Graywater is inherently unsanitary. It may contain bacteria, fungi, viruses, or other life. To disinfect graywater, and avoid funky aromas, mix in Concentrated Clorox Regular Bleach or other chlorine-based bleach periodically. For some antibiotic-resistant pathogens, chlorine bleach is the only reliable killer.
The active ingredient in chlorine bleach is sodium hypochlorite. It's a powerful disinfectant that self-decomposes into water and salt. Age, sunlight, warmth, and graywater impurities (including playa dust) all increase the rate of decomposition, and eventually exhaust the disinfecting power. So, make sure your bleach is newly purchased, not left over from last year. The following rule of thumb will help compensate for other uncertainties.
The No-Measure Rule of Thumb for Disinfecting: Follow Your Nose
Loosen the cap of your bleach jug one-half turn: no farther, or it may come off. While using your thumb to restrain the cap, invert the jug over your graywater container and dribble for about two seconds per gallon. Stir well. After half an hour, sniff the container. If it doesn't smell like chlorine, dribble more bleach into the container, about five seconds' worth, and stir. Check the aroma again, in another half hour. Repeat as needed; make it a morning and evening habit.
Graywater left to stand untreated eventually becomes blackwater as microbial populations increase. People who haul out all their graywater at the end of the week have worse stuff to manage than you do, unless they also treat it periodically with bleach. They also have leakage worries on the drive home, and containers that can't be reused safely for anything else.
When buying chlorine bleach, check the label for the concentration of sodium hypochlorite. Concentrated Clorox Regular Bleach has about 9%; it's the only brand adhering to EPA guidelines and thus qualifying for EPA registration. If another brand has 6%, it should cost two-thirds as much and you'll need to add 1.5 times as much. Scented products are always lower in sodium hypochlorite. Bleach has a limited shelf life, and this year's leftover jug won't last til next year.
Disclaimer: I am not an expert, but an expert at Clorox Corp. reviewed this text. Following the guidelines above is not certain to fully disinfect your water. I will update this page as I gain new information.