Anaerobic digestion of paper and pulp mill sludge and subsequent hydrothermal treatment of the produced digestate

Abstract

The P&P industries are the sixth largest polluting and most energy and water-intensive industries in the world and generate significant amounts of wastewater. The wastewater undergoes primary, secondary, and tertiary treatments before discharging into aquatic environments. In this process, a large amount of primary and secondary sludge is generated, which contains high organic contents, and nutrients like nitrogen and phosphorous. These sludges, if not treated properly have an adverse effect on the environment. Traditional sludge treatment methods include incineration and landfilling, which leads to the release of harmful greenhouse gases.

Biochemical processes like anaerobic digestion (AD) and thermochemical processes like hydrothermal carbonization (HTC) have been investigated to treat bio sludge efficiently and aim for value-added products. Anaerobic digestion (AD) is a promising solution suitable for high organic load wastewater and sludges and the production of methane-rich biogas and HTC is a thermochemical process that converts wet biomass into a valuable material under heat and pressure. Therefore, the integration of AD and HTC can be a potential complementary process for obtaining value-added products. This thesis aims to characterize the solid and liquid parameters of the bio sludge, followed by the quantification of the digestate resulting from AD with a subsequent HTC of the produced liquid. The results showed that the integration of AD with HTC showed better biogas yield and the process was optimized with principal component analysis to analyze its applications.