Abstract
The present study addresses the coupling of hydrothermal carbonization (HTC) with anaerobic digestion (AD) in wastewater treatment plants. The improvement in biomethane production due to the recycling back to the anaerobic digester of HTC liquor and hydrochar generated from digested sludge is investigated and proved. Mixtures of different compositions of HTC liquor and hydrochar, as well as individual substrates, were tested. The biomethane yield reached 102 ± 3 mL CH4 g−1 COD when the HTC liquor was cycled back to the AD and treated together with primary and secondary sludge. Thus, the biomethane production was almost doubled compared to that of the AD of primary and secondary sludge (55 ± 20 mL CH4 g−1 COD). The benefit is even more significant when both the HTC liquor and the hydrochar were fed to the AD of primary and secondary sludge. The biomethane yield increased up to 187 ± 18 mL CH4 g−1 COD when 45% of hydrochar, with respect to the total feedstock, was added. These results highlight the improvement that the HTC process can bring to AD, enhancing biomethane production and promoting a sustainable solution for the treatment of the HTC liquor and possibly the hydrochar itself.
Highlights
Anaerobic digestion (AD) is a widely used process, successfully applied for stabilizing the sewage sludge produced from municipal wastewater treatment plants (WWTPs) [1]
In the framework of a aim of this study is to evaluate the percentage of hydrothermal carbonization (HTC) liquor and hydrochar which should be added to sewage sludge in order to assure the highest biomethane production during AD: this in order to develop a process scheme that could be applied to full scale WWTPs, in develop a process scheme that could be applied to full scale WWTPs, in particular to the sewage sludge particular to the sewage sludge treatment line, in order to optimize it
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Summary
Anaerobic digestion (AD) is a widely used process, successfully applied for stabilizing the sewage sludge produced from municipal wastewater treatment plants (WWTPs) [1]. In the last years a lot of progress has been made in the development and implementation of pre-treatment processes capable of increasing the anaerobic biodegradability of sewage sludge. The traditional use of sewage sludge digestate as soil amendment and organic fertilizer is becoming more and more restricted due to environmental and economic reasons [4,5]. As an alternative digestate could be used as an interesting feedstock for thermochemical conversion [6,7]: the digested sludge resulting from the AD process contains a large amount of non-biodegradable organic matter usable for additional energy production [8,9]
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