If you have solids or rag in ...
Published by Jeremy Biddle, Engineering Director at Bluewater Bio
If you have solids or rag in the wastewater (likely) then a centrifugal pump won't be reliable at such low flow rates. The openings are too small.
If you want a reliable pump for low flows with solids present, use a peristaltic pump. It will generate more than 1 bar but if this is a problem you could fit a pressure relief valve or 10m high overflow pipe. Peristaltic pumps are not cheap though.