PERMIT RATIONALE FOR REISSUANCE
Harrison County Utility Authority, Long Beach and Pass Christian WWRF
Harrison County
Pass Christian, Mississippi
Water NPDES No. MS0043141
January 2025

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Domestic/Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Domestic / Municipal wastewater is biologically treated by means of an oxidation ditch.  The treatment process consists of grit and screening removal followed by biological treatment using two oxidation ditches operated in series.  The wastewater then passes through four clarifiers prior to disinfection and post-aeration prior to discharge. 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 for a flow diagram. The current design capacity of the facility is 7.0 MGD. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

IV.	RECEIVING WATERS – Unnamed ditch into Bayou Portage. 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 tributaries of St. Louis Bay (June 2007). The TMDL does not propose any reductions to existing permit limitations. The facility utilizes chlorine disinfection, which complies with the Pathogen TMDL for St. Louis Bay (July 2001). 

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, 7.0 MGD or 10.84 cfs
	Qr = Receiving stream 7Q10, 0 cfs

IWC = {(10.84 cfs)/(10.84 cfs + 0 cfs)} * 100 = 100 %

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

V.	SUMMARY OF LIMITATIONS – 
Parameter				Value			Basis
Flow					7.0 MGD		Design
	BOD5 (Summer)			10 mg/l 		WLA
	Ammonia Nitrogen (Summer)	6.6 mg/l 		WLA
BOD5 (Winter)			20 mg/l 		WLA
	Ammonia Nitrogen (Winter)		9.7 mg/l	 	WLA
	TSS					30 mg/l 		Technology
Dissolved Oxygen			> 6.0 mg/l 		WLA
	Chlorine, Total Residual		0.008 mg/l		MS WQS
Enterococci				35 Col./100 ml	MS WQS
	pH					6.0 – 9.0 SU		MS WQS
	Total Nitrogen 			Report lbs/day		WLA
	Total Phosphorus 			Report lbs/day		WLA
	Copper					0.0031 mg/l		Chem Screen, MS WQS
	Silver					0.0019 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.

Based on the Synopsis of Metals Results (Attachment 6) from the re-application data submitted, Total Recoverable Copper limits remain in the draft permit. A calculation of the Copper limit is shown in Attachment 7.

VI.	CHANGES FROM PREVIOUS PERMIT – 

Based on the Synopsis of Metals Results (Attachment 6) from the re-application data submitted, Total Recoverable Silver limits have been placed in the draft permit. A calculation of the Silver limit is shown in Attachment 7.

Based on the results of the screening for Volatile Organics, Acid-Extractables, and Base-Neutrals, limitations have been removed for 1,2,4-Trichlorobenzene, 3,3’-Dichlorobenzidine, Benzidine, and Benzo(a)pyrene.

Total Ammonia Nitrogen Limits have been updated based on the Wasteload Allocation to comply with EPA approved document Ambient Water Quality Criteria for Ammonia- (Saltwater)-1989 at 20°C and 25°C (Attachment 3).

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).  

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

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

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

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

	All of the inhibition concentrations were greater than the largest predicted IWC of 100%. 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, Long Beach and Pass Christian WWRF, the chemical specific data results submitted with the reapplication indicated the reasonable potential for violation of the appropriate criteria (Copper and Silver- acute) in Table III of Appendix D of CFR 122. See Attachment 6 for Chemical Specific Screening sheet and results, and Attachment 7 for a calculation of the Copper limit included in the draft permit.

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

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

Chlorodibromomethane, Chloroform, Dichlorobromomethane, and Toluene.

The average daily concentrations reported for the above compounds were compared to the EPA’s National Recommended Water Quality Criteria for Human Health Criteria for consumption of organism only (See Attachment 8) to determine their potential to violate Human Health Criteria. 

The results of the screening shown in Attachment 8 demonstrate that the average concentrations for Chlorodibromomethane, Chloroform, Dichlorobromomethane, and Toluene do not present a reasonable potential to violate Human Health Criteria. 

ATTACHMENTS

Attachment 1 – Flow Diagram
Attachment 2 – Discharge Location Map
Attachment 3 – Wasteload Allocation 
Attachment 4 – Chemical Specific Tabulated Results
Attachment 5 – MS Water Quality Criteria
Attachment 6 – Chemical Specific Screening for Metals
Attachment 7 – Copper and Silver Limitation Calculations
Attachment 8 – Volatile Organic, Acid-Extractable, and Base-Neutral Screening

Attachment 1 – Flow Diagram

Attachment 2 – Discharge Location Map

Outfall 001

Plant Location

Attachment 3 – Wasteload Allocation

Attachment 4 – Chemical Specific Tabulated Results

Attachment 5 – MS Water Quality Criteria

Attachment 6 – Chemical Specific Screening

Attachment 7 – Copper and Silver Limitation Calculations

     

Calculation of Acute Limit 

Cu

A

 = C

A 

/ IWC

Where:

Cu

A

 = Copper limit (acute) to be placed in draft permit

C

A

 = Acute Allowable Criteria (Copper), 0.0048 mg/l

IWC = Instream Waste Concentration, 100% or 1.00

Cu

A

 = (0.0048 mg/l) / (1.0) = 

0.0048 mg/l

     

Calculation of Chronic Limit 

Cu

C

 = C

C 

/ IWC

Where:

Cu

C

 = Copper limit (chronic) to be placed in draft permit

C

C

 = Chronic Allowable Criteria (Copper), 0.0031 mg/l

IWC = Instream Waste Concentration, 100% or 1.00

Cu

C

 = (0.0031 mg/l) / (1.00) = 

0.0031 mg/l

Parameter	Average Daily Concentration (µg/L)	10X Multiplier (µg/L)	EPA Water Quality Criteria (µg/L)	Pass/Fail	
Chlorodibromomethane 	2.47	NA	21	PASS	
Chloroform	11.54	NA	2000	PASS	
Dichlorobromomethane	6.50	NA	27	PASS	
Toluene	0.343	NA	520	PASS	
Attachment 8 – Volatile Organic, Acid-Extractable, and Base-Neutral Screening