Control and Monitoring of Hydrogen Sulfide in Digester Gas

1. Control and Monitoring of Hydrogen Sulfide in Digester Gas Steve Walker Craig Barnes Metro Wastewater Reclamation District

2. Metro District’s Central Treatment Plant

3. What to Use to Control H2S • Equipment • Iron vs. Aluminum Salts • Dewatering Issues • 503 Regulations – Aluminum under scrutiny in Round 2 • Iron beneficial for crops and other plants

4. District’s Reasons to Reduce the 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

5. Reasons for Alternatives • Cost Comparison • Flexibility • Impacts on Processes

6. Results • Cost – active pounds/digester loading – Same • Reaction time slower with ferrous • Flexibility proven – could use either product

7. Addition Points Impacts • Primary Influent • Ferrous • Ferric • DAF Conditioning Box • Ferrous • Ferric

8. Use of Ferric After Testing • H2S impacts on cogeneration equipment • Equipment reliability • Destruction of yellow metal parts • Acidification of engine oil • Struvite reduction

9. 12 cyl. Turbo 2000 hp w/1200 kW gen - 1 of 4

10. Engines required extensive maintenance

11. Trigen’s Turbine control panel with PLC

12. Trigen’s pride and joy - Centaur 40 Turbine

13. Current 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

14. Current Dosing Points • Digester Feed Line • DAF Conditioning Box • NSEC Influent Channel

15. Ni/Deni Aeration Basin - 1 of 12

16. Current Results • Gas production remains at 4MSCFD • Holding H2S levels at 900 ppm +/- 100 with consistent feed rate • Minimal Process Impacts • Foaming • Struvite

17. NOTE • 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.

18. Dosage 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

19. Calculation • 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

20. Note the floating cover on Digester 6

21. Monitoring • 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)

22. Digester Flares

23. Turbine Exhaust

24. Continuous Monitoring System (CMS) Requirements • Continuous gas stream H2S monitoring • Continuous data transmission and recording • Instrument reliability • Relative Accuracy Test Audit (RATA) compliant

25. LasIR Unit

26. Control Unit

27. Control Unit Display

28. Modifications Needed After Initial Installation • Inability to meet accepted calibration protocol • Extractive method required • Water vapor in digester gas impacted reliability

29. White Cell

30. LasIR Reconfiguration • Sensor upgrade - aka “White Cell”

31. Gas Out (1/2” or ¼” Teflon tubing) Fiber-optic cable From LasIR Analyzer Diaphragm Pump Infrared detector Pressure/Flow Controller Manual Valve 12-m multiple reflection (White) cell Cal. Gas IN ~ 15” Coaxial cable To LasIR Analyzer Three-way valve Stack Gas IN (¼” or 1/8” Teflon tubing) Layout of the Multi-pass Extractive Monitor Head Alak Chanda – 04January 2001

32. LasIR Upgrades • Laser modified for extractive analysis • Nitrogen gas used for purge and zeroing • H2S calibration gas incorporated

33. Cal and Purge Gas

34. Results • Instrument has proven reliable • Permit compliance achieved

35. Future Use • Instrument will control ferric dosing • ANY QUESTIONS?