Hydrothermal co-carbonization of sewage sludge and whey: Enhancement of product properties and potential application in agriculture

Abstract

The impacts of operating conditions on hydrothermal carbonization of sewage sludge (SS) in water media were investigated in this study. Hydrothermal co-carbonization (co-HTC) of SS and cheese whey was also carried out under selected conditions (T = 250 °C, t = 5 h), to investigate the effects of replacing water with whey on product properties. Besides, the potential application was also tested of HTC products for agricultural use.The HTC treatment increased the content of nutrients (P, Ca, and Mg) in hydrochars compared to untreated SS, but fuel properties were not improved. Increasing the operating temperature and time increased the contents of nitrogen and volatile fatty acids in the process liquids. The replacement of water with whey resulted in remarkable improvement of the fuel properties of hydrochar, as the hydrochar yield increased from 57.10 to 64.14 wt%, the calorific value from 14.75 to 18.29 MJ/kg, and carbon content from 36.89 to 45.24 wt%. The ash content decreased from 46.55 to 40.06 wt%. The content of N, P, Ca, Mg and K increased significantly. SEM-EDS, FTIR and thermogravimetric analysis revealed the strong influence of the co-carbonization on the thermogravimetric behaviour of the hydrochars and their chemical properties. The HTC products proved efficient as fertiliser in a germination test with plants. Process liquids can be used as an N fertiliser, while hydrochars as a P fertiliser. Undiluted process liquids were toxic to the freshwater organisms Daphnia magna.