Greywater Recycling – An Untapped Water Resource Inside Buildings

In this article, we will cover the basics of greywater recycling, that is what it is, how, it works, specific applications, and regulatory considerations.  The demand for greywater recycling technologies is expanding as more regions of the US and the world experience water supply challenges.  And, innovations in greywater recycling technology is making it more viable for both large and small scale projects in terms of capital cost, maintenance requirements, and complexity.

What is Greywater Recycling?

Greywater Use in the HomeGreywater is a source water just like ground water from wells or surface water from reservoirs is a source water.  Greywater comes from inside buildings.  Almost all of it comes from showers and laundries.  Additional but lesser amounts come from bathroom sinks (lavatories) as well as drinking fountain and ice machine drainage.  That means that prime candidates for greywater recycling are:

  • Residences – Large apartments, condos, dorms, barracks, and single family homes with shower/baths, laundry, and bathroom sinks
  • Hotels – Similar to above.
  • Health Clubs and Gyms – A lot of showers!
  • Car Washes – In a sense a car wash is a type of shower.

Greywater is not treated rainwater or stormwater which is used indoors.  Again, greywater comes from inside the building.  Rainwater and stormwater come from outside!  We see this confused categorization a lot.  See comparison of the water quality of raw greywater to rainwater below.  There is a day and night difference between these types of source water and thus require totally different treatment regimens to reach any given end water quality.

Greywater is differentiated from blackwater by its lack of fecal coliform from human waste and relatively low organic loading (as measured by BOD, biological oxygen demand).  Water from kitchens has higher BOD from food waste and tends to be classified as blackwater.  Treating it in a greywater system may be possible, but generally is not considered worth the effort.

As with any source water, greywater should be treated to the extent required by its end use.  After treatment, the resulting water may or may not be logically classified as greywater.  In your “laundry to landscape” greywater applications, there is not any appreciable treatment, so you are basically watering your garden with raw greywater.  This is fine for that application as long as the water is captured and used quickly (i.e. within 24 hours by many regulations). In many cases the organic load provides nutrients to the plants.  However, left untreated, bacteria in stored raw greywater rapidly multiplies and the water becomes black water.

For most applications that readers like yourselves care most about, a higher level of treatment is necessary because we want to use treated greywater water for things like:

  • Toilet flushing
  • Laundry
  • Cooling Tower Make Up
  • Car Washing.

For these types of end uses, there are a range of technologies that allow for these types of end uses.  For water to be reliably used indoors without causing problems over time, it must be treated so that there will not be appreciable biological growth after treatment.  One measure of whether treated water will meet this objective is whether the process used to treat it is certified under NSF350 C. With NSF350 C, a summary of raw greywater and treated water quality is as follows:

NSF350 C, Raw Greywater & Treated Water Quality

  Raw Greywater
(Bath & Laundry Water)
Teated Water
(NSF350 C Required Average)
TSS (ppm) 80-160 <10
BODs (ppm) 130-180 <10
Turbidity (NTU) 50-100 <2
E.Coli (CFU/ml) 100-1000 <2.2

Water treated to this level of quality will not experience appreciable re-growth of bacteria to cause problems in downstream plumbing or create any health hazards if used for non-potable uses.  End users expect clear, odor free, and safe (but not potable) water for these types of end uses and NSF350 C system provide this.

In addition, achieving NSF350 C certification requires these average numbers to be achieved for 26 weeks of operation (6 months) and that no human intervention is allowed (i.e. maintenance), so achieving certification is an indication of reliability of the process.  Meeting the target water qualities in a short term test is far different than doing it over a 6 month span or longer.  This requirement will be welcomed by most building managers, where systems requiring more frequent hands on maintenance (e.g. monthly) are far less desirable.

Why use greywater recycling?

Greywater recycling is a viable way to manage water.  We are forced to rethink 20th century water supply and management philosophies.  With aquifer and surface water depletion from ever increasing demand, we can no longer assume unlimited water supply from nature.  On the other side, we have an ever increasing load on our crumbling water and wastewater treatment infrastructure.  Greywater recycling helps reduce demand for municipal water from wells and reservoirs and also reduces the load on large scale wastewater treatment plants.

Greywater recycling can reduce outside household water demand by over 40%, which is a huge number that goes a long way toward being water independent and alleviating water stress in drought stricken areas.  Combined with rainwater collection, use of municipal or well water can be eliminated altogether for many geographies.

There may be energy recovery opportunities using warm greywater where a heat exchanger is used to pre-heat incoming fresh water or where warm treated greywater is used for laundry.  This more than offsets the energy to treat the water which is worst case in the 1.5 to 4 KWH/1000 gallon range.

Reduction of municipal water demand can translate into thousands of dollars saved on water bills over the course of a year.  In fact, in places like Atlanta, or Seattle, or San Francisco, ROI’s for commercial scale greywater systems can beat long term stock market returns.

How much greywater is there?

In a residence, we estimate that around 33 gallons per person of greywater is generated each day which may be around 40% to 70% of indoor water demand in a typical condo complex, apartment, or single family home.  As you may guess, there can be quite a range around this estimate due to many factors such as types of plumbing fixtures, inhabitant profile, and regional effects.  Of this amount, we see about 75% comes from showers and baths, 15% from laundry, and 10% from bathroom sinks (lavatories).Here is a table listing out our standard assumptions:

Greywater Supply Sources & Assumptions

Greywater Supply Sources Measure
Inhabitants 1
Showertime Average (mins) 8
Showerflow (GPM) 2.5
Showers/Day 1.25
Bathroom Sink Use PPPD (GPM) 3
Laundry Gallons/Load 15
Total Shower Flow 0.33
Total Sink Use (GPD) 25
Total Laundry Use (GPD) 4.95
Total Water Supply (GPD) 33

Based on These Assumptions

  • An apartment building with 200 units and 350 residents can generate  4-5 million gallons per year.
  • A hotel with 200 rooms may generate more than this due to the added laundry of cleaning sheets and towels.
  • A health club or gym with 500 users per day may generate 3 million gallons per year.
  • This is a significant amount of water that can far exceed what can be captured by rainwater collection in dry regions.

Types of Greywater Treatment:

There are many different technologies for greywater treatment.  When used in the right application all can work great.  As with most things, it is important to know the advantages and limitations of any choice and to make sure the right technology is chosen for the application.

To aid the discussion, we will use a comparison of water quality values, E.Coli bacteria and BOD5 (Biological Oxygen Demand), as metrics of comparison.  There are of course many other measures, but these are among the most important in terms of predicting system performance.


– This is a well known bacteria which is used as an indicator of fecal coliforms and pathogenic bacteria.  The current range of available greywater treatment equipment can remove it through filtration, neutralize it through UV, or kill it with chlorine, ozone, or advanced oxidation processes (AOP). NSF350 C requires an average of less than 14 CFU/ml for greywater systems in single family residences and less than 2.2 CFU/ml for commercial scale systems.


– This is a measure of the amount of nutrients (food) available for micro-organisms to metabolize (eat).  It is important to reduce BOD low enough so that treated water can be stored.  Otherwise, the water will putrefy over time even if it is heavily disinfected in the treatment process.  NSF350 C certification sets this limit at 10 ppm.  Below that level, water can be stored for extended periods.

Different Greywater Systems Available

  1. Direct reuse – In commercial scale applications, this type of recycling of greywater is rare unless the collected water is used almost immediately. This is more common in residential ‘laundry to landscape” et ups or in industrial processes where process streams are recycled for reuse.

Many local regulations are set up with this type of application of greywater in mind because of relatively high BOD and e.coli.  It is normal to require that untreated greywater be used within 24 hours to prevent growth.

This type of application would not be appropriate for use in any of the indoor applications mentioned above.  Direct and immediate irrigation may be a possibility.

  1. Filtration – There are many systems for large and small scale applications which use sediment filtration only where, again, water is used within 24 hours. Common type of filters are bag screen filters sometimes in combination with multi-media filters.  Filters may range from over 100 microns effective pore size down to as low at 5 microns, which will have a definite effect on water quality and frequency of maintenance required.  The choice is between clearer water and more frequent filter maintenance and lower water quality and less maintenance required.  In either case, BOD would not reach a point that water could be stored for any length of time and bacterial counts would remain closer to raw greywater than to NSF350 C treated water standards.

The main application for this type of system is almost always for outdoor irrigation only with collected water being used as it is generated each day.  There are several good products on the market which do this in a well controlled way.  One additional maintenance item that may be required periodic cleaning of irrigation lines due to slime build up.

  1. Filtration and Disinfection – Several systems add disinfection to sediment filtration to bring bacteria counts down. Chlorine addition or UV or both are the most common methods.  Ozone is another relatively common method.  Chlorine and ozone give a residual that may help prevent biofilm build up in pipes.  BOD levels are not brought down below the NSF350 C standard but in most cases bacteria is low enough if water is not stored too long.  As long as regular maintenance is done on these systems and the proper controls and alarms are in place with steps like oxidation-reduction potential (ORP) monitors, these systems can be used for indoor plumbing applications like toilet flushing with excess used for irrigation.

One watch out is that since BOD is not brought down below NSF350 C levels, there may be biological growth in plumbing over time.  The diagram below shows a study done by Kohler Corporation which reported black toilet tank walls and odor when chlorine levels went too low.

impact of greywater on toilet tanks

Taken from from a study authored by Bill Kuru and Mike Luettgen for the Kohler Company – see here (Grey Water Report ) for entire report

Use in cooling towers is typically not done with these systems.  There are special systems for laundry and car wash recycling which have special filters and ozone injection.

  1. Biological Treatment with filtration – The most common type of process in this category is the membrane bioreactor (MBR) process. These processes may add disinfection after the MBR process although this is not always necessary.  With or without additional disinfection, these processes can meet NSF350 C water quality levels.  The MBR process consists of biological stage using aeration which greatly reduces BOD and contamination followed by water transfer through fine membrane filtration of 0.2 microns or smaller.  Since 0.2 microns is smaller than bacteria, this process is very good at achieving low bacteria counts in the resulting water.   And, it is virtually impossible for bacteria to transfer, unlike a chlorine dosing failure which would send unsanitized water to the treated water holding tank.

The table below shows that with regard to e.coli and BOD, MBR treated greywater has water quality closer to drinking water than to greywater.  In fact, this water has been shown to be able to be stored for extended periods of several weeks with no appreciable deterioration. This water is also suitable for spray irrigation.

MBR Treated Greywater Comparison To Drinking Water Quality

  E.Coli (CFU/ml) BODs (ppm)
Raw Black Water 1-100 million 350
Raw Greywater (Bath & Laundry Water) 100-1000 130-180
EPA Recreational Water Quality Standard <100 NA
Treated Water NSF-350 Average (Residential) <14 <10
Raw Rainwater <10 <5
Treated Water NSF350c (Commercial) <2.2 <10
Aqualoop MBR Treated Greywater <2 <5
EPA Drinking Water <1 0

Some observations from this data:

  1.  Note the comparison of recreational water to treated greywater or rainwater.  Both are far lower in e.coli than recreational water deemed safe to swim in by the EPA.  This means there should be little problem using this water for virtually any non-potable use up to and arguably including bathing and showering.
  2. Raw rainwater meets NSF residential water quality standards with no post treatment.  This indicates that rainwater should not require additional treatment for indoor plumbing use and extended storage and that added disinfection with UV or chlorination may be a waste of money.  regulations requiring disinfection of rainwater for non-potable uses should be reviewed and challenged.
  3. MBR treated greywater has BOD and e.coli results close to EPA drinking water standards.  While this water may not be safe to drink due to existence of non-biological contaminants, both organic and inorganic, it is safe to store and use for the aforementioned end uses.


MBR treatment sometimes has a higher installed cost than other types of treatment, but some MBR technologies are coming on line which are very cost competitive.  In addition, MBR process can require far less maintenance and have less control complexity than other systems.  With the water quality, maintenance, and reliability benefits, the MBR process may be the better choice for overall life cycle cost.

Regulatory and Permitting Considerations:

As greywater recycling becomes better known and applied, regulations are gradually changing to reflect the concept of matching end water quality requirements to the needs of the end use.  The idea of permitting simple capture, filter, use in 24 hour systems for irrigation makes sense as does requiring standards like NSF350 C for indoor use.  As this sort of thinking comes one line, we will probably see higher adoption rates for greywater recycling and better understanding of where it fits in our overall water management plans.

In some areas, greywater recycling is becoming mandatory for new construction on certain building sizes.  We can probably expect this to expand over time in areas that are prone to water shortages or where water supply and wastewater treatment capacity may constrain economic growth.

It is our responsibility as industry professionals to provide safe, reliable, and cost effective systems that building operators can understand and trust. Requiring NSF350 C certification for gray water systems accomplishes that, while allowing for continued innovation and different owner priorities.

Another aspect of NSF350 C is that no particular technology is prescribed, only requirements for challenge and end water quality, reliability, and other factors to be met.  This allows manufacturers to innovate to find cheaper, more reliable, longer lasting, lower maintenance, and smaller footprint over time. This is a good thing and we think it will cause more adoption of greywater recycling over time as old regulations that categorize all greywater, treated or untreated as raw greywater give way to those recognizing greywater as a source water and prescribe water quality standards to be met for particular end uses.  Requiring NSF350 C for indoor plumbing applications and having other requirements for irrigation makes a lot of sense in our opinion.