Hybrid treatment plants: maintaining production

It is known that large and complex water treatment plants are most efficient when production levels are maintained at or near the design capacity over the long term. As production rates rise and fall dramatically, it is required to change operations and startup/shutdown major pieces of equipment, costing the plant more money to operate and maintain.

One of the major problems with treating stormwater is that the water rushes through during storm events then virtually disappears at other times, usually at times that are unpredictable. Regulating these flows using storage is most likely uneconomical because of the high cost to build huge storage facilities. Another problem with treating stormwater for reuse is that the water quality of the stormwater is possibly different than the traditional water supply and so a totally different treatment process might be necessary.

What if a treatment plant was able to quickly change the type of treatment process but maintain overall production level? Would this be economical? If this was found to be viable, then a treatment plant could divert it's traditional inflow stream and accept stormwater in it's place while the stormwater is available, then switch back after the storm is past.

Has this ever been done?

An article has recently been published describing how treatment plants in Singapore have been built that can switch from treating fresh water to saline water while maintaining production.  The abstract of the article says:

 When the river water has high salinity or dries up, treatment is maintained with the plant operating in seawater desalination mode instead of remaining idle, thus affording high plant utilisation. The treated water is of high grade with energy consumption half that of seawater desalination plants. The average unit production and capital cost of such variable salinity desalination plants is significantly lower than that of seawater desalination plants. (Sheah, 2010)

 So the question I have is whether or not this concept might be applied to a water supply system that contains stormwater collection facilities and catchments. Are there possibilities out there that would allow for a treatment plant that typically treats cleaner river water on a regular basis, but changes operating modes to treat stormwater on an as-need basis? If the plant could be maintained at the design capacity through the operational switch, could it be economical?