Kinetic optimization of the activated sludge denitrification process

Transcription

1 book Kinetic optimization of the activated sludge denitrification process Author: Benedek György Plósz Supervisor: Dr. Andrea Jobbágy Ph.D., associate professor BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS Department of Agricultural Chemical Technology BUDAPEST, 2004

2 1. INTRODUCTION Biological nitrogen and phosphorus removal have become increasingly important requirements for wastewater treatment, because influent nutrients can severely deteriorate the water quality of lakes through eutrophication. In order to achieve effective nutrient (N and P) removal, in addition to aerated vessels, anoxic basins (with nitrogen oxides as terminal electron acceptor) and anaerobic bioreactors have also been incorporated into activated sludge wastewater treatment systems. Kinetic control of the biological processes utilized under different conditions has special importance in both system design and operation. In the process of denitrification, in the absence of oxygen nitrogen oxides are reduced to nitrogen gas by heterotrophic, facultatively aerobic bacteria using organic substrate as electron donor. Design and configuration of anoxic bioreactors of the activated sludge treatment systems represent a complex problem in both pilot- and full-scale practice. With regards to theoretical principles and practical experience, uncontrolled oxygen penetration can influence both the direction and rate of nitrogen removal. However, neither the kinetic evaluation of these biodegradation processes, and consequently, nor the assessment of the impacts of dissolved oxygen in full-scale systems had been described. 1

3 2. APPLIED METHODS The aim of the study was the kinetic analysis of the processes occurring in an anoxic bioreactor in order to develop optimization methods for full-scale design and operation, including the following particular aspects: Development of an experimental method, while excluding oxygen, for the investigation of the anoxic processes; Investigation of the inhibitory impact of oxygen on denitrification processes in anoxic reactors using different types of substrates; Development of a new mathematical model in order to discover the effects of oxygen entering an anoxic reactor through the liquid surface; Demonstration of the impact of different operational parameters through mathematical simulation studies to optimize anoxic bioreactors performance; Performing a comparative, continuous-flow, pilot-scale experiment in order to study the inhibitory impact of dissolved oxygen entering the liquid surface. 2

4 3. RESULTS Kinetic optimization of activated sludge bioreactor configuration becomes extremely important in system design, when the wastewater to-be treated contains insufficient amount of biodegradable carbon source compared to the denitrifiable nitrogen. It has been verified that the impact of oxygen entering through liquid surface is considerable in such cases, and it must be taken into account in order to allow efficient nitrogen removal. This hypothesis was supported by both batch and pilot-scale experiments as well as through mathematical modeling. Finally, suggestions were made for optimizing activated sludge denitrification through bioreactor configuration. An experimental procedure based on the application of a Zero Head-space Reactor was developed for investigating the biological processes under oxygen-free conditions as well as for estimating the impact of dissolved oxygen on denitrification. It has been experimentally shown that the impact of oxygen entering the anoxic reactor through the liquid surface depends strongly on the actual denitrification rate. Experimental verification revealed that while having an almost unchanged oxygen transfer rate through the liquid surface, the different dissolved oxygen levels caused by different consumption rates are responsible for the dynamic impact resulting in different measures of kinetic inhibition on denitrification processes. A mathematical model was developed in order to simulate the performance of open anoxic bioreactors. The model incorporated aerobic and anoxic microbial growth as well as death and lysis of heterotrophic bacteria, hydrolysis of particulate organics and O 2 mass transfer through the liquid surface. The overall anoxic growth was formulated using nitrate-, nitrite- and nitrous-oxide as terminal electron acceptors. Model identification and validation were proceeded using experimental data obtained from experiments applying both municipal and synthetic wastewaters. 3

5 The different inhibitory impact of different oxygen levels on denitrification, caused by different consumption at an unchanged oxygen transfer rate through liquid surface, was also verified by simulation studies. The effect of oxygen penetration on biological nitrogen removal was shown in simulation studies using different values of particular process key parameters, i.e. influent readily biodegradable substrate- and biomass-concentration, as well as oxygen mass-transfer coefficient and temperature. It has been pointed out that the bioreactor configuration becomes irrelevant at increasing influent readily biodegradable substrate concentration at given denitrifiable nitrogen content. However, coverage of anoxic basins surface provides a cost-effective optimization when having external substrate in shortage. Simulation studies have pointed out that the inhibitory impact of dissolved oxygen increases significantly when temperature decreases, thus application of anoxic reactor coverage is useful especially at low temperatures and can also overcome the problem of heat-lost, ensuring nitrogen removal at higher temperatures. Our studies showed that when external substrate was marginal, staging of anoxic reactors provided significant benefits on denitrification efficiency. This can provide a cost-effective optimization method for treatment plants realizing inadequate nitrogen removal or for the reduction of total required anoxic volume in newly established activated sludge installations. It has been shown that bioreactor staging represents the most effective optimization technique when increased levels of biomass concentration in either open or covered anoxic vessels are applied. In case of 4 gl -1 initial biomass concentration, simulation studies revealed that the reactor coverage resulted only a slight improvement on removal efficiency, even if readily biodegradable substrate concentration was either marginal or very low. 4

6 The impact of reactor configuration on denitrification efficiency was also verified by comparative pilot-scale experiments, including systems with both staged and unstaged anoxic reactors. The obtained results revealed a good correlation between consumption rates and resulting dissolved oxygen concentrations in different unaerated vessels. 5

7 LIST OF THE PUBLICATIONS Publications in international journals with peer review: 1. Jobbágy, A., Simon, J. & Plósz, B. (2000) The impact of oxygen penetration on the estimation of denitrification rates in anoxic processes. Water Research 31(9), Plósz, B.Gy., Jobbágy, A. & Grady, C.P.L., Jr. (2003) Factors influencing deterioration of denitrification by oxygen entering an anoxic reactor through the surface. Water Research 37(4), Jobbágy, A. & Plósz, B.Gy. (2004) Impact of low-rate substrate removal on the performance of denitrifying systems. Periodica Polytechnica Chemical Engineering (Accepted for publication) Posters and oral presentations: 1. Plósz, B. & Jobbágy, A. (2001) The impact of oxygen penetration into anoxic bioreactors. (in Hungarian) Budapesti Műszaki és Gazdaságtudományi Egyetem, Ipari Nyíltnapok. (Poster presentation) 2. Jobbágy, A., Plósz, B. & Tardy, G. (2002) Impact of low-rate substrate removal on the performance of denitrifying systems. ENVIRO 2002 / IWA World Water Congress, April 7-12, 2002, Melbourne, Australia. (Poster presentation) 3. Jobbágy, A., Tóth, I., Varga, Gy. I. Simon, J., Turay, O. & Plósz, B. (2002) Conquering odour and corrosion by controlled anoxic conditions in sewer systems. ENVIRO 2002 / IWA World Water Congress, April 7-12, 2002, Melbourne, Australia. (Oral presentation) 6