Energy consumption is a major contributor to the operation cost of wastewater systems and therefore is an important parameter for choosing a treatment technology. The costs for energy usually amount up to 10–30 % of the total operation costs. Energy costs include the consumption (and internal production) of electricity, gas, oil and district heating. In sewer collection systems energy is used for transportation by pumping stations in case of a lack of sufficient hydraulic gradients. Aeration is considered to be the main energy consumer in the wastewater treatment process.

Due to different tariff structures all over the world, average costs for energy consumption of wastewater treatment systems can not be given. There are many different combinations of off-rates and peak-rates of the power utilities existing. For example in Israel the peak rates are fourth times the rates of off-peak rates, resulting in high incentive for energy consumption during off-peak period [Kadar and Siboni]

For a first calculation energy costs can be derived from the total amount of pumped wastewater and the population equivalent of the treated wastewater as follows [Bohn, 1993, NRW, 1999]:

The above given assumptions for energy demand are very roughly and have to be specified for each particular project. They mainly depend on the size and the type of the chosen treatment process (possibly including digestion with the utilization of produced biogas). Target values for energy consumption are set up by Halbach (Figure 7), which demonstrates that the specific energy demand per p.e. remarkably declines from small to large treatment plants.

The operator has to challenge with energy as one of the main cost components of the treatment plant. Thus, for best results of energy cost reduction

• energy conservation,

• process efficiency,

• aeration devices and oxygen transfer,

• process flow configuration,

• bio-gas quantities,

• bio-gas utilization and

• time of day consumption of energy

should be optimized.

Modification of existing wastewater treatment plants can reduce the current energy costs remarkably. Optimization of oxygen transfer, primary sedimentation, utilization of bio-gas from the primary and secondary waste sludge, while considering in both cases the tariff of the power utilities, can reduce energy cost to a minimum. The self-produced electricity is best used during peak tariff hours. Because in a typical wastewater treatment plant there are other energy consumers such as pumps, electrical motors etc., the electricity produced can be used during peak time. In some nations e.g. in Germany or Israel it is also possible to sell electricity back to the power utilities.

Because gas production is some what regulated during the day, it seems a good idea to buffer bio-gas by the use of a gasholder. A multitude of energy efficient equipment, technologies and operating strategies like energy-efficient motors, variable frequency drivers or electrical load management systems are available to reduce energy costs in wastewater facilities.

Automation and control is an other important factor that influences the overall energy demand. On-line alterations to the aeration times according to the on-line requirements of the system can save a substantial amount of energy.

Firstly the implementation of measures and steps toward reduction of energy cost will generate investment costs, but those costs will likely be amortisised in a few years.

Figure 7: Target values for the specific energy demand subject to the treatment capacity (EW ~ E ~ p.e.) [Halbach, 2003]