How many municipal wastewater treatment plants, by size, there are in
the world?
How many are in the US, Europe, Asia, etc.? 
How many of them are incompliant with the environmental regulations
(and thus need to be apgraded)?  

While this is not a direct answer to the question you asked, various
breakdowns by country (both rural and urban) on how much of the
population is served by water sanitation is avialable from the World
Health Organization publication, Global Water Supply and Sanitation
Assessment 2000 Report, which is avilable at
http://www.who.int/water_sanitation_health/Globassessment/GlobalTOC.htm
              turnip-ga

Thanks Turnip!

Regarding my question, trying to focus a bit, will it be possible to
obtain the data I seek in the following countries? :
US, Canada, Mexico, Germany, UK, France, Holland, Italy, Spain,
Greece, Turkey, Brazil, Japan, Australia and eastern Europe countries.

Thanks,

Eytan

Thanks Turnip!

Regarding my question, trying to focus a bit, will it be possible to
obtain the data I seek in the following countries? :
US, Canada, Mexico, Germany, UK, France, Holland, Italy, Spain,
Greece, Turkey, Brazil, Japan, Australia and eastern Europe countries.

Thanks,

Eytan

I did some research and e-mailed some people. This is one of the
e-mails I received:

"There are approximately 15,000 publicly owned wastewater treatment
facilities in the U.S. of those the majority of them treat industrial
waste.

If you are looking for specific numbers of municipal and industrial -
you may want to call the EPA - we just deal with the municipal guys.

Hope this helps! 
Robin 
Robin Davis 
Marketing & Membership Development Coordinator 
Association of Metropolitan Sewerage Agencies 
Tel: 202/533-1802 
Visit us on the web at www.amsa-cleanwater.org"

That's all I got.  You might want to try contacting the EPA yourself.