The Effects and Processes for Removal of Chromium in Activated
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Transcript The Effects and Processes for Removal of Chromium in Activated
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
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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