Greetings. I'm a CA architect living in Costa Rica. I confess I don't know plumbing logistics too well. Neither do the locals. I'm living in a jungle house that's about 15 years old, and all fixtures were installed without roof venting. It's on a septic tank with a leach field. There is a downstairs apartment and an upstairs one. The downstairs studio has a kitchen sink, a bathroom sink, a shower and a toilet. Upstairs has a sink and a toilet and we are about to add a shower and a kitchen sink. None of the system seems to drain well, and the toilet burps sewer gases. It's quite lovely. NOT! The structure is made of steel and concrete. I've been researching AAV's but don't really know where to install them. Is anyone out here able to help lend some advice?