For many reasons, it may be beneficial to keep greywater separated from the remaining sources of wastewater (blackwater and dark water). Technically speaking, it will require separated plumbing throughout the house (what practically means – adding an independent greywater line pipe). While it may be a demanding task in houses with already existing plumbing, in new ones it will be rather a low-cost effort.
Concept of the modern, eco-friendly wastewater system separating black and greywater. Source: “Greywater as a sustainable water source: A photocatalytic treatment technology under artificial and solar illumination”, by Sophia Tsoumachidou, Theodora Velegraki, Apostolos Antoniadis, and Ioannis and Poulios, Journal of Environmental Management, (Science Direct).
Note, that thanks to the advanced greywater treatment system, dark water (kitchen sink) is also directed to the greywater system. This leaves the possibility to replace flushing-toilets by composting ones and fully eliminate the source of blackwater!
For details see: Dry Toilets
When it comes to exterior installations – note, that potential environmental and health hazards caused by the release of greywater are lower compared to those created by blackwater. Lack of fats and cooking oils prevents the clogging of the treatment system, lack of solid waste – almost eliminates the need for service and periodic removal, while the potentially lower level of microorganisms and pathogens minimizes health risks. As a result, legislation-imposed restrictions are easier to meet, the treatment process is less demanding, and the disposal system is smaller. Other benefits: treated greywater can be used to a larger extent throughout the house for non-potable applications (laundry, flushing toilets, etc) as well as outdoors for landscape irrigation.
The concept of mechanical filtering (here in its crude form symbolized by a “basket-type filter). Source: “Wastewater Reuse” – (Your Home – Australia’s guide to environmentally sustainable homes).
In the simplest form, the greywater drain-field can be located on any area of the landscape, preferably near ornamental trees, bushes.
Basic Greywater Disposal System. Source: “Low-tech Greywater Treatment”, Waldeneffect.org (quoted from the book “The Good House Book” by Clarke Snell)
The typical garden soil (provided it is not clay) acts as a big, natural filter. As greywater passes through different layers of soil, contaminants are trapped and subsequently broken-down by microorganisms. Note, that the upper layer of soil is usually oxygen-rich, so full of “biological life” – not only microorganisms but also fungi, worms, plants’ roots, etc. Most of what we call contaminants, especially after being digested or decomposed by microorganisms, represent the food that will be absorbed by mentioned above living organisms. They efficiently prevent build-ups of raw matter and chemical compounds. The greywater itself, mostly decontaminated and stripped from most pollutants percolates through deeper layers of soil to recharge groundwater.
Greywater distribution hub (Note a large amount of mulch in this mini-drain-field). Source: “Greywater Systems” – a fragment of the system installed at Casa Juliana (south Texas), (Omic.net)
Quoting authors of Casa Juliana, (Omic.net): The composting mulch surrounding the infiltration chambers teems with worms, arthropods, fungi, molds, roots, etc. As compared with a conventional septic system and leach field, the level of water treatment in this system is almost certainly higher due to greater oxygenation through the wood chip mulch and to high levels of biological activity within the mulch. Also, scientific research has shown that tree roots are effective in absorbing significant quantities of nitrogen, one of the two pollutants health authorities are most concerned with in residential wastewater systems.
Practical implementation of the mechanical filtering: Grey Water Diverter Device – Aqua2use (model GWDD). Source: Water Wise Group (CA, USA)
A close look at the interior structure of Aqua2use Gravity (left) and GWDD pump-assisted (GWDD) mechanical-filtering systems. Source: Water Wise Group (CA, USA)
According to the manufacturer, the above Aqua2use greywater diverting systems use 4-stage filtration to remove lint and debris. They can be operated by forces of gravity or an electrical pump (the former offers better cleaning efficiency at the cost of lower water flow). The outlet water is suitable for subsurface drip-irrigation of the ornamental landscape.
The Legality of Greywater Reuse
Before going further, let’s “touch” the legal aspects of reusing greywater. Frankly, in many countries, it is still an open issue with more questions than answers. In countries known for shortages of freshwater like for example Australia, the relevant legislation is already in place, and the government offers support for local initiatives to increase greywater reuse. In the US, it is still a work in progress, so often you will face contradictory opinions.
The map below shows the US states where the use of greywater is permitted (blue) and the states that do not have greywater regulations or where its reuse is illegal (white). The map is probably 20 years old, so certainly, it does not reflect the current situation (2021). It shows however that in the US, the reuse of greywater still belongs to a controversial (if not “Grey”) area of business ☹.
Source: Treatment, Public Health, and Regulatory Issues Associated with Greywater Reuse (“Greywater Reuse – Is it Right for Your Facilities?”, EPA, 2015).
We strongly suggest contacting the relevant local authority before making any decision! It’s especially worrying that even in states where the reuse of greywater is legalized, local requirements may differ compared to those in the neighboring state as well as be subject to personal interpretations! Unfortunately, so far there is no relevant Federal-level legislation!
Despite all these uncertainties, you may expect the following general guidelines for greywater reuse:
- The permit for greywater reuse applies to private, residential applications.
- You may be able to use untreated greywater for indoor applications (flushing toilets) ONLY under some conditions (check imposed restrictions and legality with local authorities).
- Pre-treated greywater can be used for flushing toilets, washing machines, and outdoor irrigation (trees and ornamental vegetation). The irrigation is often permitted under the following conditions:
- а. Only sub-surface irrigation (no sprinklers),
- b. Тhe discharge (irrigation) area must be inaccessible to the public
- c. Мaximum daily flow of reused greywater is limited (typically 400 gallons/day)
At the initial stage of the Septic System’s design (especially if your soil cannot handle the projected amount of sewage), you may consider recirculating the household’s greywater for “less demanding” applications. While reusing greywater for laundry and/or shower will require a specified level of its purification (hence complexity and extra cost), in many countries/states it can be used almost “as it is” for flushing toilets.
Provided that it is authorized by local laws, the raw (untreated) greywater recirculation system will need a mechanical filtration stage to prevent clogging of the inlet valve (hairs, pieces of soap…). While it is suggested to use it immediately, for practical reasons, greywater is often temporarily stored in a dedicated retention tank. To minimize the risk of bacterial growth and generation of biogases in the stored greywater, it is required by law to reuse it within the next 24 hours. While under normal circumstances, it may be difficult to fully comply with this requirement, the practical (albeit only approximative) solution is to limit the size of the retention tank to the average volume of water used per day in a given household (as indicated earlier – around 14 gallons per person).
The bottom line: The legislation underlines the importance of the fact that the collected greywater cannot be retained for a longer time (let alone indefinitely) as it will become a breeding ground for all sorts of pathogenic microorganisms. This will transform a relatively Low-Risk Greywater into the High-Risk one with all health-hazardous consequences.
For safety, the retention tank must have an overflow outlet connected either to the Sewer (Septic) System or to an authorized outdoor discharge area. On the other side – for a “comfort” of use, you may need a bypass installation to be able to flush the toilet when the greywater retention tank is empty (remember the Murphy’s Law – if something can happen it certainly will).
Given the fact, that toilets are responsible for about 24% of used water, reused greywater can substantially lower the amount of generated sewage (and as a bonus – of the freshwater needs).
Warning: Recirculation of resources is good for the environment and in general – for our Planet and its inhabitants. Certainly, the recirculation of greywater is the move in the right direction. However, from the point of view of small, onsite Septic Systems, it may be counterproductive. By recirculating greywater, we inadvertently increase the concentration of “toxins” (disinfectants) in the wastewater. They are designed to kill microorganisms, also including those, necessary for the subsequent biological treatment. This, in turn, may significantly slow down (or even stop) the crucial process of biological decomposition of organic matter in the Septic System! In other words, it’s not a straightforward decision at an off-grid location!
Note that in some states/countries, local legislation may require the disinfection of any water reused in indoor applications. If disinfection is done by chlorination, it will put even more stress on the downstream septic system!
Example of an in-line UV Sterilizer (water is disinfected as is flows through the pipe). Photo: David Johns, Source: “Wastewater Reuse” – (Your Home – Australia’s guide to environmentally sustainable homes).
Wastewater from a bath, shower, sink, and washing machine is collected in a dedicated greywater tank, filtered, and reused for flushing toilet(s). Source: “Septic Smart – Understanding Your Home’s Septic System”; Rideau Valley Conservation Authority (Government of Ontario (Canada)).
Greywater treatment systems range from simple physical filters (called Diversion System) allowing the disposal of water over the drain-field, through household Pre-treatment Systems allowing to reuse output water for permitted by law indoor and outdoor applications, to complex municipal plants being able to recycle the water into nature (rivers, groundwater ….). Obviously, for economical and practical reasons, greywater treatment systems designed for individual households represent the low-end of the process, hence we will call them “pre-treatment” modules.
All greywater treatment systems must be approved by the national EPA as well as local authorities. The associated certificate must clearly identify conditions for their use (including permissible sources of the greywater) as well as authorized end-use of the processed water.
Grey Water Treatment System – Aqua2 (model GWTS) provides mechanical filtration, biological treatment, and disinfection. Source: Water Wise Group (CA, USA)
Depending on the certification of a particular module and local laws, pre-treated water can be used for some indoor or outdoor applications, for example:
a. Toilet flushing
As mentioned earlier, the use of raw greywater may be allowed for flushing toilets, but only under certain conditions. However, in many states, even the reuse of pre-treated greywater As As it was mentioned earlier, in some states/countries the untreated greywater can be used for flushing toilets. However (and here comes “messy” legislation at the national levels), there are states and countries where even pre-treated greywater cannot be reused for flushing toilets. It’s because of the potential exposure to “greywater aerosols” created during the flushing process. To make it legal, the greywater disinfection stage may be mandatory!
b. Cold-water supply to the washing machine
c. Irrigation systems
Mostly subsurface (drip-type) irrigation, although some models may be also certified for surface irrigation (sprinklers)
d. Washing cars, driveway, paths…
Once again, the reuse of greywater is a poorly regulated business. For certainty, you have to check what is permitted and what not with the local authority!
Domestic on-site greywater internal reuse recycling system based on the Grey-Wise Treatment Unit. Source: Water Wally (Australia)
A good example of such a greywater pre-treatment system can be the Grey-Wise unit. The pre-treatment process consists of three stages:
Wise Water – Greywater Recycling System. Source: Kerry Flanagan Wastewater (Australia)
- Coarse filtering (used to remove solids) followed by aeration in a “bioreactor” to decompose eventual organic components in an aerobic process,
- Fine filtering (using a submerged 0.2-microns pore-size, self-cleaning membrane filters ensuring, only pure water passes through),
- UV-disinfection neutralizing microorganisms.
According to the manufacturer, it’s a chemical-free and odor-free process generating 99.999% germ-free output water.
In houses where flush-toilets are replaced with dry-ones, it may be beneficial to divert the High-Risk dark water into to greywater system, eliminating this way the need for the septic system. However, to meet environmental and health standards, a more powerful greywater treatment system may be needed.
The concept of a grey-and-dark water treatment system. Source: “Treatment and effective utilization of greywater” by Dhanu Radha Samayamanthula, Chidambaram Sabarathinam & Harish Bhandary; Applied Water Science, 2019, (Springer Link)
Reuse of Pre-treated Greywater
If allowed by local legislation, greywater, after adequate pre-treatment, can be used for irrigation of landscape. Thanks to the high content of nutrients, it will serve as fertilizer while the dedicated (ornamental) landscape, will operate as a much-needed drain-field.
Suggestions for greywater ornamental gardening. Source: “The Complete Beginner’s Guide to Greywater Systems”, (Elemental Green)
The mentioned earlier restrictions (inaccessibility of watered landscape for the public, and sub-surface irrigation only) are imposed to avoid any direct contact with greywater (especially by children and pets). The fact that greywater is considered the “low-risk”, does not mean that it is risk-free. Contacts with the skin or human respiratory system (inhaling aerosols) must be avoided. For similar reasons, pre-treated greywater cannot be used for veggies and fruit gardens (if consumed raw) as well as washing car(s), driveway, etc …
If your network of irrigation pipes is more complex, you may need to use the system of Diverting Valves. It will allow you to release pre-treated greywater more uniformly over the landscape (drain-filed) and protect it from water (and chemical) overload!
The following steps can lower the stress on your onsite greywater pre-treatment system (well, our Eco-friendly approach should make it part of our everyday life even in houses connected to municipal sewer systems).
- Minimize the use of household chemicals to the necessary level,
- As much as possible, chose natural, bio-degradable household chemicals (detergents, cleaners, soaps, shampoos, ….) with low content of sodium and phosphates (phosphorus), as well as pH-neutral. The high concentration of certain salts may lead to their build-up in the soil, causing discoloration, burnt and ultimately, destruction of plants.
- Use (and periodically clean) lint filters in washing machines,
Sacow Lint filter.
4. Do not dispose to drains any chemicals (paints, oils, etc… They must be disposed of at dedicated Municipal Collection Centers!