FR2814453A1 - Biological treatment of aqueous effluents with elimination of nitrogen compounds comprises subjecting effluents to alternating aerobic-anoxic sequence of treatment - Google Patents

Abstract

Biological treatment of aqueous effluents comprises nitrification and de-nitrification stages during which effluents are subjected to alternating anoxic and aerobic stages, forming aerobic/anoxic sequence, with aerobic stages involving contact with oxygenated gas, and, for at least one stage, with super-oxygenated gas; this contact being stopped when dynamic analysis indicates end of nitrification point.

Description

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The invention relates to a process for the biological treatment of wastewater comprising nitrogenous and carbonaceous residues and aiming to eliminate these nitrogenous compounds.

bactéries nitnfiantes. Ce mélange est soumis à des cycles successifs enchaînant des étapes aérobies et des étapes anoxiques, correspondant respectivement à des étapes de nitnfication et de dénitrification
Ainsi, au cours de l'étape aéroble, une réaction. de nitrification des composés azotés en nitntes, puis nitrates, par les bactéries et au moyen de l'oxygène dissous a

lieu. La vitesse de nitrification est influencée par la valeur du pH et les variations de ce pH. Selon les eaux traitées, les meilleurs taux de nitrification sont obtenus pour un pH compris entre 6,5 et 9
Au cours de l'étape anoxique, les nitrates issus de l'étape aérobie précédente sont réduits en azote moléculaire gazeux. This type of process consists of mixing in a reactor the aqueous effluents to be treated with a biomass such as activated sludge or fixed culture, containing

nitrifying bacteria. This mixture is subjected to successive cycles linking aerobic steps and anoxic steps, corresponding respectively to nitrification and denitrification steps
Thus, during the aerosol stage, a reaction. nitrification of nitrogen compounds to nitntes, then nitrates, by bacteria and by means of dissolved oxygen a

location. The rate of nitrification is influenced by the pH value and the variations in this pH. Depending on the treated water, the best nitrification rates are obtained for a pH between 6.5 and 9
During the anoxic stage, the nitrates from the preceding aerobic stage are reduced to gaseous molecular nitrogen.

 These two stages can be implemented either in two successive basins, or in a single basin. In the latter case, aeration sequencing is carried out. the activation of the aeration corresponds to an aerobic step and the stopping of the aeration corresponds to an anoxia step The aerabon is generally done by bringing the effluent to be treated into contact with air , generally artificially, for example by stirring the surface of the effluent and the biomass or by injecting air at the bottom of the reactor. It can also be done by introducing an oxygenated gas into the effluent. The latter method improves the removal efficiency of nitrogen compounds and increases the treatment capacity of stations compared to simply contacting with air.

baisse du pH est un inconvénient, car il y a un risque de diminution de la cinétique de nitrification, voire d'tnhibition totale ou partielle de la réaction de nitrification. However, it has been observed that the introduction of an oxygenated gas during the aerobic stages could also lead to a sharp lowering of the pH. This

lowering the pH is a drawback, since there is a risk of a reduction in the kinetics of nitrification, or even of total or partial inhibition of the nitrification reaction.

 An object of the present invention is to provide a process for the biological treatment of aqueous effluents with elimination of the nitrogen compounds putting in

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work of aerobic stages with introduction of an oxygenated gas which does not have the above drawbacks and for which in particular the pH is controlled
For this purpose, the invention relates to a process for the biological treatment of an aqueous effluent comprising steps of nitrification and denitrification, where the aqueous effluent is alternately subjected in a reactor: * to an anoxic step, and * to a step aerobic during which the effluent is brought into contact with at least one oxygenated gas, the sequence of these two stages forming an aerobic / anoxic sequence.

The method therefore implements a succession of sequences alternately linking an anoxic step, then an aerobic step, an oxygenated gas being injected during this last step. By oxygenated gas is meant any type of gas comprising oxygen molecules
According to a first essential characteristic of the invention, during the aerobic stages, the contacting of the effluents with the oxygenated gas is interrupted, as soon as it is revealed, by dynamic analysis, of the characteristic point of the end of the nitrification.

 This dynamic analysis can be carried out in different ways depending on the mode of introduction of the oxygenated gas.

parmi le potentiel d'oxydo-réduction des effluents en cours de traitement et la concentration en oxygène dissous mesurée au sein du réacteur. L'analyse consiste à calculer en continu les valeurs des dérivées première et seconde de la courbe potentiel d'oxydo-réduction = f (temps) et/ou de la courbe concentration en oxygène dissous = f (temps). L'injection de gaz oxygéné est arrêtée dès que ces valeurs sont positives et que la valeur de la dérivée seconde est supérieure à un seuil prédéterminé Ce seuil doit être ajusté pour chaque effluent traité et chaque installation utilisée : un paramétrage des conditions opératoires permet de le déterminer. Ce paramétrage est à la portée de l'homme du métier. L'analyse dynamique à débit fixe peut être mise en oeuvre selon l'enseignement de la demande de brevet FR-A-2756273. Thus, in a first case, the oxygenated gas can be injected at a fixed rate. For a fixed flow, dynamic analysis can be carried out on at least one selected signal

among the oxidation-reduction potential of the effluents during treatment and the dissolved oxygen concentration measured within the reactor. The analysis consists in continuously calculating the values of the first and second derivatives of the redox potential curve = f (time) and / or of the dissolved oxygen concentration curve = f (time). The injection of oxygenated gas is stopped as soon as these values are positive and the value of the second derivative is greater than a predetermined threshold This threshold must be adjusted for each effluent treated and each installation used: a setting of the operating conditions makes it possible to determine. This configuration is within the reach of those skilled in the art. The fixed rate dynamic analysis can be implemented according to the teaching of patent application FR-A-2756273.

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In a second case, the oxygenated gas can be injected at a regulated variable rate.

In this case, the flow rate of the oxygenated gas is regulated so that at least one quantity characteristic of the biological activity reaches a given set value. This quantity characteristic of the biological activity measured within the reactor can be: the oxidation-reduction potential, at a dissolved oxygen concentration, the pH the concentration in pollutants elements such as NH4 or N0, fluorescence or any other parameter correlable to biological activity Depending on the value of these quantities, the flow of oxygenated gas is increased or decreased. For example, the flow rate of oxygenated gas can be regulated so that the dissolved oxygen concentration constantly tends towards a set value. Thus, if a drop in the dissolved oxygen concentration is measured, the flow rate of the oxygenated gas is increased and vice versa The dynamic analysis can then be carried out on any signal linked to the injectino of oxygenated gas such as the signal linked to the opening of the valve for adding oxygenated gas or the value of the flow of oxygenated gas added.

 According to the same principle, during the process according to the invention, the aerobic stages are preferably started as soon as the characteristic point of the end of biological denitrification is revealed, by dynamic analysis.

Generally, the dynamic analysis is carried out on the redox potential of the effluent during treatment: the values of the first and second derivatives of a redox potential curve = f (time) are measured continuously and injection of the oxygenated gas is started as soon as these values are negative and the value of the second derivative is less than the predetermined threshold defined above.

 In addition. according to a second essential characteristic of the invention, during at least one of the aerobic stages of the process, the aqueous effluent is brought into contact with an oxygenated gas. By oxygenated gas is meant a gas having an oxygen concentration of at least 21% by volume, preferably at least 30% by volume. It can be technical oxygen (at least 90% by volume of oxygen) or oxygen-enriched air. The oxygenated gas can also come from a later stage of recovery of gaseous vents from a post-ozonization of the treated effluents.

 Quite preferably, the method comprises at least the succession of two aerobic / anoxic sequences for which the oxygenated gas brought into contact with the aqueous effluent in the two aerobic stages has

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en oxygène du gaz oxygéné mis au contact de l'effluent à l'étape aérobie précédente ou suivante. Ainsi, au cours d'au moins une des étapes aérobies du procédé. un gaz suroxygéné est mis en contact avec l'effluent Ce gaz suroxygéné est mis au contact avec l'effluent en plus de l'air utilisé comme gaz oxygéné de base à chaque étape aéroble. De préférence, ce premier mode est mis en oeuvre de manière à ce que l'enchaînement de ces deux séquences aérobie/anoxlque (où une étape aérobie met en oeuvre de l'air et la deuxième étape aérobie met en oeuvre de l'air et un gaz suroxygéné) se répète tout au long du procédé.

different oxygen contents in these two aerobic stages. This preferred implementation of the invention can be carried out according to at least three different modes
According to a first mode, the method according to the invention is implemented so that during all of the aerobic stages of the method at least air is brought into contact with the effluent, preferably artificially , that is to say by agitation or buttape. In this first mode, air is the basic oxygenated gas brought into contact with the effluent at each aerobic stage. In addition, as indicated above, during at least one aerobic stage of the process, the oxygenated gas brought into contact with the effluent has an oxygen content different from the content

in oxygen of the oxygenated gas brought into contact with the effluent in the preceding or following aerobic stage. Thus, during at least one of the aerobic stages of the process. an oxygenated gas is brought into contact with the effluent This oxygenated gas is brought into contact with the effluent in addition to the air used as basic oxygenated gas at each aerosol stage. Preferably, this first mode is implemented in such a way that the sequence of these two aerobic / anoxic sequences (where an aerobic step uses air and the second aerobic step uses air and an oxygenated gas) repeats itself throughout the process.

According to a second mode, the method according to the invention is implemented so that during all of the aerobic stages of the method at least one oxygenated gas is injected. In this second mode, the oxygenated gas is the basic oxygenated gas brought into contact with the effluent at each aerobic stage. In addition, according to the preceding principle, during at least one aerosol stage of the process, the oxygenated gas brought into contact with the effluent has an oxygen content different from the oxygen content of the oxygenated gas brought into contact with the effluent. in the previous or next aerobic stage. Thus, during at least one of the aerobic stages of the process, air can be brought into contact with the effluent. This air is therefore brought into contact with the effluent in addition to the oxygenated gas used as basic oxygenated gas at each aerobic stage. Preferably, this second mode is implemented in such a way that the sequence of these two aerobic / anoxic sequences (where an aerobic step uses an oxygenated gas and the second aerobic step uses air and a oxygenated gas) is repeated throughout the process.

According to a third mode, the method according to the invention is implemented so that during the aerobic stages of the method, uent is implemented

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suroxygéné. Dans le cas de l'air, la mise en contact est généralement réalisée de manière artificielle
Les deuxième et troisième modes sont particulièrement recommandés lorsque le procédé s'applique à une unité de traitement d'effluents présentant soit une surcharge polluante par rapport à sa capacité initiale de dimensionnement, soit une sous-capacité d'oxygénation. contact either only with an oxygenated gas, or only with air. Preferably, according to this third mode, the method mainly comprises aerobic steps using only an oxygenated gas. The majority is understood to mean at least 50% of the aerobic stages. According to a particular variant, at least two out of three aerobic stages use only one gas

oxygenated. In the case of air, contacting is generally carried out artificially
The second and third modes are particularly recommended when the method applies to an effluent treatment unit having either a polluting overload relative to its initial design capacity, or an oxygenation undercapacity.

 Any artificial means of bringing the effluent to be treated and the oxygenated gases into contact can be used in the context of the invention. They may be turbines, brushes or surface agitators projecting the effluent to be treated into the oxygenated gaseous sky. Oxygenation can also be obtained by injecting oxygenated gas in the form of bubbles: it is thus possible to use porous diffusers, electors placed at the bottom of the basin, submerged turbines and supplied with both effluent to be treated and oxygenated gas.

 According to the invention, the pH of the effluent to be treated can be maintained at an optimal value, which makes it possible to optimize the kinetics of the biological reactions. The method according to the invention also ensures good adaptation to a polluting load: rejection standards can be met even in the event of load surges. The method according to the invention also allows optimization of energy consumption by eliminating unnecessary aerobic times and complete complementarity of systems for bringing air and oxygenated gas into contact with the effluent.

du bassin dans lequel ont lieu des étapes de nitrification/dénitrification, soit dans le même bassin. Dans ce dernier cas, une étape anaérobie est ajoutée à l'enchaînement des étapes aérobie/anoxique, de préférence après l'étape anoxique
Le procédé selon l'invention est particulièrement adapté au traitement des effluents aqueux contenant des composés azotés sous forme d'azote organique If the effluent to be treated requires a step of elimination of phosphor compounds by biological means. this step can be carried out either in a separate tank

of the basin in which the nitrification / denitrification stages take place, ie in the same basin. In the latter case, an anaerobic step is added to the sequence of aerobic / anoxic steps, preferably after the anoxic step.
The method according to the invention is particularly suitable for the treatment of aqueous effluents containing nitrogenous compounds in the form of organic nitrogen

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et/ou d'ammonium, tels que les effluents urbains, les effluents issus des industries agro-alimentaires et certains effluents chimiques, notamment ceux dont le ratio DCO/NTK (Demande Chimique en Oxygène/Azote Total Kjeldhal) présente une valeur comprise entre 10 et 15.

and / or ammonium, such as urban effluents, effluents from the food industry and certain chemical effluents, especially those whose COD / NTK (Chemical Oxygen Demand / Total Kjeldhal Nitrogen) ratio has a value between 10 and 15.

EXAMPLES
The invention is implemented in the aeration basin of a biological treatment station for industrial wastewater of the agro-food type with a capacity of 3,500 equivalent inhabitants, on an activated sludge with low load. The COD / NTK ratio of the water to be treated is 14, which is favorable for good detection of the significant point at the end of the aerobic nitrification stage by dynamic analysis of the signals of the redox potential and of the dissolved oxygen concentration.

 The aeration tank includes: * a porous system placed at the bottom of the tank and supplied with air by blowers blowing fine balls of air into the effluent,. an oxygenation turbine provided with a motor, a drive shaft and stirring blades, arranged on the surface so that the pa. are below the surface of the effluent, connected to a storage of liquid oxygen and delivering gaseous oxygen having an oxygen concentration greater than 99% by volume, called pure oxygen, * a probe for measuring dissolved oxygen, # a sensor for measuring the oxidation-reduction potential, # a probe for measuring pH, # the means of acquisition, continuous processing of data and control, including in particular an automaton.

 The basin, in which the activated sludge is located, is continuously supplied with the water to be treated. The mixture is subjected in this basin to a succession of aerobic and anoxic stages. The aerobic stages are obtained by starting up the boosters and / or the oxygenation turbine. The anoxic stages are obtained by stopping the blowers and the turbine. The basin is continuously fed. The flow rates of introduction of air or pure oxygen are fixed for each implementation. Example 1-Comparative example

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In this example, during the aerobic stages, the oxygenated gas is introduced only by the turbine introducing pure oxygen. Consequently, the method consists in implementing alternately: * an aerosol stage with the introduction of pure oxygen, then * an anoxic stage.

anoxiques ont la même durée D1. Toutes les étapes aérobies ont a même durée D2. Une mesure continue du pH montre que sa valeur diminue à chaque étape aérobie, jusqu'à une valeur d'équilibre de 6, 2. La cinétique de nitrification s'en trouve affectée, tout comme les performances d'élimination de l'azote : le taux d'azote dans les eaux traitées est supérieur aux normes en vigueur pour le site. The changeover from the anoxlque step to the aerobic step and vice versa is done automatically by setting a timer imposing an oxygen introduction time, then a stop time for oxygen introduction. All stages

anoxic have the same duration D1. All aerobic stages have the same duration D2. A continuous measurement of the pH shows that its value decreases at each aerobic stage, up to an equilibrium value of 6, 2. The kinetics of nitrification are affected, as are the nitrogen removal performance: the nitrogen level in the treated water is higher than the standards in force for the site.

Example 2-First Mode According to the Invention
In this example, during all the aerobic stages, air is systematically introduced via the porous system by switching on the boosters during all the aerobic stages. In addition, pure oxygen is injected by the oxygenation turbine during one out of three aerobic stages. The moment of switching from an aerobic step to an anoxic step, and vice versa, is determined by dynamic analysis of the redox potential curve = f (time).

 Under these conditions, a continuous measurement of the pH shows that its value varies between 6.8 and 7. The nitrogen removal performance is good: the nitrogen level in the treated water is lower than the standards in force for the site.

Example 3-Third Mode According to the Invention
In this example, the water to be treated is injected with: * one aerobic step in three: only air via porous diffusers, * two aerobic steps in three: only pure oxygen via the oxygenation turbine.

 The moment of switching from an aerobic step to an anoxic and vice versa step is obtained by dynamic analysis of the redox potential curve = f (time).

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 Thanks to this operating mode, it is observed that the pH oscillates between 6.85 and 7, which is satisfactory for the nitrifying activity.

Claims (16)

 1. A method of biological treatment of aqueous effluents comprising steps of nitrification and denitrification, where the aqueous effluents are alternately subjected in a reactor: * to an anoxic step, and

 '' at an aerobic stage during which the aqueous effluent is brought into contact with at least one oxygenated gas, the sequence of these two stages forming an aerobic / anoxic sequence, characterized in that: * during the aerobic stages, bringing the effluent into contact with the oxygenated gas is interrupted, as soon as it is detected, by dynamic analysis. of

 characteristic point of the end of nitrification, and 'during at least one of the aerobic stages of the process, the oxygenated gas is an oxygenated gas. 2. Method according to claim 1, characterized in that the oxygenated gas is injected at a fixed rate 3. Method according to claim 1 or 2, characterized in that the dynamic analysis is carried out on at least one signal chosen from the redox potential and the dissolved oxygen concentration measured within the reactor.  4. Method according to claim 1, characterized in that the oxygenated gas is injected at a regulated variable flow 5. Method according to claim 4, characterized in that the variable flow rate of the oxygenated gas is regulated so that at least one quantity characteristic of the biological activity reaches a given set value.  6. Method according to claim 5, characterized in that the quantity characteristic of the biological activity measured within the reactor is chosen from

 . the oxidation-reduction potential, the concentration of dissolved oxygen, the pH of the effluent, the concentration of pollutants such as NH4 + or N0, fluorescence. <Desc / Clms Page number 10>

7. Method according to claim 5 or 6, characterized in that the dynamic analysis is carried out on a signal related to the injection of oxygenated gas such as the opening dz da valve for adding oxygenated gas or the value of the flow of oxygenated gas added. 8. Method according to any one of the preceding claims, characterized in that the aerobic stages are started as soon as the dynamic, characteristic analysis of the end of denitrification bio) og) is revealed. 9. Method according to claim 8, characterized in that the dynamic analysis is carried out on the redox potential measured within the reactor 10. Method according to any one of the preceding claims, characterized in that the method comprises at least the succession of two aerobic / anoxic sequences for which the oxygenated gas brought into contact with the aqueous effluent in the two aerobic stages has contents in different oxygen in these two aerobic stages.  11. Method according to claim 10, characterized in that during all the aerobic stages of the process at least air is brought into contact with the aqueous effluent.  12. Method according to claim 11, characterized in that the sequence of the two aerobic / anoxic sequences (where an aerobic step uses air and the second aerobic step uses air and an oxygenated gas) repeats itself throughout the process.  13. Method according to claim 10, characterized in that during all the aerobic stages of the method, at least one oxygenated gas is brought into contact with the aqueous effluent. <Desc / Clms Page number 11>

14. Method according to claim 13, characterized in that during at least one of the aerobic stages of the process, air is brought into contact with the aqueous effluent.  15. The method of claim 10, characterized in that the aerobic steps of the process involve contacting the aqueous effluent either only with an oxygenated gas, or only with air.  16. The method of claim 15, characterized in that the method mainly comprises aerobic steps implementing) the contacting of an aqueous uent only with an oxygenated gas.