2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

5.4.25 Oil/Water Separators

5.4.25.1 BMP Description

The purpose of oil and water separator BMPs is to remove oil and other water-insoluble hydrocarbons, and settleable solids from stormwater runoff. Oil and water separator BMPs typically consist of three bays: a forebay, a separator bay, and an afterbay.

There are two general types of oil and water separators:

  • The American Petroleum Institute (API) type (also called baffle type) (API 1990)

    • API separators are composed of three bays separated by baffles. The efficiency of API separators is dependent on detention time in the center bay and on droplet size. API type separators rarely treat stormwater to reduce oil levels below 10 milligrams per liter (mg/L). The use of API separators should be limited to protection from large oil spills and not for small amounts of oil on the pavement surfaces. See BMP T11.10: API (Baffle type) Separator in Volume V, Chapter 13 of the Ecology Manual

  • The coalescing plate (CP) type

Both types of oil/water separators use gravity to remove floating and dispersed oil.

A spill control (SC) separator (see Spill Control Separator [not for oil treatment] figure in Volume V, Chapter 13 of the Ecology Manual]) is a simple catch basin with a T‑inlet for temporarily trapping small volumes of oil. The spill control separator is included here for comparison only and is not designed for, or to be used for treatment purposes.

5.4.25.2 Performance Mechanism

Oil and water separators are designed to meet the Oil Control Performance Goal, which is intended to achieve the following:

  • No ongoing or recurring visible sheen;

  • A 24‑hour average Total Petroleum Hydrocarbon (TPH) concentration ≤10 mg/L; and

  • A maximum of 15 mg/L for a discrete sample (grab sample).

See Volume III, Section 1.2 of the Ecology Manual for more information regarding the Oil Control Performance Goal.

5.4.25.3 Application and Limitations

The following are potential applications of oil and water separators where free oil is expected to be present at treatable high concentrations and sediment will not overwhelm the separator.) For low concentrations of oil, other treatments may be more applicable. These include sand filters and manufactured treatment devices and BMPs.

  • Commercial and industrial areas including petroleum storage yards, vehicle maintenance facilities, manufacturing areas, airports, utility areas (water, electric, gas), and fueling stations.

  • Facilities that would require oil control BMPs under the high-use site threshold described in Volume II, Chapter 2 including parking lots at convenience stores, fast food restaurants, grocery stores, shopping malls, discount warehouse stores, banks, truck fleets, auto and truck dealerships, and delivery services.

  • Without intense maintenance oil/water separators may not be sufficiently effective in achieving oil and TPH removal down to required levels.

  • A pretreatment BMP (see Volume V, Chapter 9 of the Ecology Manual) should be considered if the level of TSS in the inlet flow would cause clogging or otherwise impair the long-term efficiency of the oil and water separator BMP.

  • For inflows from small drainage areas (such as fueling stations, maintenance shops, etc.), BMP T11.11: Coalescing Plate (CP) Separator is typically considered, due to space limitations. However, if plugging of the plates is likely, then a new design basis for BMP T11.10: API (Baffle type) Separator may be considered on an experimental basis.

Oil/water separators may only be used to provide oil control and may not be used to partially or fully satisfy any other minimum requirement (see Oil/Water Separator Applicability.).

Table II-5.28. Oil/Water Separator Applicability.
BMP

MR #5: On-site Stormwater Management

MR #6: Runoff Treatment

MR #7: Flow Control

List

LID Performance Standard

Basic

Enhanced

Oil Control

Phosphorus

Oil/water separators

  

 

    

X

 

  

5.4.25.4 Site Considerations

Consider the following site characteristics:

  • Oil/water separators shall be installed upstream of other Runoff Treatment BMPs (except wetvaults), pumps, and conveyance structures that introduce turbulence and as close to the source of oil generation as possible.

  • Oil/water separators may be located upstream or downstream of Flow Control BMPs.

  • Oil/water separators shall be located off-line from the primary conveyance/detention system, bypassing flows greater than the off-line 15‑minute water quality design flow rate multiplied by the ratio indicated in the Ratio of SBUH Peak/WQ Flow (Offline) figure in Volume V, Chapter 7 of the Ecology Manual. If it is not possible to locate the separator off-line (e.g., roadway intersections), try to minimize the size of the area requiring oil control, and use the on-line water quality design flow rate multiplied by the ratio indicated in the Ratio of SBUH Peak/WQ Flow (Online) figure in Volume V, Chapter 7 of the Ecology Manual.

  • Oil/water separators shall not be used for removal of dissolved or emulsified materials such as coolants, soluble lubricants, glycols (anti-freeze), and alcohols. If practicable, determine the oil/grease (or TPH) and TSS concentrations, lowest temperature, pH; and empirical oil rise rates in the runoff, and the viscosity, and specific gravity of the oil.

  • Oil/water separators are best located in areas where the contributing drainage area is nearly all impervious and a fairly high load of TPH is likely to be generated.

  • Excluding unpaved areas helps to minimize the amount of sediment entering the vault, which reduces the need for maintenance.

  • Sufficient land area is available for siting the oil/water separator.

  • Adequate TSS control or pretreatment capability upstream of the oil/water separator. Pretreatment shall be considered if the level of TSS in the influent would cause clogging or otherwise impair the long-term efficiency of the separator.

  • Adequate influent flow attenuation and/or bypass capability upstream of or integrated with the oil/water separator.

  • Sufficient access for operation and maintenance.

5.4.25.5 Design Information

There is concern that oil/water separators used for stormwater treatment have not performed to expectations. Therefore, emphasis shall be given to proper application, design, operation and maintenance, (particularly sludge and oil removal) and prevention of CP fouling and plugging. Other treatment systems, such as sand filters (Vol II–5.4.19 Sand Filters) and manufactured treatment devices as BMPs (Vol II–5.4.30 Manufactured Treatment Devices as BMPs), shall be considered for the removal of insoluble oil and TPH.

See BMP T11.10 in Volume V, Chapter 13 of the Ecology Manual for detailed design guidance and criteria on API oil/water separators, and BMP T11.11 in Volume V, Chapter 13 of the Ecology Manual for detailed design guidance and criteria on CP oil/water separators.

5.4.25.6 Minimum Construction Requirements

The following are construction requirements associated with the construction of an oil/water separator:

  • Follow the manufacturer’s recommended construction procedures and installation instructions, as well as any applicable Kitsap County requirements.

  • Upon completion of installation, thoroughly clean and flush the oil/water separator prior to operation.

  • Specify appropriate performance tests after installation and shakedown, and/or provide certification by a licensed engineer that the separator is functioning in accordance with design objectives.

5.4.25.7 Operations and Maintenance

See the Operation and Maintenance section in Volume V, Chapter 13 of the Ecology Manual and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for oil/water separators.