stormwater interception - oil removal

[snafuman]

The site is a 1.5 acre parking lot draining into an existing natural detention basin (wetland). The regulatory authority would like to see a stormwater interceptor installed for separation of oils and sediments. The main problem is only 1.2 feet of available fall from pavement to wetland. The next problem is shallow groundwater at the site, which causes us more hoops to jump through with the regulatory agency, thus we would like to keep the system less than 5' deep. The final problem is keeping a minimal footprint in the wetland to minimize environmental damage.

The good news is that because the existing wetland area already acts as a better sediment remover than any commercially available unit, the regulatory agency will probably waive those requirements, leaving us to deal with oil capture only.

We've looked at a number of the commercially available interceptors, and while some could be forced into this site, we haven't found any that work great. It is difficult to make the site work vertically through a catch basin and an interceptor. We have considered eliminating the catch basin and routing directy into an interceptor through a scupper. We have also considered catch basin inserts or oil pillows, but are concerned about the high maintenance.

Any ideas, experience or input, good, great or wacky, are appreciated.

 

[MS1128]

Visit

http://www.stormdrains.com.

They manufacture catch basin inserts that are the least labor intensive; for their filter has an effective term of 8-10 months and is extremely hydrophobic.

 

[bltseattle]

I think you might have to use the scupper approach to keep the HGL high enough to treat the runoff. I've seen perimeter sand filter devices that would almost meet your need, but your hydraulic head may be inadequate to push effectively drive water through the sand media. I think your scupper idea is good, you could have a 6-inch opening probably within about 6 inches of the top of a precast vault.

Can you use a scupper/side inlet into a vault, so that the lid of the vault is above the parking lot grade slightly, and the design water surface, outlet IE would be about the same as the wetland water surface elevation? A baffle style separator doesn't take much head to drive. You could probably put on a metal grate for the top to reduce the height of the cover, is that is an issue.

Your local precast vault supplier may be able to help you. The 1992 Washington Department of Ecology Stormwater Manual has equations to use for sizing baffle-style separators, if you decide to go that route and need a reference.

In my opinion, most owners would not maintain catch basin inserts enough to make them an effective solution. A larger facility would hold more oil/sediment/crud so that maintenance intervals could be longer, and could be sized to provide the desired level of oil removal.

A biofiltration swale can be shallow, but not necessarily effective for oil removal. A bioretention approach may be better; you could sheetflow the runoff into a landscaped area, using say 12-inches of sandy loam to filter the runoff, and underdrains to collect and discharge from the retention area. You'd have to plant it with wetland compatible vegetation that is native to your area.

 

[snafuman]

Thanks for the responses. Between being sick and busy I haven't really looked at them until now.

 

[dik]

In our locale, they permit using an inverted elbow to act as an oil trap... surprising little oil comes off a parking lot these days.