Microwave-assisted hydrothermal carbonization of rabbit manure

Hydrothermal carbonization (HTC) is an efficient process capable of treating organic waste without the need for a pre-drying step. Animal manure is among the waste that can be treated using HTC. Manure is a nutrient-rich waste produced in large quantities and contains pathogens; therefore, proper management is required. The use of microwave irradiation in HTC process is considered as a fast and efficient method to treat manure. Rabbit manure (RM) represents the main waste stream generated by rabbit farming activities. In this study microwave-assisted HTC of RM was performed and the effect of operating conditions on the characteristic of the products was evaluated. Hydrochars are produced by varying temperature (180 °C and 230 °C), residence time (30, 60 and 120 min) and solid-to-liquid (S/L) ratio (1/10 and 1/20). Moreover, the potential application in agriculture was evaluated. Results indicate that varying operation conditions have different effect on process products. Specifically, increasing treatment temperature from 180 °C to 230 °C leads to hydrochar yield reduction, while increasing severity of the treatment increases fixed carbon and total phosphorus content in the hydrochar. Thus, by applying the microwave-assisted HTC process in RM at 230 °C for 120 min and with an S/L ratio of 1/10, a hydrochar with an increased content of fixed carbon (17.5 %) and phosphorus (16.4 g/kg) was obtained. Subsequently, germination assays performed using the abovementioned hydrochar confirm a phytostimulatory effect at low application rates (5 %), while applying the HTC process water induces phytotoxicity, at all concentrations examined.