Greywater: A 'New' Low Energy Water Supply

Many states are turning to greywater - typically defined as the wastewater that comes from clothes washers, showers and bathroom sinks - as an alternative water supply to help them address local water shortages. Using greywater is illegal in many states but lifting these out-dated bans can help communities create 'new,' low energy water supplies that can be used for a variety of purposes.

Over the last couple of years a number of states including Oregon, Texas, New Mexico and Arizona have passed laws allowing residents to apply greywater from their homes to beneficial uses. The most common uses of greywater are for toilet flushing or outdoor irrigation of non-edible plants.

California has joined other greywater-using states by adopting new, less stringent regulations that make greywater systems easier and more affordable to install. From a recent article on SFGate.com:

Whereas California property owners previously were required essentially to install costly mini leach fields (those are usually associated with septic systems) and obtain pricey permits, the new codes allow residents to install basic, relatively inexpensive graywater systems themselves with no permits.

Under the old regulations, a graywater system cost as much as $10,000, versus as little as $200 now.

The water savings that can be obtained by using greywater are significant: Doug Hensel from California’s Department of Housing and Community Development estimates that a typical household can save 22,000 gallons of water per year from a laundry greywater system alone.

The energy benefits of these systems are two-fold since using greywater for outdoor irrigation avoids the energy needed to pump water to the home as well as the energy that would go into treating it at a wastewater treatment plant.

Assuming national averages (1,500 kWh/MG for supply and 1,800 kWh/MG for wastewater treatment) and a direct energy savings from avoided wastewater treatment, a typical house beneficially using 22,000 gallons of greywater per year would save about 73 kWh of electricity per year – equivalent to the annual electricity use of a laptop computer.

Although this might not seem like a lot of energy savings, the numbers add up quick. If just 1000 homes installed simple greywater systems annual energy savings would be enough to power over 7 households and reduce CO2 emissions by about 52 metric tons.

It is important to keep in mind that actual energy savings and environmental benefits could be even greater. As I mentioned in a previous post, outdoor irrigation – which takes place primarily in the summer months for much of the country - is a major factor in peak water demand. Peak water demand - like peak energy demand - usually drives the development of new supplies and leveling the peaks can avoid the costs of new dams/diversion, or the development of new energy intensive supplies, such as desalinated seawater or imported water.

Furthermore, the marginal water supplies used to meet peak demands are often more energy and carbon intensive than the least cost supplies used throughout the year, therefore, in many communities, saving a gallon of water in the summer saves more energy than a gallon in the winter.

Finally, peak water demand and peak energy demand usually occur at the same time - during extremely hot weather when air conditioners are turned on high, cooling towers are working overdrive, crops and landscapes need irrigation, etc. Saving water can reduce peak energy demand, as evidenced by Idaho Power's partnership with farmers. To add one more twist, the oldest, dirtiest power plants get kicked into overdrive to meet peak electricity demand, which means that when you water your lawn on the hottest day of summer, the dirtiest, most carbon intensive energy was likely used to deliver that water from the most energy intensive source.

Below is a 30 minute instructional YouTube video on how to design and install a greywater system at your home: