Conventional systems rely on natural anaerobic decomposition. Aerobic systems use a pump to oxygenate the tank, achieving cleaner effluent and tolerating smaller drain fields. Aerobic costs more upfront ($12-$20K) but works on lots where conventional fails the perc test.
More detail
Conventional systems are simpler: solids settle, anaerobic bacteria slowly decompose them, liquid effluent flows by gravity to a drain field where soil microbes complete treatment. Lifespan 25-50 years if maintained. Aerobic treatment units (ATUs) inject oxygen via a continuous aerator pump, which accelerates bacterial decomposition and produces cleaner effluent. The cleaner effluent enables smaller drain fields (or in some cases surface-discharge to a stream under Ohio EPA permit), which makes ATUs the right choice on tight or marginal lots. Tradeoffs: ATUs require quarterly inspection, annual aerator service, electricity to run the aerator continuously (~$150-$250/year), and have shorter component lifespans (aerator pumps typically last 3-7 years). Cincinnati-area lots that drain toward streams or wells often require aerobic treatment under Ohio EPA HSTS rules. Lots with good perc rates and ample setback distance can use conventional gravity systems. Cincinnati lot-conditions decision tree: if the perc test passes standard residential criteria (10-60 mpi) and the lot has adequate setback distance, conventional gravity is almost always the right choice on cost and longevity grounds. ATU systems make sense when perc fails (slow soils, high water table, small lots near streams) or when local code requires surface discharge. The on-site assessment plus the perc test result drives the recommendation.