Graywater Reuse Survey: Data & Evaluation of results
Mailings and Response Rates
Surveys were mailed to owner-occupied single family residences (SFRs) in the service areas of six water providers ( Avra Water Co-op, Flowing Wells Irrigation District, Green Valley Water, Town of Marana, Metro Water Improvement District, Town of Oro Valley, Ray Water Company, and Tucson Water.) Residents of two additional service areas, Green Valley and Avra Valley, also were mailed surveys, despite the dearth of SFRs. In Green Valley, surveys were mailed to owner-occupied townhomes and condos. In Avra Valley, surveys were mailed to the owners of residential parcels which had improvements valued at more than $20,000. This approach was necessitated by Pima County's classification of manufactured homes as something other than permanent structures, as nearly all these are lots with manufactured homes and only a few are traditionally constructed residences.
Recipients were identified from the October 1999 Pima County ARCVIEW data base. All SFR properties within service area boundaries where the owner and the occupant were identical were identified and recipients randomly drawn from those pools. This sampling methodology resulted in each home owner within a service area having an equal chance of receiving a questionnaire. Between service areas, chances of receiving a survey varied considerably. For example, a much smaller percentage of home owners in Tucson Water's service area received surveys than in the Marana service area. For this reason, some results presented in this report are weighted so that some tentative conclusions can be drawn regarding graywater reuse throughout Pima County. (The County is used as a proxy for the Tucson Active Management Area.)
Survey mailings and response rates are summarized in Table 1. Just under 2,000 surveys were mailed, with some 600 usable responses returned. Response rates vary from 11 percent in Ray Water to 46 percent in Green Valley. These differences between service areas are likely due to different socio-demographic characteristics. For example, this type of survey is more likely to be returned by retirees. The overall response rate, including incomplete surveys, is around 33 percent. This is quite high for a fairly long mail-out survey with no follow-up phone calls or postcards.
Reuse of Graywater
Survey recipients were asked if they reuse any graywater. Responses to that question are summarized in Table 2.
A total of 49 respondents, or 8.4 percent of all respondents, indicated they reuse some of their graywater. Reuse rates by service areas vary considerably, from less than 2 percent to 25 percent. Some of the variability is probably due to the relatively small numbers involved. Nevertheless, some of the differences are statistically significant.
Service areas break down into three groups. Green Valley, Metro, and Oro Valley all show less than 4 percent reusing graywater. Differences between these reuse rates are not statistically significant; however, differences in rates between these three service areas and the other four service areas are statistically significant, suggesting that they may share characteristics that are reflected in little graywater reuse. These are all relatively affluent areas with relatively new, high-value housing. Relatively few homes are on septic tanks.
By contrast, Flowing Wells, Marana, Ray, and Tucson Water all show much higher graywater reuse rates of between 13 and 16 percent. Differences among the graywater reuse rates of survey respondents in these four service areas are statistically insignificant. Three of these service areas are characterized by relatively low incomes and older, lower-value housing; Tucson Water serves a wide range of neighborhood types, but contains considerably older neighborhoods and low income areas as well. Septic tanks are somewhat more frequent in these service areas.
A startling 25 percent of Avra Co-op respondents report some graywater reuse. Because of the relatively few surveys mailed to the Avra Co-op service area, one cannot be confident that the reuse rate is significantly higher than in some other service areas. For example, the difference between Avra Co-op's reuse rate and the reuse rate in Ray Water is not statistically significant. Nevertheless, surveys were sent to Avra Co-op customers because of certain systematic differences in housing. The dwelling units are overwhelmingly manufactured housing and virtually all Avra Co-op customers are on septic tanks. Income levels tend to be fairly low, as well.
All this suggests factors that may increase the likelihood of graywater reuse:
- older homes
- lower value homes
- manufactured housing
- lower income levels
- septic tanks
These factors appear consistent with assumptions about what motivates some people to reuse graywater. Motivations presumably include:
- environmental sensitivity
- water conservation ethic
- desire to reduce one's water bill
- desire to reduce one's sewer bill or prolong the life of a septic system
The survey was conducted in such a way that it is possible to extract respondents' street addresses from survey forms. Street addresses can then be used to extract housing age and value from Pima County records and water demand levels and sewer system connections from water providers and Pima County Wastewater. Such analysis is not part of this study or report, but will be undertaken in the future.
Estimating TAMA-wide Reuse
To better estimate Tucson AMA-wide graywater reuse, a weighted average is calculated. Each service area's graywater reuse rate is multiplied by an approximation of its share of owner-occupied single family residences within Pima County (See Table 3, last two columns). The result is graywater reuse at an estimated 13 percent of owner-occupied SFRs and manufactured homes in Pima County. This corresponds roughly to between 20,000 and 30,000 households, or 40,000 to 80,000 persons.
This suggests the enormity of the public policy issue that graywater reuse represents. If graywater reuse often causes public health problems, then a very large number of people are at risk. If, on the other hand, the great majority of graywater reuse situations pose little if any health risk, then the lost opportunities for greater water conservation are enormous.
Household Size and Composition
Another apparent difference between the service areas where graywater reuse is rare and those where it is more prevalent is household demographics. Households in Green Valley, Oro Valley, and Metro Water are, on average, smaller and older than households in the other five service areas. Fortunately, we have direct evidence on this issue from the survey.
Respondents were asked how many people lived in their household. They also were asked to indicate how many of these people fell into various age brackets. (Unfortunately, some respondents mis-read this follow-up question. For example, several checked the 0-4 bracket, apparently indicating that there were between 0 and 4 persons in the household, rather than writing how many persons in the household were between the ages of 0 and 4.)
There was no strong a priori belief concerning the impact of household size and ages on graywater reuse. On the one hand, the more people in a household, the more graywater is generated thereby increasing potential water conservation and utility bill savings. On the other hand, graywater reuse requires some time and effort. Heads of households with multiple children might not have the time or energy to devote to constructing and maintaining a graywater system.
Data for respondents' household size as a function of service area and whether the household reuses graywater is summarized in Table 3.
The data support the view that households with more residents, and therefore more graywater on average, are somewhat more likely to reuse some graywater. Differences in household sizes between graywater reusers and others within service areas are generally not significant because of the small number of cases involved. However, the differences for all respondents is significant, although not terribly large.
Sources of Graywater
Those respondents reporting graywater reuse were asked from which sources they put graywater to use. Responses are summarized in Table 4.
The most frequently tapped source of graywater by far is clothes washers, accounting for 66 percent of all graywater sources. Next are bathroom tubs/showers (15 percent) and kitchen sinks (10 percent). Bathroom sinks account for only 5 percent of graywater sources, and "other" accounts for the remaining 4 percent. The relatively small numbers of graywater reusers does not allow any conclusions to be drawn about types and numbers of graywater sources by service areas.
The great majority of graywater reusers tap a single source, with a minority tapping two or three sources. On average, reusers tap 1.2 sources of graywater.
Age of Graywater Systems
Respondents were asked to report the age of their graywater systems. About 20 percent of those who reported reusing graywater did not report an age for the system. This could be due in part to some respondents not considering themselves to have a "system"; for example, many apparently simply direct washing machine drainage water onto vegetation via a hose. Others may have bought homes that already had graywater systems. For those 80 percent who did report an age, it ranged from a few months to 30 years.
Responses are summarized as mean (average) age and median (typical) age in Table 5. The average or mean age of all systems is 9.2 years. The typical, or median age of all systems is 7.5 years. Comparisons across service areas are not meaningful because of the small numbers involved.
Storage of Graywater
Only two respondents reported storing any graywater. Both reported aboveground storage, with capacities of 20 and 165 gallons.
Treatment of Graywater
Five respondents, or approximately 10 percent of graywater reusers, reported they treat their graywater. Two filter it, one uses bleach in the clothes washer, and one uses no-phosphate detergent. One did not describe the method of treatment.
Application of Graywater
Respondents were asked how they apply graywater to its various uses. Responses are summarized in Table 6 below.
Graywater reusers resort to an average of 1.3 methods to apply graywater. The most common method is surface application (34 percent), followed by garden hose (20 percent) and by bucket (15 percent). Other application methods reported include below ground (11 percent), soaker hose (8 percent), bubbler (3 percent) and "other" (9 percent). In this latter category were large, fixed pipes, trench, and bucket-like containers.
No one reported using a drip system to apply graywater to vegetation. Also, there is some suspicion that the "below ground" response was given by some persons who apply water to the surface, but then it sinks "below ground."
Uses of Graywater
How respondents use graywater is summarized in Table 7. Graywater reusers indicate an average of nearly two uses each. Irrigation of shade or ornamental trees is by far the most common, accounting for 32 percent of all reported uses. This is followed by irrigation of shrubs (19 percent) and grass (14 percent). These three uses account for two-thirds of all graywater uses.
Relatively few graywater reusers apply it to potential food sources, with 9 percent reporting irrigation of fruit/nut trees and 4 percent irrigating vegetable/herb gardens.
Why Graywater is Not Reused
Over 90 percent of respondents indicated they do not reuse any graywater. Reasons offered for this are reported in Table 8. Responses varied considerably, with about 1.7 reasons offered per respondent.
The top reason offered for not reusing graywater is "don't know how." Another common reason is "need info./assistance." This suggests that if the legal barriers were lowered and public education and incentives offered, graywater reuse might increase considerably.
Several of the reasons offered are similar, by design. The 13 specific reasons can be grouped into five categories. For example, "don't know how" and "need info./assistance" both indicated the respondent needs help. These two categories account for 30 percent of all reasons given.
Similarly, "isn't worth the trouble", "not enough time" and "not worth the cost" all indicate a similar lack of motivation. These three reasons account for 20 percent of all responses. "Water not near use" and "no use for water" indicate reuse is not practical, and account for 19 percent of responses. "Not sure safe/sanitary" and "water is salty/chemicals" suggests health or environmental concerns. These account for 15 percent. Legal, permitting, and permitting issues account for 7 percent of responses, and "other" accounts for the remaining 10 percent
Survey Summary
It appears that 20,000 to 30,000 households in Pima County might currently be reusing graywater. These households contain 50,000 to 80,000 persons. Graywater reuse is a major issue in terms of the number of persons involved.
It appears that graywater reuse is more common in older homes and lower-income areas. Residents of manufactured homes may be particularly likely to reuse graywater because of the greater access to wastewater plumbing. Septic system preservation may be a factor in some graywater reuse.
Whether most of these systems are being operated in a safe and sanitary manner is not known. However, the survey results also suggest that if graywater reuse is determined to not be a public health issue, and if permitting requirements are relaxed, graywater reuse might increase substantially. Whether and how ADWR and municipal water providers should encourage graywater reuse are issues that should be addressed.
