PERMIT RATIONALE FOR REISSUANCE
Pearl River County Utility Authority, PRCUA Airport Road POTW
Pearl River County
Picayune, Mississippi
Water NPDES No. MS0042161
March 11, 2025

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Domestic / Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is collected and treated biologically using an aerated lagoon followed by constructed wetlands and chlorine disinfection prior to discharge. See Attachment 1 for a flow diagram. The current design capacity of the facility is 3.075 MGD. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

IV.	RECEIVING WATERS – Pearl River. The receiving stream is located in the Pearl River drainage basin and is classified as Recreation. At the point of discharge, the receiving stream does not appear on the 2018 State of Mississippi 303(d) List of Impaired Waterbodies. The discharge does appear on a completed Total Maximum Daily Load (TMDL) for Total Nitrogen and Total Phosphorus in the Pearl River (June 2009). Permit limitations were developed in accordance with the Wasteload Allocation in the TMDL.

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 1160 cfs. The Instream Wastewater Concentration (IWC) at the point of the discharge is determined by the following calculation:

IWC = {(Qw)/(Qw + QR)} * 100 
	Where:
	Qw = Design flow of the wastewater treatment facility 3.075 MGD or 4.8 cfs.  
	QR = Receiving stream 7Q10, 1160 cfs.

IWC = {(4.8 cfs)/(4.8 cfs + 1160 cfs)} * 100 = 0.41 %

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Mississippi Water Quality Standards (MSWQS), Wasteload Allocation (WLA, Attachment 3), Municipal Facility Guidelines (Technology), TMDL

VI.	SUMMARY OF LIMITATIONS – 
Parameter				Value				Basis
Flow					3.075 MGD			Design
	BOD5					30 mg/l 			Technology
	TSS					30 mg/l 			Technology
	Dissolved Oxygen			> 6.0 mg/l 			WLA
	Chlorine, Total Residual		0.09 mg/l	 		MSWQS
E. coli					126 Col. /100 ml		MSWQS
	pH					6.0 – 9.0 SU			MSWQS
	Nitrogen (Total)			295.13 lbs/day			TMDL
	Phosphorus (Total)			70.73 lbs/day			TMDL
	Ammonia Nitrogen			Report mg/l			WLA

VII.	CHANGES FROM PREVIOUS PERMIT – 

Effluent monitoring and reporting for Total Ammonia Nitrogen has been added to the draft permit in accordance with Waste Load Allocation.

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance with State Regulation 11 Mississippi Admin Code pt. 6, VI.B.2b., PRCUA, Airport Road POTW is required to perform a Whole Effluent Toxicity (WET) evaluation as part of the renewal application. The applicant was to have tested in accordance with Short – Term Methods for estimating the Acute Toxicity of Effluents and Receiving Water to Freshwater Organisms.  Short – term acute WET tests were performed on Ceriodaphnia dubia (invertebrate) and Pimephale promelas (vertebrate).  The results of the wet tests are as follows:

October 2022
Ceriodaphnia dubia		48-hour LC50 		>1.64%
Pimephales promelas		96-hour LC50		>1.64%

March 2023
Ceriodaphnia dubia		48-hour LC50 		>1.64%
Pimephales promelas		96-hour LC50		>1.64%

May 2023
Ceriodaphnia dubia		48-hour LC50      	>1.64%
Pimephales promelas		96-hour LC50		>1.64%

August 2023
Ceriodaphnia dubia		48-hour LC50 		>1.64%
Pimephales promelas		96-hour LC50		>1.64%

All of the inhibition concentrations were greater that the predicted IWC of 0.41%.  Based upon the results of the WET testing, bioassay requirements will not be placed in the permit.

CHEMICAL SPECIFIC ANALYSIS

	Chemical specific analysis has been performed in accordance with State Regulations 11 Mississippi Admin Code pt. 6, 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 4.  The reported values are then analyzed and compared to State of Mississippi and/or National water quality criteria to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment 5.  A column by column description of the calculations in Attachment 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 III 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).

PRCUA, Airport Road POTW, the chemical specific data results submitted with the reapplication did not indicate a reasonable potential for violation of the criteria in Table III of Appendix D of CFR 122.

See Attachment 5 for Chemical Specific Screening sheet and results.

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

For Base-Neutral Compounds, the permittee had an average daily concentration for the following compound: 
	
	Bis (2-ethlyhexyl) Phthalate - Average Daily Concentration: 8.5 µg/L

According to EPA’s National Recommended Water Quality Criteria, the Human Health Criteria (consumption of organism only) for the following compounds are shown below:

		Bis (2-ethlyhexyl) Phthalate - Human Health Criteria: 0.37 µg/L
			
Multiplying the average daily concentration by the IWC for the discharge at the mean annual flow of the receiving stream further determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. The mean annual flow at the discharge point for the Pearl River is 1160 cfs (NHD), resulting in a mean annual IWC of (0.40%)
	
	Bis (2-ethlyhexyl) Phthalate - [8.5 µg/L*0.004] = 0.034 µg/L < 0.37 µg/L

The above results demonstrate that the average concentration for Bis (2-ethlyhexyl) Phthalate does not present a reasonable potential to violate Human Health Criteria.

The chemical specific data results submitted had no detects for all Volatile Organic Compounds and Acid-Extractable Compounds.

ATTACHMENTS:

Attachment 1 – Flow Diagram
Attachment 2 – Location Map
Attachment 3 – Wasteload Allocation 
Attachment 4 – Chemical Specific Tabulated Test Results
Attachment 5 – Chemical Specific Screening

Attachment 1 – Flow Diagram

Attachment 2 – Location Map

Outfall 001

Plant Location

Attachment 3 – Wasteload Allocation

Attachment 4 – Chemical Specific Tabulated Results

Attachment 5 – Chemical Specific Screening