Difference between revisions of "Treatment trains"
(Created page with "File:(LSRCA Logo) Atherley Narrows annual chloride concentrations (1971 - 2020).png|thumb|650px|A graph showing increasing average levels of chloride found in Atherley Narrows, (a rural sampling location, between Lake Couchiching and Lake Simcoe), over the past few decades, due in part to increased use of rock salt in parking lots, roadways and commercial and residential properties. From 2005 - 2020 the amount of chloride increase per year has doubled when compared to...") |
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Revision as of 20:00, 25 October 2023
_Atherley_Narrows_annual_chloride_concentrations_(1971_-_2020).png)
Overview[edit]
A treatment train uses a combination of source (LID), conveyance and/or end-of-pipe practices to meet water quality, water quantity, water balance, and erosion design criteria for the site. These may be implemented to reduce the burden of facility maintenance, address a broader range of design criteria, increase overall treatment system performance, and/or control the rate of flow through downstream facilities. Examples of applications designed to address specific objectives include the following listed in the corresponding sections below.
1. Treatment trains designed to reduce maintenance burdens[edit]
These are the most common types of treatment trains. They typically involve installation of one or more pretreatment devices upstream or at the inlet of the primary stormwater treatment facility.
Example: Runoff into larger treatment practices such as bioretention or stormwater tree trenches may be pretreated by concrete sumps at curb cut inlets, forebays or catch basin inserts, which are designed to capture coarse sediment, debris and trash. Centralizing sediment and trash captured at the inlet or entrance to the facility reduces maintenance by preventing filter media clogging and limiting the area over which sediment and trash needs to be removed. In some cases, pre-treatment device clean-outs may be incorporated into existing municipal catch basin cleaning programs.
Performance calculation: If the downstream facility provides filtration, such as bioretention or stone filled trenches, the overall water quality performance of this facility would not increase with pretreatment because the coarse sediment and debris captured by the pretreatment device would be removed by the downstream facility even in the absence of pretreatment. The purpose of adding pre-treatment is to prevent clogging or filling and thereby reduce the cost and effort of long term maintenance and delay requirements for major facility rehabilitation or replacement.
2. Treatment trains designed to address one or more design criteria[edit]
These types of treatment trains combine practices that address different design criteria, in recognition that most individual stormwater facility types do not meet all design criteria as stand-alone facilities. For instance, stormwater wet ponds may provide water quality, erosion and flood control but not water balance control (i.e. runoff volume control). Bioretention provides good water quality and water balance control but are rarely designed for flood control.
Example: Proprietary filtration treatment device (providing water quality) draining to an underground infiltration trench or chamber system (providing water balance control). Overflows from the trench or chamber system could drain to a dry pond or other flood control facility to provide water quantity and erosion control). Another example may be to direct low flows from a stormwater management pond outlet to an infiltration practice.
Performance calculation: Treatment trains designed to address multiple design criteria may improve overall water quality performance by, for example, reducing water quality concentrations in the first facility and reducing water quality loads (through infiltration/evapotranspiration) in a second facility. Even if the effluent concentration from facility one and facility two are the same, the overall load reduction of the treatment train may be greater than provided by any one of the facilities alone.
3. Treatment trains designed to enhance overall treatment system performance[edit]
The design intent of these treatment trains is to enhance overall system performance. The previous category of treatment train may enhance performance, but the objective may not always be to address a broader range of stormwater criteria.
Example: A bioretention and infiltration chamber in series. Adding the second facility increases the overall area available for infiltration, which in turn reduces the overall load of pollutants to the receiver. This type of treatment may be advantageous where surface space is limited, but water balance requirements can not be met with available space allocated to the surface bioretention cell alone.
Performance calculation: In the example above, the bioretention facility would provide water quality load reductions through filtration (water quality concentration reductions) and infiltration (volume reductions). Since the second facility would receive effluent from the underdrain of the bioretention, no further reduction in TSS concentrations would be expected (ie. the TSS concentration would already be at the ‘irreducible’ level). The TSS water quality load would be reduced in the second facility only by further reductions in volumes through infiltration. If the parameter of interest was total phosphorus (TP) rather than TSS, there is the potential that the second facility may further reduce TP through filtration/adsorption, especially if the second facility contained reactive media designed to remove phosphorus.
- ↑ LSRCA. 2018. Parking Lot Design Guidelines: Municipal Policy Templates to Promote Salt Reduction in Parking Lots. https://www.lsrca.on.ca/Shared%20Documents/Parking-Lot-Design-Guidelines/Parking%20Lot%20Design%20Guidelines.pdf.