The major parameters that influence the number of operational staff employed are:

• the size of installation,

• the treatment processes and systems,

• the degree of automation,

• productivity efficiency of personnel,

• managerial efficiency and

• others.

In principle, it can be noticed that small wastewater treatment plants need more personnel per p.e. than larger ones. This is a result of economy of scale achieved in larger sizes of installations. In addition, in small and medium size plants, conventional systems are more costly in terms of personnel in comparison with extended aeration plants. An other factor that dominantly influences personnel costs is the degree of automation. Although automation requires considerable capital cost as well as specialized personnel to operate the plant, there is a concomitant reduction in the number of employees needed.

A good example is the analysis of economic data pertinent to existing municipal wastewater treatment plants (MWTP) in Greece, which has been carried out by Tsagirakis et al (2003). On the basis of data on personnel from 66 activated sludge systems the relationship between p.e. and t.p.e. and the number of personnel employed in MWTP has been analyzed (Figure 5). The figure shows the number of the p.e. equating to one person employed in a MWTP. It can be noticed that small installations need more personnel per p.e. than larger ones. This is a result of economy of scale achieved in larger plants. In addition, in small and medium size plants, conventional systems are more costly in terms of personnel in comparison with extended aeration plants.

Figure 5: Employees per p.e. and t.p.e. in relation to their size; (EA = Extended Aeration; AS = Activated Sludge) [Tsagarakis et al, 2003]