CSO and SEWERAGE CONVEYANCE TUNNEL DESIGN NACE CORROSION 2010

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1 UNDERSTANDING CORROSION PROTECTION/PREVENTION NEEDS IN CSO and SEWERAGE CONVEYANCE TUNNEL DESIGN NACE CORROSION 2010 By Randy Nixon Senior Consultant Corrosion Probe, Inc. Centerbrook, CT

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3 OVERVIEW Types of Wastewater Tunnels Corrosion Mechanisms Predicting Biogenic Sulfide Corrosion Rates Corrosion Protection Options Factors Affecting Material Selection Summarize with Recommendations

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5 MAJOR TYPES OF WASTEWATER TUNNELS Interceptor/Conveyance Conveyance Combined Sewer Overflow (CSO) Storage Storage to Conveyance For Future Capacity (CMOM & CSO/SSO Regulations)

6 MAIN FACTORS RESPONSIBLE for GREATER H 2 S LEVELS Pretreatment of Metals Clean Water Act Building Large Regional Systems: Longer transport times Longer detention times More force mains 360 O slime layer Odor control & covered structures

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8 Corroded tunnel crown

9 Typical tunnel concrete deterioration

10 Corroded hole in concrete tunnel wall

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12 Dynamic Chemical Equilibrium Turbulence - H 2 S stripped out of solution Dissolved H 2 S replaced by HS - converted to aqueous by H 2 S HS - replaced by S = converted to HS - H 2 S gas - condensation on concrete

13 Predicting and Estimating Biogenic Sulfide Corrosion Rates of Concrete Empirical data in the same system is the best approach correlation of H 2 S levels, surface ph, and concrete cement paste depth loss measurements (prediction is possible).

14 Modeling EPA 832 R Joint Publication by EPA, ASCE, and ACPA (only estimation is possible). Model calculates C avg and C max at turbulent areas. Turbulence strips H 2 S gas from solution faster - which increases the rate of H 2 S O 4 production by Thiobacillus bacteria.

15 Estimating Corrosion Rates C max compared to concrete cover thickness. H 2 S collects on walls and is consumed in distinct areas for certain distances away from turbulent zones. Corrosion will occur in the direction of air flow in the headspace if the ventilation exceeds the pull of the air flow with the direction of flow.

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17 USING EPA MODEL CASE 1 Conveyance Tunnel: C max compared to concrete with a cover thickness of 2 C max =.019 in./yr 105 years for corrosion the reach rebar

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19 USING EPA MODEL CASE 2 Conveyance Tunnel: C max compared to concrete with a cover thickness of 2 ½ C max =.052 in./yr 48 years for corrosion the reach rebar

20 Actual corrosion rate of approximately lost a maximum of ½ in 30 years

21 Researchers have developed recommended distances for lining protection upstream and downstream of turbulent drop shafts or connections.

22 Transition from concrete portion of tunnel to T Lock lined section T Lock lined drop shaft area

23 The illustrated conveyance tunnel was operated at constant negative pressure. Shafts were designed to minimize turbulence and PVC anchored lining installed 200 feet downstream and 75 feet upstream.

24 PROPENSITY FOR TUNNEL CORROSION Interceptors & conveyance tunnels generally have corrosion problems due to sulfide generation & turbulence, etc. (design dependent) CSO tunnels can be corrosive due to biosolids build-up with subsequent stripping due to turbulent flow or not so corrosive. SSO tunnels are corrosive. Storage to conveyance tunnels become corrosive over time at least in turbulent zones. Outfall tunnels from plant.

25 CORROSION PROTECTION OPTIONS Liquid applied polymeric linings mostly epoxy, polyurethane, polyurea formulations. Anchored thermoplastic linings Anchored PVC sheet linings Acid resistant cementitious shotcrete linings Deformed pipe linings Slip-linings Segmental thermoplastic sheet linings Spiral wound pipe Cured-in-place pipe

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28 This system can also be anchored into new concrete

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32 This system is fusion welded in pit as inserted

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36 FACTORS AFFECTING MATERIAL SELECTION DIAMETER limits choices 10-0 and larger can use liquid applied polymeric, anchored thermoplastic, adhered PVC sheet, segmental thermoplastic, shotcrete, or spiral wound PVC linings 6-0 to can use all of the above plus slip linings (HDPE, FRP mortar pipe, etc.) 4-0 to 6-0 slip-line, CIPP, anchored or adhered sheet linings, deformed pipe linings, and liquid applied. Can reduction in tunnel diameter/capacity be tolerated? If so, how much?

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38 CORROSION SEVERITY Length of transit/detention time upstream sulfide production Extent of turbulence turbulent zones Drop shaft design turbulence reduction Flow conditions velocity Temperature of wastewater Extent of ventilation number of air changes, headspace under negative pressure Biosolids build-up in invert or low areas What is the projected concrete corrosion rate expected to be?

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40 Corroded concrete (note exposed reinforcing bar) at a Siphon Box

41 LINING MATERIAL CORROSION RESISTANCE Resistance to H 2 SO 4 Resistance to CH 4 and CO 2 Resistance to permeation

42 ACCESS IN FUTURE Can tunnel be viably and economically dewatered in the future? (depends on dimension capability, leak-by of gates, and number and type of connections) Can humidity, condensing conditions, and temperature be effectively and economically controlled in the future? How frequent are access manholes, shafts, or structures apart? Distances to work safely and efficiently costs! Does the utility have redundancy or will they not meet permit requirements when the tunnel is shut down? Can lining maintenance, repair, or relining work be effectively performed in the future?

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44 INSPECTION CAPABILITY Can the tunnel be safely and effectively inspected while on line? Or, must flow be lowered or stopped dewatered? Safety risks or hazards must be mitigated or controlled. Is negative pressure maintained can additional airflow be provided? Some corrosion protection systems require less inspection because they perform longer.

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46 DEGREE OF GROUND WATER LEAKAGE IN TUNNEL How extensive is ground water leakage in the tunnel? Is leakage at joints or in concrete what is the leak path? Can leakage be stopped via pressure grouting or controlled by relief diversion while lining work is performed? Does the tunnel have segmental joints, grout ports, etc.?

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48 FUTURE CHANGE IN TUNNEL FUNCTION Will the new CSO tunnel become a conveyance tunnel in the future? Will connections be added to the tunnel to handle future urban sprawl and wastewater capacity needs? Will ventilation systems for new tunnel be upgraded when new turbulent zones are added in the future? Will corrosivity increase in the future?

49 TYPE OF TUNNEL CONSTRUCTION Bored and concreted tunnels generally run full. TBM segmental concrete tunnels have miles of gasketed joints, thousands of grout ports, and bolt pockets. Two pass slip-lined tunnels are very good for handling leakage and corrosion protection. Precast, reinforced pipe is limited by diameter therefore affecting lining options.

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51 COST Initial cost of lining option (is it affordable?) Net present value cost of lining option over tunnel design life Does the tunnel need to be completely lined or only in turbulent affected zones plus ventilation? Can the tunnel be lined or slip-lined in the future back to the access issue?

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53 PROJECT SCHEDULE Can lining work be performed within project schedule? Consent decree deadlines driven by state DEPs and US EPA.

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55 LINING OPTION TRACK RECORD OF SUCESSFUL PERFORMANCE How long has lining performed without degradation or need for maintenance? What is the projected service life without maintenance?

56 OTHER FACTORS Resistance to hydraulic scour Abrasion erosion resistance Degree of difficulty to repair in the future Requirement for tunnel substrate surface preparation Sensitivity of lining to tunnel conditions Quality of lining warrantee

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58 OTHER FACTORS Degree of difficulty to tie lining into other materials such as stainless steel, PVC, anchored linings, FRP, pipe, etc. Ability of lining to negotiate or handle tunnel bends Requirements for insertion pits or trenches, etc. Was the tunnel constructed from high strength concrete with silica fume addition?

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64 Summary and Recommendations Many factors affect appropriate material selection for wastewater tunnel linings. Better knowledge of those factors averts lining problems during construction and premature lining failures.