PERMIT RATIONALE FOR REISSUANCE
Tupelo POTW
Lee County
Tupelo, Mississippi
Water NPDES No. MS0036111
Date: March 2026

I.	CLASSIFICATION – Municipal NPDES Major
							
II.	DESCRIPTION OF WASTEWATER – Domestic Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Domestic wastewater is collected and treated biologically by means of an oxidation ditch process.  No increase in treated effluent flow is proposed within the Form 2A application.  Please see attachment no.1 for flow schematic.  

IV.	RECEIVING WATERS – Town Creek.  The receiving stream is located within the Tombigbee River Basin and is classified as Fish and Wildlife.  At the point of discharge the receiving stream does appear on the most recent State of Mississippi 303(d) list for Biological Impairment.  The discharge is also listed in the Total Maximum Daily Load (TMDL) for Nutrients in Town Creek (October 2009).  The TMDL limits have been place into the permit. In addition, the discharge is also denoted in the Aliceville Pool TMDL.  The 5 – day Biological Oxygen Demand and Ammonia limitations are in compliance with the issued Aliceville Pool TMDL.  As permitted, the discharge from this facility will comply with the conditions of the pH TMDL for Town Creek as well as the pesticide/sediment TMDLs for Town Creek. Only one discharge to surface waters is described within the application with no increase in treated effluent flow proposed.  The existing discharge should not further impair water quality.

At the point of discharge, the 7-day, 10 year minimum flow, 7Q10, for Town Creek is 1.2 cfs.  The instream wastewater concentration at the point of discharge can be determined from the following calculation:

IWC = {(Qw)/(Qw + QR)} * 100

Where:
Qw = Design flow of the wastewater treatment facility, 10.5 MGD or 16.24 cfs.
QR = Receiving stream 7Q10, 1.2 cfs

IWC = {(16.24 cfs)/(16.24 cfs + 1.2 cfs)} * 100 = 93.12 %

V.	APPLICABLE GUIDELINES –11 Miss. Admin. Code Pt. 6 Ch. 1 &2 , Municipal Guidelines, 2025 Wasteload Allocation

VI.	SUMMARY OF LIMITATIONS –
	Parameter			Value				Basis
	Flow				10.5 MGD			Design
	BOD5				10 mg/l (Monthly Avg.)	Modeled & TMDL
	TSS				30 mg/l (Monthly Avg.)	Technology
	Ammonia Nitrogen (Phase I)	2.0 mg/l (Monthly Avg.)	Admin Code & TMDL
	Ammonia Nitrogen (Phase II)	1.4 mg/l (Monthly Avg.)	Ammonia Toxicity
	Dissolved Oxygen		> 6.0 mg/l 			Admin Code
	E. Coli				126 Col./100 ml		Admin Code
	Chlorine, Total Residual	0.012 mg/l 			See Calculation
	pH				6.0 – 9.0 SU			Admin Code
	Nitrogen, Total		1191.8 lbs/day 		TMDL
	Phosphorus, Total		153.7 lbs/day 			TMDL

VII.	CHANGES FROM PREVIOUS PERMIT
Phased Total Ammonia Nitrogen limitations have been added to the permit to comply with ammonia toxicity limitations.  Cadmium limitations have been removed per Attachment No. 6. 

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to 11 MS Admin Code pt. 6, VI.B.2b, a whole effluent toxicity (WET) evaluation was performed as part of the renewal application.  The applicant tested in accordance with Short-Term Methods for estimating the Chronic Toxicity of Effluents and Receiving Water to Freshwater organisms.  Short-term chronic WET tests were performed on Ceriodaphnia dubia (invertebrate) and Pimephales promelas (vertebrate). 

The City of Tupelo in the WET testing used a dilution ratio of 93.1 %.  The WET tests on the species were performed.  The results of the WET tests are as follows and are included in attachment no. 4.

March 2024
Species				IC25 Results
Ceriodaphnia dubia			> 93.1 %
Pimephale promelas			> 93.1%

May 2024
Species				IC25 Results
Ceriodaphnia dubia			> 93.1 %
Pimephale promelas			> 93.1 %

October 2024
Species				IC25 Results
Ceriodaphnia dubia			> 93.1 %
Pimephale promelas			> 93.1 %

January 2025
Species				IC25 Results
Ceriodaphnia dubia			> 93.1 %
Pimephale promelas			> 93.1 %

All of the inhibition concentrations were greater than the predicted IWC of 93.1% for all species in all tests, demonstrating that the effluent is not toxic to aquatic species.  Additional whole effluent toxicity tests will not be required as a permit condition.

CHEMICAL SPECIFIC ANALYSIS

Chemical specific analysis has been performed in accordance with 11 MS Admin Code pt. 6, VI.B1.  Municipalities shall determine the toxic characteristics of their wastewater by analyzing for the toxic pollutants listed in Table III of appendix D of 40 CFR 122.  The results of this analysis are tabulated in attachment no. 4.  The tabulated results are then analyzed and compared to water quality criteria to determine any possible toxic effect to the receiving waters in attachment no. 5.  Synopses of these calculations are listed in attachment no. 6.  A column by column description of the calculations in attachment no. 6 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 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 the NPDES 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).

For the City of Tupelo POTW, the chemical specific data results submitted with the reapplication indicates the reasonable potential for violation of the appropriate criteria in Table III of Appendix D of CFR 122.  See results in Attachment #6.

Metals Results
The application submitted demonstrated a violation of acute criteria for Cadmium; therefore, the following limitations have been added to the draft permit:

CC=CCA /IWC

Where: 
CC = Chronic Metal Limitation (mg/L)
CCA = Chronic Allowable Criteria
IWC = Instream Waste Concentration

CA=CAA /IWC

Where: 
CA = Acute Metal Limitation (mg/L)
CAA = Acute Allowable Criteria
IWC = Instream Waste Concentration
	
IWC= 93.1% or 0.931

Chronic/Monthly Avg.

CCA = 0.00015 mg/L
CC = CCA / IWC = 0.00015/ 0.931 = 0.00016 mg/L

Acute/Max Weekly Avg.:

CAA= 0.00103 mg/L
CA = CAA  / IWC = 0.00103/0.931 = 0.00112 mg/L

Volatile Organics Compounds, Acid-Extractable Compounds, and Base-Neutral Compounds Results

The following Volatile Organic compounds were detected and evaluated for a potential to violate Human Health Criteria:

Bromodichloromethane:
Average Daily Concentration = 3.47 ug/L
Recommended Water Quality Criteria= 27 μg/L
 
Multiplying the average daily concentration above by 10 (due to a data set of less than 12 samples) determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. 
[3.47 ug/L * 10]= 34.7 μg/L 
34.7 μg/L > 27 μg/L 

The above results demonstrate that the average concentration(s) present a reasonable potential to violate Human Health Criteria. An additional sampling requirement will be added to the permit (see below).

Chlorodibromomethane:
Average Daily Concentration =1.08 ug/L
Recommended Water Quality Criteria= 21 μg/L
 
Multiplying the average daily concentration above by 10 (due to a data set of less than 12 samples) determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. 
[1.08 ug/L * 10]= 10.8 μg/L 
10.8 μg/L < 21  μg/L 

The above results demonstrate that the average concentration(s) do not present a reasonable potential to violate Human Health Criteria.

Chloroform:
Average Daily Concentration = 4.71 ug/L
Recommended Water Quality Criteria= 2,000 μg/L
 
Multiplying the average daily concentration above by 10 (due to a data set of less than 12 samples) determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. 
[4.71 ug/L * 10]= 47.1 μg/L 
47.1 μg/L <  2,000 μg/L 

The above results demonstrate that the average concentration(s) do not present a reasonable potential to violate Human Health Criteria.

The above results demonstrate that the average concentration(s) presents a potential to violate Human Health Criteria. The following condition has been added to the permit:
“The Permittee shall submit results of 9 effluent samples of Bromodichloromethane pollutant (s) listed in Section D of the application. Sampling of the wastewater treatment facility's influent and effluent shall be performed twice per month for a total of 9 samples. Sampling shall begin 10 days after the issuance date of permit and continuing for 9 consecutive months. Sample results shall be submitted by the 28th day of the month following completion of sampling.”

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

Attachments

Attachment No. 1 – Process Schematic
Attachment No. 2 – Discharge Location Map 
Attachment No. 3 – Waste Load Allocation 
Attachment No. 4 – Chemical Specific Tabulated Results
Attachment No. 5 – Water Quality Criteria
Attachment No. 6 – Toxicity Screening