Biodegradation of pesticide-contaminated wastewaters in denitrifying sequencing batch reactors.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This research investigated the potential for industrial-strength 2-methyl-4- chlorophenoxyacetic acid (MCPA) degradation by activated sludge microorganisms in a sequencing batch reactor (SBR) under nitrate-reducing conditions. The research was divided into four phases consisting of Phase I (a “proof-of-concept” phase); Phase II (an initial “tolerance” exploration phase); Phase III (an “effect of hydraulic retention time (HRT)” phase) and Phase IV (a “limits” phase). Prior to addition of the MCPA, baseline data was collected to ensure a stable operation of the SBR in terms of COD and nitrate removal. The SBR successfully and simultaneously removed the nitrates completely and around 98 % of the MCPA up to an initial concentration of 50 mg/L MCPA in the dimethylamine salt form (DMCPA) (Phases I, II and III); however, it took approximately 28 days to observe a steady, high-level removal of MCPA. When the concentration of DMCPA was increased to 75 mg/L (Phase IV) the MCPA removal efficiency dropped to 85 % but removal was observed only for a relatively short period of time, since the biomass appeared to eventually become saturated with the herbicide, stopping conversion of DMCPA to its acid form and halting biodegradation. The bio-kinetic parameters for nitrate and acetate (COD) were quantified when the concentration of herbicide increased from 20 to 50 to 75 mg/L. The biodegradation kinetic model of COD changed from a first-order (baseline data) to a second-order kinetic model by the addition of increasing concentrations of the herbicide. The rate constant values (k₂) decreased from 1.51 ± 0.82 to 0.57 ± 0.14 to 0.25 ± 0.11 h⁻¹ from 20 mg/L to 75 mg/L respectively. In regards to nitrate, the order of reaction remained the same as the baseline data (i.e. a first-order kinetic model) but the rate constant values (k1) decreased from 2.58 ± 0.76 to 2.14 ± 0.40 to 1.24 ± 0.16 h⁻¹ from 20 mg/L to 75 mg/L. Similarly, specific COD and nitrate uptake rates also decreased from 0.60 ± 0.12 to 0.39 ± 0.04 to 0.26 ± 0.07 mg/mg VSS d and 0.14 ± 0.01 to 0.12 ± 0.02 to 0.11± 0.01 mg/mg VSS d from 20 mg/L to 75 mg/L respectively. Further to this, the bio-kinetic rate constants of DMCPA and MCPA were estimated by solving first-order modified differential equations (MDEs) using the function ode45 in MATLAB. This function implements a Runge-Kutta method with a variable time step for efficient computation after the initial conditions at time to, are specified. Thus, the “apparent” reaction rate constants for DMCPA and MCPA for 20 mg/L of herbicides were found to be kD= 0.27 h⁻¹ and kM = 0.97 h⁻¹ respectively; whereas, a three-fold decrease (kD = 0.09 h⁻¹) in the apparent rate of DMCPA degradation and a two-fold decrease (kM = 0.47 h⁻¹) in the rate of MCPA degradation was observed when the concentration increased from 20 to 50 mg/L. The results of this study produced additional information on the biodegradability potential, limits and kinetics of MCPA under anoxic conditions; thereby providing supplementary information to an overall integrated pesticide-nitrate removal strategy.