Sanitary Sewer Infiltration & Inflow (I/I) Study City of Beatrice, Nebraska IAWEA Collection System Specialty Conference Presented by: Brian Mulinix, P.E. March 10, 2011
Discussion Topics Definition of Infiltration & Inflow Criteria for Evaluating Background and Current System Review & Discussion of Results What Has Been Done Planning Your System Study
Infiltration Definition Enters sewer system from ground Defective pipes, joints and connections, manhole walls Expect year-round, but influenced by groundwater fluctuations Broken Pipe Tree Root Bad Joint
Inflow Definition Enters sewer system as direct connection Sump pumps, roof leaders, foundation/surface drains Roof Leader Not linked to groundwater levels Misalignment Creek Inlet
Infiltration/Inflow Illustration
Evaluation Criteria
Infiltration Evaluation Criteria Population Based 1991 EPA criteria Non-excessive 130 gal/capita/day Sum of domestic base-flow & infiltration over 7-14 day average at high groundwater levels
Infiltration Evaluation Criteria (cont.) Sewer Based Developed from Federal grant programs Leakage quantified as gal/day/inch-diameter- mile Non-excessive < 1,500 gpd/idm, but as high 10,000 gpd/idm Standardized units for ranking or prioritizing subsystems for further study
Inflow Evaluation Criteria Population Based 1991 EPA criteria Non-excessive 275 gal/capita/day total daily average storm flow Regardless, considered excessive if sewer collection system or treatment plant experiences hydraulic overload
Inflow Evaluation Criteria (cont.) Rainfall / Inflow Volume Relationship Massachusetts Department of Environmental Protection Total inflow volume correlated to total rainfall produced from the storm No threshold; expressed as gal/lineal foot sewer Inflow Severity, used to prioritize subsystems for evaluation
Background of System
Background City of Beatrice 12,600 population Sewers date back to 1888 570,000 lineal feet sewer 570,000 lineal feet sewer Seven lift stations, six inverted siphons I/I Problems exhibited and studied early- 1970 s Extreme WWTP Hydraulic Overloading
Drainage Basin Map D E C B A
Lid of Manhole D8 lifted off from rim due to excessive flow during a July 17, 2008 storm. Evidence of significant flow discharged from manhole. Located on a gravity sewer downstream of L.S. #1. Excessive Storm Events
Direct Connections Discovered Water flowing from river Flow entering Manhole D11 directly from river. Overflow pipe effluent flap valve was closed and completely submerged in river.
Equipment Evaluated Lease versus Purchase Consider length of study Regular maintenance Flow Monitor ISCO 2150 Area Velocity Meter Rain Logging ISCO 674 Rain Gauge
Installation Considerations
Discussion of Results
Basin D Infiltration Rate: 166 gpm Criteria: 234 gpcd 941 gpd/idm
Sub-Basin D 20 Infiltration Rate: 68 gpm Criteria: 345 gpcd 3,661 gpd/idm
Basin E Infiltration Rate: 99 gpm Criteria: 202 gpcd 1,904 gpd/idm
Basin D Inflow 1-yr Storm: 0.33 Mgal Aver. Storm: 358 gpcd Severity: 2.1 gal/lf
Sub-Basin D 20 Inflow 1-yr Storm: 0.20 Mgal Aver. Storm: 632 gpcd Severity: 14.0 gal/lf
Basin E Inflow 1-yr Storm: 0.33 Mgal Aver. Storm: 283 gpcd Severity: 4.4 gal/lf
Results Summary Basin C E D D 20 Area (acres) 815 540 2,420 Population 4,687 1,067 2,152 376 Length (lf) 89,700 52,800 157,000 14,300 IDM 143 83 282 30 Infiltration (120 gpcd, 1,500-3,000 gpd/idm) gpcd 112 202 234 345 gpd/idm 1,741 1,904 941 3,661 Inflow (275 gpcd) gpcd 138 283 358 632 gal/lf 1.7 4.4 2.1 14
Basin E1-E5 Evaluations Basin I.D. E1 E2 E3 E4 E5 Population 14 163 340 253 346 Length (lf) 2,580 6,200 12,900 17,700 12,800 IDM 5.2 10.9 19.2 27 19 Infiltration (120 gpcd, 1,500-3,000 gpd/idm) gpcd 514 521 70 94 46 E3 gpd/idm 2,478 3,808 270 431 270 Inflow (275 gpcd) gpcd 1,234 755 95 85 87 gal/lf 5.5 3.3 0.8 0.2 0.7 E4 E2 E5 E1
Basin C1-C5 Evaluations Basin C1 C2 C3 C4 C5 Population 888 800 460 1,397 1,142 Length (lf) 20,131 17,412 8,755 25,099 18,332 IDM 36 28 13 38 28 Infiltration (120 gpcd, 1,500-3,000 gpd/idm) gpcd 330 105 106 36 41 gpd/idm 4,287 1,231 2,198 436 467 Inflow (275 gpcd) gpcd 478 79 154 25 47 gal/lf 3.9 1.6 3.3 0.5 0.5 C3 C1 C5 C4 C2
Continued Basin C Evaluations C1a C1c C2a C2b Population 628 3,259 1,397 1,142 Length (lf) 10,828 61,491 10,152 7,260 IDM 22 93 16 12 C1c Infiltration (120 gpcd, 1,500-3,000 gpd/idm) gpcd 280 50 379 46 C1a gpd/idm 6,048 648 2,798 134 Inflow (275 gpcd) gpcd 267 62 368 71 gal/lf 2.0 1.0 4.5 2.7 C2b C2a
What About a 2-inch Rain Event? Create 800,000 gallons $600-800 extra cost Backups: $15,000? NPDES permit violation? SRF requires I/I Minimization Hydraulic sizing considerations for future expansions/replacements
Impacts of Infiltration/Inflow Lift Stations Additional Sewer Maintenance Influent Headworks Equipment Wear/Tear RBC Building Flooding Potential for Sewer Backups NPDES permit
Realized Benefits Eliminated tremendous WWTP flows Excluded large percentage of City for detailed investigation Avoided unneeded capital expansions/replacements
Follow-Up Activities Re-implement flap gate maintenance Selected Basins Smoke testing Camera inspection Basin B Monitoring Anticipate selective repairs at manholes or short sewer segments Budget Sensitive
Planning Your I/I Study Determine your system needs: Objectives Personnel resources, schedule Available budget Available budget Divide into 10,000 LF sub-basins Identify known issues Sanitary sewer map Be patient!
Questions Brian Mulinix, P.E. (402) 475-8588 www.wla-consulting.com brian.mulinix@wla-consulting.com
250 200 150 100 50 0 gallon/capita/day Basin C Basin D Basin E 2000 1800 1600 1400 1200 1000 800 600 400 200 0 gpd/idm Basin C Basin D Basin E
Rehabilitation Solutions Disconnect inflow sources Sewer line Sliplining Cured-in-place-pipe Spirally wound pipe Segmental lining On-line replacement Spot Repair Manhole Replacement Chemical grouting Structural lining Coating systems Lateral Removal/replacement Lining External grouting
Physical Evaluations Smoke testing Dyed water testing Closed circuit television inspection Sewer scanner and evaluation technology Sonar technology Man-entry inspection Manhole inspection