Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion

In this work, the influence of a low temperature (70–90 °C) thermal treatment on anaerobic digestion is studied. Not only the increase in biogas production is investigated, but attention is also paid to the solubilisation of the main organic (proteins, carbohydrates and volatile fatty acids) and inorganic (heavy metals, S and P) sludge constituents during thermal treatment and the breakdown of the organic components during the subsequent anaerobic digestion. Taking into account the effects of the treatment on the sludge composition is of prime importance to evaluate its influence on the subsequent anaerobic digestion and biogas production using predictive models. It was seen that organic and inorganic compounds are efficiently solubilised during thermal treatment. In general, a higher temperature and a longer treatment time are beneficial for the release. The efficiency of the subsequent anaerobic digestion slightly decreased for sludge pre-treated at 70 °C. At higher pre-treatment temperatures, the biogas production increased significantly, up to a factor 11 for the 60 min treatment at 90 °C.