In most 'sanitary' systems the vent discharges to the HIGHEST part of the system ..... and is protected by a 'bio-blocking' filter. This is to protect the entire system from aspiration of biological spores, spores that are bacterial, or fungal. Mildew (fungal) spores are the most prevalent contamination to a boat's water tankage, many of these are very toxic or dangerous especially to one with a weakened immune system.
The vent filter can be a commercial/industrial 'capsule' filter made from hydrophobic/teflonic membrane @ 0,2µM (100% - absolute) retention and approx. 1-2 sq. ft. surface area and mwith 3/8 or 1/2" hose barb connectors. These are available from those filter suppliers/distributors who serve the bio-pharm industry. Approx. $75-100 ea.
If thats too much $$$ then take a fist sized 'wad' of bandage cotton placed over the terminal end of the vent tube, cover with bandage gauze, and tape in place. Keep dry and change every 6-12 months. The vent filter is always at the 'highest' elevation in the entire system.
Suggest you inspect and visualize the vent line 'all-the-way-to-the-tank'. 99% of the time such an unprotected vent line will contain massive colonies of bacteria/mold/mildew, etc. .... and 99% of the time this is why your tank eventually become totally fouled with biological growth. Typically if the vent hasnt been protected it usually needs replacement if not 'totally' free of discoloration/growth. To a microbiologist, most boat water tank 'vent systems are cause for projectile vomiting.
Last edited by RichH; 10-18-2010 at 08:22 PM.