Fundamental of Nitrogen Removal
Content
Fundamentals of Nitrogen Removal
Introduction Fundamentals Nitrification Denitrification Optimized Design In this section the fundamental theoretical principles underlying the process of biological nitrogen removal are presented. More specific, the following issues will be discussed:
• • •
Forms and reactions of nitrogenous material in wastewater and in the activated sludge process Mass balance of nitrogenous material Stoichometry of reactions with nitrogenous material
(1) Forms and reactions of nitrogenous material in wastewater and in the activated sludge process Nitrogen is present in wastewater in several forms, the important ones being organic nitrogen (both soluble and particulate), ammonium/ammonia and possibly some nitrate. In the activated sludge process several reactions may occur, that will change the form of the nitrogenous matter. Figure 4.1 shows the various possibilities: ammonification, nitrification and denitrification. Click here to download this section.
Figure 4.1
Schematic representation of the different forms of nitrogenous material in wastewater and their subsequent reactions in the activated sludge process
(2) Mass balance of nitrogenous matter In this section it will be demonstrated how to set-up a mass balance for nitrogen for a steadystate activated sludge system. The nitrogen mass balance recovery factor is introduced, which can be used to check whether the mass balance "closes". Click here to download this section. In Example 4.1 the use of the nitrogen mass balance and recovery factor are demonstrated. (3) Stoichiometry of reactions with nitrogenous matter The biochemical reactions involved in nitrification and denitrication result in the transfer of electrons to- and from the nitrogen atom. The oxygen consumption for nitrification and recovery of "equivalent oxygen" during denitrification are explained and quantified. Click here to download this section and here to download Example 4.2. A second effect of the biochemical reactions in the nitrogen removal process is the consumptionand production of alkalinity. During nitrification alkalinity will be consumed and during ammonification and denitrification it will be produced. The stoichiometry of these processes, as well as the effect this will have on mixed liquor pH, will be discussed. Click here to download this section and here to download Examples 4.3 and 4.4.
Nitrification
Introduction Fundamentals Nitrification Denitrification Optimized Design
This section deals with the first of the two processes involved in biological nitrogen removal: i.e. nitrification, where ammonium is converted into nitrate by autotrophic bacteria. The theoretical basis of this biological process will be presented. Furthermore model equations are developed which allow the steady state model for the activated sludge process, to be extended with the nitrification process. More specific, the following issues will be discussed:
• • • • •
Nitrification kinetics Operational factors influencing the nitrification process Determination of nitrification kinetics Nitrification in activated sludge systems with unaerated zones Nitrification capacity and -potential
(1) Nitrification kinetics Nitrification can be modelled with simple Monod kinetics, i.e. with the specific growth- and decay rates and the half saturation constant for ammonium. For the purpose of advanced modelling (for instance for simulation or process control), supplementary control functions may be included: e.g. for alkalinity and oxygen. However, when the objective is to design a nitrogen removal system, this will not be required. The effluent ammonium concentration of a completely mixed process is predicted from the value of the kinetic parameters and the applied (aerobic) sludge age. The minimum aerobic sludge age required for nitrification is calculated. Based on an extensive review of literature, values and temperature dependencies for the kinetic parameters are suggested. Click here to download this section. (2) Operational factors influencing the nitrification process Several factors influence the nitrification process and more specific the value of the specfic nitrifier growth rate: the mixed liquor temperature, -dissolved oxygen concentration and - pH value. Click here to download this section. (3) Determination of nitrification kinetics A method is presented to determine the values of the main kinetic parameters of the nitrification process: the specific growth rate, the decay rate, the half saturation constant of ammonium and the half saturation constant of oxygen. The method relies on the cultivation of a nitrifying sludge under steady state conditions and the subsequent application of respirometrics (oxygen uptake rate measurements). An extensive application example of the method is presented in Example A4.1. To download this section, click here.
(4) Nitrification with unaerated zones In activated sludge systems designed for nitrogen removal, part of the reactor volume will not be aerated to allow for denitrification. The effect of this unaerated zone on the nitrification kinetics and the residual ammonium concentration is determined. A new parameter is defined: the maximum anoxic sludge mass fraction. This value is not to be exceeded if a certain nitrification efficiency is to be maintained. Other factors will set a maximum to this mass fraction other than that set by the requirements for nitrification. Click here to download this section (5) Nitrification capacity and -potential Two important parameters are introduced that relate to the nitrogen removal performance of an activated sludge system. The first one is the nitrification potential, defined as the Total Kjeldahl Nitrogen (TKN) concentration in the influent that is available for nitrification. The second one is the nitrification capacity, defined as the influent TKN concentration that is effectively nitrified in the activated sludge system. Click here to download this section.
Denitrification
Introduction Fundamentals Nitrification Denitrification Optimized Design
In this section the focus will be on denitrification, the second of the two processes involved in the biological removal of nitrogen, in which nitrate is converted into nitrogen gas. The theoretic principles of denitrification are discussed and model equations are developed to extend the steady state model for the activated sludge system. More specific, the following issues will be discussed:
• • • •
Prerequisites for the denitrification process Configurations for nitrogen removal Denitrification kinetics Denitrification capacity
(1) Denitrification prerequisites The necessary conditions for the denitrification process to develop in an activated sludge process can be summarised as:
• • •
Presence of a facultative bacterial mass, capable of using oxygen and nitrate or nitrite Presence of nitrate and absence of dissolved oxygen in the mixed liquor (i.e. an anoxic environment) Suitable environmental conditions for bacterial growth
•
Presence of an electron donor (nitrate reductor): i.e. organic material
Click here to download this section (2) Configurations for nitrogen removal Denitrification requires organic substrate for the reduction of nitrate to nitrogen gas. The first distinction is whether the source of the organic substrate is internal (i.e. present in the wastewater) or external (i.e added to the wastewater). The second distinction is between predenitrification (pre-D) systems, where the denitrification reactor precedes the nitrification reactor, and post-denitrification (post-D) systems, where it is the other way around. Combinations of pre- and post denitrification are also quite common, as in the popular Bardenpho configuration. In this section the design principles, advantages and disadvantages of the main configurations for nitrogen removal will be discussed. Click here to download this section. (3) Denitrification kinetics The kinetics of the denitrification process are determined for both pre-D and post-D anoxic reactors. The minimum anoxic sludge mass fraction is determined, which maximizes nitrate removal with the easily biodegradable material present in the influent. Click here to download section. (4) Denitrification capacity Similar to the nitrification capacity, the denitrification capacity indicates the amount of nitrate that can be removed per litre of influent. The value of this parameter is different for pre-D and post-D anoxic reactors and is a function of the kinetic parameters of the denitrification process, the influent COD composition and the applied sludge age. Click here to download this section. Several examples illustrate the application of the concepts of nitrification- and denitrification capacity: Example 4.5. and Example 4.6
Optimized Design of Nitrogen Removal
Introduction Fundamentals Nitrification Denitrification Optimized Design In this section the theory presented in the previous sections will be used to assess the nitrogen removal capacity of an activated sludge system, for a given combination of influent characteristics and kinetic parameters values. Furthermore an optimized design procedure is developed which allows the optimal configuration of the activated sludge system to be determined, maximizing nitrogen removal at minimum total reactor volume. The following items will be discussed: • Determination of the nitrogen removal capacity • Optimised design of activated sludge systems for biological nitrogen removal (1) Determination of nitrogen removal capacity The concepts of nitrification- and denitrication capacity will be used to determine the extent of nitrogen removal possible in an activated sludge system, given the influent nitrogen- and COD composition and the values of the kinetic parameters. First a new parameter will be introduced, i.e. the amount of nitrate available for denitrification. This parameter is defined as the fraction of the nitrification capacity that will be effectively available for denitrification in the anoxic reactor for the selected values of the mixed liquor- and sludge recirculation factors. Now, as a function of the applied sludge age, the values of nitrification capacity, nitrate available for denitrification and denitrification capacity will be compared, taking into account the maximum anoxic sludge fraction and minimum required aerobic sludge age. It will then be possible to predict the mimimum effluent nitrogen concentration- and composition, as a function of the applied sludge age. Click here to download this section. (2) Optimised design of nitrogen removal A variable of crucial importance is introduced: the ratio between nitrogenous and organic (COD) material in the influent. It will be demonstrated that for each value of the sludge age it can be established whether it is possible achieve complete nitrogen removal and if not, what configuration will be superior: pre-D or Bardenpho. Now, it will be easy to select the operational sludge age for which compliance to the specified effluent limits is possible and to finalize the design of the activated sludge system. Click here to download this section. Refer also to the section on the integrated cost-based design of an activated sludge system for nitrogen removal and see Example 10.5 for a design case in which the application of the design method is detailed, in conjunction with the design of the other main treatment units of the activated sludge system.
ntegratd Cost-Based Design
Introduction Design Preparations Optimized CostBased Design So far, various examples of designing and optimising the different units of the activated sludge systems have been discussed. In this section a conceptual method is presented, which can be used for the integrated cost-based design optimisation of the activated sludge treatment configurations presented in the previous section. This optimisation method uses the same body of theory as already presented in the earlier sections, but for the benefit of the reader the whole procedure is now presented in an integrated form, considering all components included in the design. The following configurations will be discussed:
• • •
Configuration A1: Conventional secondary treatment Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration Configuration C2: Tertiary treatment - nitrogen and phosphorus removal in an UCT configuration
Furthermore it will be demonstrated that selection of the minimum sludge age required to meet the treatment objectives will indeed result in a minimal costs design. Finally, an example is given of the application of the optimised design procedure to optimisation of existing systems.
• •
Influence of the sludge age on treatment costs Operational optimisation of existing activated sludge systems
(1) Configuration A1: Conventional secondary treatment The most elementary configuration of the activated sludge system, shown in Figure 10.1, consists of a completely mixed aerobic reactor treating the influent, followed by a final settler for solids/liquid separation and equipped with a gravity thickener and anaerobic digester for stabilisation of the produced excess sludge. In practice this system will also be equipped with a pre-treatment capable of removing large debris (rags, paper, plastics), sand and if required oil, fat and grease. For optimised system design the following data are required:
• • • •
Sludge age at which the system should be operated; Values of the parameters of the ideal model of the activated sludge system; Influent characteristics; Cost and financial parameters.
Click here to download the optimised design procedure of configuration A1 and here to download Example 10.2
Figure 10.1
Basic process flow diagram of system configuration A1
(2) Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration If nitrogen removal is to be achieved, the system configuration has to be modified to include non-aerated zones for denitrification, as is shown in Figure 10.2.
Figure
Basic process flow diagram of system configuration C1
10.2 Another important change is that the sludge age is no longer set by the requirements for organic material removal: now the sludge age will depend on the constraints from the nitrification and denitrification processes. In the section on optimized design for nitrogen removal, a method was presented to calculate the minimum sludge age required to achieve complete removal of nitrate in an activated sludge system. Click here to download the optimised design procedure of configuration C1 and here to download Example 10.5. (3) Configuration C2: Tertiary treatment - nitrogen and phosphorus removal In order to effect biological removal of phosphorus, it is required to incorporate a completely anaerobic zone in the system configuration, as shown in Figure 10.3.
Figure 10.3 Basic process flow diagram of system configuration C2 Based on the theory presented in the sections on bio-P removal and optimization of nutrient removal, it was concluded that a relatively small anaerobic zone is sufficient and that preferably the sludge age is low. On the other hand, the necessity to remove nitrogen as well requires a relatively long sludge age. There is no analytical solution for this problem: one has to find an optimised solution iteratively while using expert judgment regarding the values of several operational and design variables. Click here to download the optimised design procedure of configuration C2 and here to download Example 10.6. (4) Influence of the sludge age on treatment costs
In the optimised design procedure as presented in the previous examples, it was assumed that the selected sludge age is always equal to the minimum required sludge age required for proper functioning of the processes involved. The design procedure will now be used to demonstrate that this is in fact a correct assumption. For several sludge ages the system parameters will be calculated in order to determine the quantitative effect of the sludge age on system design and costs. To download this section, click here. (5) Operational optimisation of existing activated sludge systems The previous examples all refer to the design of an activated sludge system based on an expected or experimentally determined waste water flow or composition. Once the treatment system has been designed, the actual quantity and quality of the waste water will probably differ from those expected, as well as the values of the operational parameters. In this case the theory presented in this book can be used for another type of optimisation: for a given configuration determine the optimal operational conditions, characterised by production of the specified effluent quality at minimal costs. To download this section, click here.
Bio-P Removal
Introduction Bio-P Removal Optimisation of Nutrient Removal As the phosphorus mass fraction in volatile sludge is about 2.5% of the VSS concentration in a conventional activated sludge proces, the discharge of excess sludge will also result in the partial removal of phosphorus from the wastewater. However, it will in general be required to lower the effluent phosphorus concentration to a value ≤ 1 mg P/l and when discharge of organic phosphorus with the excess sludge is the only mechanism of phosphorus removal, this is only possible under favourable conditions: i.e. a low P/COD ratio combined with a short sludge age. In waste waters with a higher level of nutrients and/or activated sludge systems operating at a higher sludge age, additional methods of phosphorus removal will be necessary. Apart from phosphorus removal by chemical precipitation, the other main method applied is biological excess phosphorus removal (or bio-P removal). Under appropriate operational conditions a sludge mass will develop that contains a significantly increased phosphorus content. Using artificial substrate (acetate), phosphorus mass fractions up to 38% weight have been attained. In systems designed for bio-P removal, a mixed population will develop with a mixture of “normal” sludge mass with 2.5% phosphorus content and “enriched” bio-P sludge mass containing 38% phosphorus. In this section the following aspects of bio-P removal are discussed:
• • •
Theoretic principles of bio-P removal Configurations for bio-P removal Model of bio-P removal
(1) Theoric principles of bio-P removal The conditions required for and the behaviour observed in biological excess phosphorus removal are discussed. These conditions are explained by a biochemical model of the metabolism of phosphate accumulating organisms (PAO). This model is presented in Figure 5.1. Click here to download this section.
Figure 5.1
Metabolism of PAO under anaerobic and oxic conditions, Smolders et al (1994)
(2) Configurations for bio-P removal Various system configurations have been developed for bio-P removal, all of which have been extensively applied in practice. The main difference between these configurations is the way in which an anaerobic zone is maintained and how this zone is protected against the introduction of nitrate. In this section several common system configurations are discussed, such as modified Bardenpho, UCT and modified UCT. A general system layout is presented that allows a single wastewater treatment plant to be operated in different bio-P removal configurations. Click here to download this section. (3) Model of bio-P removal
Based on the concepts presented in the previous sections, a model was developed at the University of Cape Town (UCT) to quantatively describe the processes involved in biological phosphorus removal, including the release of phosphorus in the anaerobic zone and the absorption of excess phosphorus in the subsequent aerobic zone. The model was derived from the results of an extensive study by Wentzel et al (1986), who operated activated sludge systems using acetate as the only source of COD in the influent, resulting in a culture enhanced with PAO. Refer to Example 5.1. In later research it was confirmed that the model could also be applied to a mixed culture of PAO and "normal heterotrophs, as will be encountered in systems designed for nutrient removal. The steady state activated sludge model has been extended to include the PAO biomass fraction. A design procedure for bio-P removal systems is presented and demonstrated in Example 5.2. The issue of denitrifying PAO is discussed in Example 5.3. To download this section, click here.
Introduction Fundamentals Nitrification Denitrification Optimized Design In this section the fundamental theoretical principles underlying the process of biological nitrogen removal are presented. More specific, the following issues will be discussed:
• • •
Forms and reactions of nitrogenous material in wastewater and in the activated sludge process Mass balance of nitrogenous material Stoichometry of reactions with nitrogenous material
(1) Forms and reactions of nitrogenous material in wastewater and in the activated sludge process Nitrogen is present in wastewater in several forms, the important ones being organic nitrogen (both soluble and particulate), ammonium/ammonia and possibly some nitrate. In the activated sludge process several reactions may occur, that will change the form of the nitrogenous matter. Figure 4.1 shows the various possibilities: ammonification, nitrification and denitrification. Click here to download this section.
Figure 4.1
Schematic representation of the different forms of nitrogenous material in wastewater and their subsequent reactions in the activated sludge process
(2) Mass balance of nitrogenous matter In this section it will be demonstrated how to set-up a mass balance for nitrogen for a steadystate activated sludge system. The nitrogen mass balance recovery factor is introduced, which can be used to check whether the mass balance "closes". Click here to download this section. In Example 4.1 the use of the nitrogen mass balance and recovery factor are demonstrated. (3) Stoichiometry of reactions with nitrogenous matter The biochemical reactions involved in nitrification and denitrication result in the transfer of electrons to- and from the nitrogen atom. The oxygen consumption for nitrification and recovery of "equivalent oxygen" during denitrification are explained and quantified. Click here to download this section and here to download Example 4.2. A second effect of the biochemical reactions in the nitrogen removal process is the consumptionand production of alkalinity. During nitrification alkalinity will be consumed and during ammonification and denitrification it will be produced. The stoichiometry of these processes, as well as the effect this will have on mixed liquor pH, will be discussed. Click here to download this section and here to download Examples 4.3 and 4.4.
Nitrification
Introduction Fundamentals Nitrification Denitrification Optimized Design
This section deals with the first of the two processes involved in biological nitrogen removal: i.e. nitrification, where ammonium is converted into nitrate by autotrophic bacteria. The theoretical basis of this biological process will be presented. Furthermore model equations are developed which allow the steady state model for the activated sludge process, to be extended with the nitrification process. More specific, the following issues will be discussed:
• • • • •
Nitrification kinetics Operational factors influencing the nitrification process Determination of nitrification kinetics Nitrification in activated sludge systems with unaerated zones Nitrification capacity and -potential
(1) Nitrification kinetics Nitrification can be modelled with simple Monod kinetics, i.e. with the specific growth- and decay rates and the half saturation constant for ammonium. For the purpose of advanced modelling (for instance for simulation or process control), supplementary control functions may be included: e.g. for alkalinity and oxygen. However, when the objective is to design a nitrogen removal system, this will not be required. The effluent ammonium concentration of a completely mixed process is predicted from the value of the kinetic parameters and the applied (aerobic) sludge age. The minimum aerobic sludge age required for nitrification is calculated. Based on an extensive review of literature, values and temperature dependencies for the kinetic parameters are suggested. Click here to download this section. (2) Operational factors influencing the nitrification process Several factors influence the nitrification process and more specific the value of the specfic nitrifier growth rate: the mixed liquor temperature, -dissolved oxygen concentration and - pH value. Click here to download this section. (3) Determination of nitrification kinetics A method is presented to determine the values of the main kinetic parameters of the nitrification process: the specific growth rate, the decay rate, the half saturation constant of ammonium and the half saturation constant of oxygen. The method relies on the cultivation of a nitrifying sludge under steady state conditions and the subsequent application of respirometrics (oxygen uptake rate measurements). An extensive application example of the method is presented in Example A4.1. To download this section, click here.
(4) Nitrification with unaerated zones In activated sludge systems designed for nitrogen removal, part of the reactor volume will not be aerated to allow for denitrification. The effect of this unaerated zone on the nitrification kinetics and the residual ammonium concentration is determined. A new parameter is defined: the maximum anoxic sludge mass fraction. This value is not to be exceeded if a certain nitrification efficiency is to be maintained. Other factors will set a maximum to this mass fraction other than that set by the requirements for nitrification. Click here to download this section (5) Nitrification capacity and -potential Two important parameters are introduced that relate to the nitrogen removal performance of an activated sludge system. The first one is the nitrification potential, defined as the Total Kjeldahl Nitrogen (TKN) concentration in the influent that is available for nitrification. The second one is the nitrification capacity, defined as the influent TKN concentration that is effectively nitrified in the activated sludge system. Click here to download this section.
Denitrification
Introduction Fundamentals Nitrification Denitrification Optimized Design
In this section the focus will be on denitrification, the second of the two processes involved in the biological removal of nitrogen, in which nitrate is converted into nitrogen gas. The theoretic principles of denitrification are discussed and model equations are developed to extend the steady state model for the activated sludge system. More specific, the following issues will be discussed:
• • • •
Prerequisites for the denitrification process Configurations for nitrogen removal Denitrification kinetics Denitrification capacity
(1) Denitrification prerequisites The necessary conditions for the denitrification process to develop in an activated sludge process can be summarised as:
• • •
Presence of a facultative bacterial mass, capable of using oxygen and nitrate or nitrite Presence of nitrate and absence of dissolved oxygen in the mixed liquor (i.e. an anoxic environment) Suitable environmental conditions for bacterial growth
•
Presence of an electron donor (nitrate reductor): i.e. organic material
Click here to download this section (2) Configurations for nitrogen removal Denitrification requires organic substrate for the reduction of nitrate to nitrogen gas. The first distinction is whether the source of the organic substrate is internal (i.e. present in the wastewater) or external (i.e added to the wastewater). The second distinction is between predenitrification (pre-D) systems, where the denitrification reactor precedes the nitrification reactor, and post-denitrification (post-D) systems, where it is the other way around. Combinations of pre- and post denitrification are also quite common, as in the popular Bardenpho configuration. In this section the design principles, advantages and disadvantages of the main configurations for nitrogen removal will be discussed. Click here to download this section. (3) Denitrification kinetics The kinetics of the denitrification process are determined for both pre-D and post-D anoxic reactors. The minimum anoxic sludge mass fraction is determined, which maximizes nitrate removal with the easily biodegradable material present in the influent. Click here to download section. (4) Denitrification capacity Similar to the nitrification capacity, the denitrification capacity indicates the amount of nitrate that can be removed per litre of influent. The value of this parameter is different for pre-D and post-D anoxic reactors and is a function of the kinetic parameters of the denitrification process, the influent COD composition and the applied sludge age. Click here to download this section. Several examples illustrate the application of the concepts of nitrification- and denitrification capacity: Example 4.5. and Example 4.6
Optimized Design of Nitrogen Removal
Introduction Fundamentals Nitrification Denitrification Optimized Design In this section the theory presented in the previous sections will be used to assess the nitrogen removal capacity of an activated sludge system, for a given combination of influent characteristics and kinetic parameters values. Furthermore an optimized design procedure is developed which allows the optimal configuration of the activated sludge system to be determined, maximizing nitrogen removal at minimum total reactor volume. The following items will be discussed: • Determination of the nitrogen removal capacity • Optimised design of activated sludge systems for biological nitrogen removal (1) Determination of nitrogen removal capacity The concepts of nitrification- and denitrication capacity will be used to determine the extent of nitrogen removal possible in an activated sludge system, given the influent nitrogen- and COD composition and the values of the kinetic parameters. First a new parameter will be introduced, i.e. the amount of nitrate available for denitrification. This parameter is defined as the fraction of the nitrification capacity that will be effectively available for denitrification in the anoxic reactor for the selected values of the mixed liquor- and sludge recirculation factors. Now, as a function of the applied sludge age, the values of nitrification capacity, nitrate available for denitrification and denitrification capacity will be compared, taking into account the maximum anoxic sludge fraction and minimum required aerobic sludge age. It will then be possible to predict the mimimum effluent nitrogen concentration- and composition, as a function of the applied sludge age. Click here to download this section. (2) Optimised design of nitrogen removal A variable of crucial importance is introduced: the ratio between nitrogenous and organic (COD) material in the influent. It will be demonstrated that for each value of the sludge age it can be established whether it is possible achieve complete nitrogen removal and if not, what configuration will be superior: pre-D or Bardenpho. Now, it will be easy to select the operational sludge age for which compliance to the specified effluent limits is possible and to finalize the design of the activated sludge system. Click here to download this section. Refer also to the section on the integrated cost-based design of an activated sludge system for nitrogen removal and see Example 10.5 for a design case in which the application of the design method is detailed, in conjunction with the design of the other main treatment units of the activated sludge system.
ntegratd Cost-Based Design
Introduction Design Preparations Optimized CostBased Design So far, various examples of designing and optimising the different units of the activated sludge systems have been discussed. In this section a conceptual method is presented, which can be used for the integrated cost-based design optimisation of the activated sludge treatment configurations presented in the previous section. This optimisation method uses the same body of theory as already presented in the earlier sections, but for the benefit of the reader the whole procedure is now presented in an integrated form, considering all components included in the design. The following configurations will be discussed:
• • •
Configuration A1: Conventional secondary treatment Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration Configuration C2: Tertiary treatment - nitrogen and phosphorus removal in an UCT configuration
Furthermore it will be demonstrated that selection of the minimum sludge age required to meet the treatment objectives will indeed result in a minimal costs design. Finally, an example is given of the application of the optimised design procedure to optimisation of existing systems.
• •
Influence of the sludge age on treatment costs Operational optimisation of existing activated sludge systems
(1) Configuration A1: Conventional secondary treatment The most elementary configuration of the activated sludge system, shown in Figure 10.1, consists of a completely mixed aerobic reactor treating the influent, followed by a final settler for solids/liquid separation and equipped with a gravity thickener and anaerobic digester for stabilisation of the produced excess sludge. In practice this system will also be equipped with a pre-treatment capable of removing large debris (rags, paper, plastics), sand and if required oil, fat and grease. For optimised system design the following data are required:
• • • •
Sludge age at which the system should be operated; Values of the parameters of the ideal model of the activated sludge system; Influent characteristics; Cost and financial parameters.
Click here to download the optimised design procedure of configuration A1 and here to download Example 10.2
Figure 10.1
Basic process flow diagram of system configuration A1
(2) Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration If nitrogen removal is to be achieved, the system configuration has to be modified to include non-aerated zones for denitrification, as is shown in Figure 10.2.
Figure
Basic process flow diagram of system configuration C1
10.2 Another important change is that the sludge age is no longer set by the requirements for organic material removal: now the sludge age will depend on the constraints from the nitrification and denitrification processes. In the section on optimized design for nitrogen removal, a method was presented to calculate the minimum sludge age required to achieve complete removal of nitrate in an activated sludge system. Click here to download the optimised design procedure of configuration C1 and here to download Example 10.5. (3) Configuration C2: Tertiary treatment - nitrogen and phosphorus removal In order to effect biological removal of phosphorus, it is required to incorporate a completely anaerobic zone in the system configuration, as shown in Figure 10.3.
Figure 10.3 Basic process flow diagram of system configuration C2 Based on the theory presented in the sections on bio-P removal and optimization of nutrient removal, it was concluded that a relatively small anaerobic zone is sufficient and that preferably the sludge age is low. On the other hand, the necessity to remove nitrogen as well requires a relatively long sludge age. There is no analytical solution for this problem: one has to find an optimised solution iteratively while using expert judgment regarding the values of several operational and design variables. Click here to download the optimised design procedure of configuration C2 and here to download Example 10.6. (4) Influence of the sludge age on treatment costs
In the optimised design procedure as presented in the previous examples, it was assumed that the selected sludge age is always equal to the minimum required sludge age required for proper functioning of the processes involved. The design procedure will now be used to demonstrate that this is in fact a correct assumption. For several sludge ages the system parameters will be calculated in order to determine the quantitative effect of the sludge age on system design and costs. To download this section, click here. (5) Operational optimisation of existing activated sludge systems The previous examples all refer to the design of an activated sludge system based on an expected or experimentally determined waste water flow or composition. Once the treatment system has been designed, the actual quantity and quality of the waste water will probably differ from those expected, as well as the values of the operational parameters. In this case the theory presented in this book can be used for another type of optimisation: for a given configuration determine the optimal operational conditions, characterised by production of the specified effluent quality at minimal costs. To download this section, click here.
Bio-P Removal
Introduction Bio-P Removal Optimisation of Nutrient Removal As the phosphorus mass fraction in volatile sludge is about 2.5% of the VSS concentration in a conventional activated sludge proces, the discharge of excess sludge will also result in the partial removal of phosphorus from the wastewater. However, it will in general be required to lower the effluent phosphorus concentration to a value ≤ 1 mg P/l and when discharge of organic phosphorus with the excess sludge is the only mechanism of phosphorus removal, this is only possible under favourable conditions: i.e. a low P/COD ratio combined with a short sludge age. In waste waters with a higher level of nutrients and/or activated sludge systems operating at a higher sludge age, additional methods of phosphorus removal will be necessary. Apart from phosphorus removal by chemical precipitation, the other main method applied is biological excess phosphorus removal (or bio-P removal). Under appropriate operational conditions a sludge mass will develop that contains a significantly increased phosphorus content. Using artificial substrate (acetate), phosphorus mass fractions up to 38% weight have been attained. In systems designed for bio-P removal, a mixed population will develop with a mixture of “normal” sludge mass with 2.5% phosphorus content and “enriched” bio-P sludge mass containing 38% phosphorus. In this section the following aspects of bio-P removal are discussed:
• • •
Theoretic principles of bio-P removal Configurations for bio-P removal Model of bio-P removal
(1) Theoric principles of bio-P removal The conditions required for and the behaviour observed in biological excess phosphorus removal are discussed. These conditions are explained by a biochemical model of the metabolism of phosphate accumulating organisms (PAO). This model is presented in Figure 5.1. Click here to download this section.
Figure 5.1
Metabolism of PAO under anaerobic and oxic conditions, Smolders et al (1994)
(2) Configurations for bio-P removal Various system configurations have been developed for bio-P removal, all of which have been extensively applied in practice. The main difference between these configurations is the way in which an anaerobic zone is maintained and how this zone is protected against the introduction of nitrate. In this section several common system configurations are discussed, such as modified Bardenpho, UCT and modified UCT. A general system layout is presented that allows a single wastewater treatment plant to be operated in different bio-P removal configurations. Click here to download this section. (3) Model of bio-P removal
Based on the concepts presented in the previous sections, a model was developed at the University of Cape Town (UCT) to quantatively describe the processes involved in biological phosphorus removal, including the release of phosphorus in the anaerobic zone and the absorption of excess phosphorus in the subsequent aerobic zone. The model was derived from the results of an extensive study by Wentzel et al (1986), who operated activated sludge systems using acetate as the only source of COD in the influent, resulting in a culture enhanced with PAO. Refer to Example 5.1. In later research it was confirmed that the model could also be applied to a mixed culture of PAO and "normal heterotrophs, as will be encountered in systems designed for nutrient removal. The steady state activated sludge model has been extended to include the PAO biomass fraction. A design procedure for bio-P removal systems is presented and demonstrated in Example 5.2. The issue of denitrifying PAO is discussed in Example 5.3. To download this section, click here.
