Optimization of supercritical carbon dioxide explosion for sewage sludge pre-treatment using response surface methodology

The present work was conducted to assess whether the implementation of Supercritical Carbon dioxide Explosion (SCE) is an efficient approach for sewage sludge pre-treatment. In this context, SCE was optimized with the aim to develop a method attempting to increase the biodegradability of sewage sludge's organic matter content, and thus, to enhance the subsequent anaerobic digestion and methane production. The statistical tool of response surface methodology was applied to evaluate the effects of the main pre-treatment parameters (i.e. temperature and time) and their interactions on methane yield, which was defined as the response. Temperature was found to be the most significant variable, having the greatest effect on methane yield. Following this, an optimum set of pre-treatment conditions corresponding to a temperature of 115 degrees C and time of 13 min, was determined. Under these optimum conditions, the predicted response value was 300 mL CH4/g of volatile solids. The corresponding experimental value obtained from the validation experiment fitted well with this value, clearly demonstrating the effective use of response surface methodology in optimizing SCE. Additionally, under optimum conditions, the methane yield presented a statistically significant increment of 8.7%, compared to untreated sludge. This revealed the impact of SCE as an effective and alternative way for the efficient pre-treatment of sewage sludge. Finally, thermal pre-treatment, alkaline and acidic hydrolysis were also applied to the already pre-treated sludge. It was concluded that the combined pre-treatment techniques contributed to a further increase of methane production compared to raw (untreated) substrate.

Automated summary

The present study aimed to assess the efficiency of Supercritical Carbon dioxide Explosion (SCE) as a pre-treatment method for sewage sludge. The optimization of SCE led to an increase in the biodegradability of organic matter in the sludge, enhancing subsequent anaerobic digestion and methane production. Response surface methodology was used to evaluate the effects of temperature and time on methane yield, with temperature being the most significant variable. An optimum set of pre-treatment conditions was determined, resulting in a significant increase in methane yield compared to untreated sludge. The combined pre-treatment techniques also contributed to further methane production.