Since there was more than a month gap since my previous blog posts, I’ll review my previous blogs. In my last blog, I talked about the first step in the calculation of local limits which is Determination of Pollutants of Concern (POCs).

Pollutants are designated as POCs, if they are:

  • On EPA’s list of 15 POCs;
  • A pre-existing local limits;
  • Limited by a permit or environmental criteria;
  • A source of an operational problems in the past;
  • Important for the protection of the treatment works, collection system, or health and safety of POTW workers.

EPA recommends that POTW check with their state approval authority for methodologies to screen out certain POCs, before expending resources on sampling.

Now, let’s dig into the Collection and Analysis of data, which is the second step in the local limits development process.

Without the collection of site specific data, accurate and defensible local limits cannot be developed. Because the NPDES permit monitoring is unlikely to provide all the data needed for local limits calculation, EPA recommends that POTWs that have approved pretreatment programs routinely sample at other sites within the treatment works, both for local limits development and to remain up to date on their loadings of each pollutant.

So, what kind of data is needed?

Sampling and Flow data needed to calculate local limits are:

  • Pollutant concentration data – influent, effluent, sludge data. Collection system, receiving stream and IUs data
  • Flow data – Total POTW flow, POTW sludge flow to the digester, POTW sludge flow to disposal, IU flows, receiving stream, hauled waste, domestic flows, and commercial flows.

It is recommended to have sampling locations within the treatment works and the collection system. EPA provides guidance on suggested sampling locations in Local Limits Development Guidance, July 2004.

POTW should sample for all the pollutants to be included in the calculation of MAHLs and Local Limits including 15 national POCs, any POTW specific POCs, CWA organic priority pollutants, TCLP pollutants – if the POTW disposes or is likely to dispose of its sludge in landfills.

If the POTW conducts influent, effluent and sludge sampling as part of its pretreatment program, the data may be used in subsequent local limits reviews and headworks analyses. Local limits are scrutinized during their initial development, reviews, NPDES permit renewals and when detailed re-evaluations are conducted. These efforts have different data requirements and consequently different results, over different time periods. Initial development of local limits may require rapid data collection whereas constant reviews and detailed reevaluations should be based on data collected as part of routine, long term sampling effort. Detailed sampling frequencies for initial program development and ongoing evaluation are suggested in Local Limits Guidance, July 2004.

Note that these minimum sampling frequencies are recommendations and not requirements. It is recommended that POTWs seek input from their Approval Authority (State) on their sampling plans.

In my next blog I will be explaining regarding the same step but the Review and Analyzation of the Data collected.

Please contact me if you have any questions.

Shri Vani Sripada is a Project Engineer at Smith Management Group. Shri Vani can be reached at

Source: U.S. EPA Introduction to the National Pretreatment Program, 2011; EPA’s Local Limits Development Guidance, July 2004