1. 1 Briefing Materials Flow and Nitrogen Issues By: D.C. Water and Sewer Authority February 28, 2008 Blue Plains Regional Committee Presented to: District of Columbia Water and Sewer Authority

2. 2 Agenda  Flow Allocation  TN Allocation  Need for Safety Factor  Poundage Allocations  Impact of Wet Weather

3. 3 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

4. 4 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

5. 5 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

6. 6 LOT Nitrogen Removal  Phosphorus  Temperature  TN Components  Organic Nitrogen  Ammonia  NOx  Flow

7. 7 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

8. 8 Temperature  Reaction rates are temperature dependent  Rate cut in half with 10 degree C drop  Design is based upon 12 o C  Lower temperatures can occur  Low temperatures hinder settling  Some plants received performance waivers at low temperatures

9. 9 TN Components  Organic Nitrogen1.0 mg/L  Ammonia1.0 mg/L  NO 2 + NO 3 1.0 mg/L  Total Nitrogen3.0 mg/L

10. 10 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

11. 11 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

12. 12 Nitrate + Nitrite  Same temperature problem as ammonia  Phosphorus may be required to achieve levels below 1.0 mg/L – no tertiary clarifiers available

13. 13 Virginia ITEMD.C.MDFairfaxOtherVA TotalGRAND TOTAL BP Flow Allocations (mgd) D.C.148.0 WSSC169.6 Fairfax CO31.0 P.I. Flows ▪LCSA13.8 ▪Town of Vienna1.5 ▪Dulles Airport1.5 ▪Navy0.07 ▪NPS0.03 ▪ Future PI Users4.5 Total (mgd)148169.731.021.352.3370 TN allocation per NPDES Permit Fact Sht.2,115,0001,993,000344,379236,621581,0004,689,000 Effluent TN required @ flow allocation (mg/L)4.693.863.65 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

14. 14 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)

15. 15 TN Allocation: Impact of Wet Weather No.ItemAvg Year Very Wet Year (2003)Notes 1Rainfall (in)40.9759.3 2Base 002 Discharge in avg year (mgd)370 3Est. 002 increment for wet weather (mgd)065From experience in 2003 4Total 002 Flow (mgd)370435( 2 ) + ( 3 ) 5001 Discharge (mgd)7.317From model 6Bubble Permit (EPA Approach) 001 + 002 must meet permit 7TN Permit Limit (lbs/yr) 4,689,000 Per permit 8001 TN concentration (mg/L)8.1 Est. ECF performance 9001 TN discharge (lbs/yr) 179,491 419,173( 5 ) x ( 8 ) x conversions factors 10TN left for 002 (lbs/yr)4,509,509 4,269,827( 7 ) – ( 9 ) 11Effluent TN required at 002 (mg/L) 4.0 3.2( 10 )/( 4 ) x conversions factors 12001 as CSO (per Final TN/WW Plan) 13TN Permit Limit = TN for 002 (lbs/yr) 4,509,000 4,689,000 – 180,000 lb /avg year allocation for outfall 001 14Effluent TN required at 002 (mg/L) 4.0 3.4( 13 )/( 4 ) x conversions factors

16. 16 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