You can, of course, use published peak factors as suggested by gt5879c but unless these are specifically for prisons you can probably get a better estimate by surveying similar prison systems. Because prisoners are likely to be highly regimented I suspect that peak wastewater flows will occur at predictable times of the day and be higher than peak flows for an ordinary civilian population. If possible, get actual flow data for similar prisons in your area or State. Metered water use data should be available and will probably be reliable unless there are large demands for irrigation, process water, or similar water uses that do not discharge to the sanitary sewer. Ask around.
Inmates are famous for devising ways to clog the sewer system. Rags, clothing, trash, mop heads ( explosives if they can get them )and all similar means are seen as weapons of mass confusion and distraction at prisons, mental hospitals, etc. I live in a State capital home to about ten such facilities. For this reason your local sewer system provider may require trash racks, inspection and metering manholes and other safeguards on any system you build. Such things make a joke of even the most careful calculations.
I will add that prisons are also famous for consuming large amounts of water. Prisons may also have special plumbing systems. For example, the fixture water supply is typically controlled with push buttons instead of knobs, for obvious reasons. Newer prisons will use less water than older prisons because of more modern plumbing fixtures.
As RWF7437 suggested, you should survey nearby prisons with to obtain the most accurate water data.
Funny, my issues also relate to a prison facility, but operation not design. Rule of thumb for our design standards was 100 gallons a day per inmate. You need to add staff as used per shift as you will not have all staff there at same time. Don't forget for the contractors that come in and work with inmates also.
