Greywater is water from sinks, showers, washing machines and bathtubs. Each person generates about 35 gallons of greywater per day. An estimated 30% in the reduction of potable water use is possible if greywater is used to flush toilets, making greywater reuse an attractive conservation measure (Eriksson, 2002). Another study found that the potential potable water savings were up to 43% a based on Danish water use statistics at a greywater treatment plant in Denmark (Revitt, 2011).

Grey water is relatively clean water. The word "relative" is the rub. The water has bacteria in it and even small amounts of human feces. There are 13 million fecal coliform bacteria per gram of human feces. So even a small amount in wash water results in slightly polluted water. The real problem is that these bacteria are alive and multiplying. Storing greywater for more that 24 hours encourages an explosion of growth by the bacteria, especially in warm temperatures. Some reuse systems filter but don't treat the greywater but use it immediately so that no storage is involved. Other systems treat the greywater to remove bacteria and many other contaminants. Treatment is sometimes followed by additional disinfection.

Characteristics of Greywater

The content of the greywater varies by the source (kitchen, laundry or bathroom) and the country. The amount of organic material is much lower than in wastewater (blackwater) from toilets but the amount of heavy metals are about the same. One study identified 900 different substances potentially present in greywater. Almost all of the are man made and many are synthetic substances not found in nature. The substances in greywater include soaps, bleaches, detergents, shampoo, flavors, fragrances, softeners, preservatives, dyes, and solvents. Furthermore, all of these categories contain many different chemical formulations. The chemical oxygen demand (COD is the oxygen demand to reduce both organic and inorganic substances in a water sample) of greywater is quite high compared to blackwater but BOD is in the range of 90–360 mg/L, which is much lower than blackwater but still far above the EPA standard of 30 for secondary sewage effluent (Eriksson, 2002).

Bacteria are also present and these are of primary concern for impacts on human health. Guidelines for reuse of treated wastewater for non-potable reuse are emerging. California established the levels of total coliforms to be a maximum of 2.2 per 100 ml in reclaimed water for use in toilet and urinal flushing, commercial laundries and in decorative fountains.

In Florida, reclaimed water for toilet flushing and for the irrigation of recreation areas must contain no detected fecal coliforms per 100 ml (Crook & Surampalli, 1996).

The World Health Organization guidelines for treated wastewater used for irrigation of agricultural crops and public sports fields limit fecal coliforms to <1000 per 100 ml and nematodes to <1 per liter (World Health Organization, 1989).

In Australia, guideline values of coliforms that live in animals are set on four levels, for recreational applications these are <150 per 100 ml and for higher contacts e.g., irrigation of salad vegetables are lighter at <10 per 100 ml (Gregory, Lugg, & Sanders, 1996).

In Germany, the corresponding limits are total coliforms < 100 ml1 and fecal coliforms < 10 ml1 as well as Pseudomonas aeruginosa < 1 ml1 (Nolde, 1999) (Eriksson, 2002).

The greywater treatment system shown above is a carefully researched and monitored system. The greywater is distributed to the root zone of the plants and trickles through the media bed (made of plastic beads in this case) into a reservoir from where it is recirculated back to the root zone until the required water quality is achieved.

In the upper tank oxygen is delivered to bacteria as it trickles through a sandy media. The water regains oxygen as it drips from the top tank into the lower reservoir.

The system has several benefits:

1. Extensive passive aeration and recirculation enhances organic matter degradation. The vertical configuration reduces the area required making the system suitable for higher density residential districts.
2 Constant wetting of the upper bed maintains the microbial community. The recirculation water dilutes the raw greywater with partially treated water, thus buffering sharp fluctuations in raw greywater strength.
3. The system is modular, enabling more units to be attached, thus allowing upscaling relative to the size of the community.
4. The greywater unit is compact and aesthetic.
5. It prevents environmental nuisances such as bad odors and mosquitoes, and reduces the possibility of human contact with the raw or treated effluent.

The system performance was monitored for a year. Raw greywater was delivered to the top of the treatment system in three daily batches of 26 gallons and the water in the lower reservoir was recirculated at the rate of 20 gallons per minute. The table below illustrates the outstanding performance of the system. Ammonia levels dropped significantly in the treated water, followed by an increase in nitrate from negligible levels in the raw greywater to 25 mg/Lin the effluent. Minimizing nitrogen loss during treatment is a desirable result for water intended for drip irrigation since the nitrate in the treated water can reduce fertilizer requirements.

(Gross, 2009)

The greywater treatment system removed E. coli to below detectable levels, but two other pathogens S. aureus and
P. aeruginosa were often present in the absence of E. coli , although 99% of these other bacteria were removed by the system (Gross, 2009). Ultraviolet light disinfection of the treated water could be used to eliminate all threat of pathogenic bacteria.

Untreated greywater must be used quickly. The immediate reuse as irrigation water is a practical strategy for the homeowner who wishes to conserve water.

Simple greywater re-use irrigation systems collect and distribute water to the landscape. One of the simplest methods for harvesting and distributing greywater is by means of a pipe that diverts greywater directly from a sink, shower, or washing machine to a particular spot in the landscape (top left). Ideally the pipe would discharge the greywater onto a plant that thrives on intermittent episodes of deluge and drought. Careful selection of plants according to water requirement is therefore important. For example, a washing machine is less frequently used compared with a kitchen sink. Water coming from a washing machine produces a lot of greywater at intervals that are usually a few days apart. On the other hand, a kitchen sink is used almost every day and the plants that receive this water would have to tolerate conditions that remain wet most of the time, and that rarely dry out.

A slight variation is to connect rigid ABS to a flexible polyethylene pipe that can be moved from one tree or large shrub to another (middle image above). This has a tremendous advantage over the first system. The system can be modified to split once (above right) or many times.

Flow Splitter

An advantage of both these methods is that they can be installed by homeowners willing to do the work themselves. Depending on the complexity of the system installed, costs can range from $20 to around $2,000+ (Ludwig, 2007). Enlarge the drawings shown in the icons above and note that each case is a gravity system.

California case studies of installed greywater systems demonstrate that drip irrigation was not impacted by greywater systems unless there were very high calcium carbonate or magnesium levels in the water supply (of course, robust filter systems are necessary to allow drip irrigation to function properly). The water chemistry of raw greywater did not adversely affect the plants. Use of greywater for drip irrigation requires treatment to remove solids and soaps that would clog the drip filters, pumps and emitters.

In-home Systems

There are sink to toilet greywater reuse devices on the market, such as this product by WaterSaver Technologies.

The sink water first runs through a simplified dispenser that houses bromine and chlorine tablets. This unit can save up to 7 gallons of water per person each day. The unit cost is $300 (not including installation) and would result in water cost reduction of $20-$30 per year.

A demonstration project in Denmark illustrates the potential of greywater treatment and reuse for toilet flushing. Located in the basement of an apartment building in Copenhagen, Denmark, the Nordhavnsgarden treatment facility consists of a primary settling tank, a three-stage rotating biological contractor, a secondary settling tank, a sand filter, an ultraviolet disinfection unit, and a service-water storage tank. The system treats bathroom greywater from 84 one-bedroom apartments (117 inhabitants) are connected to this facility which treats for reuse as toilet flushing. The system is automatic and self-cleaning (Revitt, 2011). Other research indicates that the cost of adding a greywater treatment system to a five story multi-family building is one-half of one percent of the cost of the dwelling unit (Friedler, 2006). If land area is available, a constructed wetland to treat greywater is many time less expensive than the mechanical system described above.

Since the cost of potable water is inexpensive in the United States, the cost of greywater treatment systems are difficult to justify for the individual unit. Where the town or city has exceeded the carrying capacity of the environment to provide supplies of water, including treated drinking water, then reuse of greywater for non-potable is a reasonable response to resource scarcity. At the scale of the planned community, reductions in potable water costs and sewage treatment costs negotiated with the water supply and wastewater districts could make this sustainable practice economically feasible.

The image above shows a greenhouse that can be connected directly to a multi-family housing building or it could be a separate structure. From the greenhouse, the greywater could be pumped into a subsurface infiltration galley running down the length of planting beds.

Root crops, such as carrots and potatoes, cannot be irrigated with this untreated greywater because their roots could potentially make direct contact with the greywater and cause them to be contaminated before they are harvested and eaten. Optimally, the harvested vegetables should be those that supply stems or fruit for eating, such as celery and tomatoes. In the system shown above excess water drains to the native subsoil. The greenhouse system can benefit residents by providing food and a warm gathering place as a secondary benefit. It can also alleviate some of the wastewater burden that might otherwise be sent to the wastewater treatment plant (Ludwig, 2007).

Constructed wetlands are used to purify blackwater but are suitable for wastewater since greywater is less highly contaminated. A septic tank would be used to settle the solids out of greywater before the water is sent to the constructed subsurface flow wetland. Wetland treatment systems that hold the water below the surface of a gravel bed are the safest choice for treating greywater. We will study these systems in some detail in later tutorials.

However, a study demonstrated the performance of constructed wetlands in treating greywater for a nine person household in Brazil. Two wetlands were used in a sequence (series). The first was a horizontal subsurface flow wetland similar to the one shown (above, left) and the second was a vertical subsurface flow wetland. This system (above right) removed between 92% and 98% of the COD from the raw greywater. This measure is more important that BOD since there is less organic material in greywater than in wastewater and more chemical substances. The horizontal subsurface flow wetland was 1.6 m wide × 2.9 m long × 0.4 m deep and filled with fine gravel (diameter from 0 to 4.8 mm; porosity of 0.44). The vertical subsurface flow wetland was made from a round tank (diameter: 1.7 m, height: 1 m), filled with layers of (from bottom to top): coarse gravel (20 cm), fine gravel (10 cm), coarse sand (55 cm) and fine gravel (5 cm).

A patented system by ReWater Systems, Inc. (above left, www.rewater.com) involves purifying the greywater through a sand filter. The system is automated and sends the purified water into inverted pots with drip emitters. The cones are buried underground and can irrigate lawns with fairly even coverage, but it is unusual to irrigate turf with drip irrigation. The system is contains pumps and other equipment that must be inspected annually. New systems can cost from $2,000 to $6,000 (Ludwig, 2007) and supply the needs of one household that is purifying its greywater for use in the landscape.

The system shown above is residential in scale but combines rainwater harvest and on-site combined blackwater and greywater treatment and reuse. The cost is high but offset by no water supply or sewage infrastructure costs.

A water treatment system including a septic tank is the most expensive automated system. This patented system was developed by Orenco AdvanTex (www.orenco.com) and will purify greywater and blackwater. A large underground septic tank processes the wastewater through various filters and purifies it enough for drip irrigation (Ludwig, 2007). These systems cost from $5,000 to $30,000. Yearly maintenance fees of $150 to $500 are required as part of the contract. A drawback of the Orenco system is that the high cost of installation and the yearly maintenance fees make it almost impossible to recover any water bill savings over the life of the system.

Only those greywater systems that are most similar to the septic tank and leach-field system that is currently permitted under the Uniform Plumbing Code have a reasonable potential to gain approval for use in many states in the short term.

The EPIC system is a subsurface irrigation system, patented by Rehbein Solutions (www.rehbeinsolutions.com), and uses underground storage chambers that can be filled with water. The water from the chambers is slowly released to plants through upward capillary action in the soil profile. If there is not enough greywater for the irrigation, potable water can be added to the underground cells. The EPIC system has successfully been used with unfiltered greywater in many applications in the United States and the Middle East.

Stud Questions

Approximately how much greywater is generated by each person per day?

About how long can greywater be stored before it quality drops precipitously?

List three categories of synthetic substances found in raw greywater.

Compare greywater and blackwater in terms of BOD and COD.

For the image above, discuss 1) how the water is oxygenated 2) the recirculation rate 3) ammonia removal 4) nitrate gain 5) reduction in E. coli.

The cost of adding a greywater treatment system to a five story multi-family building was ________________ percent of the cost of the dwelling unit.

The Brazilian example of a constructed wetland system for treating greywater found the at lest _______% of the COD was removed from the raw greywater.

Describe the EPIC greywater reuse system and how it delivers water to the root zone of plants without drip emitters.

References

Eva Eriksson , Karina Auffarth, Mogens Henze, Anna Ledin. Characteristics of Grey Wastewater. Urban Water, 4 85–104, 2002.

Amit Gross, Drora Kaplan, Katherine Baker. Removal of chemical and microbiological contaminants from domestic greywater using a recycled vertical flow bioreactor (RVFB). Ecological Engineering 3 1:107–114, 2007.

E. Friedler, M. Hadari. Economic feasibility of on-site greywater reuse in multi-story buildings. Desalination 190 221–234, 2006.

Art Ludwig. Create an Oasis with Greywater. 5th edition 2007. 

D. Michael Revitt, Eva Eriksson, Erica Donner. The implications of household greywater treatment and reuse for municipal wastewater flows and micropollutant loads. Water Research 45 2011.