WO1999024370A1

WO1999024370A1 - Method for the control of biodegradation

Info

Publication number: WO1999024370A1
Application number: PCT/DK1998/000486
Authority: WO
Inventors: Peter NØRGAARD; Mette Risum Mikkelsen; Nicolas Heinen
Original assignee: BioBalance AS
Current assignee: BioBalance AS
Priority date: 1997-11-10
Filing date: 1998-11-10
Publication date: 1999-05-20
Anticipated expiration: 2000-05-10
Also published as: AU1144399A

Abstract

A method for controlling biodegradation of an aqueous medium containing biodegradable material comprising nitrogen-containing components, e.g. waste water in a waste water treatment plant, the method comprising assessing the value of at least one parameter that represents the ammonia concentration of the aqueous medium, e.g. NADH fluorescence, comparing the assessed value or a series of assessed values with predetermined criteria for the at least one measured parameter, and adjusting the oxygen concentration of the aqueous medium based on the comparison to optimise biodegradation of the nitrogen-containing components. The method allows automatic and accurate determination of the end of the nitrification and denitrification phases, allowing the biodegradation process to be run more efficiently.

Description

METHOD FOR THE CONTROL OF BIODEGRADATION

FIELD OF THE INVENTION

The present invention relates to a method for controlling biodegradation of an aqueous medium containing nitrogen-containing biodegradable material, e.g. waste water, and to a method for determining the concentration of ammonia in an aqueous medium.

BACKGROUND OF THE INVENTION

The removal of biodegradable material from e.g. municipal and industrial waste water is often performed by including some sort of biological treatment step in the water purification process. Normally, complex cultures of microorganisms are used to effect the biodegradation, resulting in a conversion of the biodegradable material into environmentally acceptable compounds such as CO₂ and N₂.

It is especially desired to reduce the amount of organic matter and at the same time to reduce the amount of nitrogen-containing components present in the waste water. Such elimination of nitrogen-containing components from waste water has proved to be difficult and resource consuming. The goal is to convert the nitrogen bound in nitrogen-containing components of waste water into gaseous (atmospheric) nitrogen, and this is traditionally done by the steps of nitrification (an oxidation step) and denitrification (a reduction step). Prior to these steps, complex nitrogen-containing substances are deaminated by i.a. deaminases produced by the microorganisms (or optionally supplied to the system in question) and the remaining main problem is thus to convert ammonia into gaseous nitrogen.

The general scheme for removing nitrogen from waste water is the following:

Nitrification: NH₄ ⁺ + 1 ^ΛA O₂ → NO₂ ^_ + 2 H⁺ + H₂O

NO₂- + % O₂ → NO₃ ^" Denitrification: 10 [H] + 2 H⁺ + 2 NO₃ ^" → N₂ + 6 H₂O

Both the nitrification and denitrification reactions are facilitated by the microorganisms which are responsible for the biodegradation, but as nitrification is facilitated by high oxygen concentrations and denitrification is facilitated by low oxygen concentrations, one of two principles has generally been used:

A) The biodegradation process is subjected to intermittent aeration, whereby the two processes are substantially non-simultaneous. One example of such a process is described in US 5,304,308.

B) The biodegradation is compartmentalized in such a way that some compartments have a high oxygen concentration whereas others have a low oxygen concentration. Examples of such processes are disclosed in EP-A-218 289 and in EP-B-233 466.

Alternative A) is rather time consuming and furthermore involves a high energy consumption, as the supply of oxygen to the system requires much energy for the operation of aeration pumps, etc.

Alternative B) overcomes the problem of time-consumption, but at the expense of space- consumption. The need for a large amount of space means that such processes are mainly used in large-scale water purification.

Both of the above types of processes suffer further drawbacks, such as the fact that the mixed cultures of microorganisms responsible for the biodegradation are sensitive to changes in the oxygen concentration of their environment, and difficulties with respect to determining for how long the waste water should be processed under each of the two sets of conditions.

A solution to the problems of the above-discussed approaches is proposed in WO 96/35644, which discloses a process that allows simultaneous nitrification and denitrification in an aqueous medium containing a biodegradable material such as waste water comprising nitrogen-containing components. This is achieved by controlling the living conditions of the microorganisms in such a manner that their metabolic activity is kept within a narrow range allowing simultaneous nitrification and denitrification to take place. In particular, the oxygen concentration is kept within a narrow range below 1 mg/l (below 1 ppm). The problem remains, however, how to "fine-tune" systems of this type, as well as other biodegradation systems, in order to be able to achieve the optimum result in any given system. It has been reported that the change from an anoxic to an anaerobic condition (i.e. conversion of nitrate to N₂) is represented by an increase in fluorescence (S. Isaacs and M. Henze, 1994: Fluorescence monitoring of an alternating activated sludge process. Wat.Sci. Tech. Vol. 30, pp. 229-238).

WO 96/03254 describes an apparatus and a method for real time monitoring of the biological activity of waste water and for controlling the treatment thereof by means of detection of NADH fluorescence of a waste water sample isolated from the bioreactor tank. It is explained in this document that the apparatus can be used in the oxic stage to serve as a NH₃ meter. However, this involves, in addition to a first sample chamber containing the sample being analysed, a separate second sample chamber to which a known amount of NH₃ is added, and a subsequent calculation of the amount of NH₃ in the sample being analysed based on the known amount of NH₃ added to the second sample chamber as well as certain other assumptions. This is thus an indirect and rather complicated approach to the problem of determining the NH₃ concentration and using the results of the determination to control the biodegradation process.

BRIEF DISCLOSURE OF THE INVENTION

According to the present invention, it has now surprisingly been found that measured parameters such as the concentration of NADH in the medium can be used to determine both the end of nitrification and the end of denitrification in a biodegradation process, thereby allowing more precise control of the biodegradation process. As a result, it is possible to perform such biodegradation processes in a more cost- and time-efficient manner, since the determined values for e.g. NADH concentration can be used to automatically control the oxygen level in the water being treated, thereby improving the efficiency of the waste water treatment process.

It has in addition been found that such measurements of e.g. NADH concentration can be used to determine the ammonia concentration in an aqueous medium in a simpler manner than prior art methods for determining ammonia concentration. Although it is contemplated that this finding will be generally applicable to determining the ammonia concentration of any ammonia-containing aqueous medium, it is of particular interest in its relation to possibilities for controlling and optimising biodegradation in biological waste water treatment processes.

An object of the present invention is therefore to provide a method for optimising the biological treatment of a nitrogen-containing aqueous medium, e.g. waste water, based on determinations of e.g. the NADH concentration in the medium. Another object of the invention is to provide a simpler method for determining the ammonia concentration of an ammonia-containing aqueous medium.

One aspect of the invention thus relates to a method for controlling biodegradation of an aqueous medium containing biodegradable material comprising nitrogen-containing components, the method comprising

assessing the value (assessed value) of at least one parameter (measured parameter) that represents the ammonia concentration of the aqueous medium,

comparing the assessed value or a series of assessed values with predetermined criteria for the at least one measured parameter, and

- adjusting the oxygen concentration of the aqueous medium based on the comparison to optimise biodegradation of the nitrogen-containing components.

Another aspect of the invention relates to a method for determining the concentration of ammonia in an aqueous medium subject to biodegradation of nitrogen-containing components in the medium, the method comprising monitoring fluorescence emission values from at least one characteristic biogenic fluorophore in the medium over a period of time, and

calculating an ammonia concentration based on the monitored fluorescence emission values and a predetermined rate of conversion of ammonia to nitrate in the medium.

Additional objects and aspects of the invention will be apparent from the description below. DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, a measured parameter that "represents" the ammonia concentration of an aqueous medium is a parameter whose value is related to the ammonia concentration of the medium, i.e. the measured parameter varies with varying ammonia concentrations. However, it is not necessary in the present context that there be any fixed or direct correlation between a given value of the parameter and the actual ammonia concentration.

The expression "predetermined criteria" refers to a predefined set of criteria with which the assessed value, or, more typically, a series of assessed values, is to be compared in the method of the invention or in an individual phase of the method. The predetermined criteria can comprise a predetermined single value or a predetermined range of values for the measured parameter, and/or the predetermined criteria can comprise a set of one or more conditions that, when fulfilled, trigger a response in terms of adjustment of the oxygen concentration of the aqueous medium using predefined set-points. The predetermined criteria may, for example, comprise sets of conditions for measurements of fluorescence emission that, when fulfilled, indicate that a nitrification or denitrification phase has been completed, thereby allowing the oxygen concentration of the medium to be adjusted accordingly so that the desired next phase of the biodegradation process can begin.

As used herein, the term "controlling" denotes the act of regulating or deliberately influencing one or more variables of a process on the basis of measurements of one or more of the variables of the process. The latter variable is denoted the measured variable, whereas the first-mentioned variable is conventionally denoted the controlled variable. The desired numerical value of the controlled variable is referred to as the set-point.

As used herein, the term "biodegradable material" refers to organic and/or inorganic matter which is biologically decomposable, such decomposition taking place by subjecting the organic and/or inorganic matter, especially organic matter, to a transformation process effected by cultures of microorganisms, the transformation process taking place in an aqueous environment, for example waste water, sewage, lake water, sea water, river water and the like. The microorganisms use the biodegradable material as a source of nutrition and/or energy, thus converting the biodegradable material into additional biomass and to end products of metabolism such as nitrates, gaseous nitrogen, sulphates, phosphates, carbon dioxide, etc.

The terms "nitrogen containing substances" and "nitrogen containing components" as used herein refer to ammonia, nitrates, nitrites, proteins, amino acids, purines, pyrimidines, nucleic acids, nucleosides, nucleotides and other organic/inorganic compounds that contain nitrogen.

The expression "biodegradation" (or biological treatment) refers to the process in which microorganisms metabolize biodegradable material present in an aqueous medium. In the art of waste water purification, the aqueous medium is introduced into a tank, a basin or the like normally containing mixed cultures of microorganisms, i.e. activated sludge (biomass), wherein the biodegradable material in the aqueous medium to be treated is degraded by the microorganisms present.

The expression "optimise" in the context of optimising biodegradation herein refers to controlling the biodegradation process in such a way that the overall process results in a satisfactory or desired biodegradation of organic matter, in particular nitrogen-containing components, the general aim being to obtain biodegradation that is as effective and efficient as possible under the given circumstances.

The "aqueous medium" referred to herein contains water as the basic predominant constituent, e.g. typically at least about 80% by weight, more typically at least about 90% by weight, of water.

When controlling biodegradation according to the invention, the aqueous medium will typically be selected from waste water such as municipal waste water or industrial waste water, purified waste water, surface water, especially surface water for use as tap water, sea water, polluted sea water, or other aqueous systems containing biodegradable material as defined herein. As used herein, the term "waste water" refers to aqueous effluents containing organic and/or inorganic substances which are present or formed in an environment as a consequence of the presence and/or activity of human beings.

The expression "microorganisms" refers to organisms such as autotrophic as well as heterotrophic and aerobic, anaerobic or facultative bacteria, as well as lower eucaryotic organisms such as protozoa, yeasts, fungi, and other organisms usually present in activated sludge in the biological treatment step of a waste water purification plant, for example multicellular organisms such as slipper animalcule (Paramaecium) and parasites, especially bacteria-consuming parasites.

In the art of waste water purification, the microbial system used in the biological treatment steps is normally a mixed culture of microorganisms, i.e. comprising a variety of different species. The terms "activated sludge" or "biomass" are conventionally used terms for mixed cultures of microorganisms which are present in the biological treatment step in order to degrade the biodegradable material, i.e. especially decomposable organic and/or inorganic matter. The actual composition of the mixed cultures of microorganisms may vary widely since the composition is highly dependent on the prevailing conditions.

The method according to the invention for controlling biodegradation is suitable for use in any biodegradation process that includes biodegradation of nitrogen-containing components in an aqueous medium, including processes in which nitrification and denitrification are performed separately, e.g. with intermittent aeration or compartmentalised nitrification/denitrification, and processes in which nitrification and denitrification are performed simultaneously. Regardless of how the nitrification and denitrification is performed, the present invention will be advantageous for controlling and optimising the process.

The biodegradation process may, if desired, alternate between different phases, for example by performing, in sequence, 1) a simultaneous nitrification and denitrification phase using aeration to obtain an oxygen concentration in the range of at the most about 1.0 mg/l, e.g. about 0.1-1.0 mg/l, typically about 0.2-0.8 mg/l, 2) a nitrification phase using aeration to obtain an oxygen concentration in the range of about 0.2-3 mg/l, typically above about 1.0 mg/l, and 3) a denitrification phase without aeration. In this case, the relative length of the various phases can be adjusted as needed, and/or individual phases can be eliminated in one or more cycles of the process. This approach can also be used for determining the oxygen set-point for the simultaneous nitrification/denitrification, regardless of whether the simultaneous nitrification/denitrification is ultimately to be used alone or whether it is to be used in a sequence comprising additional, separate nitrification and denitrification phases. The oxygen set-point will be determined as a function of the denitrification time, the nitrification time, the temperature of the medium and general level of fluorescence. The denitrification and nitrification times may be determined by means of NADH fluorescence measurements as described herein, the denitrification time increasing with increasing nitrate concentrations, and the nitrification time increasing with increasing ammonia concentrations.

When alternating between nitrification and denitrification phases, the depletion of nitrate is seen as a rise in fluorescence, which in a preferred embodiment triggers a computer- controlled oxygen set-point to switch to the high oxygen concentration level. Oxygen is thus automatically introduced into the waste water, resulting in oxidation of ammonia to nitrate. In the following period the nitrification proceeds at an enhanced rate until ammonia is depleted. When ammonia becomes depleted, the fluorescence attains its minimum value, which triggers a reduction of the computer-controlled oxygen set-point, whereby the oxygen concentration attains its lower level by virtue of the oxygen supply being cut off or at least reduced. Reduction of nitrate then proceeds until the nitrate is depleted, which in turn once again leads to the automatic increase in the oxygen level as described above. The biodegradation process thus proceeds in this manner with alternating periods of either enhanced or reduced oxygen concentration, i.e. periods of nitrification or denitrification, respectively.

In one presently preferred embodiment, at least one phase of the biodegradation takes place as a simultaneous effective nitrification and denitrification, e.g. as disclosed in WO 96/35644, the contents of which is incorporated by reference. The expression "simultaneous effective nitrification and denitrification" is intended to denote that the aqueous medium is subjected to a biodegradation by the microorganisms which results in the simultaneous production of 1) nitrates from nitrogen-containing substances, in particular ammonia, and 2) gaseous nitrogen from the nitrates. The term "effective" in this context denotes that the final result is an aqueous medium with a total nitrogen concentration after biodegradation of at the most about 20 mg/l, preferably at the most about 15 mg/l, more preferably at the most about 10 mg/l, such as at the most 8 mg/l or at the most 5 mg/l. It will furthermore often be desired that the content nitrogen present as ammonia will be as low as possible, and the final NH₃-N content in the aqueous medium is therefore preferably no more than about 10 mg/l, more preferably no more than about 5 mg/l, such as no more than about 3 mg/l or no more than about 1 mg/l. In this embodiment with simultaneous nitrification/denitrification, the microorganisms are all subjected to substantially the same conditions (i.e. the metabolic level is sought kept at substantially the same level in all parts of the aqueous medium), meaning that there is no intentional physical division of the aqueous medium into e.g. zones of high and low oxygen concentration, respectively. Thus, the two reactions of nitrification and denitrification take place not only at the same time, but they also take place in parallel in the same tank, with no division into zones favouring either of the two processes of nitrification or denitrification. At the employed low general oxygen level, i.e. no more than about 1 mg/l, the oxygen concentration is made to oscillate within a range whose upper and a lower level allow both nitrification and denitrification to proceed simultaneously, although with varying rates for the nitrification and denitrification, respectively. The oxygen concentration thus oscillates around a steady state concentration at which nitrification and denitrification would proceed at an equal rate. During a period with reduced oxygen concentration, accumulated nitrate is denitrified at an enhanced rate until it is depleted. Similarly, during a period with enhanced oxygen concentration, accumulated ammonia is nitrified at an enhanced rate until it is depleted.

It will be understood that the method of the invention for controlling biodegradation is aimed at providing a favourable metabolic activity of the microorganisms, i.e. a catabolic state of the microorganisms which results in a high rate of biodegradation (i.e. only small amounts of energy are "wasted" in the anabolic metabolism of the microorganisms).

In particular when employing simultaneous nitrification and denitrification of biodegradable material, it is necessary to control parameters in the environment of the microorganisms in such a way that this is made possible. The simultaneous nitrification and denitrification results in an optimum or near-optimum balance being reached between 1) biodegradation of nitrogen-free components of the biodegradable material, 2) nitrification of nitrogen- containing components of the biodegradable material, and 3) denitrification of the nitrates produced as a result the nitrification.

Examples of parameters that may be measured in a biodegradation process are fluorescence emission from biogenic fluorophores, CO₂ concentration, oxygen concentration, biomass concentration, oxygen concentration/COD ratio, biodegradable material loading, oxygen concentration, pH, temperature, turbidity, dosage rate of precipita- tion chemicals, dosage rate of additional readily biodegradable carbon-containing material, dosage rate of substances capable of converting not readily biodegradable material into readily biodegradable material, rate of recycling of activated sludge, inlet flow rate, outlet flow rate, stirring rate, oxygen dosage rate, air dosage (aeration) rate, total amount of activated sludge in the system, and other process parameters which are conventional in treatment processes of water, waste water or the like. Such parameters may be measured by methods known to the person skilled in the art. As it will be apparent from the above description, the present invention is in particular based upon measurements of fluorescence emission from at least one characteristic biogenic fluorophore.

It is preferred that the measurements of fluorescence are performed using automatic, online measurements, as this renders possible a continuous surveillance of the processes, allowing action to be taken immediately when the relevant predetermined set of criteria is fulfilled, e.g. by means of computer controlled set-point adjustments.

The term "on-line automatization system" is intended to denote a system comprising online measurement equipment which is connected to effector equipment capable of controlling a process parameter. The effector equipment is fed with the information from the online measurements and controls the process parameter in an automated manner which is dependent on the incoming signal. Examples of such systems are negative feed-back systems, wherein a registration of values of a measured parameter indicating a change in a controlled parameter leads to the automatic regulation of the controlled parameter in a direction opposite that of the observed change. It is preferred that control of controlled parameters is effected by an on-line automatization system, although manual or semi- manual surveillance of measured parameters and subsequent manual or semi-manual adjustment of controlled parameters can of course be performed alternatively or additionally, if desired. It is especially preferred to use on-line fluorescence sensor equipment to measure fluorescence emission.

As used herein, the term "biogenic fluorophore" denotes a substance synthesized by living material (living cells), the molecules of such a substance being capable of fluorescing when irradiated with light. Biogenic (biological) fluorophores include proteins, especially tryptophan- and tyrosine-containing proteins, tryptophan- and tyrosine-containing peptides, tryptophan- and tyrosine-containing derivatives of amino acids, co-factors, purines, pyrimidines, nucleosides, nucleotides, nucleic acids, steroids, vitamins and others. In this context, NADH (nicotinamide adenine dinucleotide) and NAD(P)H are preferred examples of biogenic fluorophores. Other examples of biological substances capable of fluorescing are tyrosine, tryptophan, ATP (adenosine triphosphate), ADP (adenosine diphosphate), adenine, adenosine, estrogens, histamine, vitamin A, phenylalanine, p-aminobenzoic acid, dopamine (3,4-dihydroxyphenylethylamine), serotonin (5-hydroxytryptamine), dopa (3,4- dihydroxyphenylalanine), kynurenine and vitamin B12.

The terms "fluorescence" and "fluorescence emission" refer to the emission of radiant energy by a molecule or ion in the excited state caused by absorption of radiant energy.

Each biochemical or chemical molecule (biogenic fluorophore) has a characteristic excitation and fluorescence spectrum. Usually, the fluorescence spectrum or fluorescence band is split into two or more peaks or maxima, each peak occurring at a specific wavelength. To detect the fluorescence emission of a fluorescing molecule, this emission is detected at a wavelength which is within the envelope of the fluorescence band for the fluorophore, preferably at a wavelength corresponding to a peak in the fluorescence spectrum. Also, the fluorophore should be irradiated with light emitted at a wavelength which is within the envelope of the excitation band for the fluorophore, preferably at a wavelength corresponding to a peak in the excitation band.

The term "characteristic" as used in connection with biogenic fluorophore(s) denotes that the biogenic fluorophore is one which is inherently produced by the living biological material in question, i.e. the mixed culture of microorganisms, in an amount reflecting the biological activity, for example the metabolic activity, of the living material. Typically, the biogenic fluorophores are present as intracellular substances in the microorganisms.

The excitation peak and fluorescence emission peak, respectively, of important examples of the above-mentioned fluorophores appear from Table 1 below:

TABLE 1

Examples of Biologically Important Fluorescent Substances

Excitation Fluorescence

Peak (nm) Peak (nm)

* tyrosine 275 303

3,4-dihydroxyphenylalanine 345 410

* tryptophan 287 348 kynurenine 370 490

5-hydroxytryptamine (serotonin) 295 330 phenylalanine 260 282

3,4-dihydroxyphenylethylamine

(dopamine) 345 410 histamine 340 480 vitamin A 372 510 flavins 450 535

NADH & NAD(P)H 340 460 p-aminobenzoic acid 294 345 vitamin B12 275 305 estrogens 285 325

ATP, ADP, adenine, adenosine 272 380

Responsible for protein fluorescence

It is preferred that in the practical use of the method of the invention, the light is emitted at a wavelength longer than 250 nm, especially 250-780 nm, for example about 340-360 nm, and that fluorescence emission is detected at wavelengths longer than 250 nm, preferably 250-800 nm, especially 280-500 nm, for example about 460 nm. The wavelength should of course be adapted to the particular system, in particular the kind of fluorophores present in the system. As indicated above, important embodiments of the method are embodiments wherein the fluorophore is a nicotinamide adenine dinucleotide such as NADH or NADPH. In this case, the excitation light is preferably emitted at a wavelength of about 340 nm, and the fluorescence emission is detected at a wavelength of about 460 nm. One reason for putting emphasis on measurements of these two fluorophores is that they are very susceptible to changes in the concentration of their oxidized counterparts NAD⁺ and NADP⁺; even a fractional decrease in NAD⁺ leads to a many fold increase in the concentration of NADH. Further, the concentration of NADH and NAD⁺ taken together in living cells is about 1 mM, corresponding to approximately 0.63 g/l of cells, meaning that a significant percentage of the dry matter in cells is comprised of NADH and NAD⁺.

When selecting the predetermined criteria of the at least one measured parameter for a biodegradation process or a phase thereof, it is normal practice according to the invention to employ empirical calibration, i.e. a biodegradation process is monitored with respect to its input and output values of parameters of interest, and at the same time values of the measured parameter are recorded. The chosen predetermined criteria or values for any given set of circumstances (e.g. the nature of the apparatus, the composition, temperature and pH of the water being treated, etc.) are those which will lead to a satisfactory or desired result based on such an empirical calibration.

Examples of parameters that may be controlled in a biodegradation process are oxygen concentration, biodegradable material loading, pH, temperature, turbidity, dosage rate of precipitation chemicals, dosage rate of additional readily biodegradable carbon-containing material, dosage rate of substances capable of converting not readily biodegradable material into readily biodegradable material, rate of recycling of activated sludge, inlet flow rate, outlet flow rate, stirring rate, oxygen dosage rate, air dosage (aeration) rate, total amount of activated sludge in the system, concentration of activated sludge in the aqueous medium, and other process parameters which are conventional in treatment processes of water, waste water or the like. All of these controlled parameters are well-known in the art as are the means of effecting their direct control.

As indicated above, however, the control of biodegradation according to the present invention is in particular performed by adjusting the oxygen concentration in the aqueous medium. Adjustment of the oxygen concentration is typically performed by adjusting the aeration rate, i.e. the rate at which oxygen, air or another oxygen-containing mixture is introduced into the aqueous medium. As explained above, this is preferably performed by means of automatic monitoring of the measured parameter, e.g. NADH fluorescence, the obtained values for the measured parameter being used to determine the completion of a nitrification or denitrification phase as explained below. This information regarding the status of a nitrification or denitrification phase is in turn used for regulating the inflow of air or oxygen by means of computer controlled set-points.

The determination of the end of a nitrification phase according to the invention may in general be performed as follows: Following the end of a denitrification phase (which is represented by a peak in NADH fluorescence), the oxygen concentration of the aqueous medium is increased by adding oxygen or air to the medium. At fixed intervals, e.g. every 5 minutes, the fluorescence of the medium is measured. For purposes of the invention, the end of nitrification may be defined based on a comparison of the latest 3 determined values for fluorescence and using the following criteria:

F_n-2 < F_n-1 and F_n - F_n.₂ > K

where F_n is the fluorescence measured at time n, and K is an empirically determined constant. This is explained in more detail in the examples below.

The determination of the end of nitrification in this manner is advantageous in that it is simple, inexpensive, accurate and reliable. The result is an accurate identification of the point at which the nitrate content reaches a maximum and the ammonium content reaches a minimum, so that the supply of air or oxygen to the system can be cut off as soon as the aqueous medium no longer contains any ammonia that can be converted to nitrate. One advantageous use of this knowledge is to be able to more accurately administer the addition of air or oxygen, in particular by avoiding unnecessary addition of air or oxygen for a period of time in which the ammonia has already been depleted. This in turn allows the nitrification/denitrification process to be performed more quickly and efficiently.

The present invention provides not only improved control of a biodegradation process, but also provides an improved and simplified method for determining the ammonia content of an aqueous medium. As will be further explained below with reference to the drawings, the ammonia concentration can be readily determined using information on the elapsed time of a given nitrification phase and the speed of conversion of ammonia to nitrate. This assumes zero order kinetics in the conversion of ammonia to nitrate, which, however, has been found to be the case.

This readily available information about the ammonia concentration of e.g. waste water in a waste water treatment process allows the ammonia concentration to be adjusted to provide the most efficient operation of the process. In addition, it allows the adjustment of other process parameters that also influence the overall process efficiency, for example the carbon content of the water.

In an alternative embodiment of the invention, the method for controlling biodegradation of waste water may be adapted to provide not only removal of nitrogen, but also removal of phosphorus. In this case, a phase for biological removal of phosphorus is provided following the end of a denitrification phase. Methods are known in the art for biological removal of phosphorus by an anaerobic process, typically by adding additional waste water to result in the release of bacterially bound phosphorus. Since such biological phosphorus removal takes place by an anaerobic process, the present invention is advantageous in that it makes it possible to precisely determine when the nitrate in the waste water has been depleted, as it is at this point that the necessary conditions for anaerobic removal of phosphorus are present. In addition, the optimum phase duration for a biological phosphorus removal phase may be determined as a function of the denitrification time, the nitrification time, the temperature of the medium and the general level of fluorescence. Further information is provided below in the examples.

In a further aspect, the present invention relates to an apparatus for controlling biodegradation of an aqueous medium containing biodegradable material comprising nitrogen-containing components, the apparatus comprising

means for assessing the value (assessed value) of at least one parameter (measured parameter) that represents the ammonia concentration of the aqueous medium,

means for comparing the assessed value with predetermined criteria for the at least one measured parameter, and

means for adjusting the oxygen concentration of the aqueous medium based on the comparison to optimise biodegradation of the nitrogen-containing components. A still further aspect of the invention relates to a waste water treatment plant comprising an apparatus as described above.

The invention is further illustrated in the following non-limiting examples.

EXAMPLES

Employing fluorescence technique using an excitation wavelength of 340 nm and detecting a fluorescence emission wavelength of 460 nm in a nitrogen removing plant with an alternating oxygen level, the emission pattern shown in Figure 1 was observed. Figure 1 illustrates the level of oxygen, ammonia and fluorescence, respectively, in the waste water treatment plant over a period of 12 hours. It may be seen from Figure 1 that the plant operates with intermittent addition of oxygen to the waste water being treated, each addition of oxygen leading to a decrease in the ammonia concentration due to oxidation of ammonia to nitrate (nitrification). After a fixed period of time the oxygen supply is cut off and accumulated nitrate is reduced in the resulting low-oxygen environment to molecular nitrogen (N₂) (denitrification).

The fluorescence changes over time in a manner that is the inverse of the changes in the oxygen concentration. The fluorescence increases during the non-aerated denitrification phase from a minimum to a peak level. During the aerobic nitrification phase, on the other hand, a sharp drop in the level of fluorescence from a peak level to a lower level occurs. This drop can be separated into 2 phases: in the beginning the fluorescence drops off quickly, after which it falls off more slowly. The latter phase represents a delay of the fluorescence in reaching a minimum level after the initiation of an aerobic phase. The higher the concentration of ammonia present in the medium, the longer the time that passes before the minimum level of the fluorescence is reached after initiation of the aerobic phase. Further, the minimum level of fluorescence coincides with the moment when the ammonia is depleted.

In Figure 1 , two nitrification phases differing in initial ammonia concentration have been marked as N-, and N₂, respectively. These phases can be subdivided into 2 distinct periods. The initial periods ni and n₂ are periods with vigorous nitrification, while the subsequent periods and ns₂ effectively represent surplus nitrification time. The distance between the 2 vertical unbroken lines delimiting n-i and n₂ respectively indicates the duration of the periods with vigorous nitrification. This coincides with the presence of ammonia. It is seen that a high initial ammonia concentration requires a long nitrification period (n₂), while a low initial ammonia concentration requires a short nitrification period (n^.

The corresponding denitrification phases D^ and D₂ can in a similar way be separated into periods di and d₂ characterized by vigorous denitrification. The subsequent periods dsj and ds₂ effectively represent surplus denitrification time. The distance between the corresponding vertical unbroken lines delimiting d^ and d₂, respectively, indicates the duration the corresponding denitrification period. It can be seen that a long nitrification period (n₂) is followed by a long denitrification period (d₂), the subsequent denitrification phase. This is to be expected, as much nitrate has been formed from the high amount of ammonia in the preceding phase. Similarly, a short nitrification period (n^ is seen to be followed by a short denitrification period (d-i). This information may be used as described below to control waste water treatment.

The periods ΠS_T and ns₂ are periods during which the medium was unnecessarily aerated, since there was no ammonia left that could be nitrified. Since the present invention allows rapid identification of the end of a nitrification period, it makes it possible to eliminate such non-productive periods in the biodegradation process, so that the process as a whole can be run much more efficiently. It is estimated that this increased efficiency can provide a reduction in the overall biodegradation time of up to about 40%, which of course also means correspondingly very substantial reductions in operating costs. This will also make it possible to achieve increased capacity for a given waste treatment plant size, or alternatively, to reduce waste treatment plant size for a given waste water load.

Estimation of the ammonia concentration

The concentration of ammonia at the end of the phase preceding a nitrification phase is estimated from the fluorescence measurement. This estimation is carried out in 3 steps: 1. Identification of the event of ammonia depletion

2. Calculation of the time consumed for the depletion of ammonia

3. Estimation of the initial ammonia concentration from the time consumed for the depletion of ammonia

The 3 steps used in the calculation are further explained in the following. Step 1. Identification of the event of ammonia depletion

The determination of the time when ammonia is depleted (T_n) is for example carried out in the following way:

It is assumed that i) the tank is in a N-phase and ii) the fluorescence is recorded at discrete intervals, i.e. every 5 minutes (Figure 1). At any event of fluorescence recording, the recorded value F_n at time T_n is compared to the 2 formerly recorded values F_n-ι and F_n.₂. The depletion of ammonia is recognized if the following 2 criteria are fulfilled;

F_n-₂ < F_n.ι ^Λ F_n - F_n-₂ > k_A (equation l)

where k_A is an empirical constant.

An example of the correlation between the time consumption estimated using this method (Δt_NH3,Fiuor) and the time consumption recorded using the ammonia data (Δt_NH3, colour) is shown in Figure 2. The depletion points were in this case calculated using k_A = 0.5 BPA (1 BPA = fluorescence of 1 ppb of coumarin in water). For the colourimetric ammonia determination (Δt_NH3,coio_ur), the instrument used was an "Evita in situ sensor NH4⁺" (Danfoss A/S, Denmark) employing an indophenol method.

Step 2. Calculation of the time consumed for the depletion of ammonia

The duration of the nitrification phase is calculated by subtracting the time of the start of the nitrification phase T₀ from the time when the depletion is identified according to the fluorimetric method desribed in step 1. If the conditions in equation 1 are fulfilled at time T_n , then the depletion time is T_n-2. Accordingly:

Δt_NH3,Fiuor = T_n-2 - T₀ (equation 2)

This is shown in Figure 1. Step 3. Estimation of the initial ammonia concentration from the time consumed for the depletion of ammonia

Figure 3 shows the empirical relationship between the ammonia concentration and the time consumed in depleting the ammonia (Δt_NH3,coiour)- The figure shows a linear correlation.

Figure 4 shows the empirical relationship between the initial ammonia concentration and the time consumption estimated from the local fluorescence minimum (Δt_NH3,_Ruor)- A linear correlation is also depicted in this case.

The initial ammonia concentration ([Ammonia]_ιnιtlai)is estimated using the following equation.

[Ammonia]_ιnιt,ai = Δt_NH3,Fiuor ^* r_A + c_A (equation 3)

where the rate constant r_A is the slope of the linear regression line of Figure 4 and c_A is the intercept. r_A is temperature-dependent and can be corrected using an Arrhenius expression.

Estimation of the nitrate concentration

The concentration of nitrate at the end of the phase preceding a denitrification phase is estimated from the fluorescence measurement in a manner analogous to the method described above for estimating the ammonia concentration. This estimation is carried out in 3 steps: 1. Identification of the event of nitrate depletion

2. Calculation of the time consumed for the depletion of nitrate

3. Estimation of the initial nitrate concentration from the time consumed for the depletion of nitrate

Further details are provided in the following.

Step 1. Identification of the event of nitrate depletion

The determination of the time when nitrate is depleted (T_n) is for example carried out in the following way: It is assumed that i) the tank is in a D-phase and ii) the fluorescence is recorded at discrete intervals, i.e. every 5 minutes (Figure 1). At any event of fluorescence recording, the recorded value F_n at time T_n is compared to the preceding recorded value F_n-ι. The depletion of nitrate is recognized if the following criteria is fulfilled:

F_n - Mean (F₀ F_n-ι) > k_N (equation 4)

where k_N is an empirical constant.

Step 2. Calculation of the time consumed for the depletion of nitrate

The duration of the denitrification phase is calculated by subtracting the time of the start of the denitrification phase T₀ from the time when the depletion is identified according to the fluorimetric method described in step 1. If the conditions in equation 1 are fulfilled at time T_n , then the depletion time is T_n. Accordingly:

Δt_N03,Fiuor = T_n - To (equation 5)

This is shown in Figure 1.

Step 3. Estimation of the initial nitrate concentration from the time consumed for the depletion of nitrate

The initial nitrate concentration ([Nitrate]_init,_ai) is estimated using the following equation:

[Nitrate],nB,ai = Δt_N03,Fiuor ^* r_N + c_N (equation 6)

where r_N is a rate constant and c_N is a constant. r_N is temperature-dependent and can be corrected using an Arrhenius expression.

A method for controlling nitrogen removal processes

From the observations described above, a method for the control of nitrogen removal processes employing the information given by the fluorescence measurements can be outlined. The nitrogen removal process is carried out according to the following general scheme using 3 distinct phases. The order of the 3 phases can be changed arbitrarily, and at least one of the phases can be omitted, if desired. The phases are the nitrification phase, the denitrification phase and the simultaneous nitrogen removal phase (see WO 96/35644 for a detailed description of simultaneous nitrogen removal). The nitrification phase and the denitrification phase are analytical phases in which the initial ammonia concentration and nitrate concentration may be estimated by means of the fluorescence measurements. Further information on the phases is listed in Table 2 below.

Table 2. Characteristics of different nitrogen removal phases. Abbreviation of phase names in parentheses.

Examples of the possible order of the different phases are given below:

Example 1 : N - DN - SIM - N - DN - SIM ...etc. Example 2 N - SIM - DN - SIM - N - SIM - DN etc

Example 3 N - DN - N - DN etc

In Example 3, the simultaneous denitrification is omitted This case represents a common alternating nitrogen removing plant such as the one that produced the results presented in Figure 1 This type of plant uses a fixed oxygen set-point in the nitrification phase, and the oxygen set-point of the denitrification phase is zero The duration of the individual phases in the practice of the invention is adjusted according to the concentration of nitrate and ammonia Additionally, the oxygen concentration of the two phases can be adjusted on the basis of the fluorescence measurement in a similar way as described for the SIM-phase below

Calculation of the oxygen set-point of the SIM-phase

The oxygen set-point in the SIM-phase is calculated from the above-described estimation of the initial concentration of ammonia and nitrate The temperature is used to correct the nitrification and denitrification rates Furthermore, the actual fluorescence is used

An example of a general function is given in the equation below

[O₂]set-poιnt =

+ f₂([Nιtrate]) + f₃(temperature) + f (Fluorescence) (equation 7)

fi, f₂, f₃ and f₄are functions that are fitted to empirical data fiis an increasing function of the initial ammonia concentration f₂ is a decreasing function of the initial nitrate concentration f₃ is a temperature correction expression f is a function of the fluorescence during the SIM-phase (cf WO 96/35644) These functions are optimized in order to make the treatment process cope as well as possible with the actual effluent requirements

Claims

1. A method for controlling biodegradation of an aqueous medium containing biodegradable material comprising nitrogen-containing components, the method comprising

- comparing the assessed value or a series of assessed values with predetermined criteria for the at least one measured parameter, and

adjusting the oxygen concentration of the aqueous medium based on the comparison to optimise biodegradation of the nitrogen-containing components.

2. A method according to claim 1 , wherein the assessed value of the measured parameter is determined by means of measurement of fluorescence emission from at least one characteristic biogenic fluorophore.

3. A method according to claim 2, wherein the biogenic fluorophore is selected from the group consisting of tryptophan- and tyrosine-containing proteins, tryptophan- and tyrosine- containing peptides, tryptophan- and tyrosine-containing derivatives of amino acids, purines, pyrimidines, nucleosides, nucleotides, nucleic acids, steroids and vitamins.

4. A method according to claim 2 or 3, wherein the fluorophore is NADH, NADPH or another nicotinamide adenine dinucleotide.

5. A method according to claim 4, wherein excitation light is emitted at a wavelength of about 340 nm, and fluorescence emission is detected at a wavelength of about 460 nm.

6. A method according to any of claims 1-5 wherein the oxygen concentration of the aqueous medium is adjusted by adjusting the rate at which oxygen or air is introduced into the medium.

7. A method according to any of claims 1-6, wherein the aqueous medium is waste water, and the biodegradation comprises at least one simultaneous nitrification/denitrification phase using aeration with air or oxygen to result in an oxygen concentration of at the most about 1.0 mg/l, e.g. in the range of about 0.1-1.0 mg/l.

8. A method according to any of claims 1-7, wherein the aqueous medium is waste water, and the biodegradation comprises alternating phases comprising at least one nitrification phase using aeration with air or oxygen to result in an oxygen concentration in the range of about 0.2-3 mg/l and/or at least one denitrification phase without any substantial aeration.

9. A method according to claim 8, including the steps of: identifying the end of a nitrification phase by means of assessed values for the measured parameter, adjusting aeration so as to reduce the oxygen concentration of the aqueous medium and initiate a denitrification phase, identifying the end of the denitrification phase by means of assessed values for the measured parameter, and adjusting aeration so as to increase the oxygen concentration of the medium and initiate a new nitrification phase.

10. A method according to claim 7 or 8, wherein the biodegradation further comprises at least one anaerobic phase for biological removal of phosphorus.

11. A method according to any of the preceding claims, wherein the assessment of the measured parameter is performed by on-line measurement, e.g. using on-line fluorescence sensor equipment, and wherein adjustment of the oxygen concentration is effected by means of an on-line automatization system.

12. An apparatus for controlling biodegradation of an aqueous medium containing biodegradable material comprising nitrogen-containing components, the apparatus comprising

means for assessing the value (assessed value) of at least one parameter (measured parameter) that represents the ammonia concentration of the aqueous medium, means for comparing the assessed value or a series of assessed values with predetermined criteria for the at least one measured parameter, and

means for adjusting the oxygen concentration of the aqueous medium based on the 5 comparison to optimise biodegradation of the nitrogen-containing components.

13. An apparatus according to claim 12, wherein the means for assessing the value of the measured parameter includes means for measuring fluorescence emission from at least one characteristic biogenic fluorophore.

1 0

14. An apparatus according to claim 12 or 13, wherein the means for assessing the value of the measured parameter includes means for on-line measurement of the measured parameter, e.g. on-line fluorescence sensor equipment.

1 5 15. An apparatus according to any of claims 12-14, comprising means for exciting biogenic fluorophores with excitation light having a wavelength above about 250 nm and means for detecting fluorescence emission at a wavelength of about 280-500 nm.

16. An apparatus according to any of claims 12-15 comprising means for automatically 20 adjusting the oxygen concentration of the medium using predefined set-points and automatic, on-line measurements of fluorescence emission.

17. An apparatus according to any of claims 12-16, wherein the means for comparing the assessed value with the predetermined criteria for the at least one measured parameter 5 includes means for identifying the end of a nitrification phase and the end of a denitrification phase, and the means for adjusting the oxygen concentration of the medium includes means for regulating the inflow of air or oxygen using computer controlled set- points so as to initiate a denitrification phase or a nitrification phase, respectively, at the end of a nitrification or denitrification phase, or to initiate a simultaneous

30 nitrification/denitrification phase.

18. A waste water treatment plant comprising an apparatus according to any of claims 12- 17, and/or wherein biodegradation is controlled by a method according to any of claims 1- 11. 5

19. A method for determining the concentration of ammonia in an aqueous medium subject to biodegradation of nitrogen-containing components in the medium, the method comprising

- monitoring fluorescence emission values from at least one characteristic biogenic fluorophore in the medium over a period of time, and

20. A method according to claim 19 wherein fluorescence emission values are monitored over a period of time comprising at least one nitrification phase.

21. A method according to claim 19 or 20, wherein the biogenic fluorophore is selected from the group consisting of tryptophan- and tyrosine-containing proteins, tryptophan- and tyrosine-containing peptides, tryptophan- and tyrosine-containing derivatives of amino acids, purines, pyrimidines, nucleosides, nucleotides, nucleic acids, steroids and vitamins.

22. A method according to any of claims 19-21 , wherein the fluorophore is NADH, NADPH or another nicotinamide adenine dinucleotide.

23. A method according to any of claims 19-22, wherein the assessment of the measured parameter is performed by on-line measurement of the measured parameter, e.g. using online fluorescence sensor equipment.