PERMIT RATIONALE FOR MODIFICATION
Harrison County Utility Authority, South Woolmarket WRRF
Harrison County
Biloxi, Mississippi
Water NPDES No. MS0061204
June 2021

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Domestic / Municipal Wastewater

III.	PURPOSE OF MODIFICATION – Removal of the Cyanide limitation based on additional data submitted by the permittee that demonstrated no reasonable potential to violate water quality criteria for the parameter.

IV.	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 a three-channel orbital oxidation ditch.  The wastewater then passes through two clarifiers prior to disinfection.  Disinfection is provided by gas chlorination and residual chlorine is removed by means of 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 2.0 MGD. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

V.	RECEIVING WATERS – Biloxi River.  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 facility utilizes chlorine disinfection, which complies with the Pathogen Total Maximum Daily Load (TMDL) for Back Bay (January 2002). See Attachment 2 for location map of the discharge point. 

At the point of discharge, the 7Q10 of the receiving is 11.5 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, 2.0 MGD or 3.10 cfs
	Qr = Receiving stream 7Q10, 11.5 cfs

IWC = {(3.10 cfs)/(3.10 cfs + 11.5)} * 100 = 21.21 % 
		
The permittee discharges treated effluent from a diffuser for Outfall 001 to provide appropriate dispersion of the effluent to the receiving stream.

VI.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Wasteload Allocation (WLA, Attachment 3), Municipal Facility Guidelines (Technology), TMDL 

VII.	CHANGES FROM PREVIOUS PERMIT – 

The Cyanide limitation has been removed from the permit based on the submittal of additional data in accordance with Condition L-1 of the previous permit.

VIII.	SUMMARY OF LIMITATIONS – 
Parameter				Value			Basis
Flow					2.0 MGD		Design
	BOD5					10 mg/l 		WLA
	TSS					30 mg/l 		Technology
	Ammonia Nitrogen			2 mg/l 			WLA
	Dissolved Oxygen			> 6.0 mg/l 		WLA
	Chlorine, Total Residual		0.052 mg/l		WLA, TMDL
Enterococci				35 Col./100 ml	Admin Code, TMDL
	pH					6.0 – 9.0 SU		Admin Code
	Nitrogen (Total, Summer)		191.82 lbs/day		WLA
	Phosphorus (Total, Summer)		86.74 lbs/day		WLA
	Nitrogen (Total, Winter)		Report lbs/day		WLA
	Phosphorus (Total, Winter)		Report lbs/day		WLA

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.

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).  

February 2018
		Americamysis bahia		IC25 		>57%
		Menidia beryllina		IC25		>57%

		April 2018
		Americamysis bahia		IC25 		>84.8%
		Menidia beryllina		IC25		>84.8%

July 2018
		Americamysis bahia		IC25    	 	>84.8%
		Menidia beryllina		IC25		>84.8%

		October 2018
		Americamysis bahia		IC25 		>84.8%
		Menidia beryllina		IC25		>84.8%

	All of the inhibition concentrations were greater than the largest predicted IWC of 21.21%. 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, South Woolmarket WWRF, the chemical specific data results submitted with the reapplication did not indicate the reasonable potential for violation of the criteria in Table III of Appendix D of CFR 122. See Attachment 6 for Chemical Specific Screening sheet and results.

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

For Volatile Organic Compounds, the permittee had an average daily concentration for the following compounds: 

		Bromodichloromethane - Average Daily Concentration: 16.8 µg/L
		Chloroform - Average Daily Concentration: 40.3 µg/L
		Dibromochloromethane - Average Daily Concentration: 4.27 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:

		Bromodichloromethane - Human Health Criteria: 27 µg/L
		Chloroform - Human Health Criteria: 2000 µg/L
		Dibromochloromethane - Human Health Criteria: 21 µg/L
	
Multiplying the average daily concentrations 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.

	Bromodichloromethane - [16.8 µg/L*10] = 168 µg/L > 27 µg/L
	Chloroform - [40.3 µg/L*10] = 403 µg/L < 2000 µg/L
		Dibromochloromethane - [4.27 µg/L*10] = 42.7 µg/L > 21 µg/L

Multiplying the above numbers by the highest IWC for the discharge (21.2%) further determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria.
	
	Bromodichloromethane - [168 µg/L*0.212] = 35.6 µg/L > 27 µg/L
	Dibromochloromethane - [42.7 µg/L*0.212] = 9.06 µg/L < 21 µg/L

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

The results also demonstrate that the average concentrations for Bromodichloromethane do present a reasonable potential to violate Human Health Criteria. The previous permit required a full set of twelve (12) samples of Bromodichloromethane be submitted to further determine the potential adverse effects on water quality.

The full set of twelve (12) samples submitted do not present a reasonable potential to violate Human Health Criteria for Dibromochloromethane.

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

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

Attachment 1 – Flow Diagram

Attachment 2 – Discharge Location Map

Plant Location

Outfall 001

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