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Briefing MaterialsBriefing Materials

Flow and Nitrogen IssuesFlow and Nitrogen Issues

By:

D.C. Water and Sewer Authority

February 28, 2008

Blue Plains Regional CommitteeBlue Plains Regional Committee

Presented to:Presented to:

District of Columbia

Water and Sewer Authority

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Agenda

Flow Allocation

TN Allocation Need for Safety Factor Poundage Allocations Impact of Wet Weather

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DC flow vs Rain

100

110

120

130

140

150

160

170

180

190

20 30 40 50 60 70

Rolling 12-month Rainfall (in/yr)

DC

Flo

w t

o B

lue

Pla

ins

(mg

d)

2000-2004

2005

2006

2007

DC IMA Allocation

148

Flow Allocation: DC’s Flow has Dropped Significantly Since 2004

Old avg. for 40” rain = 162± mgd

Recent avg. for 40” rain = 145± mgd

Note: Flows include captured combined sewage

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Flow Allocation: Why Have Base DC Flows Changed? 5 tide gates fixed

(completed Nov. 2003) – reduces river inflow

12 inflatable dams replaced (completed Mar. 2004) – reduces river inflow

Major pumping station rehabilitations in progress - reduces captured combined sewage

Supported by empirical evidence Disappearance of fish on screens Color of sewage to primaries (less

river color)

Inflatable Dam under Construction

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Flow Allocation: Proposed Evaluations for COG Projections Going Forward Consider new base year flow for D.C.

Year 2005 current value is 160.45 mgd Consider revising to account for effect of tide gates and

inflatable dams

Separate out captured combined sewage in unadjusted flow projections Show year by year changes as CSO controls come on line

Evaluate flow factors for households, visitors and employment given nature of new development

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LOT Nitrogen Removal

Phosphorus Temperature TN Components

Organic Nitrogen Ammonia NOx

Flow

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Phosphorus Phosphorus must also be removed to LOT Phosphorus removed upstream Nitrogen removing bacteria require

phosphorus – low level effect P deficiency may cause settling problems Phosphorus may be added to process Added phosphorus must be precipitated

on filters

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Temperature Reaction rates are temperature

dependent Rate cut in half with 10 degree C drop Design is based upon 12o C Lower temperatures can occur Low temperatures hinder settling Some plants received performance

waivers at low temperatures

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TN Components

Organic Nitrogen 1.0 mg/L

Ammonia 1.0 mg/L

NO2 + NO3 1.0 mg/L

Total Nitrogen 3.0 mg/L

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Organic Nitrogen

Organic Nitrogen - 1.0 mg/L Soluble and insoluble 10 % of VS in effluent is organic N – 0.3 mg/L Soluble organic (RDON) may increase because

of recycles from solids handling RDON can not be removed by conventional

treatment

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Ammonia Achieve 1.0 mg/L year round? More

than double the SRT or MLSS required in winter

Required High SRT may cause settling problems

Historical problems when ammonia limit was 1.0 mg/L

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Nitrate + Nitrite

Same temperature problem as ammonia

Phosphorus may be required to achieve levels below 1.0 mg/L – no tertiary clarifiers available

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Virginia

ITEM D.C. MD Fairfax Other VA Total GRAND TOTAL

BP Flow Allocations (mgd)

D.C. 148.0

WSSC 169.6

Fairfax CO 31.0 31.0

P.I. Flows

▪ LCSA 13.8 13.8

▪ Town of Vienna 1.5 1.5

▪ Dulles Airport 1.5 1.5

▪ Navy 0.07

▪ NPS 0.03

▪ Future PI Users 4.5 4.5

Total (mgd) 148 169.7 31.0 21.3 52.3 370

TN allocation per NPDES Permit Fact Sht. 2,115,000

1,993,000

344,379 236,621 581,000 4,689,000

Effluent TN required @ flow allocation (mg/L)

4.69 3.86 3.65 3.65 3.65 4.16

TN Allocation: Process to Date has Reduced Allowable TN and Increased Risk of Noncompliance

77,000 ± lbs offloaded by MD/WSSC reduces concentration from 4.0 mg/l to 3.86 mg/L

4.5 mgd to future PI users not considered in VA allocation. Reduces TN allocation for VA by 55,000± lbs and reduces concentration from 4.0 mg/l to 3.65 mg/L

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TN Allocation: Impact of Wet Weather Two Major Issues

In wet years, service area flows can increase significantly (exclusive of CSO)

Service area flows in wet years can exceed average rainfall year flows by about 65 mgd (2003)

Requires average TN at 002 of 3.4 mg/L or less regardless of how 001 is permitted

Permitting of Outfall 001 Outfall 001 volume can change dramatically depending on rainfall EPA seems to want a bubble permit: TN from 001 + TN from 002 must meet

permit allocation of 4,689,000 lbs/yr WASA proposed evaluating 001 as a CSO (based on average year

conditions/ post-construction monitoring). Fixed permit limit applies only to outfall 002 (4,689,000 – 180,000 lbs/avg yr = 4,509,000 lb/yr 002)

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TN Allocation: Impact of Wet WeatherNo

. Item Avg YearVery Wet Year

(2003) Notes

1 Rainfall (in) 40.97 59.3

2 Base 002 Discharge in avg year (mgd) 370 370

3 Est. 002 increment for wet weather (mgd) 0 65 From experience in 2003

4 Total 002 Flow (mgd) 370 435 ( 2 ) + ( 3 )

5 001 Discharge (mgd) 7.3 17 From model

6 Bubble Permit (EPA Approach) 001 + 002 must meet permit

7 TN Permit Limit (lbs/yr) 4,689,000 4,689,000 Per permit

8 001 TN concentration (mg/L) 8.1 8.1 Est. ECF performance

9 001 TN discharge (lbs/yr) 179,491 419,173 ( 5 ) x ( 8 ) x conversions factors

10 TN left for 002 (lbs/yr) 4,509,509 4,269,827 ( 7 ) – ( 9 )

11 Effluent TN required at 002 (mg/L) 4.0 3.2 ( 10 )/( 4 ) x conversions factors

12 001 as CSO (per Final TN/WW Plan)

13 TN Permit Limit = TN for 002 (lbs/yr) 4,509,000 4,509,000 4,689,000 – 180,000 lb /avg year allocation for outfall 001

14 Effluent TN required at 002 (mg/L) 4.0 3.4 ( 13 )/( 4 ) x conversions factors

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Comments on Impacts of Wet Weather 001

Flow and volume highly variable – depends on weather Capping TN regardless of climate condition means

outfall 002 must achieve higher TN removal EPA bubble permit approach subjects BP Users to much

greater risk Precedence: numerical limit on CSO outfall

002 Service area flows in wet years can exceed average

flows by about 65 mgd (2003 experience) Requires average TN at 002 of 3.4 mg/L or less

regardless of how 001 is permitted