This chapter demonstrated the possibility of using Ultrasonication (ULS) treatment and chemically assisted ULS treatment to disintegrate anaerobically digested sludge to enhance methane production. Anaerobic digestion is a commonly used technology for sewage sludge stabilization in wastewater treatment plants. Compared to pre-treatment, post-treatment would more specifically target the solids that are more difficult to biodegrade in digested sludge. However, reports on post-treatment are relatively limited in number. ULS, a proven pre-treatment method, has, however, not been reported for post-treatment. Compared to a pre-treatment, the post-treatment would more specifically target solids, which are more difficult to biodegrade in digested sludge. The methane composition in biogas was around 64% in all the tests, indicating that post-treatments did not affect the methane composition. All the post-treatments under all hydraulic retention times and the recycle ratio tested resulted in a lower cellular adenosine tri-phosphate compared to the control despite the small proportion of post-treated sludge compared to the total reactor volume.