Apartment complex

I have an apartment complex that is in the finshing stage I just got to the job the water was tested a couple weeks ago so i get there and And then on the 3 inch main that feeds 3 buildings and isolate building 1 and 3 because I’m testing 2 currently as I open the 3/4 valve for the north side of the building I have water but very little to no pressure the system itself wasn’t air locked and the building wasn’t cold so the pipes weren’t frozen I’m thinking that the system has debris in the lines because the valve is an existing valve any ideas on how to blow the debris out of the system

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