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Date of Award

1995

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Supervisor

Professor P.L. Doid

Abstract

The objective of this research was to develop and calibrate a dynamic mechanistic model for biological nutrient (nitrogen and phosphorus) removal activated sludge systems treating municipal wastewater. The IAWPRC (ASM1) model for non-polyP heterotrophic and autotrophic organisms (Henze et al., 1987a,b) and the Wentzel et al. (1989b) model for polyP organisms were merged to form a general activated sludge model. After a number of initial modifications the model was tested against literature data from laboratory-scale nitrification denitrification biological excess phosphorus removal (NDBEPR) systems. Based on the preliminary results, a number of specific areas were identified which required further study. These included (1) accounting for sludge production and oxygen utilization in BEPR systems; (2) denitrification behaviour in BEPR systems; and (3) other issues such as hydrolysis under unaerated conditions.

The main body of this thesis is presented as a series of five papers. The first paper (Chapter 4) presents a study of COD and nitrogen balances in activated sludge systems. The results suggest that systems incorporating anaerobic zones exhibit low COD balances compared to aerobic and anoxic-aerobic systems. Possible mechanisms for this "loss" of COD are discussed, including the possibility that the COD loss is related to fermentation processes occurring under anaerobic conditions.

The second paper (Chapter 5) presents a study of denitrification behaviour in BEPR activated sludge systems. Results of a review of microbiological studies and many continuous and batch reactor experimental studies indicate that a significant fraction of the polyP organisms can use nitrate as an electron acceptor in the absence of oxygen for oxidation of stored PHB and simultaneous uptake of phosphorus.

The development of a general activated sludge model for biological nutrient removal activated sludge systems is discussed in the third paper (Chapter 6). Several modifications were made to both the ASM1 and Wentzel et al. (1989b) model components, based on the results of literature review and model simulations. A fermentation process has been included for the conversion of readily biodegradable COD to short chain fatty acids (assuming a loss of COD). Hydrolysis of enmeshed slowly biodegradable COD under anaerobic conditions has been incorporated, as well as anoxic growth of polyP organisms. These modifications and others are discussed in this paper. The matrix representation and a description of the model processes are also presented, as well as a brief outline of influent wastewater characterization.

The application of the general model is demonstrated in the fourth paper (Chapter 7) for aerobic and anoxic-aerobic systems, as well as a number of nutrient removal (NDBEPR) systems for both steady state and dynamic conditions. Results of simulations show the model is capable of predicting sludge production and oxygen utilization for a range of system types and configurations, as well as tracking changes in a number of parameters including soluble phosphorus and nitrate concentrations.

In the final paper (Chapter 8) the consequences of the COD loss assumptions incorporated in the model are demonstrated for a number of experimental anoxic-aerobic and anaerobic-anoxic-aerobic systems. Results of model simulations indicate that without the assumption of COD loss, predictions of oxygen consumption and volatile suspended solids production are significantly over-estimated for NDBEPR systems (and to a lesser extent anoxic-aerobic systems). These systems apparently consume less oxygen and produce less volatile solids than aerobic systems for the same amount of COD removal.

In conclusion, the merits and weaknesses of the general model are discussed. An important feature of the model is that a single set of kinetic and stoichiometric parameters produced quite accurate predictions for the wide range of systems to which the model was applied (with the exception of the nitrifier growth rate - discussed in Chapter 6). This provides a degree of support for the model structure and integrity. Many aspects of NDBEPR modelling require further investigation, including: the COD loss phenomenon, the fermentation processes occurring under anaerobic (and possibly anoxic) conditions, the hydrolysis of slowly biodegradable colloidal and particulate organics (particularly under anoxic and anaerobic conditions), and the impact these aspects have on denitrification behaviour in NDBEPR systems.

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