Energy Efficiency for Urban Water and Wastewater Services

Electricity is a critical input for delivering municipal water and wastewater services. Electricity costs are usually between 5% - 30% of total operating costs among water and wastewater utilities (WWUs) worldwide. The share is usually higher in developing countries and can go up to 40% or more in some countries (e.g. India and Bangladesh). Such energy costs translate into high and often unsustainable operating costs, which directly affect the financial health of WWUs, puts strains on public/ municipal budgets, and can increase tariffs on their customer base.

In developing countries, WWUs are commonly owned and operated by the government. Many are run by city authorities. As such, electricity used for provision of water and wastewater services can have a significant impact on a municipal governments’ budget and fiscal outlook. In India, for example, water supply was reported to be the largest expenditure item among all municipal services. Programs designed to lead to reductions in WWU operating costs can thus become an attractive proposition for both utilities and their municipal owners, potentially creating fiscal space to grapple with other socioeconomic priorities while also lessening the upward pressure on water and wastewater tarriffs. Improving energy efficiency is at the core of measures to reduce operational cost at WWUs.

Since energy represents the largest controllable operational expenditure of most WWUs, and many energy efficiency measures have a payback period of less than five years, investing in energy efficiency supports quicker and greater expansion of clean water access for the poor by making the system cheaper to operate.

Determining Energy Efficiency for Water and Wastewater Utilities

The overall energy efficiency (EE) of WWU services can be indicated by electricity use per unit of water delivered to endusers and per unit of wastewater treated (kWh/m3-water or wastewater). For a given level of service and regulatory compliance, reduction in those energy intensity numbers indicates improvement in EE of service delivery. In practice, applying these aggregate indicators has two main difficulties:
MISMATCH OF ENERGY AND WATER/WASTEWATER FLOW DATA: This arises when end-use metering is not universal and less than 100 percent of wastewater is treated. Oftentimes, energy use per unit of water produced is used as an indicator, instead of water delivered. Doing so leaves out an important efficiency factor—physical losses in the network.
INCOMPARABLE OPERATING CONDITIONS AND PROCESSING TECHNOLOGIES BETWEEN UTILITIES: Using these aggregate indicators for inter-utility comparison is usually fraught with problems because they are significantly affected by system operation conditions (e.g., daily flow, water main length, mix of water sources, distribution elevation, use of gravity for distribution or collection, etc.) and processing technologies (e.g., level of treatment for wastewater). For example, electricity intensity of water supply in the State of New York (varying from 0.158 to 0.285 kWh/m3-water produced) is significantly below the United States national average of 0.370 kWh/m3 primarily due to the predominance of surface water sources and a large share of gravity-fed distribution in New York.

Energy Consumption Patterns

In general, larger systems (to a limit) tend to be less energy intensive than smaller ones. Electricity use in  administrative and production buildings of WWUs, such as lighting and space conditioning, is a small percentage of a WWU’s overall energy use.

With the exception of gravity-fed systems, pumping for distribution of treated water dominates the energy use of surface water-based supply systems, usually accounting for 70% - 80% or more of the overall electricity consumption. The remaining electricity usage is split between raw water pumping and the treatment process. Groundwater-based supply systems are generally more energy intensive than surface water-based systems because of higher pumping needs for water extraction (on average, about 30 percent difference in the United States). On the other hand, groundwater usually requires much less treatment than surface water, often only for the chlorination of raw water, which requires very little electricity.

Energy usage of municipal wastewater treatment varies substantially, depending on treatment technologies, which often are dictated by pollution control requirements and land availability.
Advanced wastewater treatment with nitrification can use more than twice as much energy as the relatively simple trickling filter treatment. Pond-based treatment is low energy but requires large land area. The estimated energy intensity for typical large wastewater treatment facilities (about 380,000 m3/day) in the United States are 0.177 kWh/m3-treated for trickling filter; 0.272 kWh/m3 for activated sludge; 0.314 kWh/m3 for advanced treatment; and 0.412 kWh/m3 for advanced treatment with nitrification. The ascending energy intensity of the four different processes is due mainly to aeration (for the latter three treatment processes) and additional pumping requirements for additional treatment of the wastewater. In fact, for activated sludge treatment, a commonly used process in newer municipal wastewater treatment plants, aeration alone often accounts for about 50% of the overall treatment process energy use.