the majority of P retrieved had low plant availability. HTC enhances the transformation of inorganic P into stable apatite forms, which explains the hydrochar capacity to retain heavy metals, mainly hexavalent chromium, due to the increased porosity after HTC, as revealed by SEM images. In terms of fertilizer quality, all produced hydrochars were compliant with the requirements of (PFC) 1B-(I) for solid organo-mineral fertilizers for all aspects except Cr content. Furthermore, hydrochars produced from biologically treated sludge possessed less ash content and COD recovery levels compared to those produced from chemically treated sludge. Therefore, EBPR-based hydrochars were recommended as cleaner hydrochars. Finally, the kinetic study based on real-time P concentrations revealed a first order HTC reaction with activation energies between 48 kJ/mol and 87 kJ/ mol for the chemically treated and biologically treated sludges, respectively. Finally, the simulation resulted in achieving 95% P-recovery from an annual 752 tonnes of HTC-treated dairy processing waste.

The study demonstrated the potential of HTC as a means for P-recovery from dairy processing waste, with further adjustments required regarding increasing P-availability and reducing heavy metal content of the produced hydrochars.