Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production

Hydrothermal carbonisation of primary sewage sludge was carried out using a batch reactor. The effect of temperature and reaction time on the characteristics of solid (hydrochar), liquid and gas products, and the conditions leading to optimal hydrochar characteristics were investigated. The amount of carbon retained in hydrochars decreased as temperature and time increased with carbon retentions of 64–77% at 140 and 160 °C, and 50–62% at 180 and 200 °C. Increasing temperature and treatment time increased the energy content of the hydrochar from 17 to 19 MJ/kg but reduced its energy yield from 88% to 68%. Maillard reaction products were identified in the liquid fractions following carbonisations at 180 and 200 °C. Theoretical estimates of the methane yields resulting from the anaerobic digestion of the liquid by-products are also presented and optimal reaction conditions to maximise these identified.