Treatment of hydrothermal carbonization process water by electrochemical oxidation: Assessment of process performance

Herein electrochemical oxidation (EO) is proposed as a novel path to treat the process water obtained from hydrothermal carbonization of olive tree pruning. The aim of this work is to analyze the organic matter removal achieved by the treatment along with the identification of the chemical species formed after the electro-oxidation process at different experimental conditions. Three different tests were performed in a boron doped diamond cell, using Na2SO4 and NaCl as supporting electrolytes to compare the results obtained with the raw process water. The organic matter removal was evaluated by means of total organic carbon and chemical oxygen demand, while Gas Chromatography Mass Spectrometry was used to determine the chemical species present before and after the treatment. The addition of a promoter considerably increased the organic matter removal. In fact, the experi-ments performed using supporting electrolytes showed the best results in terms of organic matter removal compared to the control experiment (30-40% vs. 17%); This reduction agrees with the volatile fatty acids' measurements. Almost all the chemical species identified in the different feedstocks were partially or totally removed after the EO treatment depending on the experimental conditions. The specific energy consumption and the cost calculated for the treatment is highly dependent on the time of electro-oxidation and the supporting electrolyte used, obtaining values from 1 to 45 euro/kg CODremoved. All in all, this work suggests an interesting path towards a further utilization of process water from hydrothermal carbonization processes.

Automated summary

This study proposes electrochemical oxidation (EO) as a new method to treat process water from hydrothermal carbonization of olive tree pruning. The organic matter removal and chemical species formed during the electro-oxidation process under different conditions were analyzed. The addition of a promoter significantly enhanced the organic matter removal, and the choice of electrolyte had a significant impact on the removal efficiency. Moreover, the study found that various chemical species identified in different feedstocks were partially or completely removed after EO treatment. Therefore, this research suggests a promising avenue for the further utilization of process water from hydrothermal carbonization processes.