it is a bit more complex than only average flow and maximum flow for a design, you have to consider minimum flow, average flow, peak flow (during wet weather) expansion of the city because design are done considering what the city will have to treat in 25 to 50 years , etc.. I don't know where you are located but if in the USA you should read the design manuals of the EPA where you have all information about a plant design.
Hi , this also depends upon the type of industry you are designing the ETP for . I mean there are some industries like edible oil where the effluent comes continuously at a fixed rate while in some textile deying industries the effluent comes in batches so here the maximum flow rate can be 10 times the normal flow rate. So it all depends upon the working process of industry . If you study that at what intervals and in what quantity each machine releases effluent , and make a 10 days chart of it , I am sure you will find the answer of maximum flow rate and Normal flow rate .