The biodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by a mixed microbial consortium enriched from soil was investigated. The specific oxygen uptake rate (SOUR) as a measure of biomass was found to be inappropriate as the characteristics and composition of the mixed culture were found to change significantly as the operating conditions changed. Both SOUR and biomass measurements revealed an expected substrate inhibition pattern up to around 1000 mg/L 2,4-D in batch incubation trials. The maximum growth rate was recorded at a substrate concentration of around 300 mg/L when the biomass was measured by gravimetrically, although maximum oxygen uptake activity was at a substrate concentration of 500 mg/L. The continuous experiments showed that 2,4-D can be degraded at inlet concentrations of up to 6362 mg/L resulting in >99.5% degradation. The flocculation characteristics of the biomass were found to change as the substrate concentration changed and a microscopic examination confirmed that the changes in SOUR and specific substrate utilisation rate (SSUR) in the continuous system were due to changes in microbial culture characteristics.

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