Dry or Malfunctioning Drain Trap
Depending on how your graywater system is set up, you may have a trap in your drain line just like a sewer drain. Drain traps prevent sewer gas, in the case of sewer lines, from backing up into the house. Graywater holding tanks can emit some odors, so many homeowners opt to install a trap between their tanks and the indoor drains. Check to make sure your trap is installed properly and has not run dry.
Improper Pipe Usage
One common mistake in graywater setups is to use corrugated piping or hose to transport water from the drains to the graywater holding tanks. This is problematic, because the corrugation in the hose or pipe allows debris to become trapped and, decomposing over time, emit an odor. Plumb your graywater system just as you would a blackwater, or traditional, home sewer system. Opt for smooth PVC or copper to prevent debris accumulation.
Holding Tank Issues
Check your holding tank for signs of bacteria growth, an issue common to standing water. Anaerobic and aerobic bacteria can thrive in graywater holding tanks and emit foul rotten egg and sulfuric odors. Be sure that you are filtering your water before it is deposited in the holding tank and that you are changing your filter regularly.
Graywater, untreated, must be used within a few days of its reaching your holding tank. A good practice is to set up a system that purges your holding tank of unused water, depositing the water into the soil, at the end of each day. The longer the water stands, the better chance you have of harboring microorganism growth that can lead to odors and other problems, such as tank damage.