Biodegradation of pesticide-contaminated wastewaters in denitrifying sequencing batch reactors.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Civil Engineering
Degree name
Doctor of Philosophy
Publisher
University of Canterbury
Journal Title
Journal ISSN
Volume Title
Language
English
Date
2016
Authors
Chouhan, Deepak
Abstract

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.

Description
Citation
Keywords
Ngā upoko tukutuku/Māori subject headings
ANZSRC fields of research
Rights
All Right Reserved