Latest greywater data and calcuations spreadsheet (Excel), with citations. Note that as we refine the data the numbers in the spreadsheet and text may get out of sync; the spreadsheet represents our latest, most refined take.

1. Greywater is orders of magnitude less dangerous than combined sewage.
   Reported values for greywater fecal indicator bacteria vary widely. The use of indicator bacteria to gauge greywater pathogens is extremely suspect.
   
   For example, in the Arizona Water CASA greywater study , fecal coliforms were 822 mpn for greywater w/o kitchen sink water, and 8400 with kitchen sink water. Unless people in Arizona have a very unusual way of using kitchen sinks, these values are reflecting something other than feces going into the water; most likely, indicators breeding in the plumbing. Whether pathogens breed also is an open question.
   
   The average value for the Arizona study was just under 5000 mpn for fecal coliform indicators. A typical value for combined sewage is 3,000,000; six hundred times as much.
   
   The WERF study shows some researchers finding 100 mpn while others found three times as many fecal coliforms in greywater as raw sewage (10,000,000 mpn). That there could be this much feces/ pathogens in greywater fails the credibility test in two ways:
   
   a) With a billion system user years of exposure, lots of people would be getting sick if these high indicator levels corresponded to levels of pathogens
   b) A reality check on the use of indicators is to calculate, for example, the amount of feces you'd expect in greywater if, say, people showered instead of wiping after using the toilet: 1000 mpn, or 1/3000th as much as raw sewage. (If people wipe, the traces of feces remaining would yield more like 50 mpn, or 1/60,000th as much). See: Water indicator bacteria levels (xls).
   
   Several studies have demonstrated that indicator organisms can persist and even multiply in stored graywater due to available nutrients and/or biofilm formation which enhances pathogen survival (Rose et al., 1991; Ford et al., 1992). Moreover, pathogens seeded into graywater are capable of reproducing during graywater storage. Salmonella typhimurium and Shigella dysenteriae , for example, survived several days when seeded in graywater at pH 6.5 and 25C (Rose et al., 1991). This raises the question of whether the typical concentrations of indicator organisms used assess the human health risk with respect to fecal contamination in wastewater are a meaningful measure of the actual human health risk posed by graywater. Many researchers think not. Ottoson et al. (2003) indicated a potential for over-estimation of the fecal lead using Coliform as bacterial indicators for enteric pathogens. The conclusions encourage use of fecal enterococci as a guideline if one must be used. But the actual risk to human health associated with graywater reuse is not known because studies have shown that indicator bacteria can actually multiply while the graywater is in storage, while studies of actual pathogenic organisms have found that the pathogen counts decrease rapidly over time.
   
   from WERF study: Long-Term Effects of Landscape Irrigation Using Household Graywater (pdf), table 3.1, Arizona Water CASA greywater study
   
   
2. Seven percent of Americans currently have greywater systems; in California, 13.9% . Soap and Detergent Manufacturer's Association Graywater awareness and usage study (pdf , 2mb): A quarter page mail survey was sent to a nationally representative sample of 100,000 households, utilizing the Home Testing Institute monthly consumer omnibus, Insta-Vue...Total usable returns were 61,377 at a return rate of 61%... Greywater usage over 50% in Santa Barbara, 1990: Oasis Design door to door survey of a few dozen houses in Hidden Valley and San Roque neighborhoods. Oasis Design direct observation in Northern California rural areas, etc.: Greywater usage 15%. Arizona WERF study: Long-Term Effects of Landscape Irrigation Using Household Graywater (pdf)
   
   303,824,640 population of US, 2008
   x 7% US average for households that have greywater diversion systems, from Soap and Detergent Association (SDA) Graywater awareness and usage study (PDF , 2mb).
   =21,268,000 greywater users
   /2.6 average people/ house (US average, WERF study: Long-Term Effects of Landscape Irrigation Using Household Graywater (pdf)
   =8,180,000 illegal greywater systems
   
   Two million ± illegal systems in CA
   36,457,549 population of CA, 2006
   x 13.9% US average for households that have greywater diversion systems, from Soap and Detergent Association (SDA) Graywater awareness and usage study (PDF , 2mb).
   =5,068,000 greywater users
   /2.6 average people/ house (US average, WERF study: Long-Term Effects of Landscape Irrigation Using Household Graywater (pdf)
   =1,949,000 illegal greywater systems
   (this assumes the proportion of greywater use has not changed significantly since 1999. Using the 1999 population of 33,418,380, the number of illegal systems was 1,786,598).
   
   System user years -CA
   This is a back of the envelope-type calculation; the point is still valid if it is off by a factor of two.
   5,000,000 greywater users 2009, 13.9% (see above)
   2,000,000 greywater users 1950, 20% (estimate) of 10,000,000 people
   3,500,000 greywater users average for 60 years =
   210,000,000 system-user-years of greywater exposure, not counting neighbors and visitors.
   
   Note: the proportion of households with greywater systems was near 100% for the first few centuries in California. The first documented greywater system in Santa Barbara is visible today in front of the mission. It is a huge stone laundry wash station, with a dozen or more stations, that fed a cornfield where the lawn is today.
   
   System user years -US
   This is a back of the envelope-type calculation; the point is still valid if it is off by a factor of two.
   21,000,000 greywater users 2009, 7% (see above)
   15,000,000 greywater users 1950, 10% (estimate) of 150,000,000 people
   18,000,000 greywater users average for 60 years =
   1,080,000,000 system-user-years of greywater exposure, not counting neighbors and visitors.
   
   The other two million plus systems have been built poorly, with lack of professional installation help or useful official guidance.
   Personal communication with myself. We're like the greywater doctors for the world; we have been getting frank confessions from greywater users on a daily basis since 1990. We've sold something like 100,000 greywater books worldwide. By the most generous estimate, myself and other CA greywater "professionals" (defined here as someone who has made more than their own illegal system) have helped a lot less than two million households. So, it's safe to assume that most of these systems are suffering from severe design information deficit.
   
   Yet, even in this worse case health nightmare scenario (the status quo), people are not getting sick because of point #1 . Anti-greywater zealots have been asked for twenty years to show us the bodies, and in twenty years have not been able to come up with one documented instance of greywater-transmitted illness in North America.
   Here's an example of the kind of lengths you have to go to to get a theoretical 50% chance of greywater-transmitted illness (this was crafted by a health department official during the greywater regulation war of the early 1990s:
   
   "According to my calculations, someone is playing golf on grass irrigated with greywater, would have a 50% chance of getting hepatitis"
   
   I asked for the calculations and assumptions, which I then dissected. Here they are:
   
   -100% of pathogens on greywater stick to the surface of the grass, none follow the water into the soil.
   -The golf ball picks up 100% of the pathogens in a swath 50 yards long and its full diameter wide.
   -The golfer gets 100% of the pathogens from the ball to their hand.
   -The golfer gets 100% of the pathogens from hand to mouth.
   -The shower water has 1,000,000 fecal conform bacteria per 100 ml in it.
   
   I could see that these were pretty conservative (OK, well, outlandish) assumptions, with the exception of the last one, which I couldn't understand at all.
   
   Some research showed that this is something like a fifth of what you might find in raw sewage. Is that reasonable for greywater? Who the hell knows? How do you convert "1,000,000 mpn fecal coliform bacteria to a unit that a non-microbiologist can understand, like ppm? A bunch more research formed the basis for our Fecal coliform measurements page, which consistently ranks high for a search on fecal coliform+whatever.
   
   Well, it turns out that these weird units (by one of those funny coincidences) equate with parts per BILLION of feces, and hidden behind these barrier to entry/ understanding units was the most preposterous assumption: The shower water has several grams of feces in it. (For the metric-challenged, that is several hundred butt wipes). This would be a high reading for bidet drain water. For shower water, that's high by a factor of several hundred, at least in our house!
   
   
   
   (click to enlarge)
   
   
   NOTE: Dog poop is strewn all over the landscape much more widely and even less consciously than greywater. If dog poop were included on this graph, the upper end of the scale would twenty two times higher. The only other bar that would even be visible would be the toilet water one. With our bodies adapted to this scale of constant fecal assault, it is no wonder that the inconsequential threat from greywater vanishes in the statistical noise.
3. Only a few hundred systems permitted Santa Barbara, California is ground zero for greywater legalization in the US. I have been personally involved in the permitting of two systems, on in the city, one in the county. According to personal communications with city and county officials, less than 20 systems have been permitted in the city and county combined, total, from 1989 to 2009. My two systems are 10% of the permitted installations in the greywater capital of the US! ReWater Systems has permitted under 100 systems, they are the only greywater system supplier to survive from the early 1990s to the present day. The balance is a generous estimate for other suppliers.
   200 ± permitted systems/
   2,000,000 unpermitted systems
   =one in 10,000 systems permitted, or 0.01% compliance rate
4. Homeowners not deterred by tight regulation, professionals deterred Based on my own experience of nearly thirty years, I'd estimate that fewer than 10% of the public have qualms about ignoring unreasonable greywater regulations. Conversely, I often get complaints that homeowners cannot get licensed professionals to help them with greywater systems due to the latter's concern about their professional status and licence. I think that few people would argue that a plumber or builder—anyone who's done more than one system of their own—is likely to do a better job than an unaided homeowner.
5. Well-designed, very simple and inexpensive greywater systems can reduce the health and environmental threats from failing septic tanks and overloaded sewage treatment plants
   Logical inference from data on sewage system failure, septic system performance, UPC appendix K separation requirements, variation of soil purification capacity by proximity to the surface; seeTreatment effectiveness vs. wastewater application depth , Sewers & water quality , The World Health Organization on pollution plumes from dry pit toilet (CIF graphic, large file)
   The World Health Organization on pollution plumes from pit toilet in groundwater flow (CIF graphic, large file) Removal rates of land treatment facilities (CIF graphic, large file)
6. The earth's life support systems are being degraded at an accelerating rate. Worldwatch institute, National Geographic, EPA, etc. etc.
   The data on this are becoming so compelling that the judiciary is starting to take note in case law.
   Activists who painted the name of a politician in huge letters on the smokestack of a new coal-fired generator in England that he'd subsidized recently got off the hook for tens of thousands of pounds of clean up costs using the English version of the necessity defense.
   Presented with the current evidence on global warming, the Judge agreed that the clear and present danger was such that this action was legally protected in a way analogous to someone trespassing and damaging property in order to save a trapped child.
   
   This has sent some shock waves out. See Cleared! Jury Decides That Threat of Global Warming Justifies Breaking The Law
   The threat of global warming is so great that campaigners were justified in causing more than £35,000 worth of damage to a coal-fired power station, a jury decided yesterday. In a verdict that will have shocked ministers and energy companies the jury at Maidstone Crown Court cleared six Greenpeace activists of criminal damage.
   
   Jurors accepted defense arguments that the six had a "lawful excuse" to damage property at Kingsnorth power station in Kent to prevent even greater damage caused by climate change. The defense of "lawful excuse" under the Criminal Damage Act 1971 allows damage to be caused to property to prevent even greater damage - such as breaking down the door of a burning house to tackle a fire.
   
   Regulators that are tardy in the shift towards more ecologically sustainable systems may well face lawsuits for reckless, negligent endangerment of human life You want to be the one to find out the hard way?
   
   Obsessive fear of theoretical greywater cooties while the state drains of groundwater, empties of surface water, dries up and blows away...is just so last administration. The prudent course of action is now the same course of action that you could explain to your grandchildren with pride. See Legalize Sustainability for more on removing institutional barriers to sustainability.
   
   
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