Multiphase flow is a complex subject if you have to treate air flowing sometimes full and sometimes partially full (open channel flow) inside the sewer.
If the sewer is pressurised as indicated and a full bore flow is ensured then the air effect may be simplified by a reduction of the wave velocity.
Air is known to greatly attenuate the wave velocity, 0.1% and the wave speed could be halved. Consult "Fluid Transients" by EB Wylie and VL Streeter books. The reduced wave speed will help you to assess the flow conditions but getting the air out and have it under control is another matter.
I'm not sure in which country you are, but in South Africa we have a sewage gas air relief valve manufactured by Vent-o-mat/IVM Hydro Projects. Contact them on +2711-918-4057, speak to Shaun.
Air is dissolved in water and can come out if the water column depressurised. Also certain operational procedures can introduce air into a pressurised sewer, for example a pump trip. To remove air by air relief valve is a design requirement but to analyse the system from one steady state to another steady state is by surge analysis or waterhammer study. A multiphase flow is very complicated surge analysis when compressibility of air is introduced.
Simplified procedure with samll trapped air pockets can be handled by traditional single-phase water hammer study treating the air pocket as a boundary condition. Air relief valve can be sized as a byproduct of such analysis.
I'm not sure if this exactly applies to your question, but pressure sewers (forcemains) act just like water system pipes. Therefore a combination of blowoffs (low sections) and air vacs (high section) is typically used to exhaust air from the pipe.
Many pressure sewer pipe and fitting products will state that they are only designed for liquids. Most installation procedures call for all air to be evacuated from a pipeline prior to acceptance pressure testing. To do this, the pipeline must be filled slowly (1 to 2 ft/s fluid velocity), and air release valves must permit the air to escape from the line during filling. Beware that small air pockets may remain at high points throughout the pipeline. These air pockets may have to be coaxed to the valve location using the fluid flow. Failing to remove air from the pressure sewer line is dangerous. Since air is compressible, a large amount of stored energy is present when the line is under pressure. As well, transient surges may be greatly amplified by the presence of air pockets within the pipeline. (The air pocket reduces the cross sectional area available for fluid flow, which in turn increases the fluid velocity.)
