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Any consideration of emergency situations means planning for potentially not having modern conveniences like running water and sewers. The following information allows you to be mindful and prevent impact on the environment as much as possible.
Gray water is the waste water from general washing. It’s not sewage but contains soaps which can affect the natural balance of bacteria in nature and certain soaps may take longer to decompose. A “biodegradable” soap is just one that takes less time to decompose and means avoiding phosphates and anti-bacterial ingredients like triclosan.
No matter what soaps you use, you should always dig a hole at least 200 feet from water sources and pour the gray water there. It doesn’t need to be incredibly deep — six inches will do. The ground will filter it and help it degrade faster.
You can find specialized soaps meant for backpackers that are supposed to be biodegradable, but they are also more expensive and not particularly necessary. If you’d like to go with that option, some brands you can look into are Wilderness Wash, Campsuds, or Wonder Wash.
If you want to go more towards regular soaps, simple ingredients are best. Dr. Bronner’s Unscented Castile Soap or Kirk’s Castile Soap are good options. Zote laundry soap, while it contains brighteners, has quite simple ingredients otherwise and is still my go to (manufacturer information also lists it as biodegradable).
Apparently blue Dawn dish soap is also biodegradable and we’ve all seen the ads for it being used to clean up wildlife. Thing is, those are emergency situations where the priority is getting oil (or whatever) off the animal and that outweighs the harms of using the dish soap. It, and most other dish soaps on the market, are largely petroleum based. I think they certainly have their place when it comes to particularly difficult clean up jobs, but it may be better to work towards other options for use the rest of the time. Other dish soap brands claiming to be biodegradable include Seventh Generation Natural Dish Liquid or Mrs. Meyer’s Clean Day Dish Soap. These tend to have a bit “greener” ingredients than more readily available options like Dawn or Ajax.
However, remember that no matter how green, all of these options still require that you dump gray water away from water sources. A biodegradable label on your products does NOT mean that you can skip on proper disposal.