Total Maximum Daily Load (TMDL) Rule
Under the CWA, the states in the United States are required to identify surface waters that do not meet specified water‐quality standards or do not meet the designated beneficial uses (USEPA 1991a). In the 1990s, a significant percentage of the nation’s waters were found to be impaired based on quality standards or beneficial uses. For the most part, these waters were not suitable for fishing, swimming, or drinking. The major causes were excess sediments, SS, nutrients (phosphorus and nitrogen) from industries and municipalities, and human pathogens. Other causes were low DO, toxic metals, pesticides, pH, colors, algae, and habitat alterations. Sources of excess pollutants are from both point and nonpoint sources.
A total maximum daily load (TMDL) is the maximum of pollutant that a waterbody can receive and still maintain the water quality standards that the state has set. Therefore, a TMDL for each pollutant of concern must be established for all impaired waters. The TMDL will include a waste load allocation (WLA) for each point source of contamination, such as WWTP discharge, and a load allocation for each nonpoint sources as well as a MOS as shown in Eq. (4.13) and Figure 4.12.
TMDL can be specified in mass per units, such as pounds per day, toxicity units, or other measures, such as percent reduction. The TMDL concept is shown in Figure 4.12 where a water quality standard of 2 mg/l of ammonia as nitrogen and a stream flow of 30 MGD give a TMDL of 500 lb/day. With a given nonpoint source load allocation for agriculture runoff of 75 lb/day and a MOS of 50 lb/day, the WLA for the treatment plant is determined to be 375 lb/day, which would be written into the operating permit for the WWTP. The discharge permit for the treatment plant would be written so that this WLA would not be exceeded.
EXAMPLE 4.10 TMDL CALCULATION
[where, mg = milligram; MG = million gallons]
Rearranging Eq. (4.13) gives
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