Hydrogen sulfide (H2S) is highly malodorous and often occurs in sewer systems during dry and warm climate periods. Sewer gas filters mounted into the shaft help purifying gases and eliminate odors through microbiological processes of bacteria settling on granules in the filter. However these bacteria require certain conditions of the granules to maintain decent gas filtering performance. Long dry periods and heavy rain falls annihilate the conditions and are fatal for the bacteria. This invention provides constant optimal conditions in the granules of sewer gas filters during heavy rain fall and long periods of drought.