Blog_AugustReviews and re-evaluations are necessary for local limits calculations.  The regulatory requirement comes from 40 CFR 122.44(j)(2)(ii), which states that POTWs must provide a written technical evaluation of the need to revise local limits under 40 CFR 403.5 (c)(1), following permit issuance or re-issuance.

EPA suggests performing periodic evaluation of local limits be tied to the permit cycle and more detailed evaluations on an as needed basis. It further recommends performing reviews annually as part of the preparation of Annual Pretreatment Report. Following are the EPA’s recommendations in reviewing and re-evaluating local limits. Please note that this is just a suggested methodology and POTWs can employ this based on site specific conditions.

According to EPA’s recommendations:

  1. During the review, regardless of whether a local limit for each pollutant of concern (POC) was adopted, identify maximum monthly average headwork loadings during the previous year for which it calculated Maximum Allowable Headworks Loading (MAHL)
  2. Next, Compare MAHLs and POCs for which local limits were not established and if the current POC loading:
  • Exceeds MAHL…if so, establish a local limit, investigate the cause, increase Industrial User (IU) monitoring, identify any non-complying industries, and adopt pollution prevention efforts.
  • Exceeds established threshold value for the first time…if so, increase monitoring for the POC or establish a local limit
  • Exceeds established threshold value for the first time… if so, establish local limit and increase POC monitoring
  • Is below established threshold… if so, review pollutant’s loading as part of its preparation of next year’s annual report.

3. Then, Compare MAHLs and POCs for local limits that have been established and if current POC loading:

  • Exceeds MAHL, you should revise local limit (unless the exceedance is due to an unusual one-time event), investigate the cause of high loading, identification of non-complying industries, increase monitoring of IUs and adoption of pollution prevention efforts.
  • Has increased significantly from previous year (e.g., 55% to 75% of MAHL) you should, investigate the cause of increased loading, increase its monitoring for the POC or revise the local limit
  • Is below established threshold, you should review POC’s loading when it prepares next year’s report. EPA recommends maximum threshold values of 60% for metals and toxic organics and 80% for non-toxic organics and conventional pollutants. However, a POTW will use threshold values consistent with the criteria used to determine local limits.

4.  Review the data used to set the local limits. If changing conditions have affected the removal efficiencies, flow rate, or other criteria on which MAHLs were based, POTW should recalculate MAHLs.

5.  Finally, look at Compliance History. If the POTWs has violated its NPDES permit or sludge disposal or caused water quality standards violation or experienced interference of its treatment process, local limits may not be adequately protective. Unless there is an unusual incident that is unlikely to recur, the POTW is required to investigate the cause and take appropriate enforcement action against any non-complying IUs and alternatively POTW may revise local limits or establish new local limits for the pollutants that cause violation.


Source: EPA’s Local Limits Development Guidance, July 2004.


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