RE: Holding tank vent line
It COULD work...but only if it it's an exhaust fan and runs 24/7/365 (except during any extended layup, of course). However, if your tank is much deeper than 18", aeration may be the only answer...'cuz 18-20" is about the max depth that can be kept aerobic using passive ventilation above the the surface. 250 gallons is a LOT of sewage--a lot of WEIGHT (over a ton!)--to try to manage. We can brainstorm some ideas if you'd like to send me an email.