Maximum growth and decay rates of autotrophic biomass to simulate nitrogen removal at 10°C with municipal activated sludge plants

The present study aims at determining most likely values for the maximum growth rate (μA, max) and the endogenous decay rate (bA) of nitrifiers for activated sludge processes treating municipal wastewater operated at low temperature (10°C). The work used nitrification rate data measured on 10 full-scale plants and 2 pilot plants fed with domestic sewage. This set of data was combined with a modelling and a theoretical approach. The unified values (μA, max = 0.45·d-1 and bA = 0.13·d-1) were obtained at 10°C for the kinetic parameters of the autotrophic biomass in the SRT range 10 to 50 d. In addition, the factors affecting the expected nitrification rate (rv, nit) were established by a theoretical approach and confirmed by experimental results. For a given SRT, a linear relationship with the nitrogen volumetric loading rate was shown. The COD/TKN ratio of the influent on the nitrification rate was demonstrated. Finally, an operational tool for the verification of the nitrification rate in the design procedure of activated sludge processes is proposed.Keywords: nitrification; kinetics; low temperature; autotrophic biomass, maximum growth rate; decay rat

Anoxic and aerobic values for the yield coefficient of the heterotrophic biomass: Determination at full-scale plants and consequences on simulations

The present study aims at optimising the nitrification and denitrification phases at intermittently aerated process (activated sludge) removing nitrogen from municipal wastewater. The nitrogen removal performance recorded at 22 intermittently aerated plants was compared to the results obtained from the simulations given by the widely used ASM1. It is shown that simulations with a single value for the heterotrophic yield with any electron acceptor over-predict the nitrate concentration in the effluent of treatment plants. The reduction of this coefficient by 20% for anoxic conditions reduces the nitrate concentration by 10 g N·m-3. It significantly improves the accuracy of the predictions of nitrate concentrations in treatedeffluents compare to real data. Simulations with dual values (aerobic and anoxic conditions) for heterotrophic yield (modified ASM1) were then used to determine the practical daily aerobic time interval to meet a given nitrogen discharge objective. Finally, to support design decisions, the relevance of a pre-denitrification configuration in front of an intermittently aerated tank was studied. It is shown that when the load of BOD5 is below the conventional design value, a small contribution of the anoxic zone to nitrate removal occurs, except for over-aerated plants. When plants receive a higher load of BOD5, the modified ASM1 suggests that the anoxic zone has a higher contribution to nitrogen removal, for both correctly and over-aerated plants

Effets toxiques du cadmium et du 3.5-dichlorophenol sur l'activité et la mortalité des bactéries nitrifiantes des boues activées - Effet des basses températures

International audienceBackground, aims, and scope: Sometimes, urban wastewaters convey a more or less significant part of toxic products from industries or the craft industry. Nitrifying activity can be affected by these substances, implying higher ammonia concentrations in the outlet effluent and contributing to toxicity for the aquatic environment. Moreover, the more stringently treated wastewater standards now require a reliable treatment for nitrogen. One of the key issues is the identification of the inhibition behavior of nitrifying bacteria facing a toxic substance. This new understanding could then finally be integrated into models in order to represent and to optimize wastewater treatment plants (WWTP) operation in cases involving 'toxic scenarios'. Materials and methods: The toxic substances studied in this work, cadmium and 3.5-dichlorophenol (3.5-DCP), are representative of chemical substances commonly found in municipal sewage and industrial effluents and symbolize two different contaminant groups. The effects of Cd and 3.5-DCP on nitrification kinetics have been investigated using respirometry techniques. Results: IC50 values determination gives concentrations of 3.1 mg/L for 3.5-DCP and 45.8 mg/L for Cd at 21±1°C. The variation to low temperature seems to have no real effect on IC50 for DCP, but induces a decrease of cadmium IC50 to 27.5 mg/L at 14°C. Finally, specific respirometric tests have been carried out in order to determine the potential effect of these toxic substances on the nitrifying decay rate b a . No significant effect has been noticed for Cd, whereas the presence of 3.5-DCP (at IC50 concentration) induced a dramatic increase of b a at 20°C. The same behavior has been confirmed by experiments performed in winter periods with a sludge temperature around 12°C. Discussion: The target substances have different modes of action on activity and mortality, notably due to the abilities of the contaminant to be precipitated, accumulated, or even to be progressively degraded. Studies realized at low temperature confirmed this assumption, and put in evidence the effect of temperature on toxic substances capable of being biosorbed. However, the change in the sludge sample characteristics can be pointed out as a problem in the investigation of the temperature effect on nitrification inhibition, as biosorption, bioaccumulation, and predation are directly linked to the sludge characteristics (VSS concentration, temperature) and the plant operating conditions (loading rates, sludge age, etc.). Conclusions: This work brings new understandings concerning the action mode of these specific contaminants on nitrifying bacteria and, in particular, on the role of temperature. The experiments lead to the determination of the IC50 values for both toxic substances on biological nitrification. The inhibition mechanisms of Cd and 3.5-DCP on nitrifying activity have been simply represented by a non-competitive inhibition model. Recommendations and perspectives: Other experiments carried out in a continuous lab-scale pilot plant should be done with a proper control of the operating conditions and of the sludge characteristics in order to better understand the mechanisms of nitrification inhibition for each contaminant. Finally, these first results show that toxic substances can have an effect on the growth rate but also on the decay rate, depending on the characteristics of the toxic substance and the sludge. This eventual double effect would imply different strategies of WWTP operation according to the behavior of the contaminant on the bacteria

Diminution du rendement hétérotrophe en anoxie: Vérification sur site réel pour le procédé boues activées par aération alternée, conséquences sur la simulation dynamique

The present study aims at understanding and optimizing the efficiency of denitrification in biological nutrient removal activated sludge systems treating municipal wastewater. The nitrogen removal performance recorded at 22 intermittently aerated BNR WWTPs was compared to results from ASM1 simulations. The reduction of the heterotrophic yield in anoxic conditions (YH,anox) to a value of 0.54 gCODproduced/gCODremoved was confirmed, while simulations with the default ASM1 value of 0.67 gCODproduced/gCODremoved for any electron acceptor over-predicted effluent nitrate. Simulations with the modified ASM1 (dual YH) was then used to determine the daily aerobic time limits to meet a nitrogen discharge objective of 10 gN.m-3 to support operational optimization. Finally, the relevance of pre-denitrification in front of an intermittently aerated tank was studied to support design decisions. The data recorded from full-scale investigations at WWTPs operated at F/M ratio lower than the French conventional design value of 0.10 kgBOD5.(kgMLVSS.d)-1 indicate a contribution of the anoxic zone to nitrogen removal for over-aerated plants (aerobic time over 18 h.d-1). For higher F/M ratios, the simulations suggest that the anoxic zone has a higher contribution to nitrogen removal whatever the aerobic time

Paramètres cinétiques de la biomasse nitrifiante permettant de simuler l'élimination biologique de l'azote à 10°C pour le procédé boues fonctionnant sous aération prolongée

International audienceThe present study aims at determining most likely values for the maximum growth rate (mA, max) and the endogenous decay rate (bA) of nitrifiers for activated sludge processes treating municipal wastewater operated at low temperature (10°C). The work used nitrification rate data measured on 10 full-scale plants and 2 pilot plants fed with domestic sewage. This set of data was combined with a modelling and a theoretical approach. The unified values (mA, max = 0.45·d-1 and bA = 0.13·d-1) were obtained at 10°C for the kinetic parameters of the autotrophic biomass in the SRT range 10 to 50 d. In addition, the factors affecting the expectable nitrification rate (rv, nit) were pointed out by a theoretical approach and confirmed by experimental results. For a given SRT, a linear relationship with the nitrogen volumetric loading rate was shown. The COD/TKN ratio of the influent on the nitrification rate was demonstrated. Finally, an operational tool for the verification of the nitrification rate in the design procedure of activated sludge processes is proposed

Amélioration des prédictions de traitement de l'azote par le modèle numérique ASM1 en utilisant un jeu de paramètres validés sur 13 stations d'épuration des eaux résiduaires urbaines

The Activated Sludge Model n°1 (ASM1) published in 1987 is still one of the main models used in simulation projects for design and optimization of processes. It was published with a set of default parameter values at 20°C and at 10°C that reflected the best knowledge twenty years ago. Several recent lab-scale studies using experiments conducted under controlled conditions have pointed out that some of these default values may be inadequate. The endogenous decay rate of nitrifiers appears to be higher than originally assumed, while the heterotrophic yield was shown to have a lower value under anoxic conditions. As these results were issued by several different lab experiments, it is difficult to define the domain of validity of the measured parameter values, and to evaluate if a simple aggregation of the new information leads to a correct set of default parameters. Moreover most of the tests were conducted at 20°C, so the parameter values have not been confirmed at 10°C. To address these concerns, simulations were carried out with the updated set of parameters to evaluate its ability to provide improved predictions compared to observed data collected at 13 nitrifying-denitrifying municipal treatment plants. The measured maximum nitrification rates and the nitrogen concentration in the effluent were selected as the output variables to assess the model predictions. The work proves that simulations using the original ASM1 default parameters tend to overpredict the nitrification rates and underpredict the denitrification st½chiometry. The updated set of parameters allows more realistic predictions, over a wide range of operating conditions

Paramétrage cinétique de la population nitrifiante à faible température (taux de croissance maximum et taux de décès) à partir de quinze ans de pratique sur installations expérimentale et vraie grandeur

The present study aims at determining most likely values for the maximum growth rate (µA, max) and the endogenous decay rate (bA) of nitrifiers at low temperature (10°C) for activated sludge processes treating municipal wastewater. The work used nitrification rate data measured on ten full-scale plants and two pilot plants fed with domestic sewage. This set of data was combined with modeling and theoretical approaches. The unified values (µA, max = 0.45 d-1 and bA = 0.13 d-1) were obtained for the kinetic parameters at 10°C for the autotrophic biomass. In addition, the factors affecting the maximum nitrification rate (rv, nit) were pointed out by a theoretical approach and confirmed by experimental results. For a given SRT, a linear relationship with the nitrogen volumetric loading rate was shown. The COD/TKN ratio of the influent strongly affects the value of rv, nit. Finally, an operational tool for prediction of the nitrification rate in the design procedure of activated sludge processes was developed on the basis of the daily nitrogen loading rate.Le travail mené a eu pour but de déterminer à faible température (10°C) la valeur du taux de croissance maximum et du taux de décès des bactéries nitrifiantes autotrophes des systèmes boues activées. Cette étude s`appuie sur des mesures de vitesses de nitrification réalisées depuis quinze ans sur 10 installations « vraie grandeur », et sur installations « pilote » traitant un effluent domestique. Une approche par modélisation dynamique et par une analyse théorique ont permis d`exploiter cette base de données. Les valeurs uniques (µA, max = 0.45 jour-1 et bA = 0.13 jour-1) ont pu être dégagées à 10°C. Par ailleurs, les facteurs de forçage de la vitesse de nitrification, mis en évidence par l`approche théorique, ont été confirmés par l`ensemble des valeurs expérimentales. Pour un âge de boue donné, la vitesse de nitrification dépend linéairement de la charge volumique en azote. Le « ratio C/N de l`eau usée» joue une influence toute particulière sur la valeur de la vitesse maximum qui s`établit. Finalement, sur la base de la charge volumique en azote, un abaque prédictif de la vitesse de nitrification en boues activées a été développé. Cet outil constitue une aide opérationnelle lors du dimensionnement d`une installation

Anoxic and aerobic values for the yield coefficient of the heterotrophic biomass: Determination at full-scale plants and consequences on simulations

The present study aims at optimizing the nitrification and denitrification phases at intermittently aerated process (activated sludge) removing nitrogen from municipal wastewater. The nitrogen removal performance recorded at 22 intermittently aerated plants was compared to the results obtained from the simulations given by the widely used ASM1. It is shown that simulations with a single value for the heterotrophic yield with any electron acceptor over-predict the nitrate concentration in the effluent of treatment plants. The reduction of this coefficient by 20% for anoxic conditions reduces the nitrate concentration by 10 g N·m-3. It significantly improves the accuracy of the predictions of nitrate concentrations in treated effluents compare to real data. Simulations with dual values (aerobic and anoxic conditions) for heterotrophic yield (modified ASM1) were then used to determine the practical daily aerobic time interval to meet a given nitrogen discharge objective. Finally, to support design decisions, the relevance of a pre-denitrification configuration in front of an intermittently aerated tank was studied. It is shown that when the load of BOD5 is below the conventional design value, a small contribution of the anoxic zone to nitrate removal occurs, except for over-aerated plants. When plants receive a higher load of BOD5, the modified ASM1 suggests that the anoxic zone has a higher contribution to nitrogen removal, for both correctly and over-aerated plants