Optimization of phthalic acid batch biodegradation and the use of modified Richards model for modelling degradation

Microbial degradation of phthalic acid was investigated using cultures of aerobic bacteria enriched from a sewage sludge. The Gompertz function and the Richards function were modified and compared to describe the phthalic acid (PA) degradation process, and both models successfully fitted the biomass growth curve when PA was used as the sole source of growth controlling substrate. However, the modified Richards model was superior in describing the depletion curve of PA. The additional parameter, m, in the modified Richards model may be corresponding to the relative importance of the substrate consumption for maintenance. Based on the maximum degradation rates calculated using the modified Richards model, the optimal degradation conditions were determined by an orthogonal test for environmental factors including initial pH, C:N:P ratio and salt concentrations of the culture medium. More than 99% of PA at an initial concentration of 4000 mgl−1 was degraded within 5 days under the optimized condition: namely initial pH 6.0, C:N:P=100:5:1, and NaCl concentration 10 gl−1. Our results suggest that both substrate depletion and microbial biomass formation can be modelled and predicted using the modified Richards model and the limitation factor of phthalic acid degradation is the initial pH of the culture.