Wastewater lift stations

I am involved in a research project, and am in urgent need of the total
number of community wastewater lift stations in operation in the United
States.  Ideally, I would like to know the total number by year for the
preceding 20 years, but would be happy with even the current (or recent)
number.

If you are able to provide me with this statistic and its source citation,
or can suggest sources I might pursue, I would be very appreciative.  Please
post replies here and cc: to [EMAIL PROTECTED]

Thanks!

Jim Haberman
E-mail [EMAIL PROTECTED]