Net-Zero Utilities Through Water and Energy Efficiency

A water utility can reduce its need for outside energy by using less to treat water and recovering more from the wastewater it already handles. Vienna’s E_OS project brings these choices together, showing how sludge preparation and plant renewal can change the energy needed to keep treatment running.

Match Energy Use to the Work Being Done

Efficient equipment needs to be run well. Equipment that pumps water or supplies oxygen for biological treatment must match demand without lowering treatment quality. Lower electricity use may simply mean less water is passing through the plant. Comparing energy use, water volumes, and treatment results explains the difference. Water conservation and leak reduction ease the workload; reducing waste within the plant leaves less energy for renewable generation to supply.

Count the Energy Left After Running the Process

During anaerobic digestion, microorganisms break down organic material in sludge without oxygen, producing biogas. Engines can use the gas to generate electricity and heat, but preparing and heating the sludge takes energy too. More gas does not necessarily mean more energy is left for the plant. Heat recovery adds to the benefit when there is somewhere to use that heat. Looking at all stages reveals whether savings in one are being lost in another.

Be Clear About What the Numbers Show

An annual surplus means a plant produces more energy than it uses over a year. Counting electricity without heat gives a different picture of the plant’s total energy balance. It does not tell us whether outside energy is needed at particular times, or whether greenhouse gas emissions have been eliminated. Methane and nitrous oxide can still be released during treatment, and emissions also arise from construction and purchased materials. To compare projects fairly, use the same reporting period and count the same activities and emissions.

Case Study: Vienna Links Plant Renewal With Energy Recovery

At ebswien’s plant in Simmering, Vienna, aging treatment tanks needed replacing. The E_OS – Energie_Optimierung Schlammbehandlung project combined this work with new sludge facilities. New tanks for preliminary and first biological treatment took up less ground space, leaving room for sludge processing on the existing site. The plant kept treating the city’s wastewater during construction. The completed project was presented on June 4, 2020 (City of Vienna, June 4, 2020), and the sludge-treatment system entered full operation at the start of 2021 after trial operation.

Removing more water from the sludge before digestion meant there was less to heat, but sludge that became too thick would be difficult to pump. Working with TU Wien, ebswien developed a process that handled a higher proportion of solids and needed six digesters instead of twelve. Preparing the sludge differently changed both the heat needed and the size of the facilities required.

Inside the digesters, microorganisms break down organic material without oxygen. Gas from this process passes through filters before being burned in engines that drive electricity generators. Heat from the same operation is used for heating and hot water. The plant makes use of both outputs, each meeting a different need.

From its first full operating year in 2021, the system enables the plant to produce more energy over a year than wastewater treatment requires, supplying surplus electricity and heat to the respective networks. The result reflects decisions made before gas reaches a generator: where to put the new facilities and how to prepare the sludge. Other utilities can consider these choices together when rebuilding, while planning how to keep treatment running.