Treatment of Graywater so you can spray irrigate in the air, or re-use inside the building for supplying commodes (toilets) and the clothes washing machine sounds attractive, BUT you need to carefully consider irrigation efficiency AND the cost of water.
Graywater treatment isn't terribly difficult, with 3 basic stages:
Biologic
Aerobic
Sterilization
Being a 3 stage process, as well as requiring storage of the treated water, State and Plumbing Codes require certification of systems to NSF350. This isn't a cheap undertaking for manufacturers, with significant initial and continuing certification costs.
The State of Texas has tried to help reduce manufacturer's costs by requiring proof that a system meets the test requirements of NSF350, without having to gain or maintain NSF350 certification, but economics remain challenging.
Regardless of which type of water is used for irrigation, smart irrigation every day, or second day, using drip tubing is by far the most efficient method, and will save 50% of water when compared with once a week or every two week spray irrigation.
Promoting graywater treatment by proposing the resident can spray irrigate instead of subsurface is true, but at $8,000 - $12,000 for a typical system, plus installation, plus a spray irrigation system, will still require additional potable water for summer irrigation to suit most residential lots in the West and South of the US.
At an incremental cost of about $4,000 above a smart untreated smart graywater system (sub surface drip irrigation) effectively halves the irrigation benefit and doubles the cost.
There is practical merit in re-using treated graywater for commode and clothes washing machine supply, but again the economics need to be reviewed. How much water can be saved per year for those uses? How much money will I save per year?
Commode Water Saving Calculations:
3.5 Residents
1.6 gallon per flush low flow toilets (less on average with a dual flush toilet)
4 flushes per person, per day (ignoring that some residents may be using the facilities at their workplace)
That's 3.5 x 1.6 x 4 x 365 (days of the year) = 8,176 gallons per year
$5.00 per 1,000 gallons Potable water cost (typical lowest pricing tier cost in a location with reasonable water prices)
= Annual Saving of about $40.88.
Clothes Washing Machine Water Saving Calculations:
Same 3.5 Residents
15 gallons per load front loader (high efficiency)
2 loads per person, per week
That's 3 x 15 x 2 x 52 (weeks of the year) = 5,460 gallons per year.
Same $5.00 per 1,000 gallons Potable water cost
= Annual Saving of about $27.30
Combined Annual Indoor Water Savings (3 Residents) = ~ $68.18
That's a very small annual return for the approximately $4,000+ incremental cost of a treatment system above the cost of a non treatment system.
Water Savings using subsurface drip irrigation:
Refer to the IrriGRAY ROI calculations for a detailed explanation
Same 3.5 Residents
San Diego / Dallas style weather, about 8" per month peak ET requirement
3,000 sf lawn, 1,500 sf beds with 50% planting density, low / medium water use
45 Gallons per day per person graywater (summer), 25 gallons per day per person (winter)
Potable Water price will not remain at $5.00 per thousand gallons for spray irrigaiton, as now the water consumption wil have moved into second or third tier pricing. Lets say $6.50 per 1,000 gallons.
Savings comparison with spray irrigation
= ~ 200,000 gallons per year saving
= $1,300 per year saving in potable water costs.
For simplicity, I've left out maintenance and compliance costs, although they are not insignificant.
So in a nutshell, for this residence, an untreated graywater system, with an incremental installed cost of about $5k (including drip irrigation incremental cost), delivers $1,300 per year savings.
However an NSF350 (or equivalent) rated graywater treatment system with an incremental cost of $10,000+, delivers internal use savings of ~$68 per year, and irrigation savings (if sprayed) of 50,000 gallons per year, $325 per year, with a total annual saving of $393.
Residential Summary
Treatment System, incremental cost $10,000+, spray irrigation
= Annual savings of ~ $393
Non Treatment System, incremental cost $5,000+, drip irrigation
= Annual savings of $1,300
From a pure economic aspect, treatment at the residential graywater level simply doesnt make dollars and sense.
Rainwater collection / storage is another source of alternate water for commode and clothes washing machine supply
This method still has a very lengthy ROI, although less energy and equipment is required because the water is much easier to sterilize.
In other countries such as Australia, which do NOT require sterilization for these types of indoor reuse (non consumption of the rain water), the economics come close to working. Water shortages are the key driver of technology adoption in such cases where the ROI isn't great, but good enough.
What about High Occupancy Commercial or Apartment Complexes?
There are a number of issues here, and modelling is very much on a case by case basis:
Public access to treated water (toilets and clothes washing machines) requires to building operator to send water samples for lab testing each month
Incremental cost is substantially higher due to the requirement for much larger treatment systems - no longer an appliance, now a treatment plant
Apartment Complexes generally have a water bill per apartment, paid by the owner or renter, so the water bill savings do not go back to the Builder / Developer.
So Treatment is Bad?
Not necessarily. Each person needs to determine their own budget and goals. For some, with dire water issues, it may be a necessity. For others, it may be a green choice.
During the depths of the Australian Drought in the mid 2,000's, a multi-million dollar house was built with a graywater treatment system for supplying water inside, and a rainwater harvesting system for landscape irrigation. Total cost was about $70,000. Instead of this, the technology could have been reversed - untreated graywater for the landscape & rainwater harvesting for internal use. Total cost would have been around $20,000 and most likely saved more water. Perhaps more good would have been obtained by donating the $50,000 difference to a green charity?
So like any green choice, carefully review the options, and review the ROI. Every site is different, so consult with experts who understand the cost and benefits of each technology.
We are not against treatment, or rainwater storage (rainwater will be another blog post!), and we design / install / project manage numerous such systems, however we always ensure the client makes informed choices.
As an Ex Australian living in Dallas, there is a very simple Australian tip for saving toilet water, with virtually no investment required at all:
If it's yellow let it mellow - If it's brown flush it down.
So simple a 3 year old can do it.
Paul James
VP Research & Product Development
WaterReNu