Hydrothermal Processing of Sludge – A Review

Hydrothermal processes such as thermal hydrolysis and wet oxidation technologies are becoming an essential part of sludge management strategies adopted in modern wastewater treatment plants. These technologies contribute not only to the reduction of waste volume, but also to the improvement of their environmental performance. Enhanced biogas production, energetically self-sufficient processes, value-added by-products, decreased greenhouse gas emissions and reduced reactor volumes are some of the benefits that can be expected from these technologies. A review was carried out to identify recent developments, challenges and some of the range of applications of these technologies. Studies on the kinetics and mechanism of thermal hydrolysis and wet oxidation were found to be sparse and still developing although many successful implementations of the techniques have been achieved in industry, despite its limited theoretical understanding. Different configurations of the thermal hydrolysis and wet oxidation within the sludge treatment line have been investigated by different researchers and it was found to produce different outcomes; depending on the desired purpose, thermal hydrolysis and wet oxidation may even both appear within a sludge treatment line. The main challenge facing thermal hydrolysis is the production of refractory compounds at higher than optimal temperature which investigation on this issue is ongoing. The other main challenges with hydrothermal technologies are energy optimization which not so many energy evaluations have been done so far. Hydrothermal processes will play a significant role in waste and sludge treatment, and research is still ongoing to find new innovative ways to improve their performance. Keywords-hydrothermal processing; thermal hydrolysis; wet oxidation; sludge treatment technologies