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→Calculations for Load Based Performance
*''B'' = Flow proportioned effluent concentration (mg/L)}}
*''B'' = Flow proportioned effluent concentration (mg/L)}}
Simple treatment train equations that apply the pollutant removal efficiency of the downstream treatment practice to the fraction of load remaining after passing through the upstream treatment practice, or those that assign downstream practices half the removal efficiency of the upstream practice are not recommended. Instead, the configuration and type of treatment train should be considered on a case by case basis, with consideration for changes in influent runoff concentration/composition and runoff volume as water drains through the different practices. Some examples are provided in the sections below.
Simple treatment train equations that apply the pollutant removal efficiency of the downstream treatment practice to the fraction of load remaining after passing through the upstream treatment practice, or those that assign downstream practices half the removal efficiency of the upstream practice '''are not recommended'''. Instead, the configuration and type of treatment train should be considered on a case by case basis, with consideration for changes in influent runoff concentration/composition and runoff volume as water drains through the different practices. Some examples are provided in the sections below.
====Treatment trains without runoff volume reduction====
As noted above, non-infiltrating stormwater facilities are often designated as enhanced or basic treatment based on their capacity to remove total suspended solids (TSS). Enhanced level facilities are credited with 80% TSS removal while basic level facilities are credited with 60% TSS removal or less.
Basic level facilities are typically placed upstream of enhanced level facilities in treatment trains because enhanced level facilities remove the coarse sediment particles that would otherwise be removed by the basic level facility, as well as finer sediment particles. Placing the level two facility downstream of the level one facility would, therefore, not enhance TSS removal, although it may enhance removal of other pollutants depending on the design components.
Since basic level facilities remove TSS particles that are effectively removed by enhanced level facilities, the removal efficiency of a treatment train consisting of two non-infiltrating facilities would be equal to the removal efficiency of the highest performing facility (i.e. 80% TSS removal), assuming that the facilities are sized appropriately.
Examples of this type of treatment train would include: (i) [[Pretreatment features|catch basin inserts]] (aka water quality inlets) upstream of a filtration facility (e.g. lined [[bioretention]], [[Pretreatment|proprietary filtration system]], [[Media filters|sand filter]]); (ii) [[OGS]] upstream of a wet pond; (iii) [[Infiltration chambers|temporary parking lot or pipe storage]] upstream of a filtration facility. If an OGS is placed upstream of a [[dry pond]], the overall treatment performance would be equal to that of the dry pond since the finer particle size fraction of TSS is not captured. As mentioned above, the major benefit of these types of treatment trains is to reduce long term maintenance cost and effort by centralizing sediment in the upstream practice where it is easier to clean out.
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]].
==References==
==References==