Monday, March 2, 2009

Numeric Effluent Limits for Turbidity - Proposed New Source Performance Standard

On November 28, 2008, US EPA published its proposed revision to the federal requirements on stormwater discharges from construction sites. The key departure in the current proposal from existing requirements is the establishment of New Source Performance Standards (NSPS) and numeric effluent limit guidelines (ELG) that will apply to construction sites.

[Read more about the difference between "effluent limit" and "stormwater benchmarks"]

[Read more about typical TSS concentrations in stormwater compared to limits]

In June 2008, US EPA published its final general stormwater discharge permit for construction sites. US EPA’s intent is to issue a revised general permit once these new effluent limits are promulgated.

The current proposed rule addresses controls based on size of the construction site:

  1. Less than 10 acres. Controls are similar to current approaches.
  2. Greater than 10 acres. Sites greater than 10 acres will need to install temporary sediment basins meeting specific design criteria.
  3. Greater than 30 acres. For large sites, discharges will need to monitor stormwater discharges and must meet a turbidity effluent limit of 13 nephelometric turbidity units (NTU). The effluent limit of 13 NTU is based on the determination that the Best Available Technology (BAT) has been demonstrated to meet this limit. In this case, the BAT is active treatment on-site using injection of polymer into the stormwater to improve precipitation of smaller particles.

Does this mean that all large construction sites will need to install active stormwater treatment systems? Not necessarily. Large sites subject to the effluent limit of 13 NTU will need to meet that limit. Sediment basins alone may not be capable of meeting this limit, and if so active treatment, including enhanced precipitation using chemical addition, may be required.

Once the US EPA finalizes the effluent limits for large construction sites, State general permits will likely include these permit limits as they are reissued. US EPA is accepting comments on the proposed Rule through February 26, 2009.

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