2.1. Sludge source and preparationWaste activated sludge (WAS) and anaerobic seed sludge were collected from a local water reclamation plant in Singapore. They were concentrated and stored at 4 °C before use. The concentrated WAS was thermally hydrolyzed with a pressure reactor (Model 4534, Parr Instrument Company, US) under 165 ± 4 °C for 30 min. Then the pretreated sludge was cooled down to the room temperature and collected as the original THP sludge. After centrifuged at 6000 g for 10 min, the supernatant and solids part of original THP sludge were collected separately. The original THP sludge, liquid and solids fractions of original THP sludge were diluted with distilled water (named THP sludge, THP-L and THP-S) before used as feed for AD. Three sets of each type of sludge were prepared. Each set was characterized separately. The characteristics of different sludge before anaerobic digestion are shown in Table 1.