PERMIT RATIONALE FOR MODIFICATION
Harrison County Utility Authority, East Biloxi POTW
Harrison County
Biloxi, Mississippi
Water NPDES No. MS0023159
March 2020

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 activated sludge process.  The treatment process consists of grit and screening removal followed by biological treatment using an activated sludge process by means of sequential batch reactors.  The wastewater then passes through a secondary clarifier.  Disinfection is provided by chlorination and residual chlorine is removed by means of sulfur dioxide.  Sludge is treated by thickeners and aerobic digesters and then dewatered by use of a belt filter press.  See Attachment 1 for a flow diagram. The current design capacity of the facility is 10.0 MGD. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

V.	RECEIVING WATERS – Back Bay of Biloxi.  The receiving waters are located within the Coastal Streams Basin and are classified as Recreational.  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 is included the Total Maximum Daily Load (TMDL) for pathogens in the Back Bay of Biloxi (January 2002).  The facility utilizes chlorine disinfection, which complies with the conditions of the TMDL. See Attachment 2 for location map of the discharge point.  

A previous permit required the facility to develop a site specific mixing zone.  The study found the Criterion Maximum Concentration (CMC) to be 23% and the Criterion Continuous Concentration (CCC) to be at 11.4%.  MDEQ surface water reviewed this study and concurred with the results.  The IWC for acute pollutants was 23% and the IWC for chronic pollutants was 11.4%

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

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					10.0 MGD			Design
	BOD5					30 mg/l 			WLA
	TSS					30 mg/l 			Technology
	Ammonia Nitrogen			15 mg/l 			WLA
	Dissolved Oxygen			> 6.0 mg/l 			WLA
	Chlorine, Total Residual		0.056 mg/l			WLA
Enterococci				35 Col./100 ml		Admin Code
	pH					6.0 – 9.0 SU			Admin Code
	Nitrogen (Total)			Report lbs/day			WLA
	Phosphorus (Total)			Report lbs/day			WLA

Residual Chlorine Calculation:

Permit Limit = (Numeric Criteria) / (IWC)

	Dilution Ratio (Acute IWC) = 0.23
	Acute Chlorine saltwater numeric criteria = 0.013 mg/l

Maximum Limit = (0.013 mg/l) / (0.23) = 0.056 mg/l

	Dilution Ratio (Chronic IWC) = 0.114
	Chronic Chlorine saltwater numeric criteria = 0.0075 mg/l

Average Limit = (0.0075 mg/l) / (0.114) = 0.066 mg/l

For the Total Residual Chlorine parameter, the chronic IWC of 11.4% and acute IWC of 23% are used to develop the concentration average limitation and concentration maximum limitation, respectively. This results in the average limitations being greater than the maximum limitations for the above parameters. Due to the chronic (average) limitation being greater than the acute (maximum) limitation, only the maximum limitation will be placed in the permit. The facility will report the average concentration.

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).  

July 2017
		Americamysis bahia		IC25 		>57%
		Menidia beryllina		IC25		>57%

		October 2017
		Americamysis bahia		IC25 		>57%
		Menidia beryllina		IC25		>57%

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

		April 2018
		Americamysis bahia		IC25 		>45.6%
		Menidia beryllina		IC25		>45.6%

	All of the inhibition concentrations were greater than the largest predicted IWC of 23%. 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, East Biloxi 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 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: 9.07 µg/L
		Bromoform - Average Daily Concentration: 7.14 µg/L
		Chloroform - Average Daily Concentration: 4.63 µg/L
		Dibromochloromethane - Average Daily Concentration: 14.4 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
		Bromoform - Human Health Criteria: 120 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 - [9.07 µg/L*10] = 90.7 µg/L > 27 µg/L
	Bromoform - [7.14 µg/L*10] = 71.4 µg/L < 120 µg/L
	Chloroform - [4.63 µg/L*10] = 46.3 µg/L < 2000 µg/L
		Dibromochloromethane - [14.4 µg/L*10] = 144 µg/L > 21 µg/L

Multiplying the above numbers by the highest IWC for the discharge (23%) further determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria.
	
	Bromodichloromethane - [90.7 µg/L*0.23] = 20.8 µg/L < 27 µg/L
	Dibromochloromethane - [144 µg/L*0.23] = 33.1 µg/L > 21 µg/L

The above results demonstrate that the average concentrations for Bromodichloromethane, Bromoform, and Chloroform do not present a reasonable potential to violate Human Health Criteria. The results also demonstrate that the average concentrations for Dibromochloromethane do present a reasonable potential to violate Human Health Criteria. The permit will require a full set of twelve (12) samples of Dibromochloromethane be submitted to further determine the potential impacts on water quality. 

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 – Topo Map
Attachment 3 – Wasteload Allocation 
Attachment 4 – Chemical Specific Tabulated Results
Attachment 5 – MS Water Quality Criteria
Attachment 6 – Synopsis of Metals Calculations

Attachment 1 – Flow Diagram

Attachment 2 – Topo 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 – Synopsis of Metals Calculations