Steven Howard Isaacs

Controlled Carbon Source Addition to an Alternating Nitrification-Denitrification Wastewater Treatment Process Including Biological P Removal

Abstract The paper investigates the effect of adding an external carbon source on the rate of denitrification in an alternating activated sludge process including biological P removal. Two carbon sources were examined, acetate and hydrolysate derived from biologically hydrolyzed sludge. Preliminary batch experiments performed in 5 liter bottles indicated that the denitrification rate can be instantaneously increased through the addition of either carbon source. The amount by which the rate was increased depended on the amount of carbon added. In the main experiments performed in a pilot scale alternating process, the addition of either carbon source to the anoxic zone also resulted in an instantaneous and fairly reproducible increase in the denitrification rate. Some release of phosphate associated with the carbon source addition was observed. With respect to nitrogen removal, these results indicate that external carbon source addition may serve as a suitable control variable to improve process performance.

Characterization of the denitrifying fraction of phosphate accumulating organisms in biological phosphate removal

Results of experimental investigations are presented that strongly support the hypothesis that PAO from activated sludge systems consist of two groups: a) denitrifying PAO (DNPAO) capable of using oxygen and nitrate and b) non-denitrifying PAO (non-DNPAO) only able to use oxygen. Batch experiments were performed in which activated sludge obtained from a pilot scale BiodeniphoTM was submitted to a sequence of anaerobic/anoxic/aerobic, anaerobic/aerobic or anaerobic/anoxic conditions while monitoring the course of NOx-N, NH4-N, PO4-P, PHB and PHV. Several methods for the determination of the two fractions of PAO are performed and compared. This study extends on previously reported results (Kern-Jespersen and Henze, 1993) in that the pH was controlled to around pH 7 to assure that phosphate precipitation was minimal, and in the measurement of PHB and PHV. With regards to the latter, the paper also examines the influence of the size of the internal PHA pool on the anoxic phosphorus uptake rate. Simulations implementing existing models for the growth of non-DNPAO and DNPAO are used to confirm the experimental results and to gain a better understanding of some of the observations.

External carbon source addition as a means to control an activated sludge nutrient removal process

Abstract In alternating type activated sludge nutrient removal processes, the denitrification rate can be limited by the availability of readily-degradable carbon substrate. A control strategy is proposed by which an easily metabolizable COD source is added directly to that point in the process at which denitrification momentarily occurs. This approach serves to increase the denitrification rate on demand, thereby allowing the accumulation of nitrate and nitrite during periods of peak nitrogen loading to be reduced or avoided. A pilot plant demonstration of the control strategy using acetate as COD source is provided, showing a marked improvement in effluent water quality as compared to the uncontrolled case. An examination of the resulting denitrification rates illustrates the direct proportionality between these rates and the rate of COD addition.

An analysis of nitrogen removal and control strategies in an alternating activated sludge process

Abstract The biological nitrogen removal in an alternating activated sludge process is described and analyzed using a simplified model of the IAWQ activated sludge model No. 1. In face of the alternating nature of the process, a new analytical approach is developed by introducing the nitrification capacity and denitrification potential concepts into the alternating process analysis. This facilitates a more obvious insight into the nitrogen removal with the development of mathematical relationships between the nitrogen removal efficiency and the process operational conditions. The process performances with different operational conditions and control strategies are presented using this approach. The results show that the total nitrogen removal is strongly dependent on the process load, nitrification rate, denitrification rate, cycle length and DO setpoint etc. and an optimal operation requires a proper match between the nitrification and denitrification. In addition, the different control strategies are evaluated using the new analytic technique and through this their mechanism and effectiveness are better understood.

Effect of continuous addition of an organic substrate to the anoxic phase on biological phosphorus removal

The continuous introduction of a biological phosphorus removal (BPR) promoting organic substrate to the denitrifying reactor of a BPR process is examined through a series of batch experiments using acetate as model organic substrate. Several observations are made regarding the influence of substrate availability on PHA storage/utilization and phosphate uptake/release. Under anoxic conditions PHB is utilized and phosphate is taken up, indicating that at least a fraction of the PAO can denitrify. The rates of anoxic P-uptake, PHB utilization and denitrification are found to increase with increasing initial PHB level. At low acetate addition rates the P-uptake and PHB utilization rates are reduced compared to when no acetate is available. At higher acetate addition rates a net P-release occurs and PHB is accumulated. For certain intermediate acetate addition rates the PHB level can increase while a net P-release occurs. Whether the introduction of BPR promoting organic substrates to the denitrifying reactor is detrimental to overall P-removal appears to be dependent on the interaction between aerobic P-uptake, which is a function of PHB level, and the aerobic residence time.

A novel control strategy for improved nitrogen removal in an alternating activated sludge process—part I. Process analysis

Abstract Increasing demands on discharged water quality have led to the development of activated sludge processes which incorporate the biologically mediated removal of nitrogen and phosphorus. A major obstacle in the development of new control strategies for such processes is the lack of variables which can effectively alter process behavior and can feasibly be manipulated. This two part paper deals with a novel means to improve the nitrogen removal in an alternating type nutrient removal activated sludge process through control of the cycle length. In this first part, an analysis of process dynamics is undertaken. Using a simple model to describe the nitrogen dynamics in the alternating process, the existence of an optimal cycle length as a function of process conditions is demonstrated and explained. A graphical technique is developed which allows quick visualization of nitrogen dynamics under constant process conditions. This also serves as a means to assess whether a selected cycle length is optimal, too long, or too short for a given set of conditions. Based on the findings of this first part, the second part of the paper develops and demonstrates control strategies which serve to automatically adjust the cycle length to compensate for changing process conditions.

Phosphate uptake kinetics in relation to PHB under aerobic conditions.

Abstract A characteristic of the biological phosphorus removal process is the interaction between different intracellular components. Therefore, an understanding of the effects of the intracellular components on biological phosphorus removal is essential for the practical application of the process. In this study, the stoichiometry of poly-hydroxybutyrate (PHB) to P-uptake and the kinetics of uptake of ortho-phosphate in relation to PHB were investigated under aerobic and substrate limiting conditions in batch experiments with activated sludge from a biological phosphorus removal process. The stoichiometry of PHB to P-uptake was investigated by determination of the degradation rate of PHB under aerobic conditions in the presence and absence of ortho-phosphate in the bulk liquid. The degradation of PHB as a function of time was described by a first-order equation. The presence of ortho-phosphate increased the rate of PHB degradation significantly, confirming that the uptake of ortho-phosphate directly required PHB. The fraction of PHB used for uptake of ortho-phosphate from the bulk liquid was estimated to be 30%. Furthermore, batch experiments confirmed that the uptake rate of ortho-phosphate under aerobic conditions was highly dependent on the concentration of PHB in the biomass. The observed uptake rate of ortho-phosphate as a function of the PHB concentration was mathematically described by a Monod equation. The maximum uptake rate of the sludge was estimated to be 14 (mg-P/g-VSS)/h. The obtained results add to the understanding of the mechanisms of the biological phosphorus removal process under the stress conditions with limiting substrate supplies present in the full-scale wastewater treatment process. Furthermore, the results indicate that control strategies to stabilise the process should focus on a maintenance and optimal utilisation of PHB.

A novel control strategy for improved nitrogen removal in an alternating activated sludge process—part II. Control development

Abstract The first part of this two part contribution dealt with an analysis of nitrification and denitrification in an alternating activated sludge nutrient removal process for the case of constant process conditions. The existence of an optimal cycle length, which is a function of the process conditions, was demonstrated and discussed. Based on these findings, this paper examines a control strategy by which the cycle length of an alternating activated sludge nutrient removal process is automatically adjusted in order to compensate for changing process conditions. Two control algorithms are proposed. One involves proportional-integral-derivative (PID) control which requires minimal process information and computational effort. The second is a model based predictive control (MBPC) technique, which introduces a feedforward element into the control strategy. The MBPC technique is examined using a relatively simple process model. The results of experimental demonstrations of these two algorithms in a pilot plant facility indicate their potential towards improving nitrogen removal in the alternating process.

Automatic monitoring of denitrification rates and capacities in activated sludge processes using fluorescence or redox potential

A novel method is described to automatically estimate several key parameters affecting denitrification in activated sludge processes: the nitrate concentration, the denitrification capacity, and the maximum (substrate unlimited) and actual denitrification rates. From these, the concentration of active denitrifying microorganisms and the quality of available organic substrate pool can be estimated. Additionally, a modification of the method allows the determination of the efficacy of various carbon substrates to enhance denitrification, and this can be used to determine optimal dosing rates of an external carbon source. The method is based on measurements of either fluorescence or redox potential (ORP) in an isolated mini-reactor, the Biological Activity Meter (BAM), situated in the anoxic zone of the wastewater treatment plant. Advantages of the method are that it is in situ, operating at the same temperature as in the measured anoxic zone, requires no pumps or pipes for mixed liquor sampling, consumes little or no reagents, and uses measurement signals which are instantaneous and low maintenance, one of which provides a direct measure of biological activity.

Monitoring and Control of a Biological Nutrient Removal Process: Rate Data as a Source of Information

Abstract A means for obtaining rates of nitrification, denitrification and phosphate uptake/release in a nutrient removal waste water treatment process from FIA measurements is demonstrated. These rates provide useful information for understanding and modelling the process, and for the development of control strategies.