The question of whether a hydrodynamic separator should be online vs. offline is something that engineers and stormwater treatment manufacturers deal with on a daily basis.  Online systems allow storm events that are greater than the design storm to be bypassed through the treatment unit, eliminating the need for a separate bypass structure.  This results in fewer structures and a smaller system footprint; which leads to a lower landed cost.  While specifying and allowing online systems may seem like a no-brainer, there are two key items that must be evaluated before moving forward.

1.  Verify that the treatment unit has adequate hydraulic capacity to handle the peak event

There are two critical capacities to evaluate with an online device; treatment capacity and hydraulic capacity.  Treatment capacity is the maximum capacity the unit can accommodate to meet specified removal requirements.  Treatment capacity should be verified whether a unit is located online or offline.  The hydraulic capacity is typically greater than the treatment capacity and represents the maximum flow a system can handle.  When peak flow rates exceed the hydraulic capacity of an inline unit, there is a risk of flooding during the peak event.  It’s important to note that while many manufacturers provide standard maximum hydraulic capacities, the specific hydraulic capacity can often be higher depending on site characteristics.  For instance, if the distance from finished grade to the inlet pipe exceeds the manufacturer’s assumption, there could be an opportunity to pass higher peak flows through the inline unit than the standards list due to the additional head provided over the internal weir.  It’s always best to consult with a manufacturer if an online unit is desired and the peak flow rate exceeds recommended hydraulic capacities. 

2.  Verify that scour will not be an issue at flows greater than the treatment flow rate

You’ve verified the unit can handle the treatment flow rate and that it has the hydraulic capacity to internally bypass the peak flow rate, so you’re all set to move forward with the system, right?  Not quite, one more key item should be evaluated to verify the system was designed to capture pollutants and protect our waterways and doesn’t lose them during the peak events.  Re-suspension of previously captured pollutants, also known as scour, could be an issue and it’s important to understand a product’s capabilities upfront to ensure the treatment device not only captures but also retains pollutants of concern.  Many manufacturers have incorporated barriers to separate the higher velocity treatment zones from the sump area where most pollutants are stored.  Practices such as this can eliminate scour of previously captured pollutants.  Many of the leading regulating agencies have put benchmarks in place for stormwater treatment devices to be approved for online use.  As such, several devices have undergone resuspension testing to evaluate the likelihood of scour at high flow rates. 

A quick check to verify the unit can handle the flows and is not susceptible to scour will ensure the investment in stormwater treatment not only meets the treatment goals but also retains previously captured pollutants and protects the downstream waterways.