Impact of Water Treatment Plant Residuals on Fixed Film Processes

1. Impact of Water Treatment Plant Residuals on Fixed Film Processes Sidney Biesterfeld1, Mark Dane2, Richard Dingeman2, Dan Freeman2, Paul Heppler2, Kurt Keilbach2, Ernie Oram2, Dr. David Paterniti2, Dan Wadas2, and Mike Lutz1 1Integra Engineering, 2City of Boulder 75th Street WWTP

2. WTP Residuals • Water treatment plants use ferric chloride, ferric sulfate, and alum as coagulants. • Resulting residuals are sometimes discharged to the sanitary sewer for WWTP co-disposal. • Increases inert fraction of MLSS; also increases total MLSS concentration. • Adds 5-20% to WWTP solids handling costs. • Can increase metals and radionuclide levels.

3. WTP residuals increase WWTP solids handling costs

4. WTP Residuals have beneficial effects on WWTP operation • Iron based residuals mitigate odors by preventing H2S formation. • Iron sulfide, an insoluble forms instead. • Minimize H2S in anaerobic digester gas. • Improve sludge settleability. • Mitigate struvite formation by precipitating ortho-phosphorus. • May aid in soluble metals and P removal. Edwards et. al. 1997

5. 75th Street WWTP, Boulder, CO

6. Too much of a good thing, isn’t always • February 2001, the Boulder Reservoir Water Treatment Plant experimented with a new operational mode. • WTP residuals were held for an extended period of time to build deep blankets. • New operational mode was not successful. • Excess residuals were discharged to Boulder 75th Street WWTP.

7. Too much of a good thing, isn’t always • Total pounds of iron delivered unknown. • Biological processes were disrupted. • Upper levels of trickling filter rocks were coated with an orange, gelatinous material. • Still visible after nearly three years. • Final effluent quality was impacted for an extended period; nitrification, TSS, and BOD5

8. 75th Street Plant implemented an aggressive control plan • WTP was issued a pretreatment permit limiting iron discharge to 400 ppd. • WTP now discharges residuals for three minutes once every 1.5 hours to minimize impact on WWTP operation. • WTP switched from FeCl2 to FeSO4 for compatibility with WWTP operations. • Control plan limited effects of residuals.

9. Control strategy was effective, but wasn’t science based • Total pounds of iron received prior to upset was unknown. • Pretreatment limit set high enough that WTP would not be impacted. • Anecdotal evidence raised concerns that pretreatment limit is too high. Where should the pretreatment limit be set?

10. Limits should be set to minimize impacts on operation. • Coating of trickling filter biofilm which reduces oxygen transfer. • Phosphorus deficiency in biological processes due to precipitation of P. • Iron toxicity. • 1.84 mg/L Fe impacts activated sludge1 • 1.68 mg/L Fe drops nitrification rates by 20-34%2 • High levels of Fe associated with nitrite accumulation in activated sludge.3 • Ciliated protozoa impacted at 2.0 mg/L Fe4 1. Lees et al. 2001a 2. Lees et al. 2001b 3. Clark et al. 2000 4. Abraham et al. 1997

11. Trickling filters may be more resistant to toxic effects • Heavy metal toxicity in activated sludge depends on: • Metal to biomass ratio • Temperature • pH • Presence of other metal ions • Plant specific • Jar testing required.

12. Experimental Design

13. Boulder 75th Street WWTP • Four filters in parallel. • 8 feet media depth. • 4 - 6” coarse rock media. • 2 @ 155 feet diameter. • 2 @ 200 feet diameter. • HLR = 0.45 gpm/sft. • Inf. cBOD5 = 74 + 9 mg/L

14. Sampling Device provides biofilms identical to full-scale TF • Sampling device placed on top of TF media for fourteen days. • Two layers of eight slide groups. • Each slide group contains five slides. Intact Biofilms

15. Bench-Scale Reactors allow for a controlled testing regime

16. Bench-Scale Reactors allow for a controlled testing regime • Contained 600 mls of primary clarifier effluent. • Aerated continuously and stirred. • Added molasses to raise COD to 900 mg/L. • Added varying amounts of WTP residuals to each reactor. • Two reactors per residual concentration.

17. Experiment repeated on two different days. Reactors run for 2.5 hours. Dissolved oxygen monitored. Samples collected at start and every 30 minutes for COD and soluble COD. Total iron measured at start and end of reactor runs. Bench-Scale Reactors

18. Results

19. # mLs START Fe, mg/L END Fe, mg/L 1 None 1.20 1.51 2 None 1.18 1.08 3 1 1.91 1.77 4 1 1.81 2.81 5 5 5.06 4.10 6 5 5.44 4.82 7 10 9.26 7.39 8 10 9.25 7.84 9 15 14.22 12.98 10 15 13.39 11.16 Iron may be partitioning into the biomass at higher levels

20. Total COD concentrations did not vary during the test • Six measurements per reactor for a total of 60 measurements. • Total COD averaged 927 + 30 mg/L • Should not change since soluble COD is converted into particulate COD (biomass)

21. Soluble COD removal was unaffected by Fe up to 15 mg/L

22. No effect concentration was higher than previously reported values • Iron toxicity observed at 2.0 mg/L and lower in activated sludge samples. • Researchers used raw chemicals, ferric chloride and ferric sulfate, not WTP residuals. • Most of iron in WTP residuals is bound as ferric hydroxide; possibly less toxic.

23. High iron tolerance likely related to properties of biofilms • Toxicity in activated sludge is a function of metal to biomass ratio. • Biofilms have more mass per volume than activated sludge. • Exopolymers may mitigate toxic effects by binding to metal ions. • Biofilms known to be more robust.

24. Conclusions and Implications

25. Acute iron toxicity was not observed • Iron concentrations between 1.2 and 14.2 mg/L did not impact soluble COD removal in 14-day old biofilms. • Chronic effect levels are typically lower than acute effect levels. • The 75th Street WWTP should be able to routinely accept influent iron up to 14.2 mg/L without impacting rock TF performance.

26. Current operating practices at WTP protect WWTP biology • At an influent iron concentration of 14.2 mg/L and an average daily flow of 16 mgd, ppd = 1890 • Pretreatment permit limit of 400 ppd of WTP residuals protects WWTP unit processes. • Assumes continuous, not slug loading. • Conclusions apply to ferric sludge only.

27. Future work……… • Test effects of WTP residuals on activated sludge. • Test effects of WTP residuals on biofilms at higher concentrations. • Look at chronic effects with longer test periods.

28. Floyd Bebler, Ernie Oram Mark Dane, Richard Dingeman, Dan Freeman, Paul Heppler, Kurt Keilbach, Dr. David Paterniti, Dan Wadas, and the rest of the Boulder 75th Street WWTP staff.

29. Questions?

30. References 1.  Abraham, J.V., Butler, R.D., and Sigee, D.C. "Ciliate Populations and Metals in an Activated Sludge Plant." Water Research 31.5 (1997): 1103-1111. 2. Clark, T., Burgess, J.E., Stephenson, T., and Arnold-Smith, A.K. "The Influence of Iron Based Co-Precipitants on Activated Sludge Biomass." Process Safety and Environmental Protection 78.5 (2000): 405-410. 3. Edwards, M., Beorn, C., Heppler, P., and Hernandez, M. “Beneficial Discharge of Iron Coagulation Sludge to Sewers.” Journal of Environmental Engineering October (1997): 1027-1032 4. Genschow, E., Hegemann, W., and Maschke, C. "Anaerobic Treatment of Tannery Wastewater: Toxic Effects of Wastewater Constituents and Dosage of Ferric Chloride." Environmental Management and Health 8.1 (1997): 2-3. 5. Gerardi, Michael H. "Effects of Heavy Metals upon the Biological Wastewater Treatment Process." Public Works.June (1986): 77-80. 6.  Johnson, D.K., Carliell-Marquet, C.M., and Forster, C.F. "An Examination of the Treatment of Iron-Dosed Waste Activated Sludge by Anaerobic Digestion." Environmental Technology 24.8 (2003): 937-945. 7. Lees, E.J., Noble, B., Hewitt, R., and Parsons, S.A. "The Impact of Residual Coagulant on Downstream Treatment Processes." Environmental Technology 22.1 (2001): 113-122. 8.  Lees, E.J., Noble, B., Hewitt, R., and Parsons, S.A.f1f1. "The Impact of Residual Coagulant on the Respiration Rate and Sludge Characteristics of an Activated Microbial Biomass." Environmental Technology 79.5 (2001): 283-290.f1