Qualities I'd be interested in (and I am probably not thinking of some things):
Maximum range of the spell
Maximum volume or weight that can be transferred
basic constraints like (no living things, nothing inside a container,
not inside a living creature)
Fine-tuning what counts as "sewage"
Other constraints (ordered from what I think is easiest to hardest):
limiting range of the spell
restricting sewage summoning to only "below ground"
excluding some specific substances
setting up boundaries that may block the effect
targeting certain areas
(anything else?)