Effect of operating conditions on the hydrothermal valorisation of sewage sludge

Abstract

The accelerated population growth, in conjunction with the rapid urbanisation rate, are the principal driving forces behind the augmented volumes of municipal sewage sludge generated worldwide. The traditional approaches of sewage sludge treatment, which include landfilling and agricultural application, are no longer within the realms of possibility due to rigorous regulations, deficiency in the capacity of land available and the environmental and health adversities associated with detrimental constituents of sewage sludge. The population and urbanisation advancements do not only influence the emergent volumes of sewage sludge, but they also instigate fundamental provocations to the global energy demand. The reliance on fossil fuels poses a significant threat, not only to sustainable development, however they are also hugely responsible for the cumulative carbon dioxide and other greenhouse gas (GHG) emissions that deteriorate the environment, trigger global warming and deleteriously impact the livelihood of all life on earth. In line with the quest for sustainable and renewable alternative energy sources, the thermochemical treatment of municipal sewage sludge has a triple advantage of valorising the abundant volumes of the sludge, addressing the injurious nature of conventional fuels to the environment and seeking to bridge the gap as their supply diminishes. This study followed a quantitative approach, with the purpose to convert municipal sewage to valuable bio-oils. The sewage sludge was subjected to hydrothermal liquefaction in 60 ml stainless steel batch reactors, where the effect of temperature, solvent composition, and solvent content were investigated, and all the other process parameters were maintained at a constant. The six temperatures that were explored were 220oC, 250oC, 280oC, 310oC, 340oC, 370oC. The two solvents investigated were de-ionised water (H2O) and ethanol (E) which were applied in the following compositions: 1:0, 1:1 and 0:1 (H2O:E). The five solvent contents investigated were 75%, 80%, 85%, 90% and 95%. The process yielded bio-oils, solid phase and gaseous products and an aqueous phase. Dichloromethane was used as an extraction medium. The obtained results revealed that the temperature, solvent type and solvent content had a significant influence on the yield of bio-oil produced while temperature was the most influential out of the three parameters. When temperatures approached supercritical conditions of water, a notable decline in the bio-oil yields was observed. For each temperature, the bio-oil yields initially increased until about 85% solvent content, and then slightly decreased thereafter. The highest bio-oil yields were achieved at 310oC and the best yields were obtained when the ratio of H2O and E were 1:1. This study found that the optimum operating conditions were obtained at 310oC, 85% solvent content and a 1:1 composition of H2O and ethanol; the bio-oil yields at those conditions was determined to be 40,6 wt%. The bio-oils were contained in the following order of prevalence, fatty acids, aliphatic hydrocarbons, N-containing compounds, O-containing compounds, aromatics and acid esters. Aliphatic hydrocarbons and fatty acids were the dominant functional groups. The following were the most abundant compounds in the 90 runs: heptadecane, pentadecane, eicosane, hexadecane 2,6,10,14-tetramethyl hexadecane and 9-octadecanoic acid.