Fresh air enters the waste system via a unique, air-tight, one-way valve. This inflow of air is required when toilets flush, sinks drain and laundry water is exhausted, etc. so that your drains maintain water in the s-trap and block sewer gas from backing into the house. Sewer gas, composed of Methane is generated as a byproduct of the microbial breakdown of solid waste. This resultant gas is vented back up through the system to the waste stack and out into the atmosphere. SweetStack's unique design forces this waste gas through the Activated Carbon Filter which scrubs the organics and volatiles (the odor) and produces fresh, filtered air which is then exhausted to the outside world.