Skip to main content
Workforce LibreTexts

2.5: Alternative Water Supplies

  • Page ID
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)

    ( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\id}{\mathrm{id}}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\kernel}{\mathrm{null}\,}\)

    \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\)

    \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\)

    \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    \( \newcommand{\vectorA}[1]{\vec{#1}}      % arrow\)

    \( \newcommand{\vectorAt}[1]{\vec{\text{#1}}}      % arrow\)

    \( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vectorC}[1]{\textbf{#1}} \)

    \( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)

    \( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)

    \( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)

    \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)

    Alternative Water Supplies are supplies other than groundwater and surface water. Frequently, they are “reused” supplies, meaning they were potable water supplies that were captured for use individually or system-wide after being used once.

    Here are a few quick definitions to begin:

    Recycled Water

    Gray water

    Storm water

    Desalinated Water

    Heavily treated wastewater that is used for irrigation, groundwater replenishment and as a subsurface barrier against seawater intrusion; typically in California, recycled water is used for irrigation

    Household wastewater, including water from the washing machine, shower, bathroom sinks, that is captured and reused, but excluding blackwater, which is water from toilets and kitchen sinks; commonly referred to as "lightly used" water

    Runoff from precipitation (rain or snowmelt) that flows overland; may mobilize pollutants and is better to capture on site to replenish groundwater

    Ocean or brackish water that has had the salt removed to make it potable; two primary methods are used worldwide, but in the United States, reverse osmosis is most frequently used

    Learning Objectives

    After reading this section, you should be able to:

    • Analyze the water supply portfolio for several geographic locales in terms of the likelihood of adding an alternative water supply
    • Differentiate among types of alternative supplies and their appropriateness, given different situations

    Recycled Water

    You may hear “recycled water” used in a variety of ways in the United States and abroad. In California, specifically, there is some confusion in terminology. Changes to a variety of codes occurred in 1995 when “recycled water” became the term of choice rather than “reclaimed water.” These are essentially the same thing. Regulations for the level of treatment for various uses are in Title 22 of the California Code of Regulations.

    What happens to wastewater when it leaves your house? It travels through a series of larger and larger pipes to a wastewater treatment facility. Alternatively, if you live rurally, your wastewater may be held and separated in a septic tank on your own property. Wastewater that has undergone primary treatment has had the solids removed. Wastewater that has undergone secondary treatment has had organic materials removed through biological processes. Secondary treated wastewater can be used for groundwater recharge and irrigation. Water that has undergone tertiary treatment has undergone sedimentation, chemical flocculation, and filtration. As you might imagine, water that has undergone tertiary treatment has a range of uses, even those involving body contact, such as recreational use in lakes, as well as irrigation.

    Recycled water has only system-wide applications in that individual homes are not creating and using recycled water. You may be familiar with recycled water for irrigation of golf courses, medians, but it also can be used for groundwater recharge, including to act as a seawater intrusion barrier. Orange County Water District treats recycled water in a three-step process of microfiltration, reverse osmosis, and ultraviolet treatment. The treated water is then injected into the groundwater basin. The water serves as a barrier against the intrusion of ocean water into the aquifer.

    Recycled water seems like an important source to add to a water supply portfolio. After all, what community in California wouldn't want a reliable source of water for irrigation of landscapes? And what coastal community wouldn't want water to inject into the ground as a barrier of seawater intrusion. Overall, recycled water can decrease the demand for potable water by providing an addition to a water supply portfolio. But recycled water is expensive. Next to desalination, it is one of the most expensive options out there (think of the treatment costs!). Like many aspects of infrastructure, recycled water is politically appealing to residents, but the added costs are not.

    Gray water

    Certainly, the water that you drink, wash your clothes in, and use to bathe needs to be potable. But what about the water that you flush your toilets with? What about the water that you irrigate with? The premise of gray water use is that not all water that we use on a daily basis needs to be potable. Keeping this in mind, you then have to consider what can be reused in your indoor water use.

    Wastewater from toilets and wastewater from the kitchen sink both contain bacteria from feces or meats. But wastewater from a washing machine can be reused without many concerns, assuming diapers are not washed or clothes are not highly soiled, greasy or contaminated.

    The easiest gray water system to construct is called “Laundry to Landscape.” Wastewater from the washing machine is directed to a drip irrigation system outside the house. Water isn’t stored in any fashion - when you run a load of clothes in the wash, you are irrigating with the wastewater directly afterwards. You can see that you would need to time your laundry - washing everything on Sunday would lead to too much water for irrigation. Doing a load of wash every other day might be enough water for irrigation in the summer.

    What do you need in order to have a simple Laundry to Landscape system?

    • Your washing machine should be located close to an exterior wall in order to run a pipe to the outside of the house
    • Your house should be slightly above the area that you are irrigating so you can use gravity to direct the water to the drip system and the plants without a pump (though pumps that remove water from washing machines can be powerful and enough to move water some distance);
    • You need to have plants that can be irrigated with drip irrigation (shrubs or trees); and
    • You need to install a diverter valve at the washing machine that would allow especially dirty loads of laundry to drain toward the sewer or septic tank and not into your irrigation system.

    There are more complicated gray water systems that involve storing gray water in tanks and using pumps and filters, but most gray water practitioners agree that simple is best.

    Photo of a diverter valve by Stephanie Anagnoson is licensed under CC BY 4.0
    Figure \(\PageIndex{1}\)

    Here are some best practices in residential gray water systems:

    • Don’t store gray water
    • Minimize body contact with gray water
    • Allow gray water to infiltrate the soil with drip irrigation, not pool on the surface
    • Simple is better. Avoid pumps and filters.
    • Install a diverter valve
    • Match needs of plants

    Please note that these are best practices, but not necessarily rules and regulations. Rules and regulations vary by city and county.

    In a recent study of graywater systems in the Bay Area, it was noted that the most common problem in gray water systems was clogs, but this wasn't much of a surprise because most people reported performing no maintenance on the system. Plants were generally just as healthy as with a standard irrigation system and some were overwatered and some were under watered. Overall, people saw an average reduction in water use of 26%. There was at least one unintended consequence - with an abundance of water to irrigate, some people planted more plants and their irrigated area increased in size.

    There are a number of institutional hurdles to expanding gray water use. The primary hurdle is one related to the construction of homes: homes are plumbed with intermingled graywater and blackwater. This means that the wastewater from the entire home is treated as blackwater and sent to the sewer or septic tank. Additionally, if you check the city, county and state code related to graywater, they are frequently contradictory. California code, Title 24, Part 6, Chapter 16 a, Part 1 establishes minimum requirements for gray water regulations. Additionally, AB 849 (Gatto) prohibits local jurisdictions from banning gray water. However, a city or county may impose additional regulations so that it becomes too complicated to establish gray water in the home. Furthermore, as you can imagine, setting up a gray water system relies on a knowledge base of basic plumbing and wastewater. It is probably not an overstatement to say that most customers try not to think about this.

    Misconception Alert!

    Many people use “graywater” to refer to all sorts of alternative supplies, including recycled water and stormwater. This is simply incorrect, but a common misuse of language. Graywater must be water from indoor use that can be reused, typically for irrigation. It is not wastewater or stormwater.


    Picture the last rain storm that you remember in Southern California. Was there gentle rain for a long time? Or was there a short burst of rainfall? When there is an entire day (or even just an afternoon) of gentle rain, the rain is usually able to infiltrate into the ground, and eventually recharge the aquifers beneath the surface. But much of the rainfall that we receive in California is in bursts with heavy downpours and then days, weeks, and even years of nothing. This type of heavy rain results in a lot of run off. Stormwater is run off that can be captured.

    Stormwater becomes a problem when it encounters a lot of impervious surfaces, such as asphalt and concrete. These surfaces may hold visible pollutants (e.g., trash, dog poop) and invisible residues (e.g., pesticides, herbicides). When stormwater encounters impervious surfaces and pollutants, it turns can drain pollutants into storm drains and eventually the ocean.

    What slows down stormwater? Vegetation and pervious surfaces. These sorts of textured surfaces allow water to infiltrate the soil. Parkways, the area in between the sidewalk and the street, can slow water from running off properties and into the street. Planter beds near downspouts can let water percolate rather than run into the street. Plants, whether shrubs, or groundcover, or even trees, can slow water down on slopes and hillsides.

    What about rain barrels? Aren't they a good way to capture stormwater? Rain barrels are typically used on a residential site to capture water that comes off the roof through the rain gutters. In many parts of the country, rain barrels work well because the water needs of the plant correspond to the times when there are heavy rains. In much of California, the rainfall occurs in the cooler times of the year when the plant water needs are minimal. This means that water in a rain barrel must be stored for lengthy periods of time. And this is where we run into issues with creating the perfect habitat to breed mosquitoes - it’s still water available for irrigation, but it may be there for more than 4-7 days, which is all mosquitoes need to breed.

    There are other (and better) ways to capture rainfall. On a small scale keeping vegetation on hillsides and parkways, directing downspouts to planter beds, and keeping lots of green plants will decrease stormwater run-off. On a larger scale, stormwater can be captured within a neighborhood. Several neighborhoods within Los Angeles are tackling this. Elmer Avenue in Los Angeles has a stormwater capture area. Catch basins with soft bottoms allow water to percolate into the groundwater rather than re-enter the stormwater system and get flushed into the ocean. There are also bioswales in the yards near the catch basin to slow the water and allow it to infiltrate rather than run off.


    You may hear desalination or “desal” frequently touted as the solution to all water supply problems. You may wonder why there are not more desalination plants around if it is such the perfect solution. Good question—read on.

    There are two primary methods of desalination: thermal and membrane. In thermal desalination, water is heated in a boiling chamber, it then condenses in a dome, and collects in a chamber leaving all the salt behind. In membrane desalination, seawater is screened, filtered, and pushed through a reverse osmosis membrane under high pressure, and then the distilled water is treated to drinking water standards. These methodologies aren’t that complicated, but they do tend to use large amounts of energy, leading to a high cost.

    For both methods of desalination, there are similar hurdles:

    Seawater intake—Seawater needs to be removed from the ocean very carefully so as not to hurt plant and animal life. Typically, a speed slower than the ocean current is best.

    Power consumption—Typically, the reverse osmosis process uses the most energy, which contributes to the highest costs

    Brine—While brine can be returned to the ocean, most plants and animals thrive in a small range of salinities. There may be environmental consequences associated with creating areas of higher than normal salinity.

    Santa Barbara, California, provides an interesting case study in desalination work. As a result of the drought in the 1980s, the City of Santa Barbara along with Montecito and Goleta constructed a desalination facility with a capacity of 7,500 acre-feet per year (AFY) with expansion to 10,000 AFY. Construction costs of $34 million were shared based on proportions of water provided for the City of Santa Barbara of 3,181 AFY, Montecito of 1,250 AFY and Goleta of 3,069 AFY.

    By the time the desalination facility was built, the drought had ended and a period of heavy rainfall had begun. The plant operated for March, April, May and June of 1992 during and after a time of heavy rainfall. Then the desalination plant was put on standby. After it was paid off over 5 years, Goleta and Montecito decided not to renew the contract and subtracted desalination from their water supply portfolio.

    Santa Barbara has recently decided to re-activate the desalination plant as the sole funder of this enterprise. The facility will produce 3,125 acre-feet per year or roughly 30% of Santa Barbara's water supply in a year. In terms of hurdles, the intake is 2,500 feet off shore with openings of 1 millimeter and takes in water at a rate of 0.5 feet per second, which is slower than the existing current. Additionally, the power consumption for reverse osmosis has decreased by 40% since the facility was designed. The brine has twice the salinity of seawater and will be discharged at an outfall shared with the wastewater treatment facility. A study was recently completed with Scripps Institute of Oceanography, which suggested the city can comply with discharge requirements.

    It probably goes without saying, but let's say it anyway: desalination is more appropriate for coastal communities, such as Santa Barbara, San Diego or Santa Cruz. For an inland community to fund desalination, the inland community would also be evaluating an expensive pipeline to transport water or negotiating a water exchange with a coastal community in order to avoid building the pipeline. Proximity to the source is everything in desalination.

    Alternative Supply

    What is it?

    What are the benefits?

    What are the drawbacks?

    System-wide Opportunities?

    Desalinated water

    Potable water that was ocean or brackish water that has the salt removed to make it potable

    A drought-proof supply

    Generally, the most expensive source of supply.

    Disposal of the brine can be problematic. High energy use.


    Gray water

    Non-potable household wastewater, including water from the washing machine, shower, bathroom sinks, that is held and reused; excludes blackwater, which is water from toilets and kitchen sinks

    Cost-effective method for residential irrigation

    Supply and demand must be balanced.

    Easy to end up with too much supply

    Water can contain pathogens.


    Recycled water

    Non-potable water that is treated wastewater that is used for irrigation, groundwater replenishment and as a barrier against seawater intrusion

    Non-potable use of water; can recharge groundwater

    Expensive; requires separate plumbing system of purple pipes



    Non-potable water that is runoff from precipitation (rain or snowmelt) that flows overland; may mobilize pollutants

    Can recharge groundwater

    Must be captured, retained and allowed to percolate


    Try It!

    1. A coastal community in Southern California is considering diversifying its water supply with an alternative water supply. Which type of water supply is considered “drought-proof”? Why?
    2. An inland community is looking for an alternative supply to use for recharge. Which type of supplies make sense to use for recharge? Why?

    Key Terms

    Desalinated water—Potable water that was ocean or brackish water that has the salt removed to make it potable; considered a drought-proof supply

    Gray water—Non-potable household wastewater, including water from the washing machine, shower, bathroom sinks, that is held and reused; excludes blackwater, which is water from toilets and kitchen sinks

    Recycled—Non-potable water that is treated wastewater that is used for irrigation, groundwater replenishment and as a barrier against seawater intrusion

    Stormwater—Non-potable water that is runoff from precipitation (rain or snowmelt) that flows overland; may mobilize pollutants

    This page titled 2.5: Alternative Water Supplies is shared under a CC BY license and was authored, remixed, and/or curated by Stephanie Anagnoson (ZTC Textbooks) .

    • Was this article helpful?