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In some cases, it may be advantageous to include two level one facilities in a treatment train. An example may be a lined [[bioretention]] with pre-treatment upstream of a [[Pretreatment#Concentrated underground flow|filter with media designed to enhance phosphorous uptake]]. The overall [[Phosphorus#Limiting excess phosphorus|phosphorus removal rate]] for the treatment train would be equivalent to that of the downstream filter, assuming that it is sized appropriately for the site in question. The bioretention facility in this instance would help to control flow rates to the downstream facility while also filtering out sediments that would otherwise cause pre-mature [[clogging]] of the downstream [[filter media]].
In some cases, it may be advantageous to include two level one facilities in a treatment train. An example may be a lined [[bioretention]] with pre-treatment upstream of a [[Pretreatment#Concentrated underground flow|filter with media designed to enhance phosphorous uptake]]. The overall [[Phosphorus#Limiting excess phosphorus|phosphorus removal rate]] for the treatment train would be equivalent to that of the downstream filter, assuming that it is sized appropriately for the site in question. The bioretention facility in this instance would help to control flow rates to the downstream facility while also filtering out sediments that would otherwise cause pre-mature [[clogging]] of the downstream [[filter media]].
==Treatment trains with runoff volume reduction facilities==
These types of treatment trains are becoming more common because they can achieve multiple [[Runoff volume control targets|stormwater control]] and [[water quality|treatment objectives]]. Since wet ponds alone do not achieve stormwater water balance criteria, they must be supplemented with facilities providing runoff volume reductions to meet regulatory requirements. If site water balance objectives require control of the 90th percentile event (roughly 25 – 30 mm in most jurisdictions), the same infiltration facilities may also be used to treat the water quality storm (typically 25 mm), allowing for a [[dry pond]] or similar temporary detention facility to be used at the end-of-pipe to meet flood and erosion control criteria.
In the example of LID practices upstream of a wet pond, the overall treatment train would meet the 80% TSS reduction target, since both facilities achieve enhanced level water quality targets. The wet pond may be downsized to account for the upstream reduction in runoff volumes, but the overall volume of water treatment to the enhanced level target (80% TSS removal) would be equal to the volume treated by the LID plus that treated by the wet pond, which together would exceed the 90th percentile water quality requirement.
In cases where the 90th percentile storm volume is fully retained (i.e. infiltrated or evapotranspired), the wet pond may be substituted with a dry pond. In this scenario, the water quality and water balance volumes are provided by the upstream facilities, and the dry provides flood and erosion control. Runoff volumes exceeding the 90th percentile storm volume would be treated at a more basic level. Lower maintenance costs would be the primary advantage of substituting a dry pond for a wet one, although the dry pond may also provide opportunities for multiple uses in some contexts.
One final and relatively common example of infiltration practice treatment trains on smaller sites would be a [[Pretreatment#Concentrated underground flow|non-infiltrating filter]] used as pre-treatment to an infiltration practice such as a [[infiltration trench|trench or [[infiltration chamber|chamber]] system. The upstream facility is sized to treat the water quality volume (or higher) and the downstream infiltration trench or chamber is sized to meet water balance criteria (e.g. infiltration of the 90th percentile storm). If the entire water quality volume is infiltrated by the downstream practice, the TSS load reduction for the treatment train would be 100%. If only 80% is infiltrated in the downstream facility, the TSS load reduction would be 96% since 80% of TSS is reduced in the first facility and 80% of the volume is reduced in facility two (0.8 + (0.8 x 0.2) = 0.96 x 100). The second facility would not further reduce TSS concentrations because only fine unfilterable sediments are left once the runoff has passed through facility one. Therefore, facility two reduces water quality loads only by reducing the volume of runoff (which was assumed to be 80% in this example).
==References==
==References==