Benefits Of A Graywater Harvesting System Environmental Sciences Essay
Greywater can be defined as any domestic wastewater produced, excluding sewage. The main difference between greywater and sewage (or blackwater) is the organic loading. Sewage has a much larger organic loading compared to greywater. Some people also categorise kitchen wastewater as blackwater because it has quite a high organic loading relative to other sources of wastewater such as bath water.
People are now waking up to the benefits of greywater re-use, and the term ;Wastewater; is in many respects a misnomer. Maybe a more appropriate term for this water would be ;Used Water;.
With proper treatment greywater can be put to good use. These uses include water for laundry and toilet flushing, and also irrigation of plants. Treated greywater can be used to irrigate both food and non-food producing plants. The nutrients in the greywater (such as phosphorus and nitrogen) provide an excellent food source for these plants.
What Are The Benefits of Greywater Re-use?
Re-using water does not diminish our quality of life; however it can provide benefits on many levels.
Two major benefits of greywater use are:
Reducing the need for fresh water. Saving on fresh water use can significantly reduce household water bills, but also has a broader community benefit in reducing demands on public water supply.
Reducing the amount of wastewater entering sewers or on-site treatment systems. Again, this can benefit the individual household, but also the broader community.
How Is The Greywater Treated For Re-use?
There are many ways by which to treat greywater so that it can be re-used. The various methods used must be safe from a health point of view and not harmful to the environment.
Some examples of natural greywater treatment systems are presented below.
The systems shown above rely on plants and natural microorganisms to treat the greywater to a very high standard so that it can be safely re-used. The main advantage with these types of systems is that they treat the greywater naturally, and also enhance the local environment because of the attractive plants used and the fauna attracted to them.
There are other natural systems available to treat greywater. The type of system selected will depend on the specific application, and selection would be considered on a case by case basis.
Planning a New Greywater System
There are two ways of planning your greywater system:
by assuming that the system is for you and your family's use (your actual situation);
or that the system is intended for the house regardless of who occupies it( i.e.,basing your calculations on the number of bedrooms the house has).
Steps to follow:
1. Take a brief inventory of the house's greywater sources and the number of uses that they get or could get.
gal / person . day
gal / person . day
gal / person . day
gal / person . day
gal / person . day
Try to determine how many gals. per cycle your appliances use--or use the short-form sizing estimator below.
Approximate water use of standard appliances
US clotheswashing machine (top-loading)
30 gallons per cycle
European (front-loading) clotheswasher
10 gallons per cycle
3 - 5 gallons per cycle
Low-flow shower head (per shower)
3 - 7 gallons per average use
Other sink use (shaving, handwashing, etc.)
1 - 5 gallons per average use
2. Use the General Site Data and Design Considerations below to determine what steps are relevant for your situation. Give special consideration to the final dispersion of the effluent, making sure that the soil can accept the amount of water that will be generated, treated and discharged (your local sanitation engineer can do a percolation test to determine the ability of the ground to accept water). If water shortage happens to be a particular restriction where you're located, note that greywater filtered through a soilbed of the sort described in this text will not become anaerobic and thus can be saved for [lawn] irrigation.
A. Overall Site Plan
B. Regulatory Requirements
Effluent Discharge Limitations:
Land Application Regulations
Local Board of Health
State Environmental Regulations
Building Code Requirements
Test Data for Approvals
C. Design Information
Existing Treatment Facilities
Influent Quality and Quantity
Number of Bedrooms
Number of Persons regulary in residence
Occupancy (i.e.,on a year-round or seasonal basis)
Type of Appliances and fittings
Effluent Reuse Goals
D. New System Specifications
Propose Treatment Train
Effluent Treatment Character -Typical Transformation
Reduction in Pollution
Production of Fertilizer
Optimum Use of Existing Facilities
New Use of Existing Equipment
Potential for Savings
Fees - Water and Sewage
Overall Improvement Evaluation
3. Check with your local authorities regarding any special/local concerns and regulations. Submit your application to the local board of health or consult your local professional engineer (P.E.) for plans and documents needed for your application (usually a topo-graphic site drawing with pertinent information about your site and the proposed solution). If your local P.E. is unfamiliar with alternative greywater pollution prevention systems (e.g., soilbed treatment), provide her/him with the name of this website or send a copy of this manual.
Aerobic Pre-treatment -- suitable for showers, hand-washing and laundry* water treatment.
The aim of this stretch filter treatment technque is simply the removal of large particles and fibers to protect the subsequent infiltration pipes from clogging and transferring it as soon as possible for treatment into a biologically active, aerobic soil-zone environment where both macro- and microorganisms can thrive. Stretch-filters are made to retain fibers and large particles and allow the rest of the organic material to travel on to the next stage of processing. This filter is suitable for public facilities where the principal source of greywater is hand-washing and showers without any food waste to speak of. If this type of filter is used to remove food wastes, these will accumulate in the filter which then becomes anaerobic and makes the effluent malodorous.The result is often that too frequent changes of the stretch-filter becomes necessary ---thus creating an undesirable, high-maintenance situation. See diagram below for a typical Clivus greywater management fabric filter configuration:
A very interesting replaceable filter technology from Finland is called Willa Eco-matic and consists of a fiber and ion-exchange media enclosed in a sack that can be lifted out of the container and replaced when slow or otherwise determined to be needed -- preferebly according to a regular service and maintenance schedule.
*Beware of the use of harsh detergents and bleach if the water is to used in or near gardens growing edible plants. Some surfactants are associated with controversial hormone-mimicking characteristics that we need to be aware of. We also need to be aware of handwashing soap containing antibacterial chemicals which are totally unnecessary and do not improve cleansing.
Anaerobic to aerobic pre-treatment
If any significant quantity of food waste enters the system from dishwashers and kitchen sinks receiving cooking grease and a fair amount of food residue, this option is recommended. A typical installation is not very different from a traditional system; but the treated effluent is of much better quality and does not pollute nearly as much. Ideally, it should consist of a three-stage septic tank for sludge and grease separation. The separated sludge can thus be removed less frequently [every fourth year instead of bi-yearly as is standard practice with many conventional systems]. The outgoing effluent in the septic system is anaerobic. Following the septic tank is a sandfilter designed for restoration of aerobic conditions. The final treatment stage leading to purified water of near potable-quality is treatment in a planter bed. This is not the most inexpensive solution. It is, however, one of the most effective, simple-to-maintain on-site treatment techniques available today.
Grease-trap/septic tank + sandfilter + sample/pump pit
Clockwise from top left:
1. Three-chamber septic tank
2. Sandfilter with a geo-textile cloth (courtesy of Civil Eng. Hans Lönn, Älgö Sweden)
3. Final result from the sandfilter (swimming-quality water)
4. Close-up of the effluent: odor-free, clear and suitable for planter irrigation.
Planter soilbox design
Soilboxes have been used for greywater purification since 1975 with excellent results. The planter bed has to be well drained to prevent the formation of a water-logged zone in any part of it. Therefore, its bottom contains a layer of polyethylene ;actifill; or pea gravel to provide effective drainage. A layer of plastic mosquito-netting on top of the actifill prevents the next layer of coarse sand from falling through. On top of the coarse sand is a layer of ordinary concrete-mix sand, while the top two feet consist of humus-rich top soil. Clay soils must not be used.
Water injection without erosion
The pressure infiltration pipes are designed to allow for even distribution of the water even on uneven terrain. They are easy to clean and should be placed on the soil surface after planting and then covered with a 2 to 4-inch-thick layer of wood chip mulch. In cold climates the change from shallow infiltration to a deeper layer can be accomplished by an automatic switch-over (as described below in Fig. 9). The pressure infiltration piping (shown above) consists of two concentric pipes, the inner one having holes pointing up and the outer sleeve fitting snugly over the inner pipe and somewhat expanded by the water pressure when ON. This causes the water to; bleed; out along the slot at the bottom. Water pressure OFF makes the sleeve close, preventing worms, insects and roots from entering the pipe and clogging it from the inside. Piping is available in 5-ft sections and can easily be coupled together through a quick-connect system. These pipes are easily cleaned without disassembly.
Gravity or pressure leaching chamber
Hanson Associates of Jefferson, Md., reports that leaching chambers have operated successfully at a loading of 2.4 gal/sq. ft. per day receiving all the greywater from a three-bedroom house. Using half a PVC pipe 6; diameter according to the figure below, this leaching chamber can be placed in a trench on a 1-2 inch mesh plastic netting (to prevent the walls from sinking into the soil). No pre-filtration is used--only a dosing pump chamber pumping every 8 hours.
Sizing for a residence
3 bedrooms @ 150 gal/b.r. = 450 gal/day. Subtract 50% for no-flush toilets and reduced-flow shower heads etc. = 225 gal/day. The minimum trench surface area is therefore around 100 sq ft using a loading rate of around 2-2.5 gal/sq. ft/day
5-20 feet long and 1 foot wide trenches.
100 sq. ft x 1; desired water depth = 62 gal per dosing
Dosing chamber can be a Clivus LPF pre-treatment filter container without the stretch filter.
Gravity switch from shallow to frost-free zones/levels
Figure 3 shows an example of automatic switching from the shallow leach chamber to one which is below the frost line. If the shallow trench freezes and becomes clogged with ice, the water will back up and spill over into the pipe for the deeper trench. It should be noted that greywater is warmer than combined sewage and that, therefore, shallow leach zones in operation tend to stay free of ice much longer than is normally expected of combined waste waters. One notes a warming of the soil and biological activity which make freezing rare even in fairly cold climates.
Automatic switching of levels using pump pressure is somewhat different from gravity pressure. In this case, a loop has to be arranged indoors where the pressure needed for the shallow infiltration normally is lower than the pressure required to force the water up to the top of the loop. The top of this loop must, of course, not exceed the shut-off head of the pump. A margin of around three feet of water is desirable. Figure 4
The system can equally be designed to be switched manually by the opening and closing of the valves feeding the different zones/levels.
Other Cold Weather Options
There are several greywater-irrigated greenhouses located in New England. Some of these have been in operation since the 1970's and feature a combination of automatically irrigated and fertilized growing beds which provide effective greywater treatment. One 12 ft x 36 ft greenhouse near Concord, NH uses as the final treatment--after the soil beds--a fish pool, which stays clear by means of a biological treatment technique involving a waterfall and bio-filter plates on the pool-bottom(21). Deep soil beds tend to store heat both from the sun and from the greywater itself. The New Hampshire greywater-irrigated greenhouse is the top cold climate producer of salad greens in the US according to a survey carried out by A. Shapiro at the National Center for Alternative Technology (22).
This greenhouse provides a family of 4 - 6 people with more than enough salad greens throughout the long New England winter. Winter vegetables grown in it include broccoli, spinach, lettuce, mustard greens and sorrel (see below) (15).
Another simple way to facilitate better distribution of the greywater in the soilbed is to make a pipe-loop that is fed from both sides -- see the sketch below:
There are several variations on the theme of outdoor raised soilbeds that effectively replace the soil needed for an effective leach field treatment of waste water. Houses on ledges or very sandy soils that would not effectively slow down effluent sufficiently to accomplish treatment can be fitted with masonry soilboxes which, in effect, serve to build up the site's soil profile. This strategy has been used in so- called mounds or Evapo-Transpiration beds (the name derived from the often erroneous assumption that all the water will evaporate to the atmosphere even in wet and cold climates).
In areas where the density of construction makes it difficult or impossible to build up a large mound or to locate planters on the property for treating the volume of effluent produced, two adjacent neighbors can agree to build property dividers and plant hedges or evergreens as their leaching area. This space-saving alternative combines privacy and aesthetics with good environmental protection. Greywater gardens as depicted above can offer the added benefit of one's being able to garden at a back-saving height...
In colder climates, treatment will be less effective during the winter season, and at times there may even be trouble as a result of freezing. However, this is often less of a technical problem than it may seem to be at first glance:
-When relatively warm greywater is injected into the soil, increased biological activity as well as a warming of the soil tend to keep the injection zone unfrozen longer than surrounding areas.
-In rural or suburban locales, raised beds/planters may often be ideal for use as leaf ;compost bins; in the fall. The leaves act as an insulator as well as a composting fuel-source that further insures that the soil underneath does not deep-freeze.
Shallow subsoil (i.e., at 2-6 inches below soil-level) irrigation is preferable to surface irrigation
whenever the water used is ;grey;( i.e. neither clean nor free of salts which leave saline deposits when applied on the soil surface);
when located in a high evaporation locale suffering from water shortage;
when one wants to produce leaf or garden waste compost fast and create and maintain an earthworm community in one's compost-pile;
for selective irrigation (a flower border, a shrub, a bush or a tree);
when you want to automatically irrigate a drained planter either indoors or outdoors.
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