VOLUME V – Stormwater Treatment BMPs

Chapter 1 – Introduction

1.1 Purpose of this Volume

Best Management Practices (BMPs) are schedules of activities, prohibitions of practices, maintenance procedures, managerial practices, or structural features that prevent or reduce adverse impacts to waters of Washington State. As described in Volume I of this stormwater manual, there are three main categories of BMPs for long-term management of stormwater at developed sites:

•    BMPs addressing the amount and timing of stormwater flows;

•    BMPs addressing prevention of pollution from potential sources; and

•    BMPs addressing treatment of runoff to remove sediment and other pollutants.

This volume of the stormwater manual focuses on the third category, treatment of runoff to remove sediment and other pollutants at developed sites. The purpose of this volume is to provide guidance for selection, design, and maintenance of permanent runoff treatment facilities.

The Manual presents BMPs with respect to controlling stormwater flows and control of pollutant sources in Volumes III and IV, respectively.

1.2 Content and Organization of this Volume

Volume V of the stormwater manual contains 12 chapters. Chapter 1 serves as an introduction and summarizes available options for treatment of stormwater. Chapter 2 outlines a step-by-step process for selecting treatment facilities for new development and redevelopment projects. Chapter 3 presents treatment facility “menus” that are used in applying the step-by-step process presented in Chapter 2. These menus cover different treatment needs that are associated with different sites. Chapter 4 discusses general requirements for treatment facilities. Chapter 5 presents information regarding on-site stormwater management BMPs. The intent of these BMPs is to infiltrate, disperse, or contain runoff on site, as well as to provide treatment. Chapters 6 through 11 provide detailed information regarding specific types of treatment identified in the menus. Chapter 12 discusses special considerations for emerging technologies for stormwater treatment.

The Appendices to this volume contain more detailed information on selected topics described in the various chapters.

1.3 How to Use this Volume

The Reader should consult this volume to select specific BMPs for runoff treatment for the Drainage Control Plans (see Volume I). After you have identified the Core Requirements from Volume I, you can use this volume to select specific treatment facilities for permanent use at developed sites, and as an aid in designing and constructing these facilities.

1.4 Runoff Treatment Facilities

1.4.1 General Considerations

Runoff treatment facilities are designed to remove pollutants contained in stormwater runoff. The pollutants of concern include sand, silt, and other suspended solids; metals such as copper, lead, and zinc; nutrients (e.g., nitrogen and phosphorous); certain bacteria and viruses; and organics such as petroleum hydrocarbons and pesticides. Methods of pollutant removal include sedimentation/settling, filtration, plant uptake, ion exchange, adsorption, and bacterial decomposition. Floatable pollutants such as oil, debris, and scum can be removed with separator structures.

Runoff treatment facilities located within public right-of-way shall be designed to treat stormwater runoff only from the public right-of-way. Treatment for stormwater runoff from private property shall be provided on private property.

1.4.2 Maintenance

Maintenance is required for all types of runoff treatment facilities. See Volume IV, Chapter 4 for maintenance standards for the treatment facilities discussed in this volume.

Design engineers should be aware that the City of Olympia (City) generally will not approve stormwater treatment practices for which the required maintenance is too difficult, costly, or needed too frequently (typically, this would be more often than once per year). Specified treatment practices shall include a demonstration that the maintenance needs and schedule are reasonable.

1.4.3 Treatment Methods

Methods used for runoff treatment facilities and common terms used in runoff treatment are discussed below:

•    Wetpools. Wetpools provide runoff treatment by allowing settling of particulates during quiescent conditions (sedimentation), by biological uptake, and by vegetative filtration. Wetpools may be single-purpose facilities, providing only runoff treatment, or they may be combined with a detention pond or vault to also provide flow control. If combined, the wetpool facility can often be stacked under the detention facility with little further loss of development area.

•    Biofiltration. Biofiltration uses vegetation in conjunction with slow and shallow-depth flow for runoff treatment. As runoff passes through the vegetation, pollutants are removed through the combined effects of filtration, infiltration, and settling. These effects are aided by the reduction of the velocity of stormwater as it passes through the biofilter. Biofiltration facilities include swales that are designed to convey and treat concentrated runoff at shallow depths and slow velocities, and filter strips that are broad areas of vegetation for treating sheet flow runoff.

•    Oil/Water Separation. Oil/water separators remove oil floating on the top of the water. There are two general types of separators – the American Petroleum Institute (API) separators and coalescing plate (CP) separators. Both use gravity to remove floating and dispersed oil. API separators, or baffle separators, are generally composed of three chambers separated by baffles. The efficiency of these separators is dependent on detention time in the center, or detention chamber, and on droplet size. CP separators use a series of parallel plates, which improve separation efficiency by providing more surface area, thus reducing the space needed for the separator. Oil/water separators must be located off-line from the primary conveyance/detention system, bypassing flows greater than the water quality design flow. Other devices/facilities that may be used for removal of oil include “emerging technologies” (see definition below), and linear sand filters. Oil control devices/facilities should be placed upstream of other treatment facilities and as close to the source of oil generation as possible.

•    Pretreatment. Presettling basins are often used to remove sediment from runoff prior to discharge into other treatment facilities. Basic treatment facilities, listed in Step 6 – Figure 2.1.1, can also be used to provide pretreatment. Pretreatment often must be provided for filtration and infiltration facilities to protect them from clogging or to protect ground water. Appropriate pretreatment devices include a pre-settling basin, wet pond/vault, biofilter, constructed wetland, or oil/water separator. A number of patented technologies have received General and Conditional Use Level Designations for Pretreatment through the Washington State Department of Ecology’s (Ecology) TAPE (Technology Assessment Protocol – Ecology) Program. A listing and descriptions are available at Ecology’s Emerging Technologies website.

•    Infiltration. Infiltration refers to the use of the filtration, adsorption, and biological properties of native soils, with or without amendments, to remove pollutants as stormwater soaks into the ground. Infiltration can provide multiple benefits including pollutant removal, peak flow control, ground water recharge, and flood control. One condition that can limit the use of infiltration is the potential adverse impact on ground water quality. You must understand the difference between infiltrating in soils that are suitable for runoff treatment and soils only suitable for flow control to protect ground water. Sufficient organic content and sorption capacity to remove pollutants must be present for soils to provide runoff treatment. Examples of suitable soils are silty and sandy loams. Coarser soils, such as gravelly sands, can provide flow control but are not suitable for providing runoff treatment. The use of coarser soils to provide flow control for runoff from pollutant generating surfaces must always be preceded by treatment to protect ground water quality. Thus, there will be instances when soils are suitable for treatment but not flow control, and vice versa.

•    Bioretention. Bioretention refers to the use of imported soils as a treatment medium. As in infiltration, the pollutant removal mechanisms include filtration, adsorption, and biological action. Bioretention facilities can be built within earthen swales or placed within vaults. Water that has passed through the Bioretention Soil Mix (or approved equivalent) may be discharged to the ground or collected and discharged to surface water.

•    Filtration. Another of a pollutant removal system for stormwater is the use of various media such as sand, perlite, zeolite, and carbon, to remove low levels of total suspended solids (TSS). Specific media such as activated carbon or zeolite can remove hydrocarbons and soluble metals. Filter systems can be configured as basins, vaults, trenches or cartridges. A number of “Emerging Technologies” filtration devices have completed or are in the process of being assessed through the “Emerging Technologies” processed described in the following bullet.

•    “Emerging Technologies.” Emerging technologies are those new stormwater treatment devices that are continually being added to the stormwater treatment marketplace. Ecology has established a program –Technology Assessment Protocol – Ecology (TAPE) – to evaluate the capabilities of these emerging technologies. Emerging technologies that have been evaluated by this program are approved at some level of use designation under specified conditions. Their use is restricted in accordance with their evaluation as explained in Chapter 12. The recommendations for use of these emerging technologies may change as we collect more data on their performance. Updated recommendations on their use are posted to the Ecology website. Emerging technologies can also be considered for retrofit situations where TAPE approval is not required.

•    “On-line” Systems. Most treatment facilities can be designed as “On-line” systems with flows above the water quality design flow or volume simply passing through the facility with lesser or no pollutant removal efficiency. It is sometimes desirable to restrict flows to treatment facilities and bypass excess flows around them. These are called “Off-line” systems. An example of an on-line system is a wetpool that maintains a permanent pool of water for runoff treatment purposes.

•    Design Flow. For information on determining the design storm and flows for sizing treatment facilities refer to Chapter 4 of this volume.