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Control and Monitoring of Control and Monitoring of Hydrogen Sulfide in Digester Hydrogen Sulfide in Digester
Gas Gas
Steve Walker
Craig Barnes
Metro Wastewater Reclamation District
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Metro District’s Central Treatment Plant
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What to Use to Control HWhat to Use to Control H22SS
• Equipment • Iron vs. Aluminum Salts
• Dewatering Issues
• 503 Regulations – Aluminum under scrutiny in Round 2
• Iron beneficial for crops and other plants
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District’s Reasons to Reduce District’s Reasons to Reduce the Amountthe Amount
• Potential offsite buyer for methane - gas contaminated with 3000 + ppm H2S
• Tried ferrous chloride from supplier’s recommendation
• Tried ferric chloride as an alternative
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Reasons for AlternativesReasons for Alternatives
• Cost Comparison
• Flexibility
• Impacts on Processes
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ResultsResults
• Cost – active pounds/digester loading – Same
• Reaction time slower with ferrous
• Flexibility proven – could use either product
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Addition Points ImpactsAddition Points Impacts
• Primary Influent• Ferrous• Ferric
• DAF Conditioning Box• Ferrous• Ferric
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Use of Ferric After TestingUse of Ferric After Testing
• H2S impacts on cogeneration equipment• Equipment reliability
• Destruction of yellow metal parts
• Acidification of engine oil
• Struvite reduction
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12 cyl. Turbo 2000 hp w/1200 kW gen - 1 of 4
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Engines required extensive maintenance
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Trigen’s Turbine control panel with PLC
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Trigen’s pride and joy - Centaur 40 Turbine
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Current Usage and WhyCurrent Usage and Why
• Goal – Title V Air Permit Compliance• Limit of 169 tons per year SO2 to atmosphere
• H2S ceiling of 2000 ppm with running 3 hour average of 1800 ppm
• Ferric onsite – used for struvite control in dewatering process
• Handling one chemical vs. two - KISS
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Current Dosing PointsCurrent Dosing Points
• Digester Feed Line
• DAF Conditioning Box
• NSEC Influent Channel
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Ni/Deni Aeration Basin - 1 of 12
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Current ResultsCurrent Results
• Gas production remains at 4MSCFD
• Holding H2S levels at 900 ppm +/- 100 with consistent feed rate
• Minimal Process Impacts• Foaming• Struvite
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NOTENOTE
• High flow in spring 2001 reduced sulfide generators in the collection system by flushing the piping. This made results seem better. Dosages are back to traditional levels now.
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Dosage RateDosage Rate
• Roughly 0.3 gpm ferric solution to 650 gpm digester feed or……
• Roughly 2000 pounds/day to 340,000 pounds TS = 12 active lb/ton
• Digesters are fed sequentially so all get equivalent dose
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CalculationCalculation
• 0.3 gal/min x 8.34 lb/gal x 1.4 (specific gravity) x % iron in solution (40%) x 1440 min/day = 2000 lb/day of iron
• Lb/day x $0.10/lb x 365 days/yr = $73,000
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Note the floating cover on Digester 6
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MonitoringMonitoring
• Effectiveness checked with GC Mass Spec – twice per week from grab sample
• This would not meet the requirements of the current air permit
• APCD permit required installation of Continuous Monitoring System (CMS)
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Digester FlaresDigester Flares
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Turbine ExhaustTurbine Exhaust
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Continuous Monitoring Continuous Monitoring System (CMS) RequirementsSystem (CMS) Requirements
• Continuous gas stream H2S monitoring
• Continuous data transmission and recording
• Instrument reliability
• Relative Accuracy Test Audit (RATA) compliant
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LasIR UnitLasIR Unit
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Control UnitControl Unit
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Control Unit DisplayControl Unit Display
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Modifications Needed After Modifications Needed After Initial InstallationInitial Installation
• Inability to meet accepted calibration protocol
• Extractive method required
• Water vapor in digester gas impacted reliability
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White Cell White Cell
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LasIR ReconfigurationLasIR Reconfiguration
• Sensor upgrade - aka “White Cell”
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Gas Out(1/2” or ¼” Teflon tubing)
DiaphragmPump
Pressure/FlowController
To LasIR Analyzer
From LasIR AnalyzerFiber-optic cable
Coaxial cable
Infrared detector
Stack Gas IN(¼” or 1/8” Teflon tubing)
Cal. Gas IN
Three-way valve
Manual Valve
~ 15”
12-m multiple reflection (White) cell
Alak Chanda – 04January 2001
Layout of the Multi-pass Extractive Monitor Head
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LasIR UpgradesLasIR Upgrades
• Laser modified for extractive analysis
• Nitrogen gas used for purge and zeroing
• H2S calibration gas incorporated
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Cal and Purge GasCal and Purge Gas
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ResultsResults
• Instrument has proven reliable
• Permit compliance achieved
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Future UseFuture Use
• Instrument will control ferric dosing
• ANY QUESTIONS?