Operation costs can be differentiated as personnel costs, maintenance costs, energy costs, chemicals and material costs, disposal costs and miscellaneous costs for administration, insurance, discharge duty (if exists), external services etc..

There are many factors affecting operation costs [Bohn, 1993], accordingly operation costs may differ widely:

• Size and load of the plant

• Topography and geographical situation of the site (e.g. affecting pumping energy costs)

• Characteristics of wastewater and the discharge norm

• Technologies and the selected treatment process

• Type of sludge treatment and way of disposal

• Energy supply and energy recycling

• Degree of automation, measurement and process control

• Organization of the plant and its management

Figure 1 shows the composition of operation costs for a selected wastewater treatment plant of a capacity of 5,000 p.e. (population equivalent). It can be noticed, that the cost for personnel, maintenance, energy and sludge disposal are decisive categories as these account for most of the operation costs. In the following sections each single category of operation costs are discussed in more detail.

Figure 1: Composition of operation costs for a wastewater treatment plant of 5,000 p.e. in Germany [Halbach, 2003]

For rough calculation it can be assumed, that operation costs may amount up to 50% of the total costs for wastewater treatment. In average for wastewater treatment plants in Europe with more than 10,000 p.e., the specific operation costs will amount up to 25–35 €/p.e. and year. Smaller installations can achieve more than twice of that number.

As it has been the case in Europe, the more restrictive the discharge norms are – for example concerning nitrogen and phosphorous content –, the more demanding the treatment must be. That means that more technical requirements (like longer treatment times, conversion of existing structures or building of new ones) are needed to achieve the limit values. Such extra requirements represent more operational expenses as well.