PERMIT RATIONALE FOR REISSUANCE
Harrison County Utility Authority, Gulfport POTW, North and South Plants
Harrison County
Gulfport, Mississippi
Water NPDES No. MS0051756
February 2025

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Domestic/Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – 

Outfall 001 - Represents the wastewater collected by Gulfport POTW North Plant. The treatment process consists of screening and grit removal followed by biological treatment using two oxidation ditches. The wastewater then passes through two clarifiers and is then filtered prior to ultraviolet (UV) disinfection and post-aeration prior to discharge. Sludge is treated by aerobic digesters and then dewatered by use of a belt press.  See Attachment 1.a for a flow diagram. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

Outfall 002 - Represents the wastewater collected by Gulfport POTW South Plant. The treatment process consists of screening and grit removal followed by primary clarification before being biologically treated using trickling filters. The wastewater then passes through aerated contact channels, then secondary clarifiers and stage two trickling filters prior to disinfection and post-aeration. Disinfection is provided by chlorination and residual chlorine is removed using Sulfur Dioxide. Sludge is treated by aerobic digesters and then dewatered by use of a belt press.  See Attachment 1.b for a flow diagram. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

IV.	RECEIVING WATERS – 

Outfall 001 - Unnamed ditch into Bernard Bayou. See Attachment 2 for location map of the discharge point. The receiving waters are located within the Coastal Streams Basin and are classified as Fish and Wildlife.  At the point of discharge, the receiving water does not appear on the most recent State of Mississippi 303(d) list of evaluated or impaired waterbodies. The discharge does appear on a completed Total Maximum Daily Load (TMDL) for Nutrients and Organic Enrichment/Low Dissolved Oxygen in Bernard Bayou segment 3 and Industrial Seaway (June 2001). The effluent limitations included in the permit comply with load allocations given in the TMDL. The facility utilizes UV disinfection, which complies with the Pathogen TMDL for the Back Bay of Biloxi and Biloxi Bay (January 2002). 

At the point of discharge, the 7Q10 of the receiving stream is 0 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, 7.5 MGD or 12.0 cfs
	Qr = Receiving stream 7Q10, 0 cfs

IWC = {(12.0 cfs)/(12.0 cfs + 0 cfs)} * 100 = 100 %

Outfall 002 - Bernard Bayou. See Attachment 2 for location map of the discharge point. The receiving waters are located within the Coastal Streams Basin and are classified as Fish and Wildlife.  At the point of discharge, the receiving water does not appear on the most recent State of Mississippi 303(d) list of evaluated or impaired waterbodies. The discharge does appear on a completed Total Maximum Daily Load (TMDL) for Nutrients and Organic Enrichment/Low Dissolved Oxygen in Bernard Bayou segment 3 and Industrial Seaway (June 2001). The effluent limitations included in the permit comply with load allocations given in the TMDL. The facility utilizes chlorine disinfection, which complies with the Pathogen TMDL for the Back Bay of Biloxi and Biloxi Bay (January 2002).

At the point of discharge, the receiving waters are tidally influenced. The permittee has completed a mixing zone study, which found the Criterion Maximum Concentration (CMC) to be 59% and the Criterion Continuous Concentration (CCC) to be at 17%.  MDEQ surface water reviewed this study and concurred with the results.  The IWC for acute pollutants is 59% and the IWC for chronic pollutants is 17%.

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Wasteload Allocation (WLA, Attachment 3.a and 3.b), Municipal Facility Guidelines (Technology), State of Mississippi Water Quality Standards (MS WQS), TMDL, EPA Nationally Recommended Human Health Water Quality Criteria (EPA WQC), TMDL

VI.	SUMMARY OF LIMITATIONS – 

Outfall 001 – North Plant
Parameter				Value			Basis
Flow					7.75 MGD		Design
	BOD5 					4 mg/l 			WLA
	Ammonia Nitrogen 			1 mg/l 			WLA
	TSS					30 mg/l 		Technology
Dissolved Oxygen			> 6.0 mg/l 		WLA
	Chlorine, Total Residual		0.013 mg/l		MS WQS
Enterococci				35 Col./100 ml	MS WQS
	pH					6.0 – 9.0 SU		MS WQS
	Total Nitrogen 			Report lbs		WLA
	Total Phosphorus 			Report lbs		WLA
	Copper					0.0031 mg/l		Chem Screen, MS WQS
	Zinc					0.09 mg/l		Chem Screen, MS WQS
	Americamysis bahia			IC25 >100%		Admin Code
	Menidia beryllina			IC25 >100%		Admin Code

	The minimum and maximum values for influent pH, Dissolved Oxygen in the aeration unit, and 30 Minute Sludge Settleability in the aeration unit must also be reported.

Outfall 002 – South Plant
Parameter				Value			Basis
Flow (Summer)			10.5 MGD		Design
	BOD5 (Summer)			11 mg/l 		WLA
	Ammonia Nitrogen (Summer)	2.5 mg/l 		WLA
Flow (Winter)				16.0 MGD		Design
BOD5 (Winter)			21 mg/l 		WLA
	Ammonia Nitrogen (Winter)		15 mg/l	 	WLA
	TSS					30 mg/l 		Technology
Dissolved Oxygen			> 6.0 mg/l 		WLA
	Chlorine, Total Residual		0.022 mg/l		MS WQS
Enterococci				35 Col./100 ml	MS WQS
	pH					6.0 – 9.0 SU		MS WQS
	Total Nitrogen 			Report lbs		WLA
	Total Phosphorus 			Report lbs		WLA
	Copper					0.008 mg/l		Chem Screen, MS WQS
	Silver					0.003 mg/l		Chem Screen, MS WQS
	The minimum and maximum values for influent pH, Dissolved Oxygen in the aeration unit, and 30 Minute Sludge Settleability in the aeration unit must also be reported.

Combined Outfall
Parameter				Value			Basis
	Total Nitrogen (Summer)		1141.1 lbs/day 	TMDL
Total Phosphorus (Summer)		972 lbs/day		TMDL
	Total Nitrogen (Winter)		3109.3 lbs/day		TMDL
	Total Phosphorus (Winter)		1430.7 lbs/day		TMDL

Combined Annual DMR represents Gulfport POTW South and Gulfport POTW North’s combined organic load. The permit reissuance requires that Gulfport POTW South and Gulfport POTW North’s facilities comply with nutrient loads established in the Total Maximum Daily Load (TMDL) for Bernard Bayou segment 3 and Industrial Seaway.

VII.	CHANGES FROM PREVIOUS PERMIT – 
Based on the Synopsis of Metals Results (Attachment 6.a) from the re-application data submitted for Outfall 1- North Plant, Copper and Zinc limits remain in the draft permit. 

Based on the Synopsis of Metals Results (Attachment 6.b) from the re-application data submitted for Outfall 2- South Plant, Copper and Silver limits have been placed in the draft permit.

Based on the results of the WET testing, chronic bioassay requirements will be placed in the permit.

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to State Regulation 11 Mississippi Admin Code pt. 6, VI.B.2b., a whole effluent toxicity evaluation was performed as part of the renewal application.  The applicant tested in accordance with the Short – Term Methods for estimating the Chronic Toxicity of Effluents and Receiving water to Saltwater Organisms.  Short-term chronic WET tests were performed on Americamysis bahia (invertebrates) and Menidia beryllina (vertebrates).   

Outfall 001 – North Plant

July 2023
		Americamysis bahia		IC25 		>100%
		Menidia beryllina		IC25		>100%

		September 2023
		Americamysis bahia		IC25 		>100%
		Menidia beryllina		IC25		>100%

November 2023
		Americamysis bahia		IC25    	 	80.3%
		Menidia beryllina		IC25		>100%
		
		December 2023
		Americamysis bahia		IC25		>100%

		March 2024
		Americamysis bahia		IC25 		>100%
		Menidia beryllina		IC25		>100%

The above submitted bioassays demonstrate a reasonable potential to violate Whole Effluent Toxicity based on the IC25 shown above being less than the IWC of the discharge (100%). Based on the results of the WET testing, chronic bioassay requirements will be placed in the permit. 

Outfall 002 – South Plant

August 2023
		Americamysis bahia		IC25 		>68.0%
		Menidia beryllina		IC25		>68.0%

		October 2023
		Americamysis bahia		IC25 		>68.0%
		Menidia beryllina		IC25		>68.0%

November 2023
		Americamysis bahia		IC25    	 	>68.0%
		Menidia beryllina		IC25		>68.0%

		March 2024
		Americamysis bahia		IC25 		>68.0%
		Menidia beryllina		IC25		>68.0%

	All of the inhibition concentrations were greater than the chronic IWC developed in the mixing zone study of 17%. Based upon the results of the WET Testing, chronic bioassay requirements will not be placed in the draft 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 water quality criteria (see Attachment 5) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment 6.  A column by column description of the calculations in Attachment 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 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).

For HCUA, Gulfport POTW North Plant, the chemical specific data results submitted with the reapplication indicated the reasonable potential for violation of the appropriate criteria (Copper and Zinc) in Table III of Appendix D of CFR 122. See Attachment 6.a for Chemical Specific Screening sheet and results. Because the IWC of the North Plant Outfall is equal to 100%, the Copper and Zinc limitations placed in the draft permit will be equal to the water quality criteria listed in Table III of Appendix D of CFR 122.

For HCUA, Gulfport POTW South Plant, the chemical specific data results submitted with the reapplication indicated the reasonable potential for violation of the appropriate criteria (Copper and Silver) in Table III of Appendix D of CFR 122. See Attachment 6.b for Chemical Specific Screening sheet and results. Because the acute-IWC of the South Plant Outfall is equal to 59%, the Copper and Silver limitations placed in the draft permit will be equal to the water quality criteria listed in Table III of Appendix D of CFR 122 divided by the IWC.

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

Outfall 001- North Plant

For Volatile Organic, Acid-Extractable, and Base-Neutral compounds, the permittee had an average daily concentration for the following compounds: 

		Chloroform – Average Daily Concentration: 3.148 µg/L
		Toluene – Average Daily Concentration: 0.423 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:

		Chloroform - Human Health Criteria: 2000 µg/L
		Toluene - Human Health Criteria: 520 µg/L

The average daily concentrations reported for the above compounds were compared to the EPA’s National Recommended Water Quality Criteria (Human Health Criteria for consumption of organism only) to determine their potential to violate Human Health Criteria.

The above results demonstrate that the average concentrations for Chloroform and Toluene do not present a reasonable potential to violate Human Health Criteria.

Outfall 002- South Plant

For Volatile Organic, Acid-Extractable, and Base-Neutral compounds, the permittee had an average daily concentration for the following compounds: 	
		Chloroform - Average Daily Concentration: 2.19 µg/L
		Toluene - Average Daily Concentration: 1.345 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:

		Chloroform - Human Health Criteria: 2000 µg/L
		Toluene - Human Health Criteria: 520 µg/L

The above results demonstrate that the average concentrations for Chloroform and Toluene do not present a reasonable potential to violate Human Health Criteria.

ATTACHMENTS

Attachment 1.a – Flow Diagram (North Plant)
Attachment 1.b – Flow Diagram (South Plant)
Attachment 2 – Discharge Location Map
Attachment 3.a – Wasteload Allocation (North Plant)
Attachment 3.b – Wasteload Allocation (South Plant)
Attachment 4.a – Chemical Specific Tabulated Results (North Plant)
Attachment 4.b – Chemical Specific Tabulated Results (South Plant)
Attachment 5 – MS Water Quality Criteria
Attachment 6.a – Chemical Specific Screening for Metals (North Plant)
Attachment 6.b – Chemical Specific Screening for Metals (South Plant)
Attachment 7.a – Metals Limitation Calculations (North Plant)
Attachment 7.b – Metals Limitation Calculations (South Plant)

Attachment 1.a – Flow Diagram (North Plant)

Attachment 1.b – Flow Diagram (South Plant)

Attachment 2 – Discharge Location Map

South Plant Outfall

North Plant Outfall

Outfall 001

Plant Location

Attachment 3.a – Wasteload Allocation (North Plant)

Outfall 001

Plant Location

Attachment 3.b – Wasteload Allocation (South Plant)

Attachment 4.a – Chemical Specific Tabulated Results (North Plant)

Attachment 4.b – Chemical Specific Tabulated Results (South Plant)

Attachment 5 – MS Water Quality Criteria

Attachment 6.a – Chemical Specific Screening (North Plant)

Attachment 6.b – Chemical Specific Screening (South Plant)

Attachment 7.a - Metals Limitation Calculations (North Plant)

Calculation of Acute Limit- Copper

	CuA= CA/IWC
Where: 
CuA = Copper limit (acute) to be placed in draft permit
CA = Acute Allowable Criteria (Copper), 0.0048 mg/L
IWC = Instream Waste Concentration, 100% or 1.00

CuA = (0.0048 mg/L) / (1.0) = 0.0048 mg/L

Calculation of Chronic Limit- Copper

	CuC= CC/IWC
Where: 
CuC = Copper limit (chronic) to be placed in draft permit
CC = Chronic Allowable Criteria (Copper), 0.0031 mg/L
IWC = Instream Waste Concentration, 100% or 1.00

CuC = (0.0031 mg/L) / (1.0) = 0.0031 mg/L

Calculation of Acute Limit- Zinc

	ZnA= CA/IWC
Where: 
ZnA = Zinc limit (acute) to be placed in draft permit
CA = Acute Allowable Criteria (Zinc), 0.09 mg/L
IWC = Instream Waste Concentration, 100% or 1.00

ZnA = (0.09 mg/L) / (1.0) = 0.09 mg/L

Attachment 7.b - Metals Limitation Calculations (South Plant)

Calculation of Acute Limit- Copper

	CuA= CA/IWC
Where: 
CuA = Copper limit (acute) to be placed in draft permit
CA = Acute Allowable Criteria (Copper), 0.0048 mg/L
IWC = Instream Waste Concentration, 59% or 0.59

CuA = (0.0048 mg/L) / (0.59) = 0.008 mg/L

Calculation of Acute Limit- Silver

	AgA= CA/IWC
Where: 
AgA = Silver limit (acute) to be placed in draft permit
CA = Acute Allowable Criteria (Silver), 0.0019 mg/L
IWC = Instream Waste Concentration, 59% or 0.59

AgA = (0.0019 mg/L) / (0.59) = 0.003 mg/L