How to Manage the Increase in Seasonal Wastewater Inflow?

What is the optimal solution for a wastewater plant facing a big difference in flow during the year?  Many coastal cities are facing the increase in the number of residents in the summer, in comparison to winter. I live in Nador, Morocco and during the summer season the number of people multiplies 10 times,  reaching 1 million.  What's the best solution for the wastewater network and plant design in this case?