We don't have synonyms for septic tanks yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are septic tank.
(n) :
A small-scale watertight treatment system for domestic sewage in which the flow is slowed to allow sedimentation and sludge digestion by bacteria to take place.
(n) :
A small-scale watertight treatment system for domestic sewage in which the flow is slowed to allow sedimentation and sludge digestion by bacteria to take place.