Hydrothermal carbonization of primary sewage sludge and synthetic faeces: Effect of reaction temperature and time on filterability

The effect of reaction temperature and time on the filterability of slurries of primary sewage sludge (PSS) and synthetic faeces (SF) following hydrothermal carbonisation (HTC) was investigated and optimised using response surface methodology (RSM). Filterability was shown to improve as the treatment temperature and reaction time at which the solids were carbonised was increased. The best filtration results were achieved at the highest temperature (200°C) and longest treatment time (240 min) employed here. The specific cake resistance to filtration of the carbonised solids was found to vary between 5.43 x 1012 and 2.05 x 1010 m kg–1 for cold filtration of PSS, 1.11 x 1012 and 3.49 x 1010 m kg–1 for cold filtration of SF, and 3.01 x 1012 and 3.86 x 1010 m kg–1 for hot filtration of SF, and decreased with increasing reaction temperature and time for carbonisation. There was no significant difference in the specific resistance of cold and hot filtration for SF. The RSM models employed here were found to yield predictions that were close to the experimental results obtained and should therefore prove useful in designing and optimizing HTC filtration systems for generating solids for a wide variety of end uses.