PERMIT RATIONALE FOR REISSUANCE
Clarksdale POTW
Coahoma County
Clarksdale, Mississippi
Water NPDES No. MS0020311
December 14, 2017

I.	CLASSIFICATION  –  Municipal Major

II.	DESCRIPTION OF WASTEWATER – Domestic/ Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is collected and treated by means of an activated sludge system followed by UV disinfection.  See attachment No. 1 for flow diagram.

IV.	RECEIVING WATERS – Big Sunflower.  See attachment for discharge location map.  The receiving waters at the point of discharge is classified as Fish and Wildlife and are in the Yazoo River Basin. This discharge is listed in the TMDL for Pathogens.  The facility utilizes UV disinfection; therefore the discharge will comply with the conditions of the TMDL. Little Tallahatchie River, in Union County, is on the 303(d) List of Impaired Water Bodies, for biological impairment.  Hence, a future TMDL may revise the above existing effluent limits for BOD5 and NH3-N. A condition has been added to the permit which allows it to be reopened for revision based on a future TMDL. Big Sunflower River. The State of Mississippi’s 303(d) list of impaired waterbodies denotes the Big Sunflower River drainage area, Waterbody ID Number: MSBIGSUNRE, located in Coahoma, Sunflower, and Sharkey counties (from headwaters at confluence with Whittaker Bayou to the Yazoo River), to be evaluated for aquatic life support due to pesticides, nutrients, and organic enrichment-low DO. The List also denotes the Big Sunflower River drainage area, Waterbody ID Number: MSBIGSUNRM, located at Clarksdale in Coahoma County (from Lyons POTW to Hopson Bridge), to have a Completed TMDL for Aquatic Life Support due to pesticides, nutrients, sediment/siltation, and for organic enrichment-low DO. It also has a completed TMDL for secondary contact recreation due to pathogens. The Big Sunflower River is classified as fish and wildlife.  The discharge point is located within the Yazoo River Basin.

	At the point of discharge the 7Q10 for the receiving streams is 30 cfs.  The instream wastewater concentration at the point of discharge can be determined using the following calculations: 

IWC = {(Qw)/(Qw + Qr)}*100

Where, Qw = Design flow of the wastewater treatment facility, 5.0 MGD or 7.75 cfs
	Qr = Receiving stream 7Q10, 30 cfs.

IWC = {(7.75 cfs)/(7.75 cfs + 30 cfs)}*100 = 20.51 %

V.	APPLICABLE GUIDELINES – Mississippi Wastewater Regulation (11 Mississippi Admin code part 6), State of Mississippi Water Quality Criteria & “Wastewater Regulations for National Pollutant Discharge Elimination System Permits, Underground Injection Control Permits, State Permits, Water Quality Based Effluent Limitations , Water Quality Criteria (MSWQC), Water Quality Certification” applicable. Wasteload Allocation (WLA), Total Maximum Daily Load (TMDL)

VI.	SUMMARY OF LIMITATIONS – 

Parameter			Value				Basis
Flow				5.0 MGD			Design
BOD5				25 mg/l (May - Oct)
				30 mg/l (Nov - Apr)		WLA
BOD5 (Percent Removal)	85% Minimum		Technology
TSS				30 mg/l (Monthly Avg.)	Technology
TSS (Percent Removal)	85 % Minimum		Technology
NH3 – N 			5 mg/l (Monthly Avg.)	WLA
Dissolved Oxygen		6.0 mg/l  Minimum		WLA
E. Coli				126 mg/l 			MSWQC
Total Phosphorus		242.0 lbs/day			TMDL
Total Nitrogen			567.5 lbs/day			TMDL
Copper				0.097/0.170 mg/l		2005 study*
Mercury			0.01/0.00006 mg/l		Sample Data
pH	 			6.0 – 9.0 S.U.			MSWQC

* The facility performed a water quality study in 2005. The hardness adjustment chemical translator study was submitted with the permit renewal application which reconfirmed that the 2005 data is still protective in  attachement No. 7.

All previously permitted parameters have remained the same in the reissuance.

VII.	TOXICITY EVALUATION

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to State Regulation WPC-1 VI.B.2b., a whole effluent toxicity evaluation was performed as part of the renewal application.  The applicant was to have tested in accordance with Short – Term Methods for estimating the Chronic toxicity of Effluents and Receiving Water to Freshwater organisms.  Short – term chronic WET test were performed on Ceriodaphnia dubia (invertebrate) and Pimephale promelas (vertebrate).  The results of the wet tests are as follows:

July 2014
Ceriodaphnia dubia					IC25	100%
Pimephales promelas				IC25	100%

August 2014
Ceriodaphnia dubia					IC25	100%
Pimephales promelas				IC25	100%

October 2014
Ceriodaphnia dubia					IC25	100%
Pimephales promelas				IC25	100%

March 2015
Ceriodaphnia dubia					IC25	100%
Pimephales promelas				IC25	100%

All of the inhibition concentrations were greater that the predicted IWC of 100%. Based upon the results of the WET testing, chronic bioassay requirements will not be placed within the permit.

CHEMICAL SPECIFIC ANALYSIS

Chemical specific analysis has been performed in accordance with State Regulations WPC-1 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 No. 2.  The reported values are then analyzed and compared to water quality criteria (attachment No. 4) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in attachment No. 3.  A column by column description of the calculations in attachment No. 5 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 II 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).
	
The chemical specific data results submitted did indicate the reasonable potential for violation of the appropriate criteria in Table III of Appendix D of CFR 122 for Total Recoverable Mercury. Chemical specific limits have been place in the permit for Total Recoverable Mercury.

Mercury

Mercury exceeded the acute water quality criteria based on the data received through the permit application; therefore, we have placed the following limitation for Mercury into the draft permit.

Monthly Average					Maximum Weekly
0.000059 mg/l                                  0.01 mg/l

The chemical specific data results submitted did indicate the reasonable potential for violation of the appropriate criteria in Table III of Appendix D of CFR 122 for Total Recoverable Copper. Chemical specific limits have been place in the permit for Total Recoverable Copper based on 2005 Copper adjustment study.

Calculations based upon Hardness for Copper

The applicant submitted six effluent hardness data samples.  The hardness averaged 121.8 mg/L for the Big Sunflower River, and 83.5 mg/l for the effluent. Based on a mass balance, the overall river hardness used for the calculations was 114 mg/l. The background concentration for total recoverable copper was 0.004 mg/l. 

	Monthly Average

	CCC = exp {Mc [ln Hardness] + bc} (Correction Factor)
	
	Mc = 0.8545
	bc = -1.702
	CF = 0.96

	CCC = exp {0.8545[ln 114] + (-1.702)}* (0.96)

	CCC = 0.010 mg/l

	Mass Balance based on the background concentration of copper in the receiving stream.

	{5x + 19.4(0.004}/24 = 0.01 

	x = allowable copper concentration in Clarksdale’s POTW effluent = 0.0333 mg/l

	Monthly Maximum

	CMC = exp {Ma [ln Hardness] + ba} (Correction Factor)

	Ma = 0.9422
	ba = -1.7
	CF = 0.96

	CMC = exp {0.9422[ln 59] + (-1.7)}*(0.96)

	CMC = 0.0152 mg/l

	Mass Balance based on the background concentration of copper in the receiving stream.
	
	{5x + 19.4(0.004}/24 = 0.0152 

	x = allowable copper concentration in Clarksdale’s POTW effluent = 0.058 mg/l

	The Background Total Copper 

	Effluent TSS Data
	The 15th percentile value for TSS is 11 mg/l.

	Kp = Kpo*TSSa

	Kpo = 1.04E+06
	a = -0.7436
	TSS (15%) = 11 mg/l

	Kp = (1.04E+06) (11)-0.7436
	Kp =174800

	C/Ct = 1/ (1+ (Kp*TSS*10E-06))

	C/Ct = 0.342

	New Copper Limits:
	Monthly Average = (0.0333 mg/l)/ (0.342) = 0.097 mg/l
	Monthly Maximum = (0.058 mg/l)/ (0.342) = 0.170 mg/l

Chemical specific analysis for Volatile Organics Compounds, Acid-Extractable Compounds, and Base-Neutral Compounds and Results

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

Attachments

Attachment 1   Wastewater treatment plant flow diagram – See Application
Attachment  2  Discharge location map – See Application
Attachment  3  Wasteload Allocation (WLA)
Attachment  4  Tabulated Metals Results 
Attachment  5  Synopsis of Metals Results 
Attachment  6  MS State Water Quality Criteria
Attachment  7  Chemical Translator Hardness Study

Attachment 3

Attachment 4

Attachment 5

Attachment 6