Ways to optimize the energy balance of municipal wastewater systems: lessons learned from Austrian applications

• Major ways to reduce the energy demand of wastewater treatment systems are emphasized.
• Energy self-sufficient wastewater treatment with nutrient removal can be achieved.
• Turning wastewater treatment plants into net energy generators.
• Utilizing the thermal energy in wastewater by heat recovery and heat pumps.
• Further measures to optimize the energy balance of wastewater systems are mentioned.

This paper discusses some of the major ways in which the energy balance of municipal wastewater systems can be optimized. In Austria, two advanced municipal wastewater treatment plants with nutrient removal are energy self-sufficient. At these plants the total consumption of electric energy is smaller than the energy production by means of Combined Heat and Power (CHP) generation using biogas from anaerobic sludge digestion. By additional measures like the addition of organic waste to the digesters (“co-digestion”), the use of the thermal energy of the wastewater for space heating and alternative wastewater and waste options using alternative processes, municipal wastewater systems can even become “energy-positive”. The studies have shown that wastewater treatment plants are capable of reaching up to 180% energy generation compared to the energy needs, while switching from wastewater to cooling water regeneration as the heat source of heat pumps for district heating can offer electricity savings of up to 45%. However, negative effects on the environment like insufficient wastewater treatment or the release of methane gas to the atmosphere have to be avoided.