The effects of high hydraulic ...

The effects of high hydraulic loading in a treatment plant due to stormwater inflow and infiltration, and what you can do to manage the excess flows, depends on the type of treatment process you are operating and the unit components.  As others have noted, suspended growth processes like conventional activated sludge can be adversely effected as the high flows reduce the hydraulic retention time in the bioreactor (reducing the level of treatment, flush suspended biomass out of the bioreactor, and overload the clarifier resulting in excessive biomass loss.  MBR processes, while they don't lose biomass, are probably the most sensitive to hydraulic overloading due to the limited flux rate that can pass through the membranes.  Fixed film and hybrid processes are much less susceptible to biomass loss, as the bacteria are attached to surfaces and are not easily flushed from the system.  If you have an activated sludge process, and are having biomass retention problems, you could consider an MBBR conversion.  Equalization storage is a good initial approach, and can usually be retrofitted or expanded for any plant design.  Regulators in North America will often allow excess flows beyond 2 times the average dry weather flow to bypass the secondary or tertiary treatment process, while requiring the water to at least be passed through a primary clarifier and may require chemical enhancement.  Generally blending enhanced primary treated bypass flows with fully treated secondary or tertiary flows for short periods will minimize the effluent degradation.  There are also a few clarifiers that can better tolerate extended periods of high hydraulic  loading, such as the Veolia Actiflow  which incorporates a carrier, chemicals and plate separators to enhance solid separation under high flow conditions.