Subcritical hydrothermal treatment for the recovery of liquid fertilizer from scallop entrails.

Scallop entrails are organic wastes containing abundant proteins and minerals but are considered difficult to recycle because of high cadmium concentrations. In this work, the current problem of scallop entrails recycling was investigated and a subcritical hydrothermal treatment (SCHT) was examined for the recovery of liquid fertilizer from scallop entrails. Scallop entrails are mainly recycled for composting and feedstuff production. However, the dilution by mixing scallop entrails with other feed waste was the sole countermeasure to reduce the cadmium concentration of compost. For feedstuff production, whole product derived from scallop entrails was exported to other countries instead of domestic utilization. Temperature, retention time (RT) at given temperature, and liquid-to-solid (LS) ratio were examined as SCHT conditions for scallop entrails processing. The extraction ratio of each constituent mainly depends on the temperature rather than the RT or the LS ratio. Upon the SCHT of scallop entrails at 200 degrees C, an RT of 20min, and an LS ratio of 10, the extraction of fertilizer constituents such as nitrogen, phosphorus, and potassium from the liquid product was optimum, whereas the release of cadmium was suppressed. The concentrations of heavy metals in the liquid product obtained using the above-mentioned SCHT conditions were below the maximum permissible concentration stipulated by the Fertilizer Control Law. SCHT is considered to be a feasible recycling method for scallop entrails to recover fertilizer components with a concomitant separation of cadmium from the product.