WEF Discussion Forums
Laboratory Management and Technical Issues
We are in the design phase of a plant upgrade to install a disinfection system (UV).
Where is the proper sampling location, before UV or after?
Our permit required testing will include BOD5, TSS, pH, Fecal (which the state is now looking at e-coli).
Our engineer shows a sampling port before UV. When questioned they say this port is for everything but the fecal (or e-coli). For that we will take a grab after the UV.
At our plant we collect our efflent sample prior to disinfection. This was originally done so that we did not have to run our CBOD5 as a seeded sample. The lack of chlorination or UV dissinfection allowed a sufficient microbial population in our effluent.
It's interesting that you're having discussions about where to sample FINAL effluent. We sampled for BOD/ TSS pre-chlorination for over 20 years before our state (and federal) regulators said we had to collect all samples post-disinfection. It's hard to argue the fact that you're not sampling FINAL effluent unless you're sampling post-disinfection. Have your regulators affirmed that it is acceptable to sample pre-chlorination (or UV) for BOD/ TSS?
Technically speaking, CBOD5 before and after UV disinfection should be the same. This would not be true for an oxidizing disinfectant such as chlorine. I am not sure about the procedural aspect.
I would say contact your regulatory agency and make sure that they are okay with that sampling point setup.
Differences are interesting indeed! I believe we will ask the engineer to confirm sampling location before finalizing design.
Thanks for everyones input!!!
The final effluent, especially with chlorination / dechlorination should be the discharge. Excess sulfite for dechlorination will be oxidized as is accepted as part of the BOD value. Suspended solids could settle out while the flow is in the contact tank and thus you would be reporting values biased high as compared to the final effluent.
Disinfection greatly reduces the nitrification process and thus BOD values will be lower without the interference. Just be sure not to use a seed material containing any significant nitrifin bacteria.