10-3 WATER QUALITY INLETS TO CONTROL STORM WATER RUNOFF

WATER QUALITY INLETS TO CONTROL STORM WATER RUNOFF

Revision Date:	01/04
Process Code:	Navy/Marines: SER-016-99; Air Force: FA08; Army: N/A
Usage List:	Navy: Low; Marines: Low; Army: Low; Air Force: Low
Alternative For:	Direct stormwater discharge
Compliance Impact:	Low
Applicable EPCRA Targeted Constituents and CAS Numbers: N/A

Overview:

Water quality inlets (WQIs) separate pollutants from the first flush of storm water using a series of chambers for pollutant sedimentation, screening, and separation. A water quality inlet typically consists of a sedimentation chamber, an oil separation chamber, and a discharge chamber. Because of their separation capabilities, WQIs are occasionally referred to as oil/grit separators or oil/water separators. These Best Management Practices (BMPs) may be constructed on site, precast, or manufactured by a vendor.

Water quality inlets are widely used in the US for improving storm water runoff quality where space is limited and funding prohibits the use of larger BMPs, such as ponds or wetlands. Water quality inlets effectively separate hydrocarbons from storm water runoff. During the first 5 years of use, over 95% of all WQIs were in operation as designed. WQIs do not manage the volume of storm water flow, due to limited capacity, and have limited removal efficiencies when not properly maintained. For these reasons, WQIs are often used to pretreat runoff prior to discharge to other BMPs.

The WQI should be located within close proximity to a storm drain network to allow for future discharge from the WQI to the sewer system. WQIs are typically used as an off-line treatment process where lower flows will be encountered; high flows result in resuspension of settled material. Construction activity should be complete and the drainage area stabilized to minimize sediment loading to a WQI. The WQI should be located in a small, impervious area and be watertight.

Storm water runoff enters the sedimentation chamber in a water quality inlet where coarse materials settle. Flow from the sedimentation chamber is conveyed to the second chamber through an orifice covered with a trash rack and located halfway down the wall separating the two chambers. The second chamber functions as an oil separation chamber. Water that enters the third sequential chamber discharges through a storm water outlet pipe. The design should include permanent pools within the chambers (to reduce sediment resuspension during storm events) and manholes above the chambers (to provide access for cleaning and inspection).

The limited capacity of most WQIs typically means that the discharge rate is high and the detention time is relatively short. Most water quality inlets have an average detention time of less than a half-hour. Efficient pollutant removal is dependent upon proper maintenance; the lack of proper maintenance may result in resuspension and discharge of settled pollutants and separated oil. The required maintenance will vary from site to site, but cleaning before the start of each season and inspection after every storm event should ensure proper functioning of the WQI.

Water quality inlets generally have minimal effect on the removal of nutrients, metals, and organic pollutants other than free petroleum products. The sedimentation chamber can be expected to reduce grit and sediments partially. Separation of dissolved or emulsified oil from water is rarely achieved, although WQIs are effective in separating free oil and grease from storm water.

Compliance Benefit:

The use of water quality inlets for treating storm water runoff may help facilities meet the conditions and requirements contained in stormwater permits and stormwater pollution prevention plans as well as 40 CFR 122.26.

The compliance benefits listed here are only meant to be used as general guidelines and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g., the amount of workload involved.

Materials Compatibility:

No material incompatibilities identified.

Safety and Health:

The safety and health issues depend on the types of contaminants in the storm water. Some hydrocarbons may be a minor irritant to mucous membranes and eyes. Handling hydrocarbons requires caution; proper personal protective equipment is recommended.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.

Benefits:

WQIs are useful for separating sediments and oil from storm water runoff. If properly maintained, these pollutants are removed in the WQI, and the quality of the downstream storm water is improved.

Disadvantages:

WQIs do not manage large volumes of storm water.
Limited removal efficiencies when not properly maintained.
WQI residuals may require disposal as a hazardous waste.

Economic Analysis:

The cost for a precast WQI ($5,000 - $35,000) is generally lower than for other units. The cost varies depending on the capacity of the inlet and the complexity of the system. A cast-in-place WQI also varies in cost. The cost of a large oil/water separator that services the air field storm drains at Westover Air Reserve Base, Massachusetts, was estimated to cost approximately $380,000 for labor and materials.

Assumptions:

Labor Rate: $45/hour.
Cost to develop procedures (estimated at 4 labor hours) for maintenance, and for disposal.
Inspect separator four times a year (0.5 hours/each).
Twice a year remove oil/water from WQI.
Remove 200 gallons of oil/water a year from WQI.
Oil/grease analysis done at each cleaning of WQI, $40/each.
Cost of disposal is $0.42/gallon to dispose of oil/water plus an additional $80 for a vacuum truck to come to the facility.
Once a year remove sediment from WQI.
Annual disposal of sediments is 350 lbs at $0.02/lb.
Requires two people four hours to clean separator when removing sediment.
Labor cost for operating a sand filter includes filter changing, gravel and sand replacement, and debris removal, which takes approximately 3 hours/year.
Material costs for the gravel layer, filter fabric, and top portion of sand for sand filter are approximately $1,700 annually.
The figures in the table are based on one impervious acre treated.

Table 1. Cost Comparison for Water Quality Inlets vs. Sand Filter

	WQI	Sand Filter
Capital and Installation Costs:	$20,000	$20,000
Operational Costs:
Labor Costs:	$630	$135
Materials:	$0	$1,700
Waste Disposal Costs:	$251	7
Permitting and Lab Analysis Costs:	$80	$0
Total Costs (not including capital and installation costs):	$961	$1,842
Total Income:	$0	$0
Annual Benefit:	-$961	-$1,842

Economic Analysis Summary:

Annual Savings for WQI: $881
Capital Cost for Diversion Equipment/Process: $20,000
Payback Period for Investment in Equipment/Process: > 20 years

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NSN/MSDS:

None identified.

Approving Authority:

Appropriate authority for making process changes should always be sought prior to procuring or implementing any of the technologies identified herein.

Points of Contact:

Vendors:	This is not meant to be a complete list, as there may be other manufacturers of this type of equipment. Jay R. Smith Mfg. Co. (Ultracept) Environmental Products Group 2781 Gunter Park Drive, East Montgomery, AL 36109-0237 Phone:(334) 277-8520 FAX: (334) 272-7396 URL: http://www.jayrsmith.com/
	XERXES Corp. 7901 Xerxes Avenue South Minneapolis, MN 55431-1253 Phone: (952) 887-1890 FAX: (952) 887-1882 E-mail: info@xerxescorp.com URL: http://www.proformcorp.com/

Related Links:	Do You Discharge Non-Point Source Pollution To Water Supplies? -- Navy Environmental Quality Initiative (EQI) Do You Maintain A Storm Water Pollution Prevention Plan? -- Navy Environmental Quality Initiative (EQI) Storm Water Phase II Final Rule Fact Sheet Series, EPA
Sources:	Mr. Matthew Jabloner, Engineering Field Activity, Northwest, November 2002. Ms. Gina Rosse-Linderme, Westover Air Reserve Base, January 1999. Berg, V.H., 1991. Water Quality Inlets (Oil/Grit Separators). Maryland Department of the Environment, Sediment and Stormwater Administration. Schueler, T.R., 1992. A Current Assessment of Urban Best Management Practices. Metropolitan Council of Governments. Metropolitan Council of Governments (MWCOG), 1993. The Quality of Trapped Sediments and Pool Water within Oil Grit Separators in Suburban Maryland. Interim Report. Mr. Jim McPhee, United Industrial Services, Wyoming, RI, July 1997. Joint Services Pollution Prevention Technical Library Fact Sheet on Sand Filter For Treating Storm Water Runoff, June 1997.