Our understanding of the ecological consequences of the use of water is greatly tempered by the thinking “it flows like water.” Water is cheap, seemingly abundant, and woven into the fabric of everything we expect from an infrastructure-rich environment. We turn a tap and expect unceasing flow of fresh, clean water, but at what cost? While we are becoming increasingly aware of the limits of water supply, we don’t generally think of water as energy.
The hidden energy related to the use of water includes the energy used to collect, convey, pump, treat, distribute, and use water, then treat the wastewater effluent. Without even taking into account the embodied energy of our water infrastructure, we see a tremendous amount of energy “going down the drain.” According to the Santa Clara Water District, the average energy use for water is 0.044 kWh/gallon. Depending on the energy involved in use, a high value might be 0.083 kWh/gal. Similarly, according to a 2008 report by the American Water Works Association, water use energy is closer to 0.085 kWh/gal.
What does this mean for us? If the average person uses about 100 gallons of water per day, that translates to 8 kWh/day. This is equivalent to running a 1,000 watt microwave or eight 100w lights for eight hours. Like turning off the lights when you leave a room, conserving water can add up to significant environmental savings.
Closer to home, looking at my family’s home energy use for the last few years, we consume under 500 therms of gas, about 3,000 kWh/year of electricity and 60,000 gallons of water per year. For water, this comes out to about 163 gallons/day for 4 of us = 41 gallons per person, less than half the typical for the US. Then, converting the all these energy uses to CO2 using the City of Berkeley’s conversion factors*, our household uses:
Gas Use: 5,862 lbs CO2e / year
Electricity Use: 1,547
Water Use: 2,402
= 9,812 lbs CO2e / year
≈ 2,500 lbs CO2e / year per person
(250 dump trucks of CO2 **)
As you can see, our water use is almost a quarter of our overall energy use, not an insignificant part of the equation. While we have already implemented a number of water conserving measures, there’s far more to go. In particular, harvesting grey water for use in irrigation could recover more than half of our outgoing wastewater (from shower, washing machine & sinks) while eliminating the water energy cost associated with using potable water for plants. This could add up to 5,000 lbs CO2e, the equivalent of driving 7,500 miles at 30 mpg.
For Ohlone Elementary School, we calculated an expected baseline water use (using national averages) of about 600,000 gallons of water per year. Plugging in the actual fixtures to be used (normal California water conserving fixtures), we expect an actual water use around 360,000 gallons per year. Translating this to CO2 equivalent, we predict a savings of about 9,600 lbs CO2e or about 14,500 miles of driving. Looking further, if we installed waterless urinals, we could expect another 2,000 lbs CO2e.
Now, imagine the installation of a Living Machine to process all the wastewater on site and return it for reuse. Even with a 20% loss due to evaporation and leaky pipes, the school could save the emission of around 20,000 lbs of CO2e per year. That’s about 2,000 dump trucks full of CO2…**
* City of Berkeley Climate Action Plan – Conversion Factors
1.0 kWh electricity = 0.50 lbs CO2e
1.0 therm natural gas = 12.0 lbs CO2e
1.0 gallon gas = 20.0 lbs CO2e
** Using WikiAnswers: At 298 Kelvin, CO2 has a density of 1.977 kilogram/cubic meter
→ 2,500 lbs = 1,134 kg = 574 m3 = 750 cubic yards. At 3 yd3/dump truck = 250 dump trucks.
→ 20,000 lbs = 9,072 kg = 4,589 m3 = 6,000 cubic yards. At 3 yd3/dump truck = 2,000 dump trucks.
