Energy needs vary depending upon a number of factors: (1) the quality of the source water and level of treatment required to produce the quality of water appropriate for the planned use of the treated water, and (2) the distance the water needs to travel from its source. In general, more treatment requires more energy.
Energy represents the largest controllable cost of providing water or wastewater services to the public. Most facilities were designed and built when energy costs were not a concern. Water treatment and wastewater treatment have different treatment and energy requirements. Expanding your question to wastewater treatment, the least energy-intensive wastewater treatment method would be facilitative lagoons (or wetland ponds) because they require no energy but they have large space demands, possible odors and may not meet applicable discharge standards. Water treatment systems using pristine source water may require little to no energy for treatment and disinfection. Both types of water systems would use little to no energy with gravity feed transmission pipes.
Reducing the amount of water entering the wastewater treatment facility also reduces the amount of energy required for treatment. Water conservation programs, such as EPA's WaterSense program, reduce the amount of water that needs to be treated. Grey water reuse for landscape irrigation and replacement of worn, cracked, or broken wastewater sewer pipes can prevent excess water infiltrating into the wastewater treatment facility and energy requirements. Recycling water at treatment facilities reduces water requirements onsite.
Wastewater treatment facilities also produce, or have the ability to produce, energy that can be used to offset the energy required for treatment. Anaerobic digesters and sludge can produce methane gas or biodiesel or electricity, and an increasing number of facilities are using renewable energy sources such as solar or wind or gas from nearby landfills to offset energy requirements. Several facilities are using excess capacity to process fats, oils and grease from restaurants to produce and sell biodiesel fuel.
The McKinsey Institute has studied different approaches for reducing energy consumption in general and has found energy efficiency to be the most cost-effective, promising approach. Wastewater treatment facilities may consume up to one-third the energy of a community, and energy utility companies such as Pacific Gas and Electric indicate these facilities can cut energy requirements by up to 25 percent with more energy efficient equipment and practices. Many power utility companies offer financial incentives to water treatment facilities to install new pumps, aeration systems, motors and disinfection systems.
For example, the Dublin San Ramon Services District had an energy audit and by following the audit recommendations they were able to treat 48 percent more wastewater while reducing the plant's annual energy use and cost savings to 2,232,000 kWh and $290,000 respectively. This savings allowed the District to earn $67,000 in incentives from PG&E. The new plant cost $2,209,000 more to build than a less efficient one, its annual energy savings and the incentives earned will result in the plant paying for itself in 6.6 years -- and then continue to save the District money and energy.