PERMIT RATIONALE FOR REISSUANCE
Jackson POTW, Trahon and Big Creek
Hinds County
Jackson, Mississippi
Water NPDES No. MS0044059
March 30, 2026
CLASSIFICATION

Municipal Major

DESCRIPTION OF WASTEWATER

Domestic/ Municipal Wastewater

DESCRIPTION OF WASTEWATER TREATMENT  

Wastewater is collected and treated by means of an oxidation ditch followed by chlorine disinfection. See attachment No. 1 for flow diagram.

RECEIVING WATERS

Big Creek. See attachment No. 2 for discharge location map. The receiving is classified as Fish and Wildlife, and is in the Pearl River Basin. This discharge is listed in the TMDLs for pH and Nutrients. The permit requires the effluent to comply with water quality criteria before discharging, which complies with the conditions of pH TMDL. Nutrient limitations are included in the draft permit to comply with the nutrient TMDL. Big Creek appears on the current 303(d) List of Impaired Water Bodies for biological impairment.  

The instream wastewater concentration (IWC) 7Q10   at the point of discharge is determined by the following calculation:

IWC calculation for the Jackson POTW, Trahon and Big Creek is as follows:

Qw = Design flow of the wastewater treatment facility = 4.5 MGD or 6.98 cfs
Qr =  Receiving stream 7Q10 = 0 cfs
IWC = (Qw/ (Qw + Qr)) * 100 =  (6.98/ 6.98 + 30) =  100 %

APPLICABLE GUIDELINES 

Wastewater Pollution Control Regulations (11 Mississippi Admin Code part 6, Chapter 1 and 11 Mississippi Admin Code part 6, Chapter 2). State of Mississippi Water Quality Criteria & “Wastewater Regulations for National Pollutant Discharge Elimination System Permits, Underground Injection Control Permits, State Permits, Water Quality Based Effluent Limitations , Water Quality Criteria, Water Quality Certification” applicable. Wasteload Allocation (WLA), Total Maximum Daily Load (TMDL)

SUMMARY OF LIMITATIONS –

	Parameter						Value				Basis
	Flow						4.5 MGD				Design
	BOD5						10 MGD				WLA
	BOD5 (Percent Removal)			85% Minimum				Technology
	TSS						30 mg/l (Monthly Avg.)			Technology
	TSS (Percent Removal)				85% Minimum				Technology
	NH3-N (Phase I)					2 mg/l (Monthly Avg.)			WLA
	NH3-N (Phase II)					1.37 mg/l (Monthly Avg.)			WLA
	Dissolved Oxygen					> 6.0 mg/l (Monthly Avg.)			WLA
	E. coli					    126 mg/l (Monthly Avg.)			11 MS code part 6
                pH						6.0 – 9.0 SU				11 MS code part 6
	Chlorine, total Residual				0.011 / 0.019 mg/l				11 MS code part 6
	Total Nitrogen Effluent				Report lbs/day (Monthly Avg.)		TMDL
	Total Nitrogen Influent				Report lbs/day (Monthly Avg.)		TMDL
	Total Phosphorus Effluent			Report lbs/day (Monthly Avg.)		TMDL
	Total Phosphorus Influent			Report lbs/day (Monthly Avg.)		TMDL
Copper				       0.005 / 0.007 mg/l			Sample Data
	Pimephale promelas					100% Minimum				11 MS code part 6
Ceriodaphnia dubia	100% Minimum				11 MS code part 6

The minimum and maximum values for Dissolved Oxygen in aeration unit and 30-Minute Sludge Settleability in aeration unit must be reported

	See attached modeling data results attachment no. 3.

Upon issuance of this permit, the permittee shall achieve compliance with the effluent limitations specified for the parameters without a phase designation and for the parameters notes as Phase I; Phase II limitations shall not apply at this time. 

Phase II limitations shall apply as soon as the treatment system has demonstrated compliance with the Phase II limitations, but no longer than three (3) years after the issuance of the permit; Phase I limitations shall no longer apply at this time. 

The permittee shall submit an engineering report that describes how the facility will come into compliance with the Phase II limitations.

The proposed draft permit contains Copper effluent limitaions. The limitations are based on the data received through the permit application. The Copper limits were determined using the sample data received from the application and the Chemical Specific Screening calculations. 

TOXICITY EVALUATION

Copper

Copper exceeded the acute and chronic water quality criteria based on the data received through the permit application; therefore, we have placed the following limitation for Copper into the draft permit. 

Monthly Average					Maximum Weekley
0.005 mg/l                                  0.007 mg/l
BOD5, NH3-N and DO limits are based on the Wasteload Allocation and the STREAM model, which accounts for oxygen depletion in the receiving stream. The toxicity limit for NH3-N was determined by choosing the more stringent of the modeled limit and the toxicity limit given in the Wasteload Allocation, to meet Water Quality Based Effluent Limitations ( WQBELs).

The TMDL requires this Trahon/Big Creek facility to meet with Total Nitrogen 510.7 lbs/day and Total Phosphorus 110.7 lbs/day. The TMDL also gives opportunity for Total Phosphorus load to be adjusted in combination and divided between the three (3) Jackson facilities (Trahon/Big Creek, Savannah Street and OB Curtis) for a total of 1297 lbs/day. These effluent values are combined together to meet the TMDL combined load in the Jackson POTW, Savanna Street permit.

The Total Nitrogen load can also be adjusted in combination and divided between the three (3) Jackson facilities (Trahon/Big Creek, Savannah Street and OB Curtis) for a total of 6031 lbs/day

This permit does contain Total Nitrogen and Total Phosphorus monitoring to demonstrate and record the Jackson POTW Trahon’s contribution to the combined load.

A reopener clause has been added to the permit which states that the NPDES MS0044059, shall be reopened and modified, or alternately revoked and reissued, accordingly to reflect any additional, or otherwise updated limitations, and any additional monitoring determined to be necessary to be consistent with the findings of any additional stream monitoring performed as a requirement of the approval of the 2015 Pearl River Nutrient TMDL of which this facility is a part.

WHOLE EFFLEUNT TOXICITY AND CHEMICAL SPECIFIC EVALUATION

WHOLE EFFLEUNT TOXICITY EVALUATION

        In accordance to 11 Miss. Admin Code pt. 6, Ch. 1, Jackson POTW, Trahon and Big Creek
is required to perform Whole Effluent Toxicity (WET) tests as part of the renewal application.  The applicant should test in accordance with Short – Term Methods for estimating the ChronicToxicity of Effluents and Receiving Water to Freshwater Organisms. Regulations require that those facilities with an IWC of greater than 1% to perform a chronic whole effluent toxicity evaluation.  This test should be performed in accordance with Short-Term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Water to Freshwater Organisms (EPA/600/4-91/002) or most recent addition.  The chronic WET tests should be performed on Ceriodaphnia dubia (invertebrate) and Pimephales promelas (vertebrate). The results of the WET tests are as follows: 

			April 2024
			Ceriodaphnia dubia		IC25    		1.72%
			Pimephales promelas		IC25   		>100%

			June 2024 (Retest)
			Ceriodaphnia dubia		IC25   		>100%

			September 2024
			Ceriodaphnia dubia		IC25   		>100%
			Pimephales promelas		IC25   		>100%

			November 2024
			Ceriodaphnia dubia		IC25     		>100%
			Pimephales promelas		IC25     		>100%

			March 2025
			Ceriodaphnia dubia		IC25  		100%
			Pimephales promelas		IC25   		5.13%

April 2025 (Retest)
			Pimephales promelas		IC25   		>100%

June 2025
			Ceriodaphnia dubia		IC25   		>100%
			Pimephales promelas		IC25   		>100%

November 2025
			Ceriodaphnia dubia		IC25   		>100%
			Pimephales promelas		IC25   		>100%

Based on the results of the WET testing, inhibition concentrations were not greater that the predicted IWC of 100%.  Chronic bioassay requirements will be placed within the permit.

Chemical Specific Analysis

Chemical specific analysis has been performed in accordance with State Regulations WPC-1 V1.B1.  Municipalities shall determine the toxic characteristics of their wastewater by analyzing for the toxic pollutant listed in Table III of appendix D of 40 CFR 122.  The results are tabulated in attachment No. 4.  The reported values are then analyzed and compared to water quality criteria (attachment No. 6) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in attachment No. 5.  A column by column description of the calculations in attachment No. 5 is hereby provided:

Column No. 1 – Maximum concentration.  The highest effluent reading of the parameter

Column No. 2 – The maximum concentration mixed with the receiving water IWC.  This     is calculated by the following equation:

{Maximum Concentration (Column 1) * IWC %} / 100

Column No. 3 – Acute allowable – Chemical Specific State Water Quality Criteria                                   
	Column No. 4 – Pass or Fail. If column 2 > column 3 water quality criteria is exceeded                              and failure occurs.

Column No. 5 – Long term average (LTA) of all tests.  The summation of the 12 samples divided by 12.

Column No. 6 – The long term average concentration mixed with the receiving water                                  IWC.  This is calculated by the following equation:

{LTA Concentration (Column 5) * IWC%} / 100

Column No. 7 – Chronic allowable – Chemical Specific State Water Quality Criteria                                   
Column No. 8 – Pass or Fail. If column 6 > column 7 water quality criteria is exceeded                                and failure occurs.

Column No. 9 – Human health determination.  Long term average (Column 5) is mixed                with the mean annual flow IWCMA .  This is calculated by the following                            equation:

{LTA Concentration (Column 5) * IWCMA %} / 100

Column No. 10 – Human health allowable – State Water Quality Criteria   Note: Organisms only column is used if receiving waters are not drinking water supply. 

	Column No. 11 – Pass or Fail.  If column 9 > column 10 water quality criteria is exceeded                        and failure occurs.

Chemical Specific Analysis for Metals and Results

Major municipalities shall determine the toxic characteristics of their wastewater by analyzing for toxic pollutants listed in Table II of Appendix D of 40 CFR 122.  Metal analysis shall be for total recoverable metals.  Municipalities shall submit twelve influent and effluent samples collected.  Chemical specific limits shall be placed in a permittee’s permit if any of the parameters evaluated in the toxic screening procedure indicate the reasonable potential for violation of the appropriate criteria (acute, chronic, and human health).
	
The chemical specific data results submitted did indicate the reasonable potential for violation of the appropriate criteria in Table III of Appendix D of CFR 122 for Total Recoverable Copper. Chemical specific limits have been place in the permit for Total Recoverable Copper.

Chemical specific analysis for Volatile Organics Compounds, Acid-Extractable Compounds, and Base-Neutral Compounds and Results

The chemical specific data results submitted had no detects for Acid-Extractable Compounds.

For Volatile Organics, the facility had an average daily concentration of 0.00493 mg/l for Chloroform. According to the 2006 National Recommended: Water Quality Criteria, the Human Health criteria for Chloroform is 0.470 mg/l, Organisms Only. The average concentration, multiplied by the power of 10 (0.0493 mg/l), due to less than 12 samples, does not present a reasonable potential to violate Human Health Criteria.  

For Volatile Organics, the facility had an average daily concentration of 0.0013 mg/l for Dichlorobromomethane. According to the 2006 National Recommended: Water Quality Criteria, the Human Health criteria for Dichlorobromomethane is 0.017 mg/l, Organisms Only. The average concentration, multiplied by the power of 10 (0.013 mg/l), due to less than 12 samples, does not present a reasonable potential to violate Human Health Criteria. 

Attachments
Attachment 1	See Application
Attachment 2	See Application
Attachment 3	Waste Load Application 
Attachment 4	Tabulated Metals Results 
Attachment 5 	Synopsis of Metals Results 
Attachment 6 	MS State Water Quality Criteria 

Attachment 3

Attachment 4

Attachment5

Attachment 6