Dear Soufiane, If you want ...

Dear Soufiane,

If you want to treat all the wastewater coming in the Summer Season which is 10 times more then in Winter Season. And as I assume you have a longer Summer Season say for 6 - 8 months.

In this case, you could have a couple of options;

1) Your Municipality needs to have additional Treatment Plants by making Modular Type System which will enable to run only one stream in Winter and in Summer it could use all the modules. 

2) Convert your existing Biological System to MBBR System, thus may increase this system capability within the same civil structure (aeration tank) to treat approximately 3 times then existing plant capacity.

3) Convert your existing Biological System to MBR System, so you could run your existing Aeration at a Higher MLSS 12,000 - 15,000mg/l thus increasing the current bioreactor to treat 4 times or more wastewater within the same reactor.

4) If your WWTP Plant is outside the City and has no population nearby the plant and ample land is available, then you could opt for Constructed Vertical Wetland which is far cheaper than all the above solutions.

Obviously in all cases, there will be needed some more essential addition of pumps/equipment as well as other alterations. 

Everything need to be looked out, I just floated the idea to you.