Chemical oxygen demand (COD) fractionation for process modelling considerations and optimization

Abstract

Wastewater treatment is a critical chain in the urban water cycle. Wastewater treatment prevents the toxic contamination of water bodies. The notable consequences of contamination are the loss of aquatic life, upsurge of eutrophication due to nutrient overload, and potential loss of human life as a result of waterborne diseases. Wastewater works (WWW) are therefore an intrinsic component of protecting the urban water cycle and ensuring that water resources are preserved for future generations. The operation of a WWW is subject to compliance with the national legislative requirements imposed by the Department of Water and Sanitation (DWS) to ensure the preservation of water resources. These requirements oblige water and sanitation departments to employ innovative design, control and optimization of WWW. Wastewater modelling packages have presented the opportunity to simulate the wastewater treatment processes in order to maintain and sustain legal compliance with the DWS. The successful implementation of a simulation package for wastewater process optimization and modelling depends on an accurate characterization also known as fractionation of the organic fractions of the WWW influents. This thesis is a result of a comprehensive study reported for Darvill wastewater work. Darvill WWW is a 60 ML/D plant which has been receiving flows of up to 120 ML/D. The importance of the study was to motivate for the upgrade of the wastewater work to account for the increased hydraulic, organic and nutrient loading into the plant. The study looked at the application of the World Engine for Simulation and Training (WEST) and all studies required to generate data that will serve as input with the understanding the current state of Darvill WWW in terms of performance. The study presents the fractionation outcomes of the primary wastewater effluent organic matter as chemical oxygen demand (COD) and the performance by assessing the biological nutrient removal process (BNR) using BNR efficiencies in addition to the development of the Darvill WWW WEST model with the aid of the probabilistic fractionator. The fractionation was achieved through the oxygen uptake rate experiments using the respirometry method. Experiments yielded the following results: biodegradable COD (bCOD) (70.5%) and inert COD (iCOD) (29.5%) of the total COD. Further characterization of the bCOD and iCOD yielded the readily biodegradable fraction (SS) at 75%, slowly degradable (XS) at 25%, particulate inert (XI) was 50.8% and the inert soluble SI at 49.2%. The COD fractions were used and served as input to the development and evaluation of the Darvill WEST model. Calculations of BNR efficiencies were used to evaluate the effects of high inflow to the biological treatability of the activated sludge for the period September 2016 - November 2017. It was found that at inflows above design capacity, the nutrient removal efficiency reduced from an expected 80-90% to an average of 40% with an average soluble reactive phosphorus (SRP) removal efficiency being 64%. A data input file for the period of January – June 2016 was created to serve as input into WEST to develop a baseline average model for the Darvill WWW plant. The model results predicted a mixed liquor suspended solids (MLSS) concentration of 6475 mg/L for the plant during the study period this was comparable with the plant MLSS concentration of 6700 mg/L at the time which was above the design concentration of 4500 mg/L. This was largely due to the plant operating under nutrient overload conditions. The final effluent (FE) concentration in the defractionation model was found to be COD = 41.28 mg/L, ammonia (NH3) = 22.02 mg/L, Total Suspended Solids (TSS) = 32 mg/L, SRP = 2.16 mg/L. Most of these results were expectedly non-compliant to the discharge limits imposed by the DWS with the exception of COD. The plant FE measurements were COD = 45.1 mg/L, NH3 = 3.4 mg/L, TSS = 20.9 mg/L, SRP= 6.67 mg/L. The COD and TSS prediction were comparable to the model prediction however there were limitations in the models ability to predict NH3 and SRP. The model does not account for changes in dissolved oxygen (DO) and temperature as these parameters are kept constant for the purpose of this study. The model assumes a temperature of 20 oC and a DO concentration of 2 mg/L for the aerobic reactor, 0.01 mg/L for the anaerobic reactor and 0.1 mg/L for the anoxic reactor. The model assumes that with the nutrient overload, oxygen compensation occurs within the reactor to maintain a constant DO concentration within the units. This limits the model in the prediction of actual instance where the overload would deplete the DO and where other competing reactions would give rise to greater non-compliances as well as biological growth’s impairment due to cold weather conditions.