PERMIT RATIONALE FOR REISSUANCE
DeSoto County Regional Utility Authority, Metro DeSoto WWTP
Desoto County
Olive Branch, Mississippi
Water NPDES No. MS0028479
February 2019

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Municipal wastewater is biologically treated by means of an activated sludge plant followed by chlorine disinfection prior to discharge.  See Attachment 1 for a flow diagram. The current design capacity of the facility is 0.537 MGD. DCRUA, Metro DeSoto WWTF plans to expand from 0.537 MGD to 1.0 MGD due to the growth rate of existing facilities that compose the plant’s influent.  The current facility is an extended aeration activated sludge plant with a treatment capacity of 0.537 MGD. With flow exceedances caused by fluctuations of industrial inflow, as well as problems with removing nutrients and ammonia, the plant’s conversion to a Sequence Batch Reactor (SBR) process would provide the hydraulic capacity for existing and future industries, as well as improving the overall treatment process. 

IV.	RECEIVING WATERS – Unnamed tributary thence into Nonconnah Creek.  The receiving waters are located within the Northern Independent 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, nor does the discharge appear on a completed Total Maximum Daily Load (TMDL). Only one discharge to surface waters is described in the application. See Attachment 2 for location map of the discharge point.  

At the point of discharge, the receiving stream’s 7Q10 is 0.0 cfs.  The instream wastewater concentration at the point of discharge is determined by the following calculation:

IWC = (Qw)/(Qw + QR) * 100

	Where:
	Qw = 0.537 MGD or 0.83 cfs
	QR = 0.00 cfs

IWC = {(0.83 cfs)/(0.83 cfs + 0.0 cfs)} * 100 = 100 %

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 (Phase I)					0.537 MGD	Design
	Flow (Phase II)					1.0 MGD	Design
	Flow (Phase III)					1.5 MGD	Design
	BOD5 (Phase I)					10 mg/l 	Modeled
	BOD5 (Phase II & III)			16 mg/l 	Modeled
	TSS						30 mg/l	Technology
	Ammonia Nitrogen			2 mg/l 	Admin Code
	Oil and Grease 					10 mg/l 	Admin Code
	E. coli						126 mg/l	Admin Code	
	Chlorine (Phase I)			0.017 mg/l	WLA
	Chlorine (Phase II & III)                     0.011 mg/l	WLA
	pH						6.0 – 9.0 SU	Admin Code	
	Copper, Total Recoverable	0.0194 mg/l	Calculated
	Cadmium					0.00046 mg/l	Calculated	
	Lead, Total Recoverable		0.0066 mg/l	Calculated 
	Mercury, Total Recoverable	0.000012 mg/l	Admin Code
	Total Nitrogen					Report (lbs/day)	WLA
	Total Phosphorus			27 (lbs/day)	WLA, calcs

Phosphorus and Nitrogen sampling shall be conducted by grab samples from 1) the receiving stream after complete mixing with the wastewater treatment facility's effluent and 2) on the Nonconnah Creek at a location by the Mississippi-Tennessee state line. Sampling shall be performed at the locations approved by MDEQ. 

The proposed limitations for Total Phosphorus were based on sampling data provided by the facility see Attachment 4.  MDEQ calculated the 90th percentile of the sample concentrations to determine the phosphorus loading at the currently permitted design flow.  The limitation was added to the proposed permit in order to prevent any further degradation of the stream segments (TN0801021100720_0410 and TN0801021100720_2000) denoted in the State of Tennessee’s 303(d) list.  

Tennessee has demonstrated that the receiving stream is not supporting its designated use due to elevated Nitrogen.  The permit currently contains nutrient monitoring. The permit will be reopened and Total Nitrogen limitations will be established upon the listing of the receiving stream on the 303(d) List of Impaired Water Bodies.

E.coli has replaced Fecal Coliform as the bacteria indicator in Mississippi’s Water Quality Standards (MSWQS) for freshwater. The permit limitations have been updated to reflect this change. The State of Tennessee has established a TMDL for E. coli for the Nonconnah Creek Watershed in Shelby and Fayette Counties. The proposed permit requires disinfection to achieve a limitation of 126 #col/100mL (weekly average) and 410 #col/100mL (weekly maximum) to meet MSWQS, which also complies with Tennessee’s TMDL for E. coli.

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 Freshwater Organisms.  Short-term chronic WET tests were performed on Ceriodaphnia dubia (invertebrate) and Pimephale promelas (vertebrate).  The results of the wet tests are as follows:

February 2018
		Ceriodaphnia dubia		IC25 		>100%
		Pimephales promelas		IC25		>100%

		May 2018
		Ceriodaphnia dubia		IC25 		>100%
		Pimephales promelas		IC25		>100%

		August 2018
		Ceriodaphnia dubia		IC25      	 5.18%
		Pimephales promelas		IC25		>100%

	The above submitted bioassays demonstrated a reasonable potential to violate Whole Effluent Toxicity during a period of four and half years prior to the submittal of the application. The facility will be required to perform 7-day chronic, static renewal, definitive (a control and five effluent concentrations) WET tests in accordance with Short-Term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Waters to Freshwater Organisms. 

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 5.  The reported values are then analyzed and compared to water quality criteria listed in Attachment 6 to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment 7.  A column by column description of the calculations in Attachment 7 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

Municipal Toxic facilities 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.  Chemical specific analysis shall be for total recoverable metals.  Chemical specific limits shall be placed in a permittee’s permit if any of the parameters evaluated in the toxic screening procedure indicated that reasonable potential for violations of the appropriate criteria (acute, chronic, and human health). See results in Attachment 6.

Copper
The acute and chronic criteria for Copper were adjusted for a hardness of 247 mg/l in accordance to EPA’s National Recommended Water Quality Criteria document. DMR data demonstrated a violation of the recalculated limitations; therefore, the following limitations shall be placed in the draft permit:
 
Monthly Average					Maximum Weekly Average
			        0.0194 mg/l	       0.0315 mg/l

Lead
The acute and chronic criteria for Lead were adjusted for a hardness of 247 mg/l in accordance to EPA’s National Recommended Water Quality Criteria document. DMR data demonstrated a violation of the recalculated limitations; therefore, the following limitations shall be placed in the draft permit:
 
Monthly Average					Maximum Weekly Average
			        0.0066 mg/l	       0.1702 mg/l

Cadmium
The acute and chronic criteria for Cadmium were adjusted for a hardness of 247 mg/l in accordance to EPA’s National Recommended Water Quality Criteria document.  The application submitted demonstrated a violation of acute and chronic criteria for Cadmium; therefore, the following limitations shall be placed in the draft permit:
 
 Monthly Average					Maximum Weekly Average
			        0.00046 mg/l	       0.00485 mg/l

Mercury
Mercury exceeded the chronic water quality criteria based on the data received through the permit application; therefore, the following limitations shall be placed in the draft permit:
 
 Monthly Average					Maximum Weekly Average
			     0.000012 mg/l                                   0.0021 mg/l   

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: 

		Tetrachloroethylene - Average Daily Concentration: 0.69 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:

		Tetrachloroethylene - Human Health Criteria: 29 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.

	Tetrachloroethylene - [0.69 µg/L*10] = 6.9 µg/L < 29 µg/L

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

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 – Phosphorus Sampling Data
Attachment 5 – Chemical Specific Tabulated Results
Attachment 6 – MS Water Quality Criteria
Attachment 7 – Toxicity Screening

Attachment 1 – Flow Diagram

Attachment 2 – Topo Map

Outfall 001

Plant Location

Attachment 3 – Wasteload Allocation

Attachment 4 – Phosphorus Sampling Data

Attachment 5 – Chemical Specific Tabulated Results

Attachment 6 – MS Water Quality Criteria

Attachment 7 – Toxicity Screening