While mixed sewers are being phased out in many developed countries for various reasons (for example declining population), the case can be made that well designed mixed sewers are not a dead-end technology.

Besides them being cheaper, it is also easier to treat the diluted effluents in pond like treatment systems and where rainfall is well distributed over the year the transport in mixed sewers is very efficient.

Except in extreme cases (when separated sewers will probably also have issues due to localized flooding) the problem of stormwater overflow has also been solved sufficiently using bioretention basins:
en.wikipedia.org/wiki/Bioretention

Overflows from Combined Sewers

Wikipedia gives the definition of combined sewers as: A combined sewer is a sewage collection system of pipes and tunnels designed to also collect surface runoff. This type of gravity sewer design is no longer used in building new communities (because current design separates sanitary sewers from runoff), but many older cities continue to operate combined sewers. Combined sewers can cause serious water pollution problems during combined sewer overflow (CSO) events when wet weather flows exceed the sewage treatment plant capacity. The discharges contain human and industrial waste, and can cause beach closures, restrictions on shellfish consumption and contamination of drinking water sources (en.wikipedia.org/wiki/Combined_sewer)

In Karachi, Pakistan, there are specific sanitary sewers, which collects wastewater and convey to the treatments (which are usually not working). In addition, there are channels with slabs on top that convey stormwater to sea. In practice, these stormwater channels are used to convey wastewater. These channels then act as “combined channels.” These channels are rectangular in shape.

A recent report from the from the New York State Comptroller, entitled: A Partially Treated Problem: Overflows From Combined Sewers, makes an interesting reading:
(www.osc.state.ny.us/localgov/pubs/resear.../combined-sewers.pdf)

The report says:

Over 10 million residents live in communities with combined sewer systems. 46 communities have combined sanitary and storm sewers, with 807 discharge points where combined sewer and stormwater are released either partially or wholly untreated into a waterbody. 6.5 billion gallons of untreated combined sewer and stormwater were released in 2017, affecting 220 waterbodies in New York State. 43 percent of combined sewer outflows are in urban upstate communities.

As happens in Karachi, and as the report notes: Numerous recent news reports have highlighted instances of systems unable to handle the increased volume stemming from major weather events. The details are troubling: raw sewage being flushed directly into rivers, streams and lakes that are also used for recreation including boating, swimming, fishing and in some cases, drinking water. For citizens who may have been unaware of the situation, this news brings into question long-held assumptions about the natural resources they enjoy on a daily basis.

During heavy rains, the combined channels get more sewage from the clogged sewers, and all this is convey to sea, causing sea pollution. The responsibility largely falls to local governments, and they must act to prevent sea pollution.

As a sort of advice, the report says: The solutions are not simple or quick. These systems are some of the largest and oldest in the State. Full scale replacement with separated sanitary and storm sewers is often unrealistic, as it would be extremely costly and disruptive. However, many systems have seen improvements through a mix of retrofitting limited sections of sewer and treatment infrastructure, conducting basic repairs on others, and applying green infrastructure techniques that slow the movement of stormwater in order to prevent overflow in the first place.

F H Mughal