If you are looking for a rule ... Published October 23, 2020 00:12 by Rafael Gay-de-Montella MSc PEng, Water Wastewater Treatment Expert, Oil-Gas, Tailings, Boilers, ZLD; Evaporators, Sludge If you are looking for a rule of thumb, for municipal sewers without heavy industrial contaminants, aeration of 6 to 8 hours, and 2ry clarification of 3 hours.