This idea comes from Practical Sailor magazine. A lot of the holding tank vent filters on the market are expensive and disposable. A notable exception is the Big Orange http://bigorangefilter.com/. This is just a pipe filled with activated charcoal with two yellow kitchen sponges on the ends to help the charcoal stay put. Not shown is the operation where a pipe stub was sharpened and used with the vise to cut out two yellow sponge disks matching the pipe diameter.