I wonder how often we think about energy when we open our tap. In the best case, if we have a sense of awareness, we would think about conserving water and therefore would be more careful by not letting the water run down unnecessarily. Energy (electricity) is required through the whole water production and distribution process including operation of pumps, filters, disinfection equipment, process control systems, etc. The same is true for waste water treatment where the amount of energy consumed depends on the technology installed and standards required for the treated effluent. The energy used by the municipal water and wastewater treatment sector in the US, on a national basis, accounts for 35 percent of the total municipality’s budget . In general a wastewater treatment plant uses 25-40 percent of its budget for electricity  and in the case of drinking water treatment and distribution system, these numbers go up to 80 percent of the total budget of the plant .
Being aware of these considerable costs, water utilities are undertaking measures and programs to make their water and wastewater systems more efficient, in this way reducing the electricity costs. Here are a couple examples of such utilities.
San Diego, California
In California, about 20 percent of the state’s electricity goes to water and wastewater sector . A recent pilot audit conducted by the San Diego Gas and Electric (SDG&E) and the San Diego County Water Authority (SDCWA) is identifying considerable savings in energy through water conservation (including reduction of water demand) and use of less energy intensive water sources and processes (gravity fed or recycled water vs. groundwater and desalination) . Since then energy-audit programs are being conducted across the state. The audits are thoroughly done analyzing the energy conservation opportunities in different aspects of the performance of a water utility. Historical energy-use data for each facility are being collected and being analyzed to identity use, cost trends, consumption. Tours and interviews with the operations staff help the auditing team to determine plant operations, equipment type and malfunctions, etc. Related to this latter one, the audit pays attention to notice if a pump starts at the wrong time. The auditors collect information about the equipment’s age and see how that might affect electricity consumption.
The SDG&E audit program has found that opportunities for energy-efficiency might apply to every utility:
- The optimization of the water treatment filter backwash cycles reduces the amount of backwash water resulting in less energy use for pumping and reprocessing this water. The energy saved in this case for pumping is estimated to be 10-15 percent.
- Installing variable frequency drives (VFD)  and resizing the pumps improves the performance of the pumps by pumping the same amount of water with 30-50 percent less energy than before. On the other hand, reconfiguration of the booster station allows pumps to operate in the ±75 percent efficiency range .
Besides increasing the efficiency of processes and facilities, the water utilities are considering producing energy as a means to offset their energy costs. The facilities installed by SDCWA include two pump storage hydro-generation facilities one of which produces about 40 MW annually and the smaller one produces 4 MW. Considering also the photovoltaics (PVs) installed at one treatment plant, the San Diego County Water Authority expects to save in total up to $1.7 million over 20 years .
The Haworth Water Treatment Plant in New Jersey, constructed in 1960, underwent a renovation of $100 million in 2009 which included installing highly efficient dissolved air flotation (DAF), and providing a backup power source of four 2 MW Caterpillar natural gas-fired generators . The plant operates the backup power source, when gas prices are relatively low, in this way avoiding buying electricity from the city provider. In addition, the water treatment and distribution is monitored by and controlled by a SCADA system .
The SCADA system allows the operators to monitor each process from the wells, to the treatment plant and to the distribution system, knowing at every moment where the energy is being used and how much energy each facility is using. Having this information continuously makes it possible for the utility to compare the consumption parameters of the facilities in time and make sure that they are working efficiently; and if not, take maintenance measures as soon as the problem is detected.
Since the upgrade, the plant has reduced its kilowatt hours by 0.5-1 percent per million gallons of water, and the utility is positive that they might achieve even better results with greater monitoring implemented in remote sites .
It is obvious that reducing energy consumption at water and wastewater treatment plants, not only reduces operational costs for the utilities, but benefits the customers as well by allowing lower rates. However, the measures to conserve energy should be made on a thorough basis considering the cost-benefit analysis accounting for capital costs, operational costs, return on investment and payback.