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.

The compliance benefits listed here are only meant to be used as a general guideline 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.

Consult your local Industrial Health Specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing any of these technologies.

• 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.

• 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.

Assumptions:

• Labor Rate: $45/hour.
• Cost to develop procedures (estimated at 4 man-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.

Cost Comparison for Water Quality Inlets vs. Sand Filter

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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*There are multiple MSDSs for most NSNs.
The MSDS (if shown above) is only meant to serve as an example.

Jay R. Smith Mfg. Co. (Ultracept)
Environmental Products Group
2781 Gunter Park Drive, East
Montgomery,  AL   36109-1405
Phone: (800) 767-0466
or (334) 277-8520
FAX: (334) 272-7396
URL:  http://www.jayrsmith.com/