Good morning Felipe. I ...

Good morning Felipe.

I believe some information is missing here. What is your range of desired flow? (you only mention a starting point of 5 l/min), What is your dynamic head?. Do you have VFDs? (Variable Frequency Drives). Do you control the level in your equalization basin or just the incoming flow to 5 l/min?

In any case, you should take a look at the point of discharge of this system (a system curve would be great) and see if your existing pump is operating within its curve, otherwise you might be operating it in a very inefficient zone, or perhaps your pump is oversized.

If you ask me, for wastewater you want a pump with two vane impeller, capable of passing 3" sphere (depending on pump size, although I figure this is a very small pump) and of the self-cleaning type (such as Flygt with N impeller). If wastewater is already screened, you don't need a self-cleaning impeller because the risk of that pump clogging is minimum.

Returning part of your flow back to the equalization basin is giving you the hardship of consuming more energy without truly conveying any flow to your final destination, reason sufficient to get a VFD, and you'll see your energy cost dropping significantly (see affinity laws).

Best regards.