Jenny Merical
Introduction
 Chromium Sources
 Biological Removal Methods
 Activated Sludge Absorption
Capacity
 Biomass Growth
 Nitrification
 COD Removal
 Toxicity of Chromium
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Sources of Chromium
 Chromium
 Cr(VI)
 Cr(III)
 Sources
 Leather tanning
 Electroplating
 Wood Preservation
 Textile manufacturing
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Activated Sludge Plants in Iowa
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Chromium Removal Methods
 Traditional:
 Chemical process
 Biological:
 Reduction of Cr(VI) to Cr(III)
 Adsorption
 Positive charged Cr(VI) attracted to negative charged
microorganism cell wall
Reduction of Cr(VI) to Cr(III)
 Most common removal mechanism
 Reduced then precipitated as Cr(OH)3
Metal Distribution for 1 mg/l Cr(III)
Precipitated
9%
Dissolved
1%
Metal Distribution for 1 mg/l Cr(VI)
Adsorbed
14%
Adsorbed
90%
Stasinakis, Thomaidis, Mamais, and Karivali et al., 2003
Precipitated
0%
Dissolved
86%
Activated Sludge Absorption Capacity
 95% Cr(III) removal efficiency
 Increased removal
 Longer SRT
 Higher pH
 96-99% chromium present in the form Cr(III) when
anoxic selector precedes aerobic tank
Stasinakis, Thomaidis, Mamais, and Karivali et al., 2003
Activated Sludge Characteristics
 Suspended Solids Concentration
 Cr(III) removal efficiency increases with a high SS
concentration
 Cr(VI) removal did not correlate with SS concentration
 Sludge Age
 Cr(III) removal efficiency decreases as age increases
 Cr(VI) removal not affected by sludge age
Activated Sludge Acclimation
 Cr(VI) and Cr(III) increase biomass lag time
 Cr(III) more inhibitive at concentrations less than 70 mg/L
 Cr(VI) more inhibitive at concentrations greater than 70
mg/L
 Lag time increases with increased chromium
concentration
 Optimum growth conditions:
 10 mg/L Cr(III) or Cr(VI)
 11 and 17 HRT, respectively
Biomass Growth
 25 mg/L Cr(VI) stimulates biomass growth
 15 mg/L Cr(III) stimulates biomass growth
 Higher concentrations limit growth
Gikas and Romanos, 2006
Nitrification
Nitrobacter sp.
 Cr(VI) interferes with nitrification
 Increases ammonium concentration
 Decreases nitrate concentration
 5 mg/L decreased ammonium removal to 30%
 System recovery of about 12 days
 Cr(III) interferes at higher concentrations
 25 mg/L or greater limit nitrification
 System recovery of about 7 days
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COD Removal
 Cr(VI) limits COD removal capacity
 No significant impact with less than 5 mg/L
 5 mg/L system required 3 days to recover from loading
 Higher Cr(VI) concentrations

More pronounced effect on COD removal

Longer system recovery time
 Cr(VI) shock loading does not impact COD
Toxicity of Chromium
 Microbiological effects
 Decrease biomass
 Decrease activity
 Decrease density
 Cr(VI) 100 times more toxic than Cr(III)
 Cr(III) less soluble
 Presence of sodium decreased Cr(VI) toxicity
Chromium Reducing Bacteria
 Acinetobacter
Acinetobacter
 Partially reduce Cr(VI) to
Cr(III)
 Assist in chromium removal
 Ochrobactrum
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 Aureobacterium
 Hydrogenophaga
 Clavibacter
Cellulomonas
 Corynebacterium
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Chromium loading on bacteria
 Nitrifying bacteria more sensitive than COD reducing
bacteria
 Longer recovery time
 Smaller quantity/diversity of nitrifying bacteria
 Cr(VI) has to be toxic to several species to impact
COD reducing bacteria
 Shock loading
 Lethal to Cr(VI) reducing bacteria 9.25-211 mg/L
 Range implies different toxicity levels
Chromium Reducing Protozoa
Vorticella
 Species:
 Vorticella
 Opercularia
 Stalked ciliates
 Free swimming ciliates
 Rotifers
plantphys.info
 Free swimming ciliates dominate
 5 mg/L Cr(VI) toxic to all
protozoa
Opercularia
in high Cr(VI) concentration
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Activated Sludge Chromium Removal
Advantages
 Self sufficient communities
 Stimulate biomass growth at
optimum concentration
 Some microorganisms assist
in chromium removal
 Possibly more economical
Drawbacks
 Inhibits nitrification process
(25 mg/L)
 Inhibits filamentous bulking
 Increased biomass growth lag
time
 Limits COD removal
 Limits microorganism
diversity
Conclusion
 Activated sludge sufficient for chromium removal
 95% removal efficiency by absorption
 Reduction of Cr(VI) to Cr(III)
 Couple with nitrification process
 Improve chromium removal:
 Lower activated sludge age
 Avoid high concentrations
 Longer SRT
 Higher pH
 Increase Suspended Solids