PERMIT RATIONALE FOR REISSUANCE
Laurel POTW, Number 2
Jones County
Laurel, Mississippi
Water NPDES No. MS0020176
March 2026

I.	CLASSIFICATION – Municipal NPDES Major

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Municipal wastewater is collected and treated biologically by means of an oxidation ditch.  No increase in treated effluent flow is proposed within the application.  Please see Attachment No. 1 for flow schematic.

IV.	RECEIVING WATERS – Tallahala Creek.  The receiving stream is located within the Pascagoula River Basin and is classified as Fish and Wildlife. See Attachment No. 2 for a discharge location map. At the point of discharge the receiving stream appears on the 2018 State of Mississippi 303(d) list of evaluated or impaired waterbodies. The receiving stream does have a Total Maximum Daily Load (TMDL) for Organic Enrichment / Low Dissolved Oxygen (DO) and the Wasteload Allocation (WLA) is in accordance with the TMDL.  Only one discharge to surface waters is described within the application with no increase in treated effluent flow proposed. 

At the point of discharge, the 7-day, 10 year minimum, 7Q10, for Tallahalla Creek is 4.4 cfs. The instream wastewater concentration at the point of discharge can be determined using the following calculation:

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

	Where:
	Qw = Design flow of the wastewater treatment facility, 6.6 MGD or 10.22 cfs.
	QR = Receiving stream 7Q10, 4.4 cfs.

IWC = {(10.22 cfs)/(10.22 cfs + 4.4 cfs)}*100 = 69.90 %

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Major Municipal Facility Guidelines (Technology), Wasteload Allocation (WLA, see Attachment No. 3), Total Maximum Daily Load (TMDL)

VI.	CHANGES FROM PREVIOUS PERMIT – Total Ammonia Nitrogen limitations (May-Oct) is being phased from 2.0 mg/l (Monthly Avg.) (Phase I) to 1.9 mg/l (Monthly Avg.) (Phase II)

Total Dissolved Copper limitations have been added: 0.00715 mg/l (Monthly Avg.)

Cyanide limitations have been added: 0.00744 mg/l (Monthly Avg.)

VII.	SUMMARY OF LIMITATIONS –

Parameter					Value			Basis
Flow						6.6 MGD			Design

BOD5
(May – Oct.)					12 mg/l (Monthly Avg.)		WLA/TMDL
(Nov. – April)					30 mg/l (Monthly Avg.)		WLA

TSS						30 mg/l (Monthly Avg.)		Technology

	Ammonia Nitrogen	
	(May – October) (Phase I)			2.0 mg/l (Monthly Avg.)		WLA/TMDL
	(May – October) (Phase II)			1.9 mg/l (Monthly Avg.)		WLA/TMDL
	(Nov. – April)						Report mg/l (Monthly Avg.)	WLA

Dissolved Oxygen				> 6.0 mg/l (Monthly Avg.)		MS WQS

Chlorine, Total Residual		0.016 mg/l			WLA

E. coli						126 (col/100ml)			MS WQS

pH						6.0 – 9.0 SU			MS WQS

Phosphorus, Total				Report mg/l 			Technology

Nitrogen, Total 				Report mg/l 			Technology

Copper, Total Dissolved		0.00715 mg/l (Monthly Avg.)	Calculations

Cyanide					0.03147 mg/l (Monthly Avg.)	Calculations

The permit limitations derived from the wasteload allocation comply with the conditions of the Tallahalla Creek TMDL. The TMDL required the NH3 reductions during the seasonal critical conditions of May through October. It did not require limitations during the non-critical periods. 

Chlorine limitations were added to the permit to allow the facility to utilize chlorine disinfection in a manner that protects water quality from chlorine toxicity in the event of disruption of UV system. 

Primary method for disinfection of treated effluent is by ultraviolet (UV) disinfection.  The permittee has proposed use of the chlorination / de-chlorination facilities in “an emergency/as needed basis”.

Laurel POTW, Number 2 submitted 12 metals samples for their reapplication. The average sample for Total Dissolved Copper was 0.006 (mg/l) and the average sample for Cyanide was 0.003 (mg/l), both samples exceeded the State Water Quality Criteria for their respective parameters. Total Dissolved Copper and Cyanide limitations have been included in this permit and the calculations have been attached. See Attachment No. 6.

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to State Regulations WPC – 1 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 Laurel POTW, Number 2 had WET test samples analyzed at six concentrations (0%, 6.25%, 12.5%, 25%, 50%, and 100%).  The WET tests on the species were performed in December 2023, February 2024, May 2024, and September 2024. The results of the WET tests are as follows and are included in attached Form 2A application.

December 2023

Species				IC25 Results

Ceriodaphnia dubia			> 100.0 %
Pimephale promelas			> 100.0 %

February 2024

Species				IC25 Results

Ceriodaphnia dubia			> 100.0 %
Pimephale promelas			> 100.0 %

May 2024

Species				IC25 Results

Ceriodaphnia dubia			> 100.0 %
Pimephale promelas			> 100.0 %

	September 2024

Species				IC25 Results

Ceriodaphnia dubia			> 100.0 %
Pimephale promelas			> 100.0 %

The IC25, the concentration of the effluent that causes a 25% reduction in survival was calculated to be greater than 100% effluent. 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 State Regulations WPC – 1 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.  Synopses of these calculations are 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 Results

Major municipalities shall determine the toxic characteristics of their wastewater by analyzing for toxic pollutants listed in Table III of Appendix D of 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 Laurel POTW, Number 2 the chemical specific data results submitted with the reapplication did indicate the reasonable potential for violation of the appropriate criteria in Table III of Appendix D of 40 CFR 122.  See attached Chemical Specific Screening sheet and results.

Chemical Specific Analysis for Volatile Organic Compounds, Acid – Extractable Compounds, and Base – Neutral Compounds and Results

The chemical specific data results submitted had no detects for Volatile Organic Compounds.

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

The chemical specific data results submitted had no detects for Base Neutral 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 – Chemical Specific Calculations
	Attachment No. 6 – Copper and Cyanide Calculation

Attachment No. 1

Attachment No. 2

Attachment No. 3

Attachment No. 4

Attachment No. 5

Attachment 6

Laurel POTW, Number 2, Instream Waste Concentration (IWC): 69.9 %

Dilution Ratio: (IWC) / 100 = 0.699

Limitation Calculation

	{(Water Quality Criteria) / (Dilution Ratio)} = Permit Limitation

Copper Chronic Limitation

EPA Chronic Allowable Criteria for Copper (Chronic, Freshwater) = 0.005 mg/l from the State’s Water Quality Criteria

	{(0.005 mg/l) / (0.699)} = 0.00715 mg/l

Copper Acute Limitation

EPA Chronic Allowable Criteria for Copper (Acute, Freshwater) = 0.007 mg/l from the State’s Water Quality Criteria

	{(0.007 mg/l) / (0.699)} = 0.01001 mg/l 

Cyanide Chronic Limitation

EPA Chronic Allowable Criteria for Cyanide (Acute, Freshwater) = 0.022 mg/l from the State’s Water Quality Criteria

	{(0.0052 mg/l) / (0.699)} = 0.007439 mg/l 

Cyanide Acute Limitation

EPA Acute Allowable Criteria for Cyanide (Chronic, Freshwater) = 0.0052 mg/l from the State’s Water Quality Criteria

	{(0.03147 mg/l) / (0.699)} = 0.03147 mg/l