While certainly not purporting to be a foremost expert in this field nor knowing if the following is applicable to your specific application, I believe for major wastewater plants in the USA it is most common to see either unlined ductile iron and/or stainless steel piping employed off the blowers of aeration systems. With regard specifically to iron piping, I suspect however a wide variety of joints, coatings, and linings have been installed in accordance with what specifiers at one time or another have required and/or contractors etc. have installed for various air applications.
It should be noted however that some air piping applications reportedly often involve quite elevated temperature service, apparently due to at least some quasi-adiabatic effect that is most pronounced nearer the blowers. This in my opinion in particular should be very carefully considered in the design and specifying process for pipe, joints, gaskets, and perhaps also any coatings and linings one thinks is applicable.
I believe push-on and/or push-on gasketed restrained joint ductile iron piping, and gasket material also acceptable for the maximum temperatures actually involved are in general satisfactory choices for air piping. Bolted mechanical joint (MJ) seals, which are typically much more labor-intensive and labor reliant than push-on, and also any joints/gaskets e.g. with rubber gaskets other than those mentioned used beyond their temperature ratings, may be most adversely affected by imperfect installation and/or what goes along with high temperatures and/or fluctuating heating and cooling cycles. For these reasons the use of mechanical joints should probably be avoided (at least where they can be), e.g. use modern, available push-on joints at least in at least straight runs of air piping. EPDM gaskets are often used for air piping, although extremely high temperatures and/or a history of past system problems may make special fluoroelastomer (FE) gasket material the best choice for some applications [see e.g. the guidance at
etc.] FE is the highest temperature and also most chemical resistant common gasket material available, although I believe FE gaskets also have the highest cost.
Incidentally, with temperatures for air piping I have seen sometimes seen specified approaching 300 degrees F. or so, I suspect some specifiers (at least those who are aware of real limits and behaviors) are not real comfortable with many aspects of most plastic pipe at that temperature.
