Select BMPs for Runoff Treatment

5.3.4 Select BMPs for Runoff Treatment

For projects that trigger Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment, this section describes the step-by-step process for selecting the type of treatment BMPs that apply to individual projects (or a TDA within the project), as well as the physical site features that can impact BMP selection. This section provides detailed information about BMP selection for the following runoff treatment performance goals: oil control, phosphorus, enhanced, and basic.

This section also applies to projects that require runoff treatment prior to infiltration. See Site Suitability Criteria (SSC)‑6 Soil Physical and Chemical Suitability in Volume V, Section 5.6 in the Ecology Manual. If the project proposes the use of Underground Injection Control (UIC), see also Volume I, Section 4.13 in the Ecology Manual for further details on source control and runoff treatment requirements pertaining to UICs.

Runoff Treatment BMPs located upstream of Flow Control BMPs can be designed as on-line or off-line BMPs, as follows:

On-Line BMPs – Runoff flow rates in excess of the water quality design flow rate can be routed through the on-line Runoff Treatment BMP provided that the BMP is sized sufficiently to treat the influent flows to the required level and that velocities are not high enough to resuspend sediments.
Off-Line BMPs – Runoff flow rates in excess of the water quality design flow rate may be bypassed around the off-line Runoff Treatment BMP. Where feasible, off-line BMPs are required to prevent resuspension and washout of accumulated sediments during storm events. During bypass events, the BMP shall continue to receive and treat all flows up to and including the water quality design flow rate. Only those flows higher than the water quality design flow rate shall be allowed to bypass around the BMP.

In most cases, the engineer may choose whether to design Runoff Treatment BMPs as on-line or off-line systems, provided they are sized sufficiently to provide the required treatment for the influent flows. However, oil/water separators shall be designed as off-line BMPs in all cases. Runoff Treatment BMPs located downstream of Flow Control BMPs are generally considered to be off-line systems, since the influent flows are moderated by the Flow Control BMPs.

Follow the steps presented in this section and in Figure II-5.2 to select the appropriate Runoff Treatment BMPs for projects that trigger MR #6 or require runoff treatment prior to infiltration. In addition, MR #5: On-site Stormwater Management and Minimum Requirement #7: Flow Control may apply (see Vol II–5.3.3 Select BMPs for On-Site Stormwater Management and Vol II–5.3.5 Select BMPs for Flow Control, respectively).

Figure II-5.2. Runoff Treatment BMP Selection Flow Chart.

Step 1: Determine the Receiving Waters and Pollutants of Concern Based on Offsite Analysis

When identifying the receiving waters and downstream conveyance as part of the minimum requirement determination, specific pollutants of concern that the project must mitigate shall be determined. Such pollutants of concern could be identified in a Watershed or Basin Plan, a Water Clean-Up Plan, a Ground Water Management Plan (Wellhead Protection Plan), a Lake Management Plan, or similar. See Vol I–3.2 Step 2 – Identify the Receiving Water and Downstream Conveyance for further discussion.

An analysis of the proposed land use(s) of the project shall also be used to determine the stormwater pollutants of concern. See Volume III, Chapter 1 in the Ecology Manual for further discussion on this

Step 2: Determine if an Oil Control BMP is Required

The use of oil control devices and BMPs is dependent upon the specific land use proposed for development. The Oil Control BMP Options (see Volume III, Section 1.2 in the Ecology Manual) applies to projects that have “high-use sites.” High-use sites are those that typically generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. High-use sites include:

An area with commercial or industrial uses subject to an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area.

Gasoline stations, with or without small food stores, will likely exceed the high-use site threshold.
An area of a commercial or industrial site subject to petroleum storage and transfer in excess of 1,500 gallons per year, not including routinely delivered heating oil.

The petroleum storage and transfer criterion is intended to address regular transfer operations such as gasoline service stations, not occasional filling of heating oil tanks.
An area of a commercial or industrial site subject to parking, storage or maintenance of 25 or more vehicles that are over 10 tons gross weight (trucks, buses, trains, heavy equipment, etc.).

In general, all-day parking areas are not intended to be defined as high use sites and shall not require an oil control BMP.
A road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use improvements.

The traffic count can be estimated using information from “Trip Generation,” published by the Institute of Transportation Engineers, or from a traffic study prepared by a professional engineer or transportation specialist with experience in traffic estimation. See the Institute of Transportation Engineers: www.ite.org.

The land uses below may have areas that fall within the definition of “high-use sites” and require oil control treatment. Further, these land uses require special attention to the oil control treatment selected. See Volume III, Chapter 1 in the Ecology Manual for more details:

Industrial machinery and equipment, and railroad equipment maintenance areas
Log storage and sorting yards
Aircraft maintenance areas
Railroad yards
Fueling stations
Vehicle maintenance and repair sites
Junkyards and areas with vehicle recycling operations
Construction businesses (paving, heavy equipment storage and maintenance, storage of petroleum products)

All stormwater runoff from hard surface areas subject to motor vehicle traffic shall flow through a spill control (SC‑type) oil/water separator prior to surface discharge off site. See Volume III and Volume IV of the Ecology Manual for additional requirements. Spill control requirements are separate from this treatment requirement.

If oil control is required for the site, see the General BMP Design requirements in Volume V, Chapter 1 of the Ecology Manual. The general requirements may affect the design and placement of BMPs on the site (e.g., flow splitting). Then see Volume V, Chapter 13 of the Ecology Manual for guidance on the proper selection of options and design details for oil and water separator BMPs.

Step 3: Determine if it is Practicable to Provide Runoff Treatment by Infiltrating into the Native Soil

Due to the hydrologic benefits of infiltration, Kitsap County recommends evaluating whether it is practicable to provide runoff treatment by infiltrating into the site's native soils before considering other Runoff Treatment BMPs. If runoff treatment by infiltrating into the native soil is practicable, it has the advantage that it is presumed to meet the Phosphorus, Enhanced, and Basic Treatment Performance Goals.

The guidance in Volume V, Chapter 5 of the Ecology Manual shall be followed for designing infiltration BMPs. Volume V, Section 5.6 (Site Suitability Criteria) in the Ecology Manual details the site conditions that shall be met for infiltration to be practicable for the site and includes conditions specific to using the native soil for runoff treatment. Runoff treatment may be provided by infiltrating into the native soil if the conditions below the infiltration BMP meet the criteria for Runoff Treatment per Volume V, Section 5.6 in the Ecology Manual.

Most infiltration BMPs should be preceded by a pretreatment BMP to reduce the occurrence of plugging. Some infiltration BMPs have pretreatment integrated into the BMP, such as permeable pavement and bioretention; and therefore it is not necessary to provide additional pretreatment prior to infiltration. Any Basic Treatment BMPs, or detention ponds, vaults, or tanks designed to meet flow control requirements, can also be used for pretreatment. If an oil/water separator is necessary for oil control, it could also function as the pre-settling basin as long as the influent suspended solids concentrations are not high. However, frequent inspections are necessary to determine when accumulated solids exceed the 6‑inch depth at which clean-out is recommended (see Volume V, Appendix V‑A: BMP Maintenance Tables in the Ecology Manual).

If infiltration is planned, also see the guidance in Volume V, Chapter 1 General BMP Design and Volume V, Appendix V‑A in the Ecology Manual. This guidance may affect the design and placement of infiltration BMPs on the site.

Infiltration through soils that do not meet the criteria for runoff treatment per Volume V, Section 5.6 in the Ecology Manual is allowable as a Flow Control BMP only. Use of infiltration through such soils is acceptable provided the appropriate type of Runoff Treatment BMP (Enhanced, Phosphorus, or Basic) is provided as directed in the following steps.

If it is practicable to provide runoff treatment by infiltrating into the native soil, select and apply a Pretreatment BMP and an infiltration BMP. You have completed the Runoff Treatment selection process.

If it is not practicable to provide Runoff Treatment by infiltrating into the native soil, proceed to Step 4.

Step 4: Determine if a Phosphorus Treatment BMP is Required

The plans, ordinances, and regulations identified in Step 1 and in Vol I–3.2 Step 2 – Identify the Receiving Water and Downstream Conveyance are a good reference to help determine if the subject site is in an area where phosphorus control is required.

Kitsap Public Health District (KPHD) conducts regular lake sampling and has determined that Kitsap Lake and Long Lake are phosphorus limited. Therefore, Phosphorus treatment is required for project sites draining into these receiving water bodies. Consult with KPHD to see if any other phosphorus limited receiving bodies have been identified.

If phosphorus control is required, select and apply a phosphorus treatment BMP. Refer to the Phosphorus Treatment BMP Options in Volume III, Section 1.2 of the Ecology Manual. This section of the Ecology Manual includes options for individual BMPs and Treatment Trains for Phosphorus Treatment (see Volume III, Section 1.2 of the Ecology Manual). Select an option after reviewing the applicability and limitations, site suitability, and design criteria of each for compatibility with the site. If you have selected a phosphorus treatment BMP, see the General Requirements in Volume V, Chapter 1 of the Ecology Manual, as they may affect the design and placement of the BMP on the site.

Project sites subject to the Phosphorus Treatment requirement could also be subject to the Enhanced Treatment requirement (see Step 4). In that event, apply a BMP or a treatment train that is listed in both the Enhanced Treatment Menu and the Phosphorus Treatment Menu.

Step 5: Determine if an Enhanced Treatment BMP is Required

Except where specified under Step 6, Enhanced Treatment for reduction of dissolved metals is required for the following project sites that:

Discharge directly to fresh waters or conveyance systems tributary to fresh waters designated for aquatic life use or that have an existing aquatic life use; or
Discharge to conveyance systems that are tributary to fresh waters designated for aquatic life use or that have an existing aquatic life use; or
Infiltrate stormwater within 0.25 mile of a fresh water designated for aquatic life use or that has an existing aquatic life use.

The types of project sites are:

Industrial project sites
Commercial project sites
Multi-family residential project sites
High Annual Average Daily Traffic (AADT) roads as follows:
- Within Urban Growth Areas:
  - Fully controlled and partially controlled limited access highways with AADT counts of 15,000 or more.
  - All other roads with an AADT count of 7,500 or greater.
- Outside of Urban Growth Areas:
  - Roads with an AADT of 15,000 or greater unless the site discharges to a 4th Strahler order stream or larger;
  - Roads with an AADT of 30,000 or greater if the site discharges to a 4th Strahler order stream or larger (as determined using 1:24,000 scale maps to delineate stream order).

Any areas of the above-listed project sites that are identified as subject to Basic Treatment requirements (see Step 6) are not also subject to Enhanced Treatment requirements. For developments (or TDAs) with a mix of land use types, the Enhanced Treatment requirement shall apply when the runoff from the areas subject to the Enhanced Treatment requirement comprises 50% or more of the total runoff from the development (or TDA).

If Enhanced Treatment is required for the site, see the Enhanced Treatment BMP Options in Volume III, Chapter 1 of the Ecology Manual. These options include individual Enhanced Treatment BMPs and Treatment Trains for Enhanced Treatment (see Volume III, Chapter 1 of the Ecology Manual).

Step 6: Select a Basic Treatment BMP

Note that if an Enhanced Treatment BMP or a Phosphorus Treatment BMP have been applied, an additional Basic Treatment BMP is not required. Phosphorus Treatment and Enhanced Treatment BMPs meet both the Basic Treatment Performance Goal as well as their own respective Performance Goals.

The Basic Treatment Performance Goal

Basic Treatment BMPs are intended to achieve 80% removal of total suspended solids for influent concentrations that are greater than 100 mg/l, but less than 200 mg/l. For influent concentrations greater than 200 mg/l, a higher treatment goal may be appropriate. For influent concentrations less than 100 mg/l, the BMPs are intended to achieve an effluent goal of 20 mg/l total suspended solids (TSS).

The Basic Treatment Performance Goal applies to the water quality design storm volume or flow rate, whichever is applicable. The incremental portion of runoff in excess of the water quality design storm volume or flow rate can be routed around the BMP (creating an off-line Runoff Treatment BMP), or can be passed through the BMP (creating an on-line Runoff Treatment BMP) provided a net TSS reduction is maintained. Ecology encourages the design and operation of Runoff Treatment BMPs that engage a bypass at flow rates higher than the water quality design flow rate as long as the reduction in TSS loading exceeds that achieved with initiating bypass at the water quality design flow rate. Note that wetpool BMPs are always designed to be online. The Basic Treatment Performance Goal assumes that the BMP is treating stormwater with a typical particle size distribution. For a description of a typical particle size distribution, see Ecology's Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies (Ecology 2018).

When is Basic Treatment Required?

Areas that must provide Phosphorus Treatment BMPs or Enhanced Treatment BMPs do NOT have to provide additional Basic Treatment BMPs to meet the Basic Treatment Performance Goal.
If Phosphorus Treatment BMPs or Enhanced Treatment BMPs are not provided, Basic Treatment BMPs are required before discharging runoff off site through either infiltration or surface flow.
For developments (or TDAs) with a mix of land use types, Basic Treatment BMPs are required when the runoff from the areas subject to the Basic Treatment Performance Goal comprises 50% or more of the total runoff from the development (or TDA).

Refer to the Basic Treatment BMP Options in Volume III, Section 1.2 of the Ecology Manual. Select an option from the list after reviewing the applicability and limitations, site suitability, and design criteria of each for compatibility with the site.

After selecting a Basic Treatment BMP, see the General Requirements in Volume V, Chapter 1 of the Ecology Manual.