This article is the second in a series focusing on the complicated linkage between our water and energy systems. This interdependency is referred to as the water-energy nexus. The last article, in October 2016, focused on the unquenchable thirst of our power grid and the massive amount of water that is withdrawn and consumed to produce electricity. This article looks the other direction at the amount of energy it takes to pump, treat, distribute and collect the water we use.
In the dialogue of energy conservation, water is often only considered when it comes to high-efficiency water heaters or tankless options. These elements are important for the energy at the end-use of the water, but what about all the energy spent on the water itself? It’s not as though this stuff just falls out of the sky … Okay, actually it does, but not treated, pressurized and available in essentially any quantity at any time, and then carried away when we’re done with it. To get the quantity, quality and reliability we expect from our municipal water systems requires a multi-step process with a multitude of energy input along the way. Since these occur at large municipal or statewide scales, energy use is generally measured in kilowatt hours required per million gallons of water or (kWh/MG).
The essential steps that our water flows through are:
1. Extraction – getting fresh water
2. Conveyance – transporting raw freshwater to a treatment plant
3. Supply Water Treatment – filtering and disinfecting raw water to potable standards
4. Distribution – supplying potable water to users
5. Use – all the cooking, bathing, drinking, flushing and irrigating that we do
6. Wastewater collection – conveying and pumping sewage to wastewater treatment plants
7. Wastewater treatment – removing solids, nutrients and pathogens from wastewater for discharge
The first two steps are related to the source of the water. Surface freshwater from lakes and rivers — which make up more than 75 percent of our national water supply — is the easiest source to draw from with minimal energy input required. The rest of our water is mostly pumped groundwater water, which depending on the depth can require 500-2,900 kWh/MG of pumping energy. Finally, a small portion of water is generated from brackish or seawater desalination plants. These are by far the most energy intensive sources, requiring up to 13,800 kWh/MG. This energy is just to get us raw water to start with.
Once the raw water is available it needs to be conveyed to a treatment plant. The energy use of conveyance systems depend on the distance and the terrain the water has to travel over. It can vary widely, from 1,000-10,000 kWh/MG. The largest example of these conveyance systems is the California State Water Project, which brings water from Northern California down to Southern California. It has 20 different pumping stations including the highest lift of any water system in the world to go almost 2,000 feet over the Tehachapi Mountains to Los Angeles and San Diego. It is the largest energy user in the state, representing up to seven percent of California’s total electrical consumption.
The conveyance system brings the water to a plant where is treated to achieve drinking water quality standards. This is actually a relatively minor component of the overall energy use. Pumping the water thought the various stages of settling, filtration and disinfection only requires about 100 kWh/MG. But then once it’s treated, it’s back to high-energy pumping again. This time the water is pumped through a network of distribution piping that has to be maintained pressurized to deliver water to users. Municipal water distribution in America is handled by more than 170,000 different private or public water systems of varying sizes in terrain. As such, pumping energy for distribution can range from greatly 400 to 2,600 kWh/MG.
All of this happens before the water even gets to the building and plumbing engineers start sending it around the building to different uses. In the overall lifespan, the “use” period is relatively brief. While the use can be very energy intensive for heated applications, this particular phase is not the focus of the article which is looking at the larger systems. Therefore, we’ll skip past the details of usage and follow the water down the drain, out to the sewer, and into a whole new conveyance system to the wastewater treatment plant. This requires another network of conveyance piping and pumping stations. The pumps are by nature less efficient because of the required space around the impeller to pump solids, but since there is no pressurization requirement often gravity assists this collection, the pumping energy for wastewater is around 150 kWh/MG.
At the wastewater treatment plant, there is a series of screening, sedimentation, filtration, aeration and biological processes to remove solids, nutrients and pathogens. There are definite economies of scale when it comes to the size of these plants and the quality of the effluent dictates the treatment processes. Very large and basic plants can process wastewater for less than 700 kWh/MG while smaller plants or those with more advanced treatment can be 2,000-3,000 kWh/MG. And this concludes our journey. Once the water is treated to an acceptable level, it’s discharged to surface — sometimes to the ocean, but more often back into a lake or river where it’s likely to start the process all over again at the next city downstream.
This whole process and the energy requirements along the way vary widely depending on the specific systems. Total water related energy (excluding end use) in low intensity areas with close and plentiful surface water can be around 4,000 kWh/MG. Arid locations, such as Southern California or Arizona, on the other hand can be three to four times this amount. Overall, water supply and wastewater systems account for three percent of annual electricity consumption in the U.S. To put that in perspective, it is more than the total residential electrical consumption of the entire state of California.
And this doesn’t even address the energy associated with the end-use in the building. In the next article, we’ll bring the topic of the Water-Energy Nexus home to the building scale and see how it plays out in our own backyard.
Calina Ferraro, P.E., CxA, CPD, LEED AP, is a principal at Randall Lamb, a consulting engineering firm specializing in mechanical, electrical and plumbing systems in San Diego. She plays a key leadership role spearheading the company’s sustainable design group, managing projects and leading design teams. Ferraro’s project experience includes commercial, science and technology, healthcare and institutional market sectors. She is an active member in a number of industry groups including the American Society of Plumbing Engineers (ASPE), currently serving on the Women of ASPE committee; ASHRAE; and the USGBC. She can be reached at CFerraro@RandallLamb.com.