Since, as you mentioned, ...

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Since, as you mentioned, screening is full in the night and water overflows,  It could be a obstruction in the screen channel.

My suggestion is to  review this section and, in case, build at least a coarse screen before entering buffer balancing tank. Moreover, if flows during night is not greater than during the day (and process works good during rain) there is no reason to build a balancing tank (that is costly) only for night overflows. It is better to spend money for automatic screens and automatic flow controlling devices.

Best regards