What’s This HWA Design Thing All About???

Design Capacity HWA Info from one POTWCITY OF GREENSBORO North Buffalo POTW

NORTH BUFFALO POTW Design Flow: 16 MGD

Current Flow = ~10.0 MGD Primary Treatment Secondary Treatment

Trickling Filters Activated Sludge with Nitrification

Tertiary Treatment Phosphorus removal

Chemical Addition/Precipitation Effluent Sand Filters

NB CURRENT MAHL CALCULATIONCBOD [Actual Removal/Actual Flow]

NPDES Permit Limit: 8.0 mg/l (summer)Removal Rate: 98.8% (270 data points/MRE)Actual Flow: 14.2 MGD** 8.0 mg/l = 8.0 = 666 mg/l max. allowable1.0-.988 .012 influent conc.

666 * 8.34 * 14.2 MGD = 78,952 pounds

**DWQ required use of actual flow..flow has dropped since then. These are the approved headworks calculations from December 2003

“DESIGN CRITERIA”NORTH BUFFALO POTW

From Hazen and Sawyer Engineers Original Design (~1980 Upgrade)

BOD = 196 mg/l TSS = 245 mg/l Total Phosphorus = 6.0 mg/l NH3-N = 18.5 mg/l

NB “DESIGN CRITERIA” per DWQMAHL CALCULATION: CBOD

Engineering Design BOD = 196 mg/l Used BOD as CBOD

Engineering Design Flow = 16 MGD 196 * 8.34 * 16.0 = 26,154 pounds MAHL Current Domestic BOD Loading: 27,615 lbs 20 of last 24 monthly average influent

concentrations exceeded 196 mg/l while resulting in average effluent CBOD of <2 mg/l

CAN YOU SPELL OVERALLOCATED?????

NB CURRENT HWA CALCULATIONTSS [Actual Removal/Actual Flow]

NPDES Permit Limit: 30.0 mg/l Removal Rate: 97.8% (271 data points/MRE)Actual Flow: 14.2 MGD

30.0 mg/l = 30.0 = 1363 mg/l max. allow.1.0 -.978 .022 influent conc.

1363 * 8.34 * 14.2 MGD = 161,492 pounds**DWQ required use of actual flow..flow has dropped since then.These are the approved headworks calculations from December 2003

NB “DESIGN CRITERIA”CALCULATIONS: TSS

Engineering Design TSS = 245 mg/l Engineering Design Flow = 16 MGD

245 * 8.34 * 16.0 = 32,693 pounds

[Current Domestic TSS Loading: 22,289] Not overallocated now, but flow is only ~10.0

MGD Domestic alone will use up the rest of the

allocation

IMPLICATIONS Underestimating POTW Conventional

Pollutant Capacities and Capabilities Potential Over-Allocation With Just Domestic

Loading! Loss of Potential New Industrial Users Potential Moratorium

“Domestic concentration” is rising due to newer plumbing fixtures

BOD Limits for All SIUs North Carolina Local Limits Policy

North BuffaloCurrent Loading Situation: DISMAL

Actual Influent CBOD Loading = 19,718 lbs 213 mg/l * 8.34 * 11.1 MGD [24 mo. avg]

DWQ Design Loading = 26,154 lbs 6436 lbs CBOD left…[“domestic” on paper is >MAHL] BOD Limits for Everyone!!!! [75.4% “loaded”]

Actual Influent TSS Loading = 32,401 lbs 350 * 8.34 * 11.1 MGD [24 mo. avg]

DWQ Design Loading = 32,693 lbs Only 292 Pounds of TSS Left…but over 4 MGD of

hydraulic capacity left… TSS Limits for Everyone!!!! [~100% “loaded”]

North Buffalo SUMMARY

CALCULATIONMETHOD

Influent mg/l

MAHL pounds

BOD

Pass-Thru Cal/Actual Flow 666 78,952

O&M “Design” (Proposed DWQ) 196 26,154

TSS

Pass-Thru Cal/Actual Flow 1363 161,493

O&M “Design” (Proposed DWQ) 245 32,693