Countries tend to choose one major wastewater treatment plant in each city or choose several secondary treatment plants(packaged units) in the same city based on several factors. The most important factors are: the availability of land for the establishment of such plants, economic feasibility, the environmental impact of the establishment of such plants, the availability of sewerage networks in all parts of the city,the location of the water source to which the treated water is discharged, the type of community to be served, .......and other factors.

Knowing that this subject needs many details for the purpose of stating the exact answer to it.

Yes, dear Isam Issa Omran. Best regards

i Like to draw your attention to the work of my former phd student xiao liang, with whom I have published a number of articles on this issue, for example 

§  Liang, X. and Van Dijk, M. P. (2012). Centralized waste water treatment in Beijing economically and financially feasible. In: Water reuse & recycle, May, pp. 48-53.

§  Liang, X. and Van Dijk, M. P. (2012).Cost benefit analysis of centralized waste water reuse systems. In: Journal of Benefit-Cost Analysis, Vol. 3 (3) Article 5 doi: 10.1515/2152-2812.1060, pp. 1-28.

kind regards meine pieter van dijk

Very interesting dear Meine Pieter van Dijk. Best regards. 

Dear

 i am with Centralized wastewater treatment plants(WWTP) in terms of efficiency,power consumption and economy

regards

Thanks a lot dear Ali hadi Ghawi. Best regards.