Grease/Oil Trap Capacity

[FarvAC91]

Hi all,

First time poster here so I apologize if I'm going about this wrong.

I've got a parking lot design where I need to determine the spacing of some grease/oil traps. For the sake of simplicity, let's say the lot is 2 acres and the capacity of the traps is 2 cfs with excess simply bypassing. We have rain totals for all storms and the main requirement is that we need to capture the first 0.5 inches of rain. How would I go about doing this? The main issue I'm having is determining if this requires a specific storm to be run or not. Obviously a 10yr, 15minute storm would produce a greater peak flow than, say a 10yr, 24hr storm.

Hopefully this question is clear enough.

Thanks

 

[psmart]

If your primary criteria is to capture (retain) the first 1/2 inch, the required volume is simply 1/2 inch times the contributing area. The rainfall event would come into the calculations only for the purpose of checking the flow capacity of the system (e.g. 2cfs) and to ensure that the traps are not flushed by the high flow rate.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[FarvAC91]

Thanks for the reply Peter,

So would running a model of various storms to show which ones the selected inlets and their tributary areas could handle be the next step? I'll have to check the local ordinances but I don't think it says anything about a standard storm for determining capacity calculations.

 

[psmart]

The need for further modeling would depend on the exact design of the traps and the associated drainage system - plus the stormwater regulations. Perhaps some of the more experienced engineers on the site will have some recommendations.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

I have done a bit of research on this and unfortunately, first flush requirements are generally vague. some agencies have clarified this in local drainage ordinance or development requirements, so the first step would be to ask. A common method is to capture all the flow in a retention basin and bleed it off to an oil / water separator and the rest infiltrates into the ground. This will reduce the possibility of bypass flows. otherwise, without a retention basin, it is difficult if not impossible to capture the first half inch in a oil/water separator which generally can only handle very low flow rates, most of the flow will bypass.

 

[FarvAC91]

What about after determining that 0.5" volume over the catchment area, seeing if that volume is available below the maximum capture rate in the first part of a given storm's hydrograph?

 

[psmart]

Are you concerned that some of the flow might bypass the traps? Otherwise the runoff hydrograph doesn't matter - all you need to do is provide the 1/2" volume.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[FarvAC91]

Yeah that's exactly what I'm trying to determine. It's just difficult to tell if that first 0.5" will actually make it into the trap or bypass it and how to prove it.

Thanks again for the continued help.

 

[psmart]

If you're concerned with bypass and/or flushing of the traps, this would require a more complex H&H study. But since most of your annual rainfall depth generally occurs in small events (less than the 1-year storm), this will presumably enter the traps without bypassing or flushing, and a full hydrograph analysis would not be needed. Of course, this depends entirely on your stormwater regulations - it's hard to design these systems without knowing the regulatory guidelines.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[CivilEngAus]

I agree with Peter with regards to capturing the first half an inch across your site.

if you have a large site then the constraint (that is, how many traps required) will be the inlet capacity and the rate at which runoff can enter the trap.