WO2008068040A1

WO2008068040A1 - Process and apparatus for the biological treatment of waste water

Info

Publication number: WO2008068040A1
Application number: PCT/EP2007/010784
Authority: WO
Inventors: Willy Verstraete; Ilse Forrez; Joseph G. M. Colsen
Original assignee: Universiteit Gent; Colsen Bv
Priority date: 2006-12-04
Filing date: 2007-12-04
Publication date: 2008-06-12
Also published as: GB0624167D0

Abstract

This invention provides a biological treatment process of waste water wherein a decrease of the phosphate concentration in water together with an ammonium ions removal step performed under controlled concentrations of dissolved oxygen and/or under controlled concentrations of nitrate and soluble manganese and/or iron ions, provides a significant decrease in the oxygen consumption and/or the sludge production of said process.

Description

PROCESS AND APPARATUS FOR THE BIOLOGICAL TREATMENT OF

WASTE WATER

FIELD OF THE INVENTION This invention relates to apparatus and a process for the biological treatment of waste water and other aqueous effluents from various types of industrial or agricultural activities. More specifically the present invention relates to apparatus and a process for the biological treatment of water containing ammonium ions, organic compounds, oxygen, phosphates and metallic ions like manganese and/or iron dissolved therein.

BACKGROUND OF THE INVENTION

Conventional methods for the treatment of waste water containing ammonium ions through activated sludge systems require a high degree of aeration contributing for a large part to operational costs. These methods of treatment typically involve a nitrification step (see equation 1 below) followed by a denitrification step (see equation 2 below).

NH₄ ⁺ + 2 O₂ -» NO₃ ^' + 2 H⁺ + H₂O (equation 1 )

6 NO₃ ^" + 5 CH₃OH -» 5 CO₂ + 3 N₂ + 7 H₂O + 6 OH^" (equation 2) While the nitrification step according to equation 1 consumes oxygen, the denitrification step according to equation 2 requires the addition of one or more organic compounds which contribute to the sludge formation and operational costs. Various attempts have been made to limit the amount of oxygen required. For instance, in U.S. Patent No. 5,078,884 a water purification process is described in which ammonium ions are oxidised by anaerobic ammonium oxidising bacteria during an anaerobic denitrification process according to the following equation: 5 NH₄ ⁺ + 3 NO₃- ^■* 4 N₂ + 9 H₂O + 2 H⁺ (equation 3)

This type of denitrification differs from the type of denitrification according to equation 2 by the fact that NH₄ ⁺ replaces the one or more organic compounds as the reducing agent.

In U.S. Patent No. 6,383,390 a water purification process is described in which ammonium ions having bicarbonate as the counter ions, are partially oxidised by aerobic ammonium oxidising bacteria during a nitrification process controlled by aeration such as to oxidise up to half of the available ammonium ions into nitrites (see equation 4) 2 NH₄ ⁺ + 3 O₂ -> 2 NO₂ ^' + 4 H⁺ + 2 H₂O (equation 4)

This type of " nitrification " will be referred to as " partial oxidation " of ammonium ions in order to avoid confusion.

The remaining ammonium ions may be oxidised by anaerobic ammonium oxidising bacteria by using the newly formed nitrite according to the following equation :

NH₄ ⁺ + NO₂- ^■* N₂ + 2 H₂O (equation 5)

In U.S. Patent No. 6,485,646, a water purification process is described in which ammonium ions are oxidised by aerobic oxidising bacteria during a partial oxidation step according to equation 4. This process occurs in a reactor having a hydraulic retention time shorter than the sludge retention time, itself being shorter than the doubling time of the nitrate-producing bacteria, with the consequence that nitrite producing bacteria are favoured over nitrate producing bacteria. Next, nitrites produced are either reduced using organic electron donors, e.g. carbohydrates or alcohols, or by providing ammonia which is itself oxidised to produce dinitrogen.

A problem encountered when organic compounds are added to water is the resulting increased sludge production. Another problem encountered when implementing a denitrification step according to equation 3 or equation 5 is the lack of sufficient activity of the ammonium oxidising anaerobic bacteria involved.

There is therefore a need in the art for improving existing waste water biological treatment methods. More specifically there is a need in the art for an improved biological ammonium ions removing process from waste water with a reduced oxygen consumption and/or a reduced sludge production. There is also a need in the art for an improved biological ammonium ions removing process with a denitrification step according to equation 3 or equation 5 with substantially increased efficiency. SUMMARY OF THE INVENTION

An object of the present invention is to provide improved apparatus and a process for the biological treatment of waste water and other aqueous effluents from various types of industrial or agricultural activities, more specifically the present invention provides apparatus and a process for the biological treatment of water containing ammonium ions, organic compounds, oxygen, phosphates and metallic ions like manganese and/or iron dissolved therein. An advantage of the present invention is that it can solve one or more of the various problems encountered by the prior art waste water biological treatment methods, especially as summarised herein-above (background of the invention).

Broadly speaking, the invention is based on the unexpected finding that a decrease of the phosphate concentration in water together with an ammonium ions removal process performed under controlled concentrations of dissolved oxygen and/or under controlled concentrations of manganese and/or iron ions, provides a significant decrease in the oxygen consumption and/or the sludge production of this biological treatment process. In the foregoing description, and unless otherwise stated, the term " manganese and/or iron ions " refers to one or more ions selected from the group of Mn²⁺, Mn⁴⁺, Fe²⁺ and Fe³⁺ preferably in a water-soluble form.

A significant advantage of the process of the present invention is to permit use of less oxygen and/or addition of less or no organic material for achieving a substantially complete removal of ammonium ions from waste water.

Another advantage of the present invention can be that less sludge is produced during the performance of the biological treatment method.

In an embodiment of the present invention the performance of a phosphate removing step accompanies or precedes, one or more ammonium ion removing steps. In one aspect, the present invention therefore relates to a multi-step water treatment method comprising a step aiming at decreasing the phosphate concentration of an aqueous effluent having phosphates and ammonium ions dissolved therein, a step aiming at oxidising said ammonium ions, and a step aiming at reducing the oxidised ammonium ions (e.g. nitrites and/or nitrates) thus produced using ammonium ions as electron donor, in the presence of controlled amounts of manganese and/or iron. In another embodiment, the present invention relates to a process for the biological treatment of water containing one or more organic compounds, oxygen, ammonium ions, one or more phosphates and one or more manganese and/or iron ions dissolved therein, said process comprising the steps of: a) decreasing the phosphate concentration of the water so as to increase the ratio chemical oxygen demand (COD) to phosphates concentration

(COD/P) above a predetermined level of about 10, b) optionally adapting the oxygen content in water obtained after step (a) such as to reach a concentration of dissolved oxygen between about 0.2 mg/l and the saturation concentration of dissolved oxygen (this step is not necessary if the oxygen level is already within the required range), c) oxidising at least part of the ammonium ions contained in water obtained after step (b) while maintaining a concentration of dissolved oxygen between about 0.2 mg/l and the saturation concentration of dissolved oxygen and optionally while maintaining the COD/P ratio above said predetermined level of about 10, d) adapting the oxygen content in water from step (c) such as to reach a concentration of dissolved oxygen below about 0.2 mg/l, and adapting the concentration of manganese and/or iron ions in water above a value (i.e. a concentration) sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, and e) converting a major portion of the oxidised ammonium ions from step (c) into dinitrogen by means of anaerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen below 0.2 mg/l and a concentration of manganese and/or iron ions sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria. A sludge produced by the process of this invention may be used as a fertiliser.

In yet another embodiment, the present invention relates to a system for the biological treatment of water containing ammonium ions and one or more manganese and/or iron ions dissolved therein, said system comprising a first reactor, a second reactor in fluidic communication with said first reactor, said second reactor comprising means for adapting the oxygen content of said water, a third reactor in fluidic communication with the second reactor, and a fourth reactor in fluidic communication with the third reactor, the fourth reactor comprising means for adapting the oxygen content in the water, wherein said system further comprises means for measuring the concentration of manganese and/or iron in the water. This embodiment involves upstream recycling from said reactors two, three and four.

The present invention also provides a processing plant for the biological treatment of water containing one or more organic compounds, oxygen, ammonium ions, one or more phosphates, and manganese and/or iron ions dissolved therein, the plant comprising: a) means for decreasing the phosphate concentration of said water such as to increase the ratio chemical oxygen demand (COD) to phosphate concentration (COD/P) above a predetermined level of about 10, b) means for oxidising at least part of the ammonium ions contained in water from the means for decreasing the phosphate concentration while maintaining a concentration of dissolved oxygen between 0.2 mg/l and the saturation concentration of dissolved oxygen and optionally while maintaining the ratio chemical oxygen demand (COD) to phosphate concentration above said predetermined level of about 10, c) means for adapting the oxygen content in water from the means for oxidising such as to reach a concentration of dissolved oxygen below 0.2 mg/l, and means for adapting the concentration of manganese and/or iron ions and nitrate in water above a value (i.e. a concentration) sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, and d) means for converting a major portion of the oxidised ammonium ions from the means for adapting the oxygen content into dinitrogen gas by means of anaerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen below 0.2 mg/l and a concentration of manganese and/or iron ions and nitrate sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria. The present invention, its advantages and embodiments will now be described with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of a process and installation according to a first embodiment of the present invention.

Figure 2 is a schematic view of a process and installation according to a second embodiment of the present invention.

Figure 3 is a schematic view of a process and installation according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described herein with respect to certain particular embodiments and with reference to a drawing, but the invention is not limited thereto but only by the claims. In the drawing, the size of some constituting elements of the installation for performing the method of the invention may be exaggerated and not drawn on scale for illustrative purposes.

Where the term « comprising » is used in the present description and/or claims, it does not exclude the presence of other elements or method steps.

Where an indefinite article is used when referring to a singular noun e.g. « a », « an » or « the », this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and/or in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. DEFINITIONS

As used herein and unless stated otherwise, the term " chemical oxygen demand " (hereinafter conventionally abbreviated as COD) refers to the mass of oxygen consumed per litre of an aqueous solution having organic compounds dissolved therein in order to oxidise all such organic compounds into (mainly) carbon dioxide, water and/or ammonia. COD is conventionally measured according to the standard ISO 6060.

As used herein and unless stated otherwise, the term " organic compound " refers to organic molecules, including macromolecules, belonging to all classes of chemicals or biochemicals, being either isolated or forming structures such as aggregates, networks, biological tissues and the like, which may be from vegetal origin, animal origin or industrial origin.

As used herein and unless stated otherwise, the term "free ammonia" refers to the gaseous form of ammonium or ammonia (NH₃) which is in a aqueous solution in equilibrium with the ammonium cation (NH₄ ⁺). This equilibrium is mainly dependent on the pH and temperature of the aqueous solution.

As used herein and unless stated otherwise, the term " nitrification " refers to the transformation of ammonium ions into nitrites and/or nitrates, e.g. with a major portion of nitrates.

As used herein and unless stated otherwise, the term " partial oxidation of ammonium " refers to the transformation of ammonium ions into nitrites and/or nitrates, e.g. with a major portion of nitrites. As used herein and unless stated otherwise, the term " denitrification " refers to the reduction of nitrites and/or nitrates to dinitrogen gas.

As used herein and unless stated otherwise, the terms " nitrogen equivalent " refers to the mass of all kinds of nitrogen atoms comprised within a nitrogen containing species.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment, the present invention relates to an improved apparatus and process for the biological treatment of waste water or aqueous effluents containing organic compounds, ammonium ions, one or more phosphates and manganese and/or iron ions dissolved therein. Water that can be treated by the process of the present invention usually also contains oxygen dissolved therein in a largely variable amount. Waste water may contain one or more organic compounds in largely variable types and/or amounts. There is no limitation concerning the concentration of ammonium ions in the waste water or aqueous effluent to be treated that is imposed for compatibility with the performance of the present invention. For instance, waste water having an ammonium ion concentration of about 50 mg/l or more, e.g. in the range between about 50 mg/l and about 4 g/l can be treated successfully according to various embodiments of the method of the invention. This range of ammonium ion concentration is useful because it corresponds to what is usually encountered in waste water from most industrial or agricultural origins.

There is no strict limitation concerning the concentration of manganese and/or iron ions in the aqueous effluent to be treated in the processing plant or according to the process of the present invention, as long as a concentration sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria can be reached in step (d) (e.g. through means (c)) and maintained in step (e), e.g. through means (d). Manganese and/or iron ions can be present at the level of traces in waste water, or more substantial amounts may be present in aqueous effluents from certain industrial origins. As a particular feature of various embodiments of this invention, the initial concentration of dissolved manganese ions in water may be within a range from the detectable limit, e.g. about 0.01 ppm (e.g. 0.01 mg/L), up to about 5,000 ppm (e.g. 5,000 mg/L). The initial concentration of dissolved manganese ions in water may be above 0.1 ppm (e.g. 0.01 mg/L), or may be below 500 ppm (e.g. 500 mg/L), for instance below 100 ppm (e.g. 100 mg/L). As another particular feature of various embodiments of this invention, the initial concentration of dissolved iron ions in water may be within a range from the detectable limit, e.g. about 0.01 ppm (e.g. 0.01 mg/L), up to about 5,000 ppm (e.g. 5,000 mg/L). The initial concentration of dissolved iron ions in water may be above 0.1 ppm (e.g. 0.01 mg/L), or may be below 500 ppm (e.g. 500 mg/L), for instance below 100 ppm (e.g. 100 mg/L).

Water that can be treated by the process of the present embodiment can have any origin. Examples thereof include water from lakes, rivers, canals and ponds, waste water from agro-industrial plants (e.g. human or animal food products such as a potato-based products factory), and all kinds of industrial activities, especially industries based on natural products such as the pulp and paper industry, leather manufacturing industry, coatings and plastics industries. Whatever the origin of water or waste water, the purpose of the treatment of the invention is to significantly reduce the amount of ammonium ions dissolved in said water.

By biological treatment according to this invention, it is meant a water treatment wherein at least one step involves the use of micro-organisms such as for instance bacteria. Suitable bacteria may be obtained from common activated sludge sources such as for instance a sludge of existing water- treatment plants, in which ammonia is degraded and/or converted into oxidised forms of ammonia.

In one specific embodiment, the process of this invention comprises the steps of: a) decreasing the phosphate concentration of water so as to increase the ratio chemical oxygen demand (COD) to phosphates concentration (COD/P) above a predetermined level of about 10, b) optionally adapting the oxygen content in water obtained after step (a) such as to reach a concentration of dissolved oxygen between about 0.2 mg/l and the saturation concentration of dissolved oxygen (unless the oxygen content is already at this level), c) oxidising at least part of the ammonium ions contained in water obtained after step (b) while maintaining a concentration of dissolved oxygen between about 0.2 mg/l and the saturation concentration of dissolved oxygen and while maintaining the ratio chemical oxygen demand (COD) to phosphates concentration above said predetermined level of about 10, d) adapting the oxygen content in water from step (c) such as to reach a concentration of dissolved oxygen below about 0.2 mg/l, and adapting the concentration of manganese and/or iron ions in water above a value (i.e. a concentration) sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, e.g. above 0.01 mg/L, and e) converting a major portion of the oxidised ammonium ions from step (c) into nitrogen by means of anaerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen below about 0.2 mg/l and a concentration of manganese and/or iron ions sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, e.g. above 0.01 mg/L.

In step (a), the phosphate concentration of waste water may be decreased by any suitable means such as, but not limited to, the use of phosphate- accumulating micro-organisms such as Accumulibacter-relaXed bacteria (e.g. Candidatus Accumulibacter phosphates), Rhodocyclus-re\ated phosphate accumulating organisms, Acinetobacter calcoaceticus,... or by precipitation. The term "decreasing the phosphate concentration", should preferably be understood as decreasing the "active phosphate concentration", i.e. the concentration of phosphate susceptible to precipitate manganese and/or iron ions at any stage of the performance of the process. For instance, any of neutralising, binding or complexing phosphate ions such as to form a phosphate species unable to precipitate manganese and/or iron ions should also be understood as a means for suitably decreasing the phosphate concentration according to the present invention.

Decreasing the phosphate concentration in step (a) by a method such as precipitating a substantial portion of the phosphates being originally present may have the advantage to be relatively easier to perform and control than other methods, for instance, the use of phosphate-accumulating microorganisms. Within this specific embodiment, it may be advantageous to obtain phosphate precipitation by adding cationic species such as for instance calcium, aluminium or magnesium ions in the form of e.g. salts or complexes, to the waste water since these can lead to the formation of insoluble phosphate salts or complexes. The use of a magnesium salt such as, but not limited to, Mg(OH)₂, MgCO₃ or MgO is particularly useful because it may form a magnesium-ammonium-phosphate salt (also named struvite) and therefore permit removal already of a part of the ammonium ions present in the waste water. An advantage of using MgO, Mg(OH)₂ or MgCO₃ is that the pH increases, which is favourable for nitrite production in step (c). A further advantage of the use of a magnesium salt is that the struvite formed can be separated for further use as a fertilizer. Without being bound by theory, reaction in the presence of a magnesium cation is likely to perform according to the following equation: Mg²⁺ + NH₄ ⁺ + HPO₄ ^2' + 6H₂O -» MgNH₄PO₄.6H₂O + H⁺ (equation 6)

The level above which the COD to phosphates concentration (COD/P) ratio is increased in step (a) is about 10, preferably about 15, more preferably about 30, for instance about 40, in particular about 50 (to form the ratio COD and phosphate concentration should both be expressed in the same unit, e.g. mg/L). As another advantageous feature of the present embodiment of the invention, the phosphate concentration is decreased during step (a) by about 60% to 85%, i.e. the remaining phosphate concentration is about 15% to 40% of the original phosphate concentration before performance of step (a). Increasing the COD to phosphate concentration ratio above any of the herein- specified predetermined levels and/or decreasing the phosphate concentration by 60% to 85% with respect to the original phosphate concentration is an advantageous feature because it has been unexpectedly observed that this operational condition is favourable to the growth, and therefore the oxidising activity of anaerobic ammonium oxidising to be used elsewhere in the process, which in turn rises the efficiency of step (e). According to another advantageous embodiment of this invention, the phosphate concentration in water is decreased below about 20 mg/L after performing step (a).

As an additional feature of the present embodiment, step (a) also results into increasing the ratio of the concentration of manganese and/or iron ions to the phosphate concentration (both being expressed in the same unit) above a predetermined level. This feature is advantageous because each of manganese and/or iron ions favours the growth and the activity of anaerobic ammonium oxidising bacteria and especially since Mn⁴⁺ and Fe³⁺ can be reduced by anaerobic bacteria or by other ways to lead to the favourable Mn²⁺ and Fe²⁺ ions, which in turn rises the efficiency of step (e).

In step (b) adaptation of the oxygen content in water (i.e. the aqueous effluent) from step (a) may be performed in order to provide suitable conditions for the oxidising action of aerobic ammonium oxidising bacteria to be used elsewhere in the process of the invention. Adaptation can consist in an increase or a decrease of the oxygen content dissolved in water or can involve no specific action if the oxygen content dissolved in the water is already within the appropriate range, e.g. from about 0.2 mg/L to oxygen saturation, or about 0.5 to 2 mg/L in a preferred embodiment. If the aqueous effluent from step (a) has a dissolved oxygen content below the appropriate lower limit, rising this oxygen content can be achieved by introducing air or oxygen into the waste water by means such as, but not limited to, stirring under an aerated atmosphere, injecting air or oxygen directly into water via a pump and a tube or pipe, and the like. Useful oxygen contents to be reached in step (b) and maintained through step (c) are between about 0.2 mg/L and the saturation concentration of dissolved oxygen at the relevant temperature (usually from about 5°C to about 35°C), for instance between about 0.2 mg/L and about 2 mg/L or between about 0. 5 and about 1 mg/L. Adapting the oxygen content in water in step (b) between about 0.5 mg/L and about 2 mg/L is useful because it permits step (c) to lead preferentially to the formation of nitrites over nitrates when step (c) is performed in the presence of aerobic ammonium oxidising bacteria. In step (c), the oxidation of ammonium ions in water is performed, e.g. chemically or biologically. If performed chemically, oxidation can for instance be obtained photocatalytically by using a combination of titanium dioxide and ultra-violet (UV) light. Preferably oxidation in step (c) is performed biologically such as by means of aerobic ammonium oxidising bacteria well known in the art. The type and amount of such bacteria, as well as other operating conditions (e.g. pH and temperature) for biological oxidation, suitable for the performance of step (c) are extensively available in the literature. Examples of aerobic ammonium oxidising bacteria include, but are not limited to, Nitrosomonas such as Nitrosomonas europaea and Nitrosospira such as Nitrosospira briensis among others. Step (c) can lead to various ammonium oxidation products, among which mainly nitrites and nitrates. Preferably, the operating conditions are selected in such a way that the oxidation products of step (c) comprise a major portion of nitrites. This feature is advantageous because the formation of nitrites is less oxygen demanding than the formation of nitrates and leads to a reduced organic electron donor demand in a downstream denitrification process such as step (e). As an optional feature, steps (b) and (c) may be performed in a continuously agitated, e.g. stirred reactor. This feature is advantageous due to a more complete mass transfer obtained in this type of reactor and the reduced complexity of a system comprising a lower number of separate reactors. As another optional feature, the sludge retention time of the effluent under treatment in step (c) is less than about 10 days.

As another optional feature, ammonium ions may be added in step (c). If ammonium ions are contained in e.g. additional waste water, or water recycled upstream from step (c), this source of ammonium ions can represent a handy alternative entry path for these aqueous effluents into the purification process of the present invention.

The pH at which step (c) is performed may be between about 6.5 and about 8.5, preferably above about 7.4. The sludge retention time suitable for the performance of step (c) may be, for example, between about 2 hours and about 120 hours, preferably between about 4 hours and about 24 hours, depending upon a series of other operating parameters such as, but not limited to, pH, concentration of ammonium ions contained in water, dissolved oxygen content, and the like.

In step (d), the adaptation of the dissolved oxygen content in water (i.e. the aqueous effluent) from step (c) consists in decreasing the oxygen content below a certain level, e.g. below about 0.2 mg/L. Decreasing the dissolved oxygen content in step (d) can be achieved by means such as, but not limited to, stopping one or more aeration means currently in use, closing the reactor and replacing at least partially the atmosphere above the waste water contained in the reactor by an inert gas or oxygen-deprived atmosphere (e.g. an atmosphere rich in nitrogen, argon, carbon dioxide and the like), pumping out the atmosphere above the waste water in order to decrease the oxygen content, and the like. Useful oxygen contents to be reached in step (d) are below 0.2 mg/L, for instance below 0.05 mg/L. Ideally, the dissolved oxygen content to be reached in step (d) should be as low as possible in order to give rise to anaerobic conditions suitable for the action of anaerobic bacteria. Adapting the dissolved oxygen content below this upper limit is useful because it improves the efficiency of step (e) and favours denitrification and anaerobic ammonium oxidation.

Adaptation of the concentration of soluble manganese and/or iron ions in waste water in step (d) may be effected either by increasing the manganese and/or iron ions concentration, or can involve no action if these concentrations of manganese and/or iron ions in waste water are already at or above a value (i.e. a concentration) sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, e.g. above about 0.01 mg/L. Even if a value sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria has already been achieved, the concentration of manganese and/or iron ions can nevertheless be increased in step (d) in order to further improve the growth, and therefore efficiency, of the anaerobic ammonium oxidising bacteria. Although the growth is stimulated, the main objective is to stimulate the activity. Due to the slow growth of anaerobic ammonium oxidising bacteria, slow growth is a result of high activity, i.e. high ammonium oxidation rates are necessary for these bacteria to grow. Anywhere stated that the growth is improved, increased or like, it should be considered as a significant increase in the activity of the anaerobic ammonium oxidising bacteria.

The adaptation of the concentration of manganese and/or iron ions, although preferably performed in step (d), can alternatively be performed in any step upstream from step (d) independently of the performance or non- performance of any recycling step, or it can be performed during the performance of hereinafter described step (f) or the hereinafter described optional step (g) if recycling of the effluent from step (g) is performed. Adaptation of the concentration of soluble Mn²⁺ and/or Fe²⁺ ions in waste water in step (d) can comprise for instance one of the following actions, among others: - adding Mn²⁺ and/or Fe²⁺ soluble ions,

- solubilising insoluble Mn²⁺ and/or Fe²⁺ salts, or regenerating Mn²⁺ and/or Fe²⁺ ions by reduction of Mn⁴⁺ and/or Fe³⁺ ions respectively; in a particular embodiment of the present invention, this reduction may be achieved at least partly by the reaction of Mn⁴⁺ and/or Fe³⁺ ions with NH₄ ⁺ and/or COD.

As an optional feature of this invention, at least part of the oxidised ammonium entering step (e) is in the form of a nitrate, preferably not exceeding a predetermined level. Adaptation of the concentration of nitrate entering step (d) can achieved for instance by adding nitrate in directly, or by nitrates formed upstream step (d), e.g. in step (b) ,or by recycling nitrate formed downstream step (d), e.g. in step (f) or (g). A minimum level of nitrate increases the activity of anaerobic ammonium oxidising bacteria.

As another optional feature of this invention, manganese and/or iron ions may be added to waste water to be treated during step (e). This feature is advantageous because soluble manganese and/or iron ions favour the growth, and consequently the activity of anaerobic ammonium oxidising bacteria, which in turn rises the efficiency of step (e). Alternatively, manganese and/or iron ions may be added to waste water to be treated during any step upstream from step (e). Whether manganese and/or iron ions are or are not added to waste water to be treated during step (e) or during any step upstream from step (e), it is useful for the effective performance of the present invention that the concentration of (Mn²⁺ + Mn⁴⁺) in waste water, either initially or during the performance of step (e), is within a range from about 0.01 ppm (e.g. 0.01 mg/L) to about 5,000 ppm (e.g. about 5 g/L), for instance up to about 100 ppm. It is also useful that the concentration of (Fe²⁺ + Fe³⁺) in waste water, either initially or during the performance of step (e), is within a range from about 0.01 ppm (e.g. 0.01 mg/L) to about 5,000 ppm (e.g. about 5 g/L), for instance up to about 100 ppm. It has been found surprisingly that such concentrations of manganese and/or iron ions are fully adequate for stimulating the growth and activity of anaerobic ammonium oxidising bacteria and solving the problems addressed by the process of the present invention. Examples of suitable anaerobic ammonium oxidising bacteria include, but are not limited to, Planctomycetes order (e.g. Candidatus Brocadia anammoxidans, Candidatus Kuenenia stuttgartensis, Candidatus Scalindua wagneri, Candidatus Scalindua brodae or Candidatus Scalindua sorokinii) among others. As another optional feature, the process of the present invention may further comprise a step of (h) oxidising part of the organic compounds present in waste water by means of organotrophic aerobic bacteria under conditions including a concentration of dissolved oxygen above about 0.2 mg/L and below or equal to the saturation concentration of dissolved oxygen in water at the relevant temperature (usually from about 5°C to about 35°C). This step (h) may be performed simultaneously with step (c) and/or with step (g), preferably simultaneously with step (c).

As another optional feature, the process of the present invention can further comprise a step (i), being performed preferably simultaneously with step (e), of reducing at least part of the oxidised ammonium ions present in waste water (i.e. the aqueous effluent) from step (c) by means of anaerobic bacteria. This optional feature may be useful in permitting to diminish the amount of organic material in waste water while simultaneously reducing nitrates into nitrites and/or reducing nitrates and nitrites into nitrogen. However, in many circumstances, it may be preferable to perform the major part of the ammonium removal by means of step (e) and a minor part of the ammonium removal by means of step (i) since step (e) permits a smaller overall oxygen consumption to be achieved. The process of the present invention may be performed discontinuously (i.e. batchwise) or, preferably, continuously.

As an optional feature of the present invention, step (a) is performed in a first reactor, steps (b) and (c) are performed in a second reactor downstream from the first reactor, and steps (d) and (e) are performed in a third reactor downstream from the second reactor. This arrangement of a cascade of reactors in series is advantageous because it permits to operate a continuous process wherein each reactor can be kept under selected and stable conditions adapted to the performance of the relevant step for which it is dedicated.

As another optional feature of the present invention, at least part of waste water (i.e. the aqueous effluent) from step (e) may be recycled upstream from step (d), preferably at step (c). This feature is advantageous because it permits to further improve in a simple manner the degree of purity of water obtained after treatment, and it permits to successively oxidise and reduce manganese and/or iron. This feature is also advantageous because it permits to dilute the waste water of any step upstream from step (d) in order to prevent toxicity of chemicals in general, and free ammonia in particular. Recycling of waste water effluent can be operated by any means known in the art such as, but not limited to, the use of pumps, pipes, tubes and the like. A sludge separator may be placed downstream from the reactor performing step (e) in order to separate the sludge from the waste water and thus to facilitate the recycling of waste water only.

The pH at which step (e) is performed may be comprised between about 5 and about 9, for instance between about 6 and about 8. The sludge retention time in step (e) may be comprised between about 2 hours and about 30 days, depending upon a series of other operating parameters such as, but not limited to, pH, concentration of ammonium ions and organic compounds contained in water, dissolved oxygen content and the like.

As another optional feature of the present invention, the process may further comprise the steps of: f) adapting the oxygen content in the water obtained in (e) such as to reach a concentration of dissolved oxygen above about 0.2 mg/L and below or equal to the saturation concentration of dissolved oxygen, and g) oxidising at least part of the ammonium ions present in water from step (c) by means of aerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen above about 0.5 mg/l and below or equal to the saturation concentration of dissolved oxygen. This optional feature has the advantage to permit elimination of part of the ammonium ions that would otherwise remain after step (e). Preferably, this optional feature is performed under conditions suitable for removing a major portion of the remaining ammonium ions still present after step (e).

In step (T), the adaptation of the oxygen content in waste water (i.e. the aqueous effluent) from step (e) preferably consists in increasing the dissolved oxygen content until the lower limit of about 0.2 mg/L is reached. Increasing the oxygen content of the effluent from step (e) can be achieved by any suitable means such as, but not limited to, introducing air, oxygen-enriched air or oxygen into waste water for instance by stirring the aqueous effluent under an aerated atmosphere, injecting air, oxygen-enriched air or oxygen directly in waste water via a pump and a tube or pipe, stopping one or more oxygen removal means currently in use, and the like. If the oxygen content in the effluent water from step (e) is already comprised between about 0.5 mg/L and the saturation concentration of dissolved oxygen, specific actions are not necessarily required.

In step (g), the oxygen content is preferably maintained between about 0.5 mg/L and the saturation concentration of dissolved oxygen.

Preferably a major portion, more preferably more than 90%, most preferably more than 95% of the ammonium ions still present in the effluent of step (e) are oxidised. Preferably, the concentration of ammonium ions remaining after step (g) is not higher than about 0.1 mg/l. Preferably the concentration of ammonium ions remaining after step (g) represent less than about 1 %, more preferably less than about 0.1 %, of the total amount of ammonium ions initially present before the performance of step (a). The pH at which the optional step (g) is performed may be comprised within a range from about 6.5 to about 8.5, preferably above about 7.4. The sludge retention time in step (g) may be comprised between about 2 hours and about 30 days, depending upon a series of other operating parameters such as, but not limited to, pH, concentration of ammonium ions and organic compounds contained in water, dissolved oxygen content, and the like.

As another optional feature of the present invention, a sludge separator may be placed downstream from the reactor performing step (g) in order to separate the sludge from waste water and thus to facilitate the recycling of waste water without the sludge. At least part of waste water (i.e. the aqueous effluent) from step (g) may be recycled upstream from step (f), preferably at step (e) or at step (c). This feature is advantageous because it permits to further improve the degree of purity of the water obtained after treatment and it permits to successively oxidise and reduce manganese and/or iron. This feature is also advantageous because it permits to dilute the water of any step upstream from step (f) in order to prevent toxicity of any compound, especially free ammonia. Recycling can be operated by any means known in the art such as, but not limited to, the use of pumps, tubes, pipes and the like.

As another optional feature of the present invention, the ammonium oxidising bacteria used in step (c) and/or step (e) are contained in a sludge, and at least part of said sludge and water from step (g) are recycled upstream from step (f) so as for instance to perform step (d) and step (e) again at least one more time. Preferably, the recycling ratio, i.e. the ratio between the recycled matter and the matter leaving the system, is larger than about 4.

As another optional feature, step (a) is performed in a first reactor, steps (b) and (c) are performed in a second reactor downstream from the first reactor, steps (d) and (e) are performed in a third reactor downstream from the second reactor, and optional steps (f) and (g) are performed in a fourth optional reactor downstream from the third reactor. This arrangement of a cascade of reactors in series is advantageous because it permits a continuous process wherein each reactor can be kept under selected and stable operating conditions best suited for the performance of the step for which it is dedicated. In another embodiment, the present invention relates to a system or apparatus for the biological treatment of water containing ammonium ions and manganese and/or iron ions dissolved therein. Referring to figures 1 and 2 described below, said biological treatment system comprising a first reactor (1 ), a second reactor (2) in fluidic communication with said first reactor (1 ), said second reactor (2) comprising means (9) for adapting the oxygen content of said water, a third reactor (3) in fluidic communication with said second reactor (2), and optionally a fourth reactor (4) in fluidic communication with said third reactor (3), the optional fourth reactor (4) comprising means (9¹) for adapting the oxygen content in the water, wherein said system further comprises means for measuring the concentration of manganese and/or iron ions in water and means for adapting the concentration of manganese and/or iron ions above a predetermined value (e.g. above 0.01 mg/L) in one or more of the reactors (1 ) to (4).

The means (9) and (9¹) for adapting the oxygen content in said water consist for instance of stirring rods and an aerated atmosphere, air or oxygen injectors, pump and tubes or pipes, and the like. The means for measuring the concentration of manganese and/or iron ions consist for instance of AAS (atomic absorption spectrometry), ICP (Inductive Coupled Plasma) and various colorimetric methods.

As an optional feature, the means for measuring the concentration of manganese and/or iron ions in water are comprised in the third reactor (3). This feature is advantageous because steps (d) and/or (e) will usually be performed in this reactor, and a sufficient concentration of manganese and/or iron ions is particularly useful during step (e).

Figure 1 schematically shows a flow-chart of a first installation or apparatus for implementing a representative embodiment of a process according to the present invention. In this illustrative apparatus, waste water (13) containing ammonium ions, phosphate ions and organic compounds is provided to a first reactor (1 ) equipped with a stirrer (12). Step (a) is performed in reactor (1 ) by adding a magnesium salt to the waste water (13) such as to precipitate phosphate ions present in water (13). The resulting waste water (14) with a decreased phosphate concentration is then transferred to a second reactor (2) equipped with an air diffuser (9) receiving air (11 ) from an air supply line (7). In reactor (2) oxygen content is monitored , and if necessary modified during step (b), in order to reach a dissolved oxygen concentration between about 0.2 and about 2 mg/l, and step (c) is then performed in such manner that part of the ammonium ions contained in waste water (14) is oxidised by means of aerobic ammonium oxidising bacteria living in a sludge present in reactor (2). Waste water (15) exiting reactor (2), being poorer in ammonium ions and richer in oxidised ammonium species such as nitrites and nitrates, is then transferred to a third reactor (3). Reactor (3) is equipped with a stirrer (12) and contains a sludge containing anaerobic ammonium oxidising bacteria and heterotrophic denitrifying bacteria. In reactor (3), the concentration of manganese and/or iron ions is monitored, and if necessary modified during step (d), and step (e) is then performed such as to convert a major portion of the oxidised ammonium ions present in waste water (15) through the action of the bacteria contained in this reactor (3), i.e. anaerobic ammonium oxidation and heterotrophic denitrication. Step (e) achieves an important decrease in the concentration in nitrogen equivalent in waste water by simultaneously eliminating ammonium ions and oxidised ammonium ions, thus leading to a waste water (19) poorer in nitrogen equivalent content. Waste water (19) is then transferred to a fourth reactor (4), equipped with air diffusers (9') for performing step (f) by increasing oxygen concentration. In this reactor (4), aerobic ammonium oxidising bacteria present in a sludge perform step (g) by oxidising a major portion of the ammonium ions remaining in waste water (19) and thus producing purified water (21 ) with a very low ammonium ions concentration. A proportion of said purified water (21 ) and the sludge exiting from reactor (4) are recycled via pipe (8) to reactor (3) and a proportion thereof is directed to a sludge separator (5) where water and sludge are separated. After separation, the sludge is recycled via pipe (10) from said separator (5) to the reactor (2), whereas water exits the system as a purified effluent (22).

Figure 2 schematically shows a flow-chart of an installation and apparatus for implementing another representative embodiment of a process according to the present invention. This apparatus is similar to that shown in figure 1 , except that after separation in the separator (5), the sludge is recycled via pipe (10) from said separator (5) to the reactor (3) whereas water exits the system as a purified effluent (22).

The following example is provided for illustrative purpose only and does not imply any restriction in the definition of the present invention.

EXAMPLE 1 This working example of the present invention is described herein with reference to tables 1 to 8, in which the following abbreviations have been used:

- " Flow in " relates to the amount of water entering the reactor involved in the relevant process step,

- " P-tot " relates to the phosphate concentration,

- " P load in " relates to the total amount of phosphate entering the system,

- " P load out " relates to the total amount of phosphate leaving the reactor involved in the relevant process step, - " COD " relates to the chemical oxygen demand,

- " COD load in " relates to the COD of the amount of organic compounds entering the reactor involved in the relevant process step,

- " COD load out " relates to the COD of the amount of organic compounds leaving the reactor involved in the relevant process step, - " N-tot " relates to the nitrogen equivalent concentration,

- " N load in " relates to the total amount of nitrogen equivalent entering the reactor involved in the relevant process step,

- " N load out " relates to the total amount of nitrogen equivalent leaving the reactor involved in the relevant process step, - " NH₄_N " relates to the nitrogen equivalent ammonium ion concentration,

- " NO₂_N " relates to the nitrogen equivalent nitrite concentration,

- " NO₃_N " relates to the nitrogen equivalent nitrate concentration,

- " NH₄-N out " relates to the nitrogen equivalent contribution from ammonium ions to the concentration of nitrogen equivalent leaving the reactor involved in the relevant process step,

" NH_4-N load out " relates to the nitrogen equivalent contribution from ammonium ions to the total amount of nitrogen equivalent leaving the reactor involved in the relevant process step,

- " NO2_N load out " relates to the nitrogen equivalent contribution from nitrites to the total amount of nitrogen equivalent leaving the reactor involved in the relevant process step,

- " NO₃_N load out " relates to the nitrogen equivalent contribution from nitrates to the total amount of nitrogen equivalent leaving the reactor involved in the relevant process step, and - " Sludge removal_N load out " relates to the nitrogen equivalent contribution from the sludge flux out of the reactor to the total amount of nitrogen equivalent leaving the reactor involved in the relevant process step.

For the sake of clarity, each of tables 1 to 8 contains experimental data pertaining to one single process step only. In the present example, a system according to figure 2 was used and its operation was monitored during a time period of seven weeks. During this period of time, an average of 4,756 m³ of waste water (influent) were fed every day to a first reactor (1 ) aiming at reducing the phosphate concentration in waste water. Before starting treatment, the incoming waste water contained in average 55.4 mg/l of phosphorus-equivalent (P-tot, see table 1 ), amounting to an average load of 262.7 kg phosphorus-equivalent per day. After treatment in the first reactor, i.e. step (a) of the process, the exiting aqueous effluent contained in average only 14.2 mg/l phosphorus-equivalent (P-tot), amounting to an average of 67.4 kg phosphorus-equivalent per day. This first step of the waste-water purification thus removed an average of 195.4 kg of phosphorus-equivalent per day which represents an efficiency of 74.2%. The ratio chemical oxygen demand (COD) (mg/l) / phosphates (mg/l) in the influent was in average 13.3 (i.e. 734.7 mg/L/ 55.4 mg/L) while the ratio chemical oxygen demand (COD) (mg/L)/phosphates (mg/L) in the effluent exiting the first reactor (1 ) was in average 42,7 (see table

2). This effluent was then fed into the second reactor (2) where it became the influent (see tables 4 and 5) for performing steps (b) and (c). The second reactor (2) is a continuous stirred-tank reactor serving as a partial oxidation of ammonium unit containing a sludge of anaerobic ammonium oxidising bacteria wherein the oxygen content of water was adapted to 0.5 mg/L in average (see table 5). After this treatment step, the exiting effluent contained in average 192.2 mg/l nitrogen-equivalents (including 106.1 mg/L originating from ammonium ions, 61.1 mg/L originating from nitrites and 25.0 mg/L originating from nitrates (see table 4), amounting to 983.6 kg of nitrogen-equivalents per day (including 504.9 kg originating from ammonium ions, 289.8 kg originating from nitrites, 118.8 kg originating from nitrates, and 70.0 kg originating from the sludge production). During this partial oxidation of ammonium step, an average of 320.6 kg of nitrogen-equivalents, i.e. 24.1 % of the initial nitrogen- equivalents were removed per day (i.e. " N Removed " / " kg N in ") (see table 4). In average 23.2% of the nitrogen-equivalents present in the influent were transformed into nitrites and 9.7% of the nitrogen-equivalents present in the influent were transformed into nitrates. Simultaneously with the nitrogen partial oxidation of ammonium, a daily average of 71.3% of the COD is removed through oxidation from the influent in this second reactor (2). This is due to the action of organotrophic aerobic bacteria.

The effluent from the second reactor (2), containing a daily average of 983.6 kg of nitrogen-equivalents, was then fed as an influent into a third reactor (3) for performing the denitrification steps (d) and (e) together with a daily average 79.8 kg nitrogen-equivalents originating from an additional waste water source, 64.8 kg nitrogen-equivalents originating from the sludge recycle stream from a downstream reactor and 139.0 kg of nitrogen-equivalents originating from water recycled from a downstream nitrification step. The effluent coming out from this third reactor (3) acting as a denitrification unit contained only an average of 91.0 kg of nitrogen-equivalents per day (including 16.8 kg originating from ammonium ions, 3.2 kg originating from nitrites and 71.0 kg originating from nitrates). Thus an average amount of 1176.2 kg nitrogen-equivalents was removed each day, representing an average diminution of 93.1 % of the nitrogen-equivalent content during this step (i.e. " N removed 7" total N load in ") (see table 6).

The effluent of this denitrification step was fed as an influent to a fourth reactor (4) for a last nitrification step were the remaining ammonium ions were transformed into nitrates. As a result only traces (less than 0.01 mg/l) of nitrogen-equivalents originating from ammonium ions remained in the purified water thus obtained (see table 8).

In this example, iron concentrations varied from 0.98 mg/L to 6.0 mg/L in the first reactor, from 0.9 mg/L to 180 mg/L in the second reactor, and from 0.96 mg/L to 169 mg/L in the third reactor. Manganese concentrations varied from 0.02 mg/L to 0.12 mg/L in the first reactor, from 0.06 mg/L to 2.3 mg/L in the second reactor, and from 0.03 mg/L to 2.2 mg/L in the third reactor.

EXAMPLE 2

This second example relates to a similar installation, but without using a phosphate removing step, as described in figure 3. The pH of water was about 8.0-8.5, while the COD/P-ratio was about 45. In a first reactor (1 ), the pH was between 7.6 and 8.4. Ammonia concentrations of water fed to reactor (1 ) varied between 700 mg N/L and 2,000 mg N/L. In reactor (1 ), a major portion of ammonia was oxidised to nitrite by nitrite-oxidising bacteria. Of the oxidised forms of ammonia, about 60-85% were nitrites and 15-40% were nitrates. This proportion was influenced by temperature, pH and recirculation from reactor (2), reactor (3) and reactor (6).

The average concentration of ammonia-N, nitrite-N and nitrate N in tank (1 ) were 76 mg N/L, 185 mg N/L and 50 mg N/L respectively. On average, 75% of ammonia was converted into nitrites and nitrates, varying between 50% up to more than 90%. In tank (2), nitrite and nitrate were denitrified to nitrogen gas, while in tank (3), remaining ammonia was oxidised to nitrates. There was a recirculation from reactor (3) to reactor (1 ) and from reactor (2) to reactor (1 ), with a flow between 0 to 50 m³/h. Due to a high recirculation flow from reactor (3) to reactor (2), the concentrations in both reactors were the same. The concentration of ammonia-N, nitrite-N and nitrate-N were 19 mg N/L, 19 mg N/L and 14 mg N/L respectively for both reactor (2) and reactor (3).

In order to treat the final ammonia and nitrogen present in the sludge of reactor (3), a conventional denitrification-nitrification installation was added, being reactor (4) and reactor (5) respectively. The sludge was separated from the effluent in a membrane bioreactor (MBR) (6). From this reactor, there was a variable recirculation flow to all previous reactors, varying from 0 to 50 m³/h.

The COD of reactor (1 ) was on average 1 ,700 mg/L. In reactor (1 ), 50- 80% of the COD is removed. The remaining part was removed in reactors (2) and (3). In reactor (2) and (3), a remaining concentration of 1 ,600 mg/L hardly biodegradable COD was still present. This COD partially passes the MBR, and partially remains in the installation. The remaining COD was slowly biodegraded by the micro-organisms of tanks (1 ) to (5).

In this example, from 50 to 350 mg/L Fe³⁺ were added to the third reactor, thus providing a minimum iron concentration of 50 to 350 mg/L, and iron-containing water from the third reactor was recirculated to the first and second reactors.

K*

Table 1

K*

OO

Table 2

K*

Table

Table 4

Table 5

K*

Table 6

Table 7

Table 8

Claims

1. A process for the biological treatment of water containing one or more organic compounds, oxygen, ammonium ions, one or more phosphates, and manganese and/or iron ions dissolved therein, comprising the steps of: a) decreasing the phosphate concentration of said water such as to increase the ratio chemical oxygen demand (COD) to phosphate concentration (COD/P) above 10, b) oxidising at least part of the ammonium ions contained in water from step (a) while maintaining a concentration of dissolved oxygen between 0.2 mg/L and the saturation concentration of dissolved oxygen and while maintaining the ratio chemical oxygen demand (COD) to phosphate concentration (COD/P) above 10, c) adapting the oxygen content in water from step (b) such as to reach a concentration of dissolved oxygen below 0.2 mg/L, and adapting the concentration of manganese and/or iron ions and nitrate in water above a value sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, and d) converting a major portion of the oxidised ammonium ions from step (c) into nitrogen gas by means of anaerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen below 0.2 mg/L and a concentration of manganese and/or iron ions and nitrate sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria.

2. A process for the biological treatment of water according to claim 1 , further comprising a step (a1 ) of adapting the oxygen content in water for use in step (b) such as to reach a concentration of dissolved oxygen between 0.2 mg/L and the saturation concentration of dissolved oxygen.

3. A process for the biological treatment of water according to claim 1 or 2, further comprising the steps of: e) adapting the oxygen content in the water obtained in (d) such as to reach a concentration of dissolved oxygen above 0.2 mg/L and below or equal to the saturation concentration of dissolved oxygen, and f) oxidising at least part of the ammonium ions present in water from step (b) by means of aerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen above 0.2 mg/L and below or equal to the saturation concentration of dissolved oxygen.

4. A process for the biological treatment of water according to claim 1 or 2, wherein step (a) is performed in a first reactor, step (b) and optionally step

(a1 ) are performed in a second reactor downstream from the first reactor, and steps (c) and (d) are performed in a third reactor downstream from the second reactor.

5. A process for the biological treatment of water according to 3, wherein step (a) is performed in a first reactor, step (b) and optionally step (a1 ) are performed in a second reactor downstream from the first reactor, steps (c) and (d) are performed in a third reactor downstream from the second reactor, and wherein steps (e) and (f) are performed in a fourth reactor downstream from the third reactor.

6. A process for the biological treatment of water according to any of claims 1 to 5, wherein step (b) is performed by means of aerobic ammonium oxidising bacteria under conditions including a concentration of dissolved oxygen between 0.2 mg/L and oxygen saturation.

7. A process for the biological treatment of water according to claim 3, wherein step (b) is performed by means of aerobic ammonium oxidising bacteria, wherein the ammonium oxidising bacteria used in steps (b) and (d) are contained in a sludge, and further comprising the step of separating sludge and water obtained in step (T).

8. A process for the biological treatment of water according to claim 7, wherein at least part of water separated from the sludge is recycled upstream from step (e).

9. A process for the biological treatment of water according to claim 3, wherein the ammonium oxidising bacteria used in steps (b) and (d) are contained in a sludge, and wherein at least part of sludge and water from step (f) are recycled upstream from step (e).

10. A process for the biological treatment of water according to any of claims 1 to 9, wherein the initial concentration of ammonium ions in water is 50 mg/L or more.

11. A process for the biological treatment of water according to any of claims 1 to 10, wherein said decrease of the phosphate concentration in step (a) is obtained by precipitation of said phosphates.

12. A process for the biological treatment of water according to claim 11 , wherein said precipitation is obtained by adding a calcium, aluminium or magnesium salt or complex to said water.

13. A process for the biological treatment of water according to any of claims 1 to 12, wherein said ratio chemical oxygen demand (COD) to phosphates is increased above 10 in step (a).

14. A process for the biological treatment of water according to any of claims 1 to 13, wherein the oxidising step (b) results in a major portion of nitrites.

15. A process for the biological treatment of water according to any of claims 1 to 14, wherein the phosphate concentration of water is decreased in step

(a) such as to increase the ratio of manganese and/or iron ions concentration to phosphate concentration above a predetermined level.

16. A process for the biological treatment of water according to any of claims 1 to 15, wherein the soluble manganese ions concentration in said water in step (c) and/or step (d) is within a range from 0.01 ppm to 5,000 ppm.

17. A process for the biological treatment of water according to any of claims 1 to 16, wherein the soluble concentration of iron ions in said water in step (c) and/or step (d) is within a range from 0.01 ppm to 5,000 ppm.

18. A process for the biological treatment of water according to any of claims 1 to 17, wherein said process further comprises the step (g) of oxidising part of said one or more organic compounds by means of organotrophic aerobic bacteria under conditions including a concentration of dissolved oxygen above 0.2 mg/L and below or equal to the saturation concentration of dissolved oxygen.

19. A process for the biological treatment of water according to any of claims 1 to 18, wherein said process further comprises the step of (h) reducing at least part of said oxidised ammonium ions present in water from step (b) by means of organotrophic anaerobic bacteria.

20. A process for the biological treatment of water according to any of claims 1 to 19, wherein manganese and/or iron ions are added to water to be treated upstream from step (e).

21. A process for the biological treatment of water according to any of claims 1 to 20, wherein water contains Mn⁴⁺ and/or Fe³⁺ ions, and wherein Mn²⁺ and/or Fe²⁺ ions are generated by reduction of said Mn⁴⁺ and/or Fe³⁺ ions in water during step (b) or step (c).

22. A process for the biological treatment of water according to claim 21 , wherein said reduction of Mn⁴⁺ and/or Fe³⁺ ions is achieved at least partly by the reaction of Mn⁴⁺ and/or Fe³⁺ ions with NH₄ ⁺ and/or COD.

23. A process for the biological treatment of water according to any of claims 1 to 22, wherein the phosphate concentration is decreased by 60% to 85% during step (a).

24. A process for the biological treatment of water according to any of claims 1 to 23, wherein the phosphate concentration is decreased to less than 20 mg/l during step (a).

25. A system for the biological treatment of water containing ammonium ions and manganese and/or iron ions dissolved therein, said system comprising a first reactor (1 ), a second reactor (2) in fluidic communication with said first reactor (1 ), said second reactor (2) comprising means (9) for checking or adapting the oxygen content of said water, a third reactor (3) in fluidic communication with said second reactor (2), and a fourth reactor (4) in fluidic communication with said third reactor (3), said fourth reactor (4) comprising means (9) for adapting the oxygen content in said water, wherein said system further comprises means for measuring the concentration of Mn²⁺ and/or Mn⁴⁺ and/or Fe²⁺, and/or Fe³⁺ in said water and means for adapting the concentration of manganese and/or iron ions above a predetermined value in one or more of reactors (1 ) to (4).

26. A system according to claim 24, wherein said means for measuring the concentration of manganese and/or iron ions in water are located within said third reactor (3).

27. A processing plant for the biological treatment of water containing one or more organic compounds, oxygen, ammonium ions, one or more phosphates, and manganese and/or iron ions dissolved therein, the plant comprising: a) means for decreasing the phosphate concentration of said water such as to increase the ratio chemical oxygen demand (COD) to phosphate concentration above 10, b) means for oxidising at least part of the ammonium ions contained in water from the means for decreasing the phosphate concentration while maintaining a concentration of dissolved oxygen between 0.2 mg/l and the saturation concentration of dissolved oxygen and while maintaining the ratio chemical oxygen demand (COD) to phosphate concentration above 10, c) means for adapting the oxygen content in water from the means for oxidising such as to reach a concentration of dissolved oxygen below 0.2 mg/L, and means for adapting the concentration of manganese and/or iron ions and nitrate in water above a value sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria, and d) means for converting a major portion of the oxidised ammonium ions from the means for adapting the oxygen content into nitrogen gas by means of anaerobic ammonium oxidising bacteria while maintaining a concentration of dissolved oxygen below 0.2 mg/L and a concentration of manganese and/or iron ions and nitrate sufficient for stimulating the growth of anaerobic ammonium oxidising bacteria.

28. Sludge obtained by the process of any of the claims 1 to 24.

29. Use of the sludge of claim 28 as a fertiliser.