I have a water pipeline (several miles with hilly terrain), which takes treated water from a sump pump and dumps it into a separator for venting off the air, the water from there goes through a diffuser into a river.  I have air release valves on the hill tops.  The line is 16" ANSI 150 (operating pressure is 80 psig at 2500 gpm).

I have the following questions:

1) What is the purpose of the diffuser?
2) I see no back pressure control at the river.  Does it need one and if yes, do I set it at one psig or so above atmospheric pressure?
3)My main question is what parameters are important to be considered in the water pipeline, as far as the hazards are concerned?filter

The purpose of a diffuser at the pipe outlet is to entrain air in the water stream going into the river. The entrained air has a bioligal action on the water and helps to purify it.
Conversely the air vents on your hill tops are to remove accumulate air in the pipeline and prevent water hammer.
 Securely support your pipeline, eliminate bends as far as possible, check for landslide possibilities,falling trees, seismic alllowance, and traffic across your pipeline if it is buried near road areas. Allow for thermal expansion /contraction ,if you have large variations in winter/summer temperatures. Insulate and heat trace if in Arctic area.

"1) What is the purpose of the diffuser?"

A diffuser is typically used in wastewater applications to dilute the wastewater effluent into the river. A model may be required so that it can be demonstrated that the effluent is thoroughly mixed and diluted within the effluent mixing zone.

"2) I see no back pressure control at the river."

I don't think a backwater pressure would be required.

Not familiar with the use of a separator tank for this application.

If the separator tank is located ny the river and is open to atmosphere, then you will probably need a valve with level controller to prevent the separator from overflowing. Because the pipeline should have a higher head than the separator.

One would  think that you have adequate velocity to force any air bubbles through the pipeline.

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