AHC/PC: Earliest Sewerage System in London

What would be the earliest POD for the City of London to be able to construct a sewer system, and is there any plausibility of a sewer system being built before Joseph Bazalgette rolled out his designs?
 
Londonium during Roman occupation may have had a sewer system as most major Roman cities had some sort of system. If somehow this system is more robust and is maintained longer it might carry over.
 
The city of York had a Roman sewer system made of stone, portions of which survive today so it's likely that the Romans had extensive works in London. However as a low lying, fairly swampy city on a tidal river it's likely that they went for an open ditch design that London was later known for rather than tunnels they used elsewhere. It wouldn't surprise me though if some of the London ditches that were later bricked over to create the modern sewers had originally started as Roman works somewhere in their past.
 
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