The role and control of sludge age in biological nutrient removal activated sludge systems

2010 ◽  
Vol 61 (7) ◽  
pp. 1645-1652 ◽  
Author(s):  
G. A. Ekama
Keyword(s):  
Activated Sludge ◽  
Nutrient Removal ◽  
Settling Tank ◽  
Biological Nutrient Removal ◽  
Hydraulic Control ◽  
Dual Purpose ◽  
Waste Sludge ◽  
Secondary Settling ◽  
And Control ◽  
Activated Sludge Systems

The sludge age is the most fundamental and important parameter in the design, operation and control of biological nutrient removal (BNR) activated sludge (AS) systems. Generally, the better the effluent and waste sludge quality required from the system, the longer the sludge age, the larger the biological reactor and the more wastewater characteristics need to be known. Controlling the reactor concentration does not control sludge age, only the mass of sludge in the system. When nitrification is a requirement, sludge age control becomes a requirement and the secondary settling tanks can no longer serve the dual purpose of clarifier and waste activated sludge thickeners. The easiest and most practical way to control sludge age is with hydraulic control by wasting a defined proportion of the reactor volume daily. In AS plants with reactor concentration control, nitrification fails first. With hydraulic control of sludge age, nitrification will not fail, rather the plant fails by shedding solids over the secondary settling tank effluent weirs.

Difficulties and developments in biological nutrient removal technology and modelling

1999 ◽  
Vol 39 (6) ◽  
pp. 1-11 ◽  
Author(s):  
George A. Ekama ◽  
Mark C. Wentzel
Keyword(s):  
Activated Sludge ◽  
Nutrient Removal ◽  
P Uptake ◽  
Biological Nutrient Removal ◽  
Filamentous Bulking ◽  
Recent Developments ◽  
Removal Technology ◽  
Hydrolysis Of ◽  
Activated Sludge Systems ◽  
P Removal

Filamentous bulking and the long sludge age required for nitrification are two important factors that limit the wastewater treatment capacity of biological nutrient removal (BNR) activated sludge systems. A growing body of observations from full-scale plants indicate support for the hypothesis that a significant stimulus for filamentous bulking in BNR systems in alternating anoxic-aerobic conditions with the presence of oxidized nitrogen at the transition from anoxic to aerobic. In the DEPHANOX system, nitrification takes place externally allowing sludge age and filamentous bulking to be reduced and increases treatment capacity. Anoxic P uptake is exploited in this system but it appears that this form of biological excess P removal (BEPR) is significantly reduced compared with aerobic P uptake in conventional BNR systems. Developments in the understanding of the BEPR processes of (i) phosphate accumulating organism (PAO) denitrification and anoxic P uptake, (ii) fermentation of influent readily biodegradable (RB)COD and (iii) anaerobic hydrolysis of slowly biodegradable (SB)COD are evaluated in relation to the IAWQ Activated Sludge Model (ASM) No.2. Recent developments in BEPR research do not yet allow a significant improvement to be made to ASM No. 2 that will increase its predictive power and reliability and therefore it remains essentially as a framework to guide further research.

A comparison of biological nutrient removal in intermittent cyclic and continuous activated sludge systems

1994 ◽  
Vol 11 (1-4) ◽  
pp. 149-159 ◽  
Author(s):  
Kin-man Ho ◽  
Paul F. Greenfield ◽  
Linda L. Blackall ◽  
Peter R.F. Bell ◽  
Andre Krol
Keyword(s):  
Activated Sludge ◽  
Nutrient Removal ◽  
Biological Nutrient Removal ◽  
Activated Sludge Systems

Sludge production and oxygen demand in nutrient removal activated sludge systems

1996 ◽  
Vol 34 (5-6) ◽  
pp. 43-50 ◽  
Author(s):  
P. S. Barker ◽  
P. L. Dold
Keyword(s):  
Oxygen Consumption ◽  
Activated Sludge ◽  
Nutrient Removal ◽  
Suspended Solids ◽  
Model Simulation ◽  
Oxygen Demand ◽  
Biological Nutrient Removal ◽  
Solids Concentration ◽  
Volatile Solids ◽  
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.

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

1994 ◽  
Vol 29 (7) ◽  
pp. 203-212 ◽  
Author(s):  
T. G. Casey ◽  
M. C. Wentzel ◽  
G. A. Ekama ◽  
R. E. Loewenthal ◽  
GvR Marais
Keyword(s):  
Nitric Oxide ◽  
Literature Review ◽  
Activated Sludge ◽  
Nutrient Removal ◽  
Phosphorus Removal ◽  
Laboratory Research ◽  
Biochemical Pathways ◽  
And Control ◽  
Aerobic Zone ◽  
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.

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