August 19, 2009
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Toilet-flush water is called blackwater. Contaminated or difficult-to-handle greywater, such as solids-laden kitchen sink water or water used to launder diapers, I call "dark greywater," though most regulators consider these blackwater. However, the level of pathogens in even the darkest greywater is a small fraction of that in blackwater.
Wastewater without added solids, such as warm-up water from the hot water faucet, reverse-osmosis purifier drain water, or refrigerator compressor drip, is called clearwater.
What to Do With Greywater?
Conventional plumbing systems dispose of greywater via septic tanks or sewers. The many drawbacks of this practice include overloading treatment systems, contaminating natural waters with poorly treated effluent, and a high ecological/economic cost.
Instead, you can reuse this water. The most common reuse of greywater is for irrigation. It can also be cascaded to toilet flushing or for laundry.
Why Use Greywater?
It is said that there is no such thing as "waste," just misplaced resources. Greywater systems turn "wastewater" and its nutrients into useful resources. Why irrigate with drinking water when most plants thrive on used water containing small bits of compost?
Unlike many ecological stopgap measures, greywater use is part of the fundamental solution to many ecological problems. The benefits of greywater recycling include:
* Reduced use of freshwater. Greywater can replace freshwater for some uses. This saves money and increases the effective water supply, especially in regions where irrigation is needed.
* Less strain on septic tanks or treatment plants. Greywater, which comprises the majority of the wastewater stream, contains vastly fewer pathogens than blackwater and 90 percent less nitrogen (a nutrient that is a problematic water pollutant). Reducing a septic system's flow by getting greywater out greatly extends its service life and capacity.
* More effective purification. Greywater is purified to a spectacularly high degree in the upper, most biologically active region of the soil. This protects the quality of natural surface and groundwaters.
* Feasibility for sites unsuitable for a septic tank. For sites with slow soil percolation or other problems, a greywater system can partially or completely substitute for a costly, over-engineered septic system.
* Reduced use of energy and chemicals. This reduction is due to the reduced amount of freshwater and wastewater that needs pumping and treatment. If you provide your own water or electricity, you'll benefit directly from lessening this burden.
* Plant growth. Greywater can support a flourishing landscape where irrigation water might otherwise not be available.
* Reclamation of nutrients. Loss of nutrients through wastewater disposal into rivers or oceans is a subtle but highly significant form of erosion. Reclaiming otherwise wasted nutrients in greywater helps maintain the land's fertility.
* Increased awareness of, and sensitivity to, natural cycles. The greywater user, by having a reason to pay more attention to the annual progression of the seasons, the circulation of water between the Earth and the sky, and the needs of plants, benefits intangibly but greatly by participating directly in the wise husbandry of vital global nutrient and water cycles.
When Not to Use Greywater
There are a number of possible reasons not to use greywater, or to use it only during certain times of year:
* Insufficient space. In some situations, neighbors are too close, or the yard too small or nonexistent.
* Inaccessible drainpipes. If all plumbing is entombed under a concrete slab, accessing most of the greywater won't be economical.
* Unsuitable soil. Soil that is either extremely permeable or impermeable may preclude the use of a greywater system.
* Legality concerns and/or permit hassles. In much of the industrialized world, the legality of greywater systems is a "grey" area. However, there seems to be movement toward a less paranoid, more realistic official attitude, concurrent with increased experience and improved systems, not to mention water shortages and pollution problems.
Elements of a Greywater System
What a greywater system does is collect greywater, then divide and distribute the flow among planted areas. Most of the systems are variations on this basic functionality, even those systems designed primarily to get rid of greywater rather than extract value from it. The "hardware" that accomplishes this generally consists of:
1. Greywater source(s): washing machine, shower, bathtub, and/or sinks.
2. Collection plumbing: pipes that transport greywater from the house to one or more points just outside the house.
3. Surge tank, filter, and pump: optional elements that add complexity and cost but make the distribution plumbing's job easier, especially for large flows.
4. Distribution plumbing: plumbing that transports greywater through the yard and divides it among plants.
5. Receiving landscape: soil, roots, plants, and mulch basins that contain, cover, purify, and use the greywater.
6. People: those who design, make, and maintain the system, generate the greywater, tend the garden, and eat the fruits.
Very Context-Dependent
A small change in your context--siting, sources, irrigation need, percolation rate, etc.--can mean drastic changes in your system selection and design. Most individual systems will follow the middle path: a carefully considered and optimized do-it-yourself residential retrofit system for gravity-flow irrigation, not too ritzy, built without a permit, in a moderate climate, with a plumber involved in the collection plumbing, and a septic or sewer backup.
However, multiple side trips from this main path may apply to you. For example, a quick and dirty (instead of carefully considered and optimized) approach is not necessarily wrong. Greywater systems can be slapped together without much thought or trouble--and the vast majority are. Greywater work is pretty forgiving, so no matter how it gets done, you'll realize a portion of the possible benefits.
If your system is to be for disposal only and not irrigation (for example, to take the pressure off a septic system), then information on optimizing irrigation efficiency will concern you only inasmuch as it helps you spread greywater over a wide area and avoid overloading the soil with water.
If your system is to be retrofitted to an existing building, you're pretty much stuck working with what you've got. But if it's for new construction, you have the opportunity to make everything better integrated--a much more challenging and potentially rewarding task.
If permits are involved, you'll need to deal with legal requirements that push you to do impractical things. To get through this maze, you may choose to arrange plumbing conventionally first, to pass inspection, and then add greywater plumbing later. If you keep blackwater and greywater lines separate, installing a greywater system later is easy.
A failing septic leach field can be restored to function by lowering or eliminating the flow of greywater through it. It is worth noting that savings on septic tank pumping and leach field replacement will pay for almost any greywater system.
A Few Assessments
A site map is a great aid for designing a system. A map with a scale of 1:100 with 1' contours is ideal. If such a map is not available, make a sketch showing the house, boundaries, irrigated areas, natural water features, utility lines, routes of plumbing under the house, and so forth.
Evaluating your water resources is important as well. The first question for any human habitation is, where does the water come from? Ideally, for minimum impact and long-term sustainability, rain should be the primary water source, followed by greywater, followed by gravity-flow spring or creek water, followed by well or municipal water. Wherever the water comes from, note its characteristics and limitations.
If you aim to live ecologically, thoroughly explore conservation options before turning to reuse. Because of the unavoidable inefficiencies inherent in greywater systems, the gain from reducing consumption with water-conserving fixtures is always greater than that from reusing the water.
For any system in any context, I recommend doing at least a rough reality check to make sure the quantities of output and intended use are in the same ballpark. It is commonly assumed that however much greywater there is and however much irrigation is needed, they'll magically match up. Even if you don't want to reuse the greywater, it's good to have an idea of how much you've got in order to know if your disposal/treatment area is big enough. For most purposes, you can just take the home's population and multiply it by an approximate daily use figure, as shown in the chart below.
Homes without water-conserving fixtures generate about 55 gal/person/day. Efficient fixtures cut this daily output to 40 gal/person/day, or enough to water four mature fruit trees or a dozen shrubs in an average climate. Extreme conservation habits can cut production to 5 gal/person/day.
On your site map, note where the greywater sources are and (if they are not combined) how much of the total flow comes from each set of sources. Group fixtures that share a common drain into subtotals (for example, the bathroom sink, tub, and shower often share a drain), and note where their plumbing goes.
Consider whether it is appropriate to move any greywater sources (such as a washing machine), add an outdoor shower, or take any other action to improve the fit of water sources to irrigation need. The places where greywater is generated should ideally be high on the property, so you can irrigate more area by gravity.
You should also note variations in the population of water users. If everyone goes away in the dry season for an extended time, this obviously affects the system's design; you won't want to set up water-loving plants to be irrigated by greywater only.
...To be continued.
Printed here by permission of Oasis Design, www.oasisdesign.net.
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TOC for Forest & River News, Summer 2009