From what I understand from ...

From what I understand from the OP, stormwater drains are seeing sewage disposed of within them.

The technology to treat waste water / sewage already exists obviously - wastewater treatment systems either at a local, cluster, or centralised scale.  However it may not be appropriate to divert stormwater flows to central WWTPs unless they are designed for irregular loading, or capacity exists to treat the additional volumes of water to the same level as that which the WWTP was designed.  Connecting stormwater systems to pre-existing WWTPs is counter productive in my understanding as the result is an overwhelmed receiving environment (and dilution is not treatment!)

At a cluster (or catchment) scale, I imagine that sewage could be collected in stormwater retention/detention basins - but this would have massive ramifications to the local environment, not to mention the additional operational costs and maintenance requirements, and the vector for diseases that this would present.

I can't see the installation of additional septic systems, nor retrofit of blackwater treatment systems to stormwater as being very feasible or practical at a local or cluster scale.

As easy as it is to say, and as difficult as it may be to employ successfully and at scale, the only "technology" that is going to prevent disposal of sewerage in stormwater is a good behavioural change program (education), with a regular and ongoing maintenance program (sewerage and stormwater networks - are septic systems overflowing into stormwater?), and perhaps punitive measures for those who insist on using stormwater as a sewer.