Post Stormwater Treatment TSS Monitoring

[Cadi1]

I'm currently reviewing post-construction monitoring data for stormwater treatment of TSS. The removal efficiency goal is 80%, however the average removal efficiency measure for the site is 62%. The average TSS leaving the site is under 20 mg/l with a few hits just below 30 mg/l. Is there any literature that defines a reasonable limit for TSS in stormwater? Based on the data the site is showing 80-90% removal efficiency at some discharge points where the incoming waters were loaded above 100 mg/l, while the incoming waters with lower TSS concentrations did not show as significant a removal efficiency. Any ideas welcomed. thanks ahead of time.

 

[DMcGrath]

Gotta love the 80% TSS removal efficiency "standard". Pretty hard to achieve when the influent flow is relatively clean to begin with. I think the limit we have with current NPDES permits in Virginia is a monthly maximum of 88 mg/L, and a maximum average (for the month) of 27 mg/L. This is for an industrial stormwater permit, not a "construction" stormwater permit.

Perhaps your regulating agency can give you a real number to shoot for rather than 80% of the incoming flow. Other influencing factors include quality of the receiving stream, volume of runoff, etc. Under 20 mg/L sounds about as good as you can get without extensive treatment. Read the article about SeaTac Airport in the September 2006 Stormwater Magazine to see how extensive treatment can get.

 

[TerryScan]

This is the problem with the common 80% removal standard. It has been know for quite some time. At a stormwater conference in 2000, a speaker from ASCE was pushing for a target concentration standard that was acheivable with current technology.

He pointed out that if he had to prove 80% removal, he would have a ruck dump a load of soil at the inlet of a practice and could probably get the % removal easily...and conversly...one would have a lot of trouble getting 80% TSS removal from tap water.

There is a great need for more real scientific studies related to stormwater. There are a lot of regulations based on very limited science. The regulations seem to be based on beliefs that change from year to year...even to contradict themselves in a short period of time.

 

[bltseattle]

You don't mention the source of the 80% reduction criteria. It sounds like what would be in a stormwater manual, which is typically a means to achieve state water quality regs. If the 80% removal efficiency is the actual state code criteria, and is doesn't address "irreducible concentrations", it is poorly written, and disregard the following comments. Realize many sites are not monitored, so you are in a better position to assess if your design complies with the actual regs on allowable discharges, and you dont' have to rely on the generic assumptions used by agencies in writing a stormwater manual.

Verify your state water quality regs. The reason you are treating is to achieve a water quality criteria on the discharge. So if your discharge complies with the criteria, who cares if it's 62% or 80% or 5% efficient - you are complying.

In Washington (where turbidity is regulated, not TSS concentration) the turbidity regs are written that you are allowed a certain exceedance over background. If background is less than 50, you are allowed a 5 NTU increase. If over 50, you are limited to a 10% increase. Again, no word on treatment efficiency.

So maybe there's room for reintepreting the site discharge as applies directly to the reg, rather than the stormwater manual. If you built the BMP per the stormwater manual, and can achieve the water quality regs, you should be able to make the case that your site is compliant.