Nutrient Removal Activated Sludge Systems Research Articles

Sludge production and oxygen demand in nutrient removal activated sludge 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 biological excess phosphorus removal (BEPR) activated sludge 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. A general model for biological nutrient removal systems has recently been presented by Barker and Dold. Three mechanisms for COD loss are suggested, based on results of COD balances for different types of activated sludge system. Model simulation results with and without the assumption of COD loss are discussed, as well as the influence of influent COD composition on predictions of volatile suspended solids concentration/production and oxygen consumption.

Phosphate uptake under anoxic conditions and fixed-film nitrification in nutrient removal activated sludge system

Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system. The aim of the experiments was to find whether poly-P bacteria are capable of taking up phosphate under anoxic conditions by utilising nitrate as an electron acceptor. The phosphate uptake in anoxic conditions was compared to that in aerobic environment in batch tests. The results of the long-term operation of continuous-flow lab-scale system as well as the results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after preceding anaerobic substrate uptake could significantly reduce the extent of competition for organic substrate between poly-P bacteria and denitrifiers. A side-stream nitrification in fixed-film reactor enabled to reduce the losses of organic carbon by aerobic oxidation and to stabilise the slow-growing population of nitrifiers in the system.

Biological anoxic phosphorus removal - the dephanox process

Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system (DEPHANOX). The aim of the experiments was to find whether it can be technologically convenient to use the capability of poly-P bacteria of taking up phosphate under anoxic condition by utilizing nitrate as electron acceptor. An experimental comparison of removal efficiencies with a more conventional treatment flow scheme (JHB), using low influent TKN/COD ratio, was also carried out. Phosphate uptake in anoxic condition was compared to that in oxic environment in batch tests. PHB anoxic and oxic degradation was evaluated by respirometric tests. Results of the long-term operation of continuous-flow lab-scale system as well as results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after a previous anaerobic substrate uptake could significantly reduce the extent of competition for organic substrate between poly-P bacteria and denitrifiers. A side-stream nitrification in fixed-film reactor reduced the losses of organic carbon by oxidation and stabilized the slow-growing population of nitrifiers in the system.

Biological anoxic phosphorus removal - the dephanox process

Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system (DEPHANOX). The aim of the experiments was to find whether it can be technologically covenient to use the capability of poly-P bacteria of taking up phosphate under anoxic condition by utilizing nitrate as electron acceptor. An experimental comparison of removal efficiencies with a more conventional treatment flow scheme (JHB), using low influent TKN/COD ratio, was also carried out. Phosphate uptake in anoxic condition was compared to that in oxic environment in batch tests. PHB anoxic and oxic degradation was evaluated by respirometric tests. Results of the long-term operation of continuous-flow lab-scale system as well as results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after a previous anaerobic substrate uptake could significantly reduce the extent of competition for organic substrate between poly-P bacteria and denitrifiers. A side-stream nitrification in fixed-film reactor reduced the losses of organic carbon by oxidation and stabilized the slow-growing population of nitrifiers in the system.

Sludge production and oxygen demand in nutrient removal activated sludge 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 biological excess phosphorus removal (BEPR) activated sludge 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. A general model for biological nutrient removal systems has recently been presented by Barker and Dold. Three mechanisms for COD loss are suggested, based on results of COD balances for different types of activated sludge system. Model simulation results with and without the assumption of COD loss are discussed, as well as the influence of influent COD composition on predictions of volatile suspended solids concentration/production and oxygen consumption.

The implementation of biokinetics and conservation principles in ASIM

The Activated Sludge SIMulation program ASIM is introduced as a didactic tool to be used in design courses for the simulation of the dynamic behaviour of modern nutrient removal activated sludge systems. It allows for free definition of the biokinetic model, the flow scheme, process control strategies and load variation. Its user interface is simple enough to be used in the classroom but powerful enough to support even professional work. The most difficult aspect of ASIM is the preparation of the biokinetic model (transformation processes) to be used. A systematic use of stoichiometric conservation principles simplifies this task and at the same time reduces the number of empirical parameters. The practical application of conservation principles for charge, elements and theoretical COD is presented in detail. Composition equations are introduced as a link to the wastewater treatment plant reality.

The implementation of biokinetics and conservation principles in ASIM

The Activated Sludge SIMulation program ASIM is introduced as a didactic tool to be used in design courses for the simulation of the dynamic behaviour of modern nutrient removal activated sludge systems. It allows for free definition of the biokinetic model, the flow scheme, process control strategies and load variation. Its user interface is simple enough to be used in the classroom but powerful enough to support even professional work.The most difficult aspect of ASIM is the preparation of the biokinetic model (transformation processes) to be used. A systematic use of stoichiometric conservation principles simplifies this task and at the same time reduces the number of empirical parameters. The practical application of conservation principles for charge, elements and theoretical COD is presented in detail. Composition equations are introduced as a link to the wastewater treatment plant reality.

The Activated Sludge Model No. 2: biological phosphorus removal

The Activated Sludge Model No. 2 is introduced as a further development of Activated Sludge Model No. 1. Model No. 2 introduces phosphorus accumulating organisms (PAO) and allows us to simulate the behaviour of biological nutrient removal activated sludge systems. Typical wastewater composition and a set of stoichiometric and kinetic parameters are provided in order to make reasonable predictions. The model has not yet been calibrated or verified in any full-scale application. This report contains a simplified version of the full Model No. 2; the full model is published in the final report of the Task Group (Henze et al., 1994).

The Activated Sludge Model No. 2: Biological phosphorus removal

The Activated Sludge Model No. 2 is introduced as a further development of Activated Sludge Model No. 1. Model No. 2 introduces phosphorus accumulating organisms (PAO) and allows us to simulate the behaviour of biological nutrient removal activated sludge systems. Typical wastewater composition and a set of stoichiometric and kinetic parameters are provided in order to make reasonable predictions. The model has not yet been calibrated or verified in any full-scale application. This report contains a simplified version of the full Model No. 2; the full model is published in the final report of the Task Group (Henze et al., 1994).

Acinetobacter cell biomass, growth stage and phosphorus uptake from activated sludge mixed liquor

Enhanced biological phosphorus removal activated sludge plants often do not remove phosphorus adequately in order to meet legal demands. Currently FeSO4 is being added to almost all South African nutrient removal activated sludge systems discharging effluents to the sensitive catchments to prevent phosphorus from entering fresh water systems. In order to understand biological phosphorus removal mechanisms in order to optimise the process, the role of growth rate and phosphorus removal in Acinectobacter was investigated. Phosphorus was accumulated in the lag phase of the normal growth cycle. Little or no phosphorus was accumulated in the logarithmic growth phase, instead phosphorus was released at the beginning of logarithmic growth. Further phosphorus accumulation took place in the stationary phase, once active growth had ceased. Cells had a limit to the amount of phosphorus that could be accumulated per cell irrespective of substrate availability. It was therefore concluded, that the number of cells (biomass) in a system and their growth stage were crucial factors governing biological phosphorus removal. Maximum cell numbers should therefore be obtained and logarithmic growth should be prevented in the aerobic zone, in order to optimize biological phosphorus removal from activated sludge.

A Hypothesis for the Causes and Control of Anoxic-Aerobic (AA) Filament Bulking in Nutrient Removal Activated Sludge Systems

From laboratory research and a literature review of the biochemical pathways of aerobic-facultative heterotrophic organisms, an hypothesis is proposed for the proliferation of anoxic-aerobic (AA) filamentous organisms in nitrification-denitrification (ND) and nitrification-denitrification biological excess phosphorus removal (NDBEPR) systems. In activated sludge, under anoxic conditions floc-forming organisms execute the denitrification of nitrate (NO3−) through each of the denitrification intermediates to dinitrogen (N2), in the process of which the intermediate nitric oxide (NO) is accumulated intracellularly. Intracellular NO is inhibitory to the utilization of oxygen in the subsequent aerobic zone. In contrast, the filamentous organisms execute only part of the denitrification pathway, i.e. the reduction of NO3− to NO2−; they do not accumulate NO and hence are not inhibited in the subsequent aerobic zone. Thus in anoxic-aerobic systems, floc-formers are placed at a disadvantage in the aerobic zone giving an advantage to the filaments in the competition for substrate. Experimental evidence to support this hypothesis is presented and a tentative proposal of a strategy for control of AA filament proliferation is described and tested experimentally.

The Effect of Incomplete Denitrification on Anoxic-Aerobic (Low F/M) Filament Bulking in Nutrient Removal Activated Sludge Systems

Experiments have been performed to investigate the hypothesis that the alternation of anoxic and aerobic conditions is the major factor influencing filamentous bulking in low F/M systems. The results provided strong supporting evidence for the hypothesis; nitrite, rather than nitrate, appears to play a dominant role in causing the bulking.

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems: T. G. Caseyet al., Wat. Res.26, 867–869 (1992)

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems: T. G. Caseyet al., Wat. Res.26, 867–869 (1992)

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems

Abstract Laboratory research has indicated that a possible cause of low F/M filament bulking in ND (nitrification-denitrification) and NDBEPR (nitrification-denitrification biological excess phosphorus removal) systems occurs as a result of competition for substrate between filamentous and floc-forming organisms which have different denitrification pathways. In ND and NDBEPR systems alternating anoxic-aerobic conditions prevail and continuous utilization of particulate slowly biodegradable COD (SBCOD) by floc-forming organisms in these systems leads to accumulation of the denitrification intermediates nitrite (NO2−) and nitric oxide (NO) under anoxic conditions. It is proposed that a cause for low F/M filament bulking is that the intermediate NO inhibits the utilization of SBCOD by floc-formers under subsequent aerobic conditions, with high concentrations of NO2− exacerbating this effect, thereby allowing filamentous organisms, which do not accumulate NO, to dominate. Some experimental evidence to support this proposal is presented.

Nitrification Kinetics in Single-Sludge Biological Nutrient Removal Activated Sludge Systems

Nitrification kinetics as a function of mixed liquor temperature were compared for a conventional fully-aerobic activated sludge system and a system accomplishing biological nutrient removal (BNR) by incorporation of anaerobic and anoxic zones using the UCT configuration. The systems treated the same municipal wastewater and both had flow rates of 151 L/day. The nitrification rates were greater in the nutrient removal system compared to the conventional system as long as the aerobic MCRT was above the minimum for complete nitrification. It was concluded that BNR systems require less aerobic volume than fully aerobic systems to accomplish nitrification because the aerobic biomass concentration is greater in the BNR systems, particularly if the UCT configuration is used. Nonetheless, BNR systems require more total volume to accomplish complete nitrification than fully aerobic systems, and the volume differential increases as mixed liquor temperatures decrease.

The influence of a particulate substrate on filamentous bulking and phosphorus removal in activated sludge systems

Particulate substrates comprise all forms of solid and colloidal particles and soluble organic molecules with complex structure and high molecular weight. According to the IAWPRC Hydrolysis Hypothesis, the particulate substrate cannot be utilized in anaerobic zones of nutrient removal activated sludge systems because the rate of its hydrolysis (i.e. solubilization) is zero under anaerobic conditions. The particulate substrate is enmeshed in the flocs of activated sludge and transferred from an anaerobic zone to an oxic zone where hydrolysis occurs and hydrolysis products become available for filamentous microorganisms. The influence of a particulate substrate on the activated sludge system with an anaerobic zone has been studied in two laboratory systems. The systems consisted of a three-compartment anaerobic zone followed by an oxic completely-mixed tank or by a three-compartment oxic cascade. The systems were fed by artificial wastewater. The soluble fraction of substrate was simulated by glucose, ethanol and acetic acid, the particulate fraction with peptone and the suspension prepared from hard-boiled eggs. It has been demonstrated that filamentous bulking caused by oxic zone growers (e.g. Types Sphaerotilus and 021N) occurred when the oxic zone was supplied either with the soluble substrate generated by fermentation in the preceding anaerobic zone; or with the products of the hydrolysis of the enmeshed particulate substrate. The compartmentalization of the oxic zone significantly reduced the bulking problems and improved the phosphorus removal with all substrates used in this study.

Filamentous Bulking in Nutrient Removal Activated Sludge Systems

Behaviour of filamentous microorganisms under anaerobic, anoxic, and oxic conditions has been reviewed from the point of view of filamentous bulking in nutrient removal activated sludge systems. It was concluded that the growth of most filamentous microorganisms is considerably suppressed under anaerobic and anoxic conditions and that the filamentous bulking in nutrient removal systems is chiefly caused by the growth of filamentous microorganisms in oxic zones. The role of particulate substrate hydrolysis was also discussed and the necessity of compartmentalization of oxic zones was shown.