Hydrothermal Carbonization Conditions Parameters Optimization Using RSM–CCD for preparation of biofertilizer materials from olive solid wastes

Olive solid wastes (OSW), a low-cost biomass, were utilized to prepare a biofertilizer by hydrothermal carbonization. In the present study the effect of temperature (ranging from 170 to 240 °C) and residence time (ranging from 17 to 103 min) on hydrochar production was studied to maximize the solid yield. The study employed a central composite design (CCD) response surface method (RSM) to optimize temperature and residence time of hydrothermal carbonization process for enhancing hydrochar yield and quality, to be used as a biofertilizer in the culture media. Response surface methodology (RSM) was applied to investigate the effect of operational parameters (temperature and residence time) on the solid yield. The results showed that a maximum hydrochar yield output of 57.4 %was obtained at 171 °C and during 35 minutes. High HTC temperature (240°C) promotes a low pH as well as the content of ash, but generates high electrical conductivity (EC). The predicted reduced cubic model for response variables was found to be significant (p < 0.01), and the analysis of variance (ANOVA) showed a high coefficient of determination with R2 being 0.99 for biomass. The three phases obtained were then characterised by various analytical techniques and the hydrochar was evaluated as a biofertiliser, via phytotoxicity tests on lettuce seeds (Lactuca sativa). Phytotoxicity tests revealed significant toxicity of the aqueous extracts, whereas washing significantly reduced the phytotoxicity of the hydrochars by eliminating the water-soluble toxic compounds.