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

Laboratory of Process Engineering and Industrial Systems (LR11ES54), National School of Engineering of Gabes, University of Gabes. Medenine Street 6029, Tunisia

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

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.