By: Jordan Kahlenberg – Sr. Stormwater Design Engineer
When designing a stormwater filtration system the two most common design constraints are treatment flow rate and sediment loading. In other words, how much flow can it filter/treat, and how much sediment (i.e. Total Suspended Solids, TSS) can it remove before maintenance is required? Both of these parameters are important to the design of a stormwater filtration system, but there is a third important item that often gets overlooked when designing these systems: Hydraulics.
In order for a stormwater filter to operate properly, sufficient hydraulic driving head must be available upstream of the device to permit gravity flow of contaminated stormwater through the filter media and discharge of treated water downstream. One significant advantage of the Jellyfish® Filter is the minimum amount of hydraulic driving head needed to operate the system. The Jellyfish Filter is most often designed to operate at full capacity with 18 inches (455 mm) of driving head. Standard design provides substantial excess filtration media surface area to ensure that design flow capacity is maintained during the period between maintenance service operations.
To ensure that a filter will operate as intended, there is one additional item which must be evaluated: Tailwater. What is tailwater? For our purposes, we define tailwater (also referred to as backwater or a submerged condition) as a condition which causes the water surface elevation downstream of the filter to be at or above the outlet invert of the filter structure. In the case of the Jellyfish Filter, this would result in standing water on top of the cartridge deck. By properly evaluating the potential for tailwater in the system, the design of the upstream flow diversion structure will accommodate the filter’s full operating capacity.
Let’s take a look at an example evaluation of tailwater on an off-line Jellyfish Filter.
The images below illustrate a typical off-line layout of a Jellyfish Filter. Notice the upstream structure includes a weir which diverts water quality flow to the Jellyfish Filter. When properly designed, flows in excess of the design water quality flow rate overtop the weir and are diverted downstream of the filter. The Jellyfish Filter includes a backwash pool weir that is 6 inches (150 mm) high, which allows the system to passively backwash the cartridges installed within the weir each time a storm event subsides. Evaluating the tailwater condition against the backwash pool weir elevation allows for determination of proper function, and sets the elevation of the weir in the upstream diversion structure.
During review of site plans, these are the questions the design engineer asks to assess potential tailwater conditions:
Is your tailwater condition at or below the outlet invert of the Jellyfish Filter structure? If so, then your system should operate as intended with no special design considerations. This goes for instances where there is NO tailwater condition, which is the desired condition.
Is your tailwater condition at or above the outlet invert of the Jellyfish Filter structure, but below the elevation of the deck’s backwash pool weir? If so, there may be impact on the passive backwash operation. Special consideration may be needed to ensure proper backwash operation, and reduced maintenance interval may be expected. Since the tailwater condition is still below the elevation of the deck’s backwash pool weir, there should not be any change to the elevation of the weir in the upstream diversion structure.
Is your tailwater condition above the elevation of the deck’s backwash pool weir? If so, the backwash operation and net available driving head of the system will be impacted, since the system will have to work against the standing water column during treatment and backwashing operation. In this case reduced maintenance interval is expected. Special design considerations should be made to ensure proper operation of the unit, or inverts raised to avoid this condition where possible.
In any instance where you are considering the use of a Jellyfish Filter to handle a project’s water quality needs and encounter a tailwater condition, reach out to your local Imbrium Systems representative for design assistance. Our team of engineers is always more than happy to help!