Low-flow, low-pressure wastewater pumps

Hello everyone, I'd like to know which pump brand/model/type is the most suitable for a continuous feeding to a wastewater treatment plant. I'm talking about very low flows, starting at 5 liters per minute, and also low working pressure, not more than 1 bar. One way I've solved this problem is by-passing the feed just in front of the pump and sending it back to the equalization tank. Any information about low flow and low pressure wastewater pumps is very appreciated. Thanks in advance