Jenny Merical

IntroductionChromium Sources

Biological Removal Methods

Activated Sludge Absorption Capacity

Biomass Growth

Nitrification

COD Removal

Toxicity of Chromium

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Sources of ChromiumChromium

Cr(VI)Cr(III)

SourcesLeather tanningElectroplatingWood PreservationTextile manufacturing

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Activated Sludge Plants in Iowa

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Chromium Removal MethodsTraditional:

Chemical process

Biological:

Reduction of Cr(VI) to Cr(III)

Adsorption

Positive charged Cr(VI) attracted to negative charged microorganism cell wall

Adsorbed90%

Precipitated9%

Dissolved1%

Dissolved86%

Adsorbed14%

Precipitated0%

Reduction of Cr(VI) to Cr(III)Most common removal mechanismReduced then precipitated as Cr(OH)3

Metal Distribution for 1 mg/l Cr(III)

Metal Distribution for 1 mg/l Cr(VI)

Stasinakis, Thomaidis, Mamais, and Karivali et al., 2003

Activated Sludge Absorption Capacity95% Cr(III) removal efficiencyIncreased removal

Longer SRTHigher 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 CharacteristicsSuspended Solids Concentration

Cr(III) removal efficiency increases with a high SS concentration

Cr(VI) removal did not correlate with SS concentration

Sludge AgeCr(III) removal efficiency decreases as age increasesCr(VI) removal not affected by sludge age

Activated Sludge AcclimationCr(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 Growth25 mg/L Cr(VI) stimulates biomass growth15 mg/L Cr(III) stimulates biomass growthHigher concentrations limit growth

Gikas and Romanos, 2006

NitrificationCr(VI) interferes with nitrification

Increases ammonium concentrationDecreases nitrate concentration5 mg/L decreased ammonium removal to

30%System recovery of about 12 days

Cr(III) interferes at higher concentrations25 mg/L or greater limit nitrificationSystem recovery of about 7 days

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Nitrobacter sp.

COD RemovalCr(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 ChromiumMicrobiological 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 BacteriaAcinetobacter

Partially reduce Cr(VI) to Cr(III)

Assist in chromium removal

Ochrobactrum

Aureobacterium

Corynebacterium

Hydrogenophaga

Clavibacter

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Chromium loading on bacteriaNitrifying bacteria more sensitive than COD reducing

bacteriaLonger recovery timeSmaller quantity/diversity of nitrifying bacteriaCr(VI) has to be toxic to several species to impact

COD reducing bacteriaShock loading

Lethal to Cr(VI) reducing bacteria 9.25-211 mg/LRange implies different toxicity levels

Chromium Reducing ProtozoaSpecies:

VorticellaOperculariaStalked ciliatesFree swimming ciliatesRotifers

Free swimming ciliates dominate in high Cr(VI) concentration

5 mg/L Cr(VI) toxic to all protozoa

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Activated Sludge Chromium RemovalAdvantages Drawbacks

Inhibits nitrification process (25 mg/L)

Inhibits filamentous bulking

Increased biomass growth lag time

Limits COD removal

Limits microorganism diversity

Self sufficient communities

Stimulate biomass growth at optimum concentration

Some microorganisms assist in chromium removal

Possibly more economical

ConclusionActivated sludge sufficient for chromium removal

95% removal efficiency by absorptionReduction of Cr(VI) to Cr(III)Couple with nitrification process

Improve chromium removal:Lower activated sludge ageAvoid high concentrationsLonger SRTHigher pHIncrease Suspended Solids