Its important the tank can breathe, you can have problems if air cannot get in and out to equalize the pressure. As you add waste water to the tank, the air thats in the tank has to escape, if it can't your drain will be very slow and could start burping. Same when you empty the tank air must be able to get in. Roof vents are mostly required as the tanks usually refer to sewage type waste where gasses can build up and be dangerous. Here's a site for RV's that explains about types of valves: http://www.rvdoctor.com/2...-waste-management.html
These are in the context of sewage, but still can apply. Take a look at the ASTVD type valve, it may fit your purpose, allowing air in but not out.