William Robert :>), I have installed several Activated Carbon Roof Vent Filters to eliminate the sewer gas odors emanating from roof vents and being carried by downdrafts into the yards of homes at which I have installed septic systems. The activated carbon adsorbs the hydrogen sulfide gas, which is a product of microbial digestion in the septic tank. When all of the sorption sites are occupied, the activated carbon must be replaced. The frequency of replacement varies greatly depending upon the volume of hydrogen sulfide produced in the system. I have had to replace the activated carbon on only one of eight installations. The initial charge lasted seven years on that particular system. The replacement procedure is very easy.
It is possible to recycle the Activated Carbon load, but the volume of activated carbon in the filter is quite small. It hardly seemed worth the effort to ship it back to the supplier for regeneration. I just added the spent carbon to my compost pile and ordered a new activated carbon load.
There have been reports of freezing water vapor, hoarfrost, clogging the roof vent filters in northern climates. I have not experienced this problem in Colorado and all of the filters that I have installed have been on systems located in the Rocky Mountains at very high elevations where extended periods of sub-zero temperatures are common.
I have used the “Sweet Air” brand, but there are other brands on the market. You can see the Sweet Air Filters, and purchase them on-line at the following web site: http://www.spotfree.net/sweet_filter.html