Septic tank explosions are possible and are dangerous. This particular explosion appears to have been heard by persons even kilometers away. Normally, methane, hydrogen sulfide and other gases like carbon dioxide are generated from these tanks.
https://globalnews.ca/video/3882959/septic-tank-explosion-in-china-kills-at-least-19-destroys-neighbourhood
https://forum.susana.org/human-tragedies-examples/21967-septic-tank-explosion-an-explosion-that-ripped-through-the-chinese-megaport-of-ningbo