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

Metro District’s Central Treatment Plant

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

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

Reasons for AlternativesReasons for Alternatives

• Cost Comparison

• Flexibility

• Impacts on Processes

ResultsResults

• Cost – active pounds/digester loading – Same

• Reaction time slower with ferrous

• Flexibility proven – could use either product

Addition Points ImpactsAddition Points Impacts

• Primary Influent• Ferrous• Ferric

• DAF Conditioning Box• Ferrous• Ferric

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

12 cyl. Turbo 2000 hp w/1200 kW gen - 1 of 4

Engines required extensive maintenance

Trigen’s Turbine control panel with PLC

Trigen’s pride and joy - Centaur 40 Turbine

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

Current Dosing PointsCurrent Dosing Points

• Digester Feed Line

• DAF Conditioning Box

• NSEC Influent Channel

Ni/Deni Aeration Basin - 1 of 12

Current ResultsCurrent Results

• Gas production remains at 4MSCFD

• Holding H2S levels at 900 ppm +/- 100 with consistent feed rate

• Minimal Process Impacts• Foaming• Struvite

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.

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

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

Note the floating cover on Digester 6

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)

Digester FlaresDigester Flares

Turbine ExhaustTurbine Exhaust

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

LasIR UnitLasIR Unit

Control UnitControl Unit

Control Unit DisplayControl Unit Display

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

White Cell White Cell

LasIR ReconfigurationLasIR Reconfiguration

• Sensor upgrade - aka “White Cell”

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

LasIR UpgradesLasIR Upgrades

• Laser modified for extractive analysis

• Nitrogen gas used for purge and zeroing

• H2S calibration gas incorporated

Cal and Purge GasCal and Purge Gas

ResultsResults

• Instrument has proven reliable

• Permit compliance achieved

Future UseFuture Use

• Instrument will control ferric dosing

• ANY QUESTIONS?