I've recently been discussing greywater reclamation with a few folks, and the topic of cleansers came up. There are a fantastic array of "natural", "green", "biodegradable", and "eco-friendly" cleansers on the market today... and almost every one of them are NOT appropriate for use in a greywater reclamation system where you use the water to irrigate food crops.
Why not? They're better than all those old harsh chemicals, right? Hey, look, they're EPA-approved!
Well, yes... and no. Most of the environmental concerns that are addressed by these cleaners are for AQUEOUS environments, not TERRESTRIAL environments. For the vocabularily-challenged, they're designed to be (or become) safe in water, not soil. Since the vast majority of our new-world sanitation is done through a combined grey/black water sewage treatment plant (everything gets flushed down the drain), and the liquid effluents from those plants are eventually returned to fresh waterways and the ocean (or back into our city water supply!), it is very important that any residues from cleansers be safe for introduction to waterways and other aquatic life (your dish soap can't be killing the fishes!) and/or breakdown rather rapidly in the presence of water or air.
To this end, phosphates were reduced because these cause horrible algae blooms; nitrates were reduced because these can also cause horrible algae blooms (as well as blue baby syndrome -- although that's normally associated with run-off from synthetically & over-fertilized fields, and some cancers); chlorine and sodium ingredients harmlessly decompose in water, and borates and other elements either don't react or are so diluted they aren't harmful. The alkalinity of most cleansers and their chemical elements aren't a major issue because seawater (the eventual end destination of most effluent waters) is alkaline and all the critters that live in it can handle the alkalinity, and there's plenty of fresh water in most rivers and lakes to dilute the alkalinity in those.
However, when you're putting water that contains cleansers directly on/in the soil it's a whole different story. Soil and it's ability to provide nutrients to healthy growing plants has an almost opposite set of rules. Chlorine is a powerful disinfectant... it also "disinfects" the good bacteria that help breakdown plant nutrients in the soil. Sodium, and all the other salts but sodium in particular, is extremely alkaline and can rapidly alter the pH of the soil either killing the plants or locking up the nutrients that the plants need to grow. Boron is a great water conditioner, pesticide and fungicide... unfortunately, these properties are extremely harmful to a happy, healthy garden. Phosphates and nitrates are generally just hunky-dory in soil (within reason of course!) since they are both plant food... which is what makes them so bad in aquatic environments. And buildup of elements is much more of a concern in soil since there certainly isn't enough water in it to dilute it, and some elements take a very long time to decompose in the absence of water and/or air (like salt). Most plants prefer neutral or slightly acidic soils, the nutrients they need are most active/available at neutral or slightly acidic pH, so high alkaline cleansers made for alkaline seawater are completely inappropriate unless you have seriously acidic soil that you're trying to neutralize.
And you never, ever, ever, want to use anything that is marketed as "antibacterial", "antimicrobial" or "disinfecting"... you're going to kill your good soil bacteria, not just that "bad" bacteria that might make you sick. I say "might" because routine exposure to low levels of "bad" bacteria is normally not harmful at all and can actually result in a stronger immune system. Hmm... maybe all these "sanitizers" are actually making us sicker?
Also try to avoid using anything that has enzymes or is labeled "biodegradable" unless you know exactly what they are, what they break down, and what they break down into... or you can totally mess up your soil's microorganisms, nutrient ability/composition, and the soil structure itself. They're not all bad, just be careful! Biodegradable and compostable aren't the same thing... most biodegradable products are designed to biodegrade anaerobically, in landfills or septic tanks, not aerobically in your compost bin or garden mulch.
OK - So what am I supposed to clean with then?
Surprisingly, a great many cleaning tasks can be accomplished with plain old soap (lye & fat) and water. For a little extra kick you can add a mild acid (lemon juice or vinegar) if you aren't cleaning marble/stone, rubbing alcohol, hydrogen peroxide, household ammonia (ammonium nitrate -- use with proper ventilation!), or a small amount of baking soda (sodium bicarbonate -- use sparingly, it is still a salt).
If you must disinfect something (like food processing tools), boil them. If you can't boil them, use steam... either an autoclave if you can afford it or a handheld unit. If steam won't work for you, citrus and many herbs and essential oils have natural antimicrobial properties (rosemary, peppermint, tea tree, etc). And sparingly, as a last resort, hydrogen peroxide or ammonia or alcohol. Instead of using chlorine to disinfect/treat your water supply, swimming pool or hot tub... use concentrated hydrogen peroxide instead, your plants will thank you.
The best bleach in the world is sunlight. Hang your whites in the sun the dry and I guarantee those stains will disappear without any chemical assistance. You'll have sparkling white socks and panties without bluing or other optical brighteners. But if you absolutely must use bleach, try pretreating with lemon juice first, and then use hydrogen peroxide instead of chlorine bleach. Which brings up "Oxygen Bleach"... yes, these don't have chlorine BUT they are chock-full of sodium. Use real hydrogen peroxide instead, your plants will thank you.
Washing your laundry rarely requires detergent. Yes, you read that right... simply soaking your clothes overnight is enough to remove most dirt and debris from your clothes because water is the universal solvent and also lifts particles from clothing fibers all on it's own. However, if you want to make this work a little better, or you have some pretty tough stains, try using something slightly alkaline (like plain soap or ammonia) because most stains on clothing are acidic (perspiration, oil/fat, blood, etc). Rubbing a little plain soap (or Fels-Naptha soap) on a stain before washing will do the trick most times, no need to add soap to the water. Adding just a half-cup of ammonia does a wonderful job cutting any body oils and deodorizing through neutralization not fragrance... and no your laundry won't stink of ammonia, the smell dissipates as soon as it's dry. The only caution is to be careful using ammonia if you pre-treat spots with Fels-Naptha because this contains a small amount of chlorine and we all know to never mix chlorine and ammonia!
There are a gajillion tips for treating stains of different origins all over the internet, I won't repeat them here ;) But remember that soap and ammonia are slightly alkaline, so do add a little vinegar or lemon juice to your laundry rinse water... it helps your clothes feel softer (gets out the last of the soap residue), reduces static cling, removes any soap scum from your laundry tubs, and helps neutralize your greywater.
A little bit of plain liquid soap in the wash water and white vinegar in the rinse water gives you squeaky clean dishes. A little vinegar, water and alcohol give you sparkling windows and counters (but not if they're stone!). A little vinegar in the mop water and your hard surface floors will shine (unless they're stone!), or a little ammonia if they're really greasy grimy.
Vitreous china, glazed ceramics, or enameled surfaces in the bathroom can be cleaned with lemon juice, vinegar, or ammonia since all three will cut through soap scum and hard water deposits. If you must scrub, make a paste with a small amount of baking soda and water, wipe up the majority of it and throw it away instead of rinsing it down the drain. If you have mildew stains, try applying straight lemon juice first, then hydrogen peroxide, and if that still doesn't work apply a paste of borax and water, apply it directly to the affected area, scrub a little and let it sit for a little while -- wipe it off and through it away rather than rinsing down the drain.
Rubbing alcohol in any formulation helps cut through oils, especially petroleum oils which are harder than biological oils to cut through, and just evaporates away into nothing so it rarely leaves any sort of residual gunk in your greywater... but I wouldn't go pouring straight rubbing alcohol on your garden because it has a high desiccant factor (it dries things out), which is also why it's not so great for cleaning wood surfaces.
Hydrogen peroxide is slightly acidic, not so much that it makes a difference unless it's super-concentrated though. Hydrogen-peroxide straight out of the bottle is a potent disinfectant, but the effect doesn't last very long. However, hydrogen peroxide rapidly decomposes into plain water and oxygen... so, by the time you drain the hot tub, or dump the laundry basin, you don't have to worry about what it's going to do to the garden (not that it would really hurt it anyway!).
Now, having said all that, if you need to or want to use chlorine bleach, borax, biological washing soda, or any other of the "no-no" cleansers as long as you capture that water separately and dispose of it somewhere that you don't want any plants to grow... like your paths and driveways.
Of course, periodically flushing your greywater system and irrigating your garden with rainwater or snow melt (which as close to pH neutral as we can naturally get) will help keep salts and other elements from building up in your garden soils and causing problems.
A Little Note on SOAP:
When I say soap, I mean soap... lye and fat. Any other cleaning product (with the exception of a saponin product like Laundry Nuts) that is not made of lye and fat is a detergent. Soap is basically just the fatty salts that are left over when the lye finishes reacting... the result of the process of saponification.
Real soap is mildly alkaline after it is properly aged... soap doesn't have to be pH neutral or pH-balanced to our skin (which means mildly acidic). In fact, in order to clean us properly, since our perspiration and body oils are mildly acidic, soap actually needs to be mildly alkaline. In a properly made and aged soap, there is no caustic lye left in the bar.
Lye in soapmaking is either sodium hydroxide (NaOH - red devil) or potassium hydroxide (KOH - potash), although sodium hydroxide is most commonly used and it's hard to find potassium hydroxide commercially (although you can make your own by leaching hardwood ashes). The biggest difference is that NaOH makes a hard bar, whereas KOH normally results in a soft gloopy "cream" texture (the original liquid soap). The type and amount of fat used in the recipe also affects the consistency, texture and lathering of the soap -- hard fat like beef tallow makes a harder bar (or very thick gloop) that barely lathers, and soft fats like olive oil makes a smooth liquid soap. Most NaOH soaps that use only oils are liquid soaps, and most soaps that use tallow or lard are bar soaps... although there are exceptions, I'm not going to get into soapmaking here either ;) Keep in mind that all soaps lather poorly in hard water, but a soap doesn't have to lather to clean well (it's a psychological thing I think, needing to foam away the dirt!).
A Final Ecological Note:
If you aren't trying to reclaim your greywater for irrigation, and are just trying to disperse it so it can be soil treated, as long as your dispersal area is large enough, you probably don't have to worry as much about "salting the earth". But, if there is any possibility that you will produce so much greywater that it will cause excess runoff, or be washed by runoff before soil treatment is completed, and thereby contaminating nearby waterways... be mindful of the fishies! In that case, you may want to consider constructing an artificial wetland with appropriate aquatic plants and beasties to treat your greywater instead of just pouring it out or spraying it on the ground.