PERMIT RATIONALE FOR REISSUANCE
Tunica County Utility District, Robinsonville
Tunica County
Robinsonville, Mississippi
Water NPDES No. MS0048691
August 16, 2024

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 a return activated sludge process.  The treatment process consists of grit and screening removal followed by biological treatment via two aeration basins. The wastewater then passes through two clarifiers prior to disinfection by chlorination and post aeration.  Sludge from the digesters is dewatered by use of a belt filter press.  See Attachment 1 for a flow diagram. The current design capacity of the facility is 3.0 MGD. Only one discharge to surface waters is described in the application with no increase in treated effluent flow proposed.

IV.	RECEIVING WATERS – Mississippi River. The receiving waters are located in the Mississippi River Basin and are classified as Fish and Wildlife.  See Attachment No. 2 for discharge location map. This section of the Mississippi River does not appear on the current State of Mississippi 303(d) List of Impaired Waterbodies, nor does the discharge appear on a completed Total Maximum Daily Load (TMDL). 

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 100,000 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 3.0 MGD or 4.64 cfs.  
	QR = Receiving stream 7Q10, 100,000 cfs.

IWC = {(4.64 cfs)/(4.64 cfs + 100,000 cfs)} * 100 = 0.005 %

MDEQ assumes rapid and complete mixing of the treated effluent in the receiving stream due to the base flow of the Mississippi River at the discharge location. Mixing is further facilitated in that the outfall structure extends into the river and discharges below the surface at the record low water level.

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

VI.	SUMMARY OF LIMITATIONS – 

Parameter				Value				Basis
Flow					3.0 MGD			Design
	BOD5					30 mg/l 			WLA
	TSS					30 mg/l 			Technology
	Ammonia Nitrogen			Report mg/l 			WLA
	Dissolved Oxygen			> 6.0 mg/l 			WLA
	Chlorine, Total Residual		1.0 mg/l (maximum)		WLA
E. coli					126 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

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 with State Regulation 11 Mississippi Admin Code pt. 6, VI.B.2b., TCUD, Robinsonville 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:

June 2023
		Ceriodaphnia dubia		IC25 		> 0.04 %
		Pimephale promelas		IC25		> 0.04 %

September 2023
		Ceriodaphnia dubia		IC25 		> 0.04 %
		Pimephale promelas		IC25		> 0.04 %

December 2023
		Ceriodaphnia dubia		IC25 		> 0.04 %
		Pimephale promelas		IC25		> 0.04 %

Since, facility was unable to submit all of the WET testing requirements according to the 2A application. Facility is required to take one more WET test in the first quarter of 2025 (January-March) and based on that data, this permit may be reopened and modified. 

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 TCUD, Robinsonville, 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: 0.391 mg/l
		Bromoform - Average Daily Concentration: 0.492 mg/l
		Chloroform - Average Daily Concentration: 0.040 mg/l
		Dibromochloromethane - Average Daily Concentration: 0.140 mg/l	
		Phenol - Average Daily Concentration: 0.030 mg/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: 0.013 mg/l
		Bromoform - Human Health Criteria: 0.140 mg/l
		Chloroform - Human Health Criteria: 0.470 mg/l
		Dibromochloromethane - Human Health Criteria: 0.017 mg/L
		Phenol – Human Health Criteria: 1700 mg/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 - [0.391 mg/L*10] = 3.91 mg/L > 0.013 mg/l
	Bromoform - [0.492 mg/L*10] = 4.92 mg/L > 0.140 mg/L
	Chloroform - [0.040 mg/L*10] = 0.40 mg/L < 0.470 mg/L
		Dibromochloromethane - [0.140 mg/L*10] = 1.40 mg/L > 0.017 mg/L
		Phenol - [0.030 mg/L*10] = 0.30 mg/L < 1700 mg/l 

Multiplying the above numbers by the highest IWC for the discharge (0.005%) further determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria.
	
	Bromodichloromethane - [3.91 mg/L*0.00005] = 0.0001 mg/L < 0.013 mg/l
	Bromoform - [4.92 mg/L*0.00005 = 0.0002 mg/L < 0.140 mg/L
	Dibromochloromethane - [1.4 mg/l *0.00005] = 0.00007 mg/L < 0.017 mg/l
	
The above results demonstrate that the average concentrations for Bromodichloromethane, Bromoform, and Dibromochloromethane do not presents a reasonable potential to violate Human Health Criteria. 

The chemical specific data results submitted had no detects for all 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 – MS Water Quality Criteria
Attachment 6 – 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 – MS Water Quality Criteria

Attachment 6 – Chemical Specific Screening