Abstract:
A process for converting ammonium (NH 4   + ) of a mainstream of a wastewater plant to dinitrogen gas (N 2 ), including the consecutive steps of i.) removing biodegradable carbon compounds in the mainstream, ii.) converting ammonium (NH 4   + ) in the mainstream to nitrite (NO 2   − ) in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitration vessel ( 133   a - 133   d ); and iii.) denitrifying the resulting stream from step ii.) to dinitrogen gas in an anammox vessel ( 200 ). Growth of nitrite oxidizing bacteria (NOB) in step ii.) is prevented by periodically subjecting the bacteria in said nitration vessel ( 133   a - 133   d ) to water suppressing growth of nitrite oxidizing bacteria (NOB).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention refers to a process for converting ammonium of a mainstream of a wastewater plant to dinitrogen gas. The process includes removing biodegradable carbon compounds in the mainstream, after which ammonium in the mainstream is converted to nitrite in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitration vessel. Nitrite in the resulting stream is then converted to dinitrogen gas in an anammox vessel. 
       PRIOR ART 
       [0002]    In the art of wastewater treatment, it is vital to reduce the content of nitrogen compounds, mainly ammonium, from the wastewater stream. Today, this is usually achieved by the aerobic biological oxidation of nitrogen from ammonium (NH 4   + ) to nitrate (NO 3   − )(nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. The denitrification process takes place under anoxic conditions and require an electron donor, usually in the form of biodegradable organic material to reduce the nitrate. 
         [0003]    Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonium (NH 4   | ) to nitrite (NO 2 ) is facilitated by so-called ammonium oxidizing bacteria (AOB), for example  Nitrosomonas  spp. (“nitroso” referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO 3   − ), is carried out by so-called nitrite oxidizing bacteria (NOB), such as  Nitrospira  spp (nitro referring the formation of a nitro functional group). 
         [0004]    Denitrification requires anoxic conditions It is facilitated by a wide diversity of bacteria. Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, biodegradable organic matter (e.g. from faeces), sulfide, or an added donor like methanol. 
         [0005]    In wastewater treatment, nitrification as well as denitrification is usually carried out in the activated sludge process but also in different kinds of biofilm processes. Nitrification, being an aerobic process, is normally operated with the supply of air which provides both oxygen for the oxidation and mixing of the reactor contents. The denitrification tanks, operated without air, are typically mixed mechanically, e.g. by using submersible mixers, in order to secure good contact between wastewater and the active biomass so that the desired denitrification is achieved. 
         [0006]    The traditional way of removing nitrogen through nitrification and denitrification is energy and resource consuming. The oxidation of ammonium to nitrate requires a large input of energy for aeration to supply oxygen. Large amounts of organic matter are needed for denitrification, which may be added in the form of organic chemicals such as methanol. Alternatively, the denitrification process may be carried out using so-called pre-denitrification, in which the organic content of the wastewater itself is used to reduce nitrate. However, even in this case, it is highly desirable to reduce or eliminate the amount of organic matter needed for nitrogen removal, as it is regarded a resource from which energy can be recovered in the form of biogas by applying anaerobic treatment. 
         [0007]    One possibility to reduce the consumption of energy and organic matter for nitrogen removal is to apply the so-called SHARON process. This process makes use of partial nitrification, i.e. nitritation, that is the oxidation of ammonium to nitrite, i.e. NO 2   − , followed by denitritation, i.e. the reduction of nitrite to dinitrogen gas, N 2 , with the consumption of organic matter. As compared with traditional nitrification/denitrification, the SHARON process reduces both the amounts of oxygen and organic matter needed. However, the application of the SHARON process has been limited to wastewater streams with high concentrations of ammonium and high temperature (&gt;20° C.) as it has not been possible to maintain a stable nitritation at lower ammonium concentrations and temperatures. Under conditions typical for mainstream municipal wastewater, i.e. low ammonium concentration and at least seasonally low temperatures, the second step of nitrification, i.e. the oxidation of nitrite to nitrate, has proven faster than the first step, i.e. the oxidation of ammonium to nitrite, so that a stable nitritation without forming nitrate has not been possible. Therefore the SHARON process is applied only for streams such as reject water from sludge dewatering that are high in ammonium concentration and temperature. 
         [0008]    A more recent development for nitrogen removal is the so-called anammox process. In this process, a wastewater stream is subjected to an oxidation of part of the ammonium content to nitrite. In a subsequent anoxic process performed by so-called anammox bacteria, the nitrite formed and the ammonium remaining react to form water (H 2 O) and dinitrogen gas (N 2 ), a reaction called anoxic deammonification. The anammox process has great advantages as it reduces the oxygen needed by 60% and eliminates completely the need for organic matter as compared with traditional nitrification/denitrification. The anammox process can be carried out either with a type of activated sludge process based on so-called granular sludge, i.e. dense aggregates of bacteria, or with different kinds of biofilm processes, the biomass growing on a carrier media most often made of plastic. The anammox process can be run with separate tanks for aerobic nitritation and anoxic deammonification. Part of the ammonium content of the waste stream is then oxidized to nitrite in the nitritation reactor after which this nitrite and ammonium are reacting to nitrogen gas in the deammonification reactor. It is theoretically possible to control the treatment in the first stage so that half of the ammonium is oxidised and thereafter transfer the water to the second stage in order to allow the NO 2   − +NH 4   + =&gt;H 2 O+N 2  reaction of the remaining ammonium and the formed NO 2   − . It is, however, preferred to divide the stream of water to be treated into two streams, wherein one of the streams is subjected to the ammonium oxidation to NO 2   −  and the other stream is bypassed. The two streams are then mixed in an anoxic tank, where the NO 2   − +NH 4   + =&gt;H 2 O+N 2  reaction of the ammonium in the bypassed stream and the NO 2   −  in the treated stream takes place. Alternatively, using a biofilm type of process, the anammox process can be performed in a single stage, wherein the oxidation of ammonium to nitrite takes place in an upper layer of a biofilm and the anoxic process forming water and nitrogen gas takes place in a deeper layer of the biofilm. 
         [0009]    Although very advantageous from an energy and resource point of view, the anammox process has, however, some severe limitations. Just as for the SHARON process, the anammox process depends upon a stable nitritation without the nitrite formed being further oxidized to nitrate by NOB. In practice, this has only been possible on wastewater with high ammonium concentrations and high temperatures (&gt;20° C.). Consequently, the anammox process has been successfully applied on streams such as reject water from dewatering of digested sludge, while previous attempts to run the anammox process with ordinary “main stream” municipal wastewater have proven unsuccessful. 
         [0010]    WO 2013/151836 discloses an anammox process, wherein main stream waste water is treated in a single step. The biofilm performing both the NH 4   + +O 2 =&gt;NO 2 +H 2 O reaction and the NO 2   − +NH 4   | =&gt;H 2 O+N 2  is grown on carrier elements allowing for a virtually unlimited biofilm thickness. In order to “rejuvenate” the biofilm, the carrier elements on which the biofilm grows is periodically subjected to waste water having a high concentration of NH 4   + . Due to the rejuvenating process, the process will function relatively well even for wastewater containing low concentrations of NH 4   + . 
         [0011]    However, it has been shown that the anammox bacteria, performing the NO 2   − +NH 4   + =&gt;H 2 O+N 2  reaction are very sensitive to high concentrations of NH 4   +  and NO 2   −  during the periodical subjection to wastewater with high concentrations of ammonium and to avoiding to kill these bacteria, the process has to be operated under conditions that will not completely suppress the NOB converting NO 2   −  to NO 3   − . Moreover, this problem has been found to increase further by the required thick biofilm developing zones with conditions favourable for the oxidation of NO 2   −  to NO 3   −  when operating on main stream wastewater. 
         [0012]    In Pusker Regmi et. al., “NOB Repression for Mainstream Nitrite-Shunt and Deammonijication: A Pilot Study”. Proceedings of the Water Environment Federation, WEFTEC 2013: Session 26 through Session 34, pp. 1959-1981(23), a pilot study for the viability of mainstream NOB suppression and deammonification is described. The publication considers Nitritation-Denitritation through Modulating Aeration (NiDeMA) CSTR with a clarifier followed by a fully-anoxic AMX moving bed bioreactor (MBBR). The NiDeMA CSTR is operated under cyclical dissolved oxygen (DO) conditions for NOB suppression. 
         [0013]    However, in the pilot study, the aeration scheme is controlled via an extensive sensor and monitoring system with aeration cycles as short and precise as 10 minutes. Furthermore, transient anoxia has been validated primarily for high strength wastes, and true use for mainstream NOB suppression has still got to be shown. 
       SUMMARY OF THE INVENTION 
       [0014]    The above and other problems are solved or at least mitigated by a process for converting ammonium (NH 4   + ) of a mainstream of a wastewater plant to dinitrogen gas (N 2 ), including the consecutive steps of: 
         [0015]    i.) removal of biodegradable carbon compounds in the mainstream; 
         [0016]    ii.) conversion of ammonium (NH 4   + ) in the mainstream to nitrite (NO 2   − ) in an aerated biological process containing ammonium oxidising bacteria (AOB) in a nitration vessel; 
         [0017]    iii.) denitrifying the resulting stream from step ii.) in an anammox vessel; 
         [0018]    wherein growth of nitrite oxidising bacteria (NOB) in step ii.) is prevented by periodically subjecting the bacteria in said nitration vessel to water suppressing growth of nitrite oxidising bacteria (NOB). 
         [0019]    Typically, the resulting stream from step ii.) is denitrified by converting nitrite together with ammonium to dinitrogen gas. 
         [0020]    In order to provide for an environment free of biodegradable carbon compounds, step i.) may comprise aerated biological treatment of the mainstream of the wastewater, such that the biodegradable carbon compounds react with oxygen (O 2 ) to form carbon dioxide (CO 2 ) and water (H 2 O). 
         [0021]    In order to use the energy comprised in the biodegradable carbon compounds, step i.) may comprise an anaerobic digestion of the wastewater, such that methane (CH4), is formed by the biodegradable organic compounds. 
         [0022]    Step i.) may comprise an aerated biological treatment of the mainstream of the wastewater, such that the biodegradable carbon compounds will react with oxygen (O 2 ) to form carbon dioxide (CO 2 ) and water (H 2 O). 
         [0023]    In order to make the energy conservation more efficient, step i.) may comprise micro filtering of the biodegradable organic compounds, wherein the filtrate is conveyed to a digester for subsequent digestion. 
         [0024]    Since most waste water treatment plants are provided with a sludge digester providing a reject water from dewatering of the digested sludge having high concentration of ammonium, it may be beneficial if the water suppressing growth of nitrite (NO 2   − ) oxidising bacteria is reject water. The ammonium concentration of the reject water is higher than that of the mainstream of a wastewater plant and may lie in the range from 300 mg(N)/1 to 1200 mg(N)/1. 
         [0025]    In another embodiment of the invention, the water suppressing growth of nitrite (NO 2   − ) oxidising bacteria (NOB) may contain high concentrations of NO 2 , higher than that of the mainstream of a wastewater plant. In such a case, the concentration of NO 2   −  may be in the range from 300 mg(N)/1 to 1200 mg(N)/1. 
         [0026]    Since tests have shown that the best suppression of nitrite oxidising bacteria takes place at elevated temperatures, the temperature of the water suppressing growth of nitrite (NO 2   − ) oxidising bacteria (NOB) may be 20-35 degrees centigrade. 
         [0027]    Tests have shown that a suitable time between subjecting the bacteria to water suppressing growth of nitrite oxidizing bacteria (NOB) is between 0.5 to 10 weeks. 
         [0028]    The denitritation step may be performed by adding an organic electron donor to the water after step ii.) and allowing bacteria to convert the nitrite to dinitrogen gas. 
         [0029]    Alternatively, the denitritation process may be performed by transferring the mainstream in which NH 4   +  has been converted to NO 2   −  is to an anammox vessel where said stream is denitrified by adding water containing ammonium (NH 4   + ) to the water after step ii.) and allowing bacteria to convert the nitrite (NO 2   − ) using ammonium as the electron donor. This alternative is beneficial since no organic electron donor, i.e. in form of biodegradable carbon compounds, is necessary for the denitritation. 
         [0030]    In one alternative, the mainstream in which NH 4   +  has been converted to NO 2   −  is transferred to an anammox vessel in which nitrite (NO 2   − ) formed in the nitritation vessel is allowed to react with residual ammonium (NH 4   + ) in the wastewater to form dinitrogen gas (N 2 ). 
         [0031]    The anammox vessel ( 200 ) is preferably operated under non-aerated or anoxic conditions. Anoxic conditions refers to water that has a concentration of dissolved oxygen of 0.5 milligrams per liter or less. 
         [0032]    In order to provide for easy suppressing of growth of nitrite (NO 2   − ) oxidising bacteria (NOB), the bacteria may grow on free-flowing carrier elements. In order to get a biofilm having a well-defined thickness, the free-flowing carrier elements may be designed such that a thickness of a film in which the bacteria grow is controlled to a limited thickness, e.g. 500 μm, 300 μm or 200 μm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Hereinafter, the invention will be described by description of preferred embodiments with reference to the appended drawings, wherein: 
           [0034]      FIG. 1  is a scheme showing a prior art wastewater plant, and 
           [0035]      FIG. 2  is a scheme of a one embodiment of a wastewater plant using the system according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    The present invention is primarily directed to purification of municipal wastewater. Such wastewater contains a variety of contaminants, e.g. nitrogen compounds (mainly in the form of ammonium salts), phosphorous compounds and different kinds of carbon compounds. 
         [0037]    In  FIG. 1 , a schematic view of an exemplary prior art wastewater treatment plant  100  is shown. It should be understood that a prior art plant not necessarily will comprise all the shown components and process steps. It could also comprise more steps and processes than the ones shown. 
         [0038]    In a first step  110 , the wastewater will be sieved, in order to remove large particles and other debris. The size of the sieve openings may vary significantly. If small sieve openings are used, a substantial portion of the compounds containing carbon will be trapped in the sieve. The particles trapped in the sieve may be transported to a digester  120 , in which the particles will digest anaerobically to form e.g. methane. However, in many cases, a very crude sieve (not shown) may be provided upstream the sieve  110 . The items stuck in the crude sieve are often not digestible, and may hence be deponated. 
         [0039]    In a second step, even more particles will be separated from the waste water by a sedimentation step  130 , in which particles denser than water will sink to a bottom of a tank, whereas particles less dense than water will float to the surface. The denser particles will form a sludge layer on the bottom and can be pumped out from the sedimentation step, whereas the less dense particles may be skimmed from the surface. The sludge and skimmed particles from the sedimentation step will also be conveyed to the digester  120 . 
         [0040]    In a third step, an “activated sludge” process will reduce the contents of carbon compounds and ammonium. 
         [0041]    Generally, the active sludge process start with an aerobic step  140 , in which bacteria contained in a sludge or provided on carrier elements (not shown) will consume organic compounds and oxidise ammonium to nitrate, NO 3   − . The aerobic step is dependent on air being pumped through the vessel containing the sludge and the wastewater, since oxygen is required for oxidising the organic compounds and the ammonium to nitrate. The first step of the activated sludge process is generally referred to as the nitrification step. 
         [0042]    The second step of the activated sludge process is a denitrification step  150 , in which the nitrate formed by the nitrification step will be reduced to nitrogen gas, i.e. N 2 . This step takes place under anoxic conditions, i.e. without oxygen, and also requires an electron donor, for example in form of biodegradable organic matter CH in order to reduce the nitrate to N 2 . The organic matter, or electron donor, is usually added to the wastewater in form of ethanol, methanol, acetone, or any other easily biodegradable carbon source. 
         [0043]    After the denitrification step  150 , there is a final sedimentation step  160  for removing sludge S from the wastewater. The majority of the removed sludge S is reintroduced into the activated sludge process, usually in the nitrification step  140 , but since new sludge is formed during the activated sludge process, some of the sludge removed after the activated sludge process is conveyed to the digester  120 , where it is allowed to digest. 
         [0044]    In some cases, the treated wastewater is filtered through a mechanical filter, e.g. a sand filter, prior to being let out in the environment. 
         [0045]    As mentioned earlier, the sludge S removed from the wastewater in the sedimentation step  160  is conveyed to the digester  120  for further treatment. In the digester, methane gas, CH 4 , is formed under anaerobic conditions. The methane forming process will continue until all of the easily biodegradable organic material in the sludge is consumed. After this organic matter is consumed, there will still be some sludge left. This sludge is drained from excess water, which has high concentrations of ammonium. This water, which generally is referred to as reject water RW, can either be conveyed back to the main stream, i.e. the aerobic step  140 , or be treated separately in order to reduce the nitrogen compounds therein. 
         [0046]    Recently, the so-called anammox process has been used for reducing the content of nitrogen compounds in the reject water RW. The anammox process differs significantly from the previously disclosed process for reducing the ammonium content of the main stream. 
         [0047]    In the anammox process, which is schematically shown in  FIG. 1 , a part of the reject water is subjected to an oxidation process  170 , in which the ammonium, NH 4   + , in the reject water is oxidised to form NO 2   − , i.e. nitrite. The other part of the reject water bypasses the oxidation process  170 . For some reason, the high ammonium content of the reject water seems to suppress bacteria forming nitrate, i.e. NO 3   − , and promote bacteria forming nitrite, i.e. NO 2   − . It also seems that high temperatures promote nitrite forming bacteria. 
         [0048]    After the part of the reject water has been subjected to the oxidising process forming nitrite, NO 2   − , i.e. the nitritation process, the part having been subjected to the oxidising process is mixed with the bypassed part, such that bacteria may use the nitrate to reduce the ammonium in the untreated water, hence forming nitrogen gas. This process is performed under anoxic, i.e. oxygen-free, conditions at 180. 
         [0049]    As can be noted above, one major difference, and benefit, with the anammox process is that no biodegradable organic compound is necessary for the reduction of ammonium to nitrogen gas. Moreover, the nitritation process consumes less oxygen than the nitrification process disclosed in connection with  FIG. 1 . 
         [0050]    As mentioned above, the oxidation process for forming NO 2   −  rather than NO 3   −  only works with high NH 4   +  concentrations; if the NH 4   +  concentrations are too low, growth of bacteria forming NO 3   −  from NO 2   −  will be promoted, and the NO 2   − =&gt;NO 3   −  process is much faster than the NH 4   + =&gt;NO 2   −  process, meaning that once bacteria performing the NO 2   − =&gt;NO 3   −  reaction has been formed, virtually all NH 4   +  will end up as NO 3   − , which makes a subsequent anammox process impossible. 
         [0051]    According to one embodiment of the invention, growth of bacteria forming NO 3   −  from NO 2   −  is inhibited by sequentially subjecting biofilm grown on biofilm carriers to a water having high concentration of NH 4   + . This embodiment is useful both for the anammox process and the Sharon process as disclosed in the prior art section, and gives the possibility to oxidise ammonium contained in water having a low concentration thereof to nitrite rather than nitrate. As mentioned in the prior art section, for the Sharon process, this is beneficial since less oxygen is required for the nitritation process compared to the nitrification process and since less organic electron donor, e.g. in form of biodegradable carbon sources, is required for the denitritation process. For the anammox process, formation of NO 2   −  rather than NO 3   −  is a necessity for the conversion NH 4   + +NO 2   − =&gt;N2+H2O. 
         [0052]    In  FIG. 2 , a schematic view of a wastewater treatment plant according to one embodiment of the invention is shown. Most components and process steps are identical to the components and process steps of the previously described prior art systems, and for identical or similar processes and steps, the same reference numerals have been used in both  FIGS. 1 and 2 . It should be noted that that the reduction of NH 4   +  differs completely between the embodiments shown in  FIGS. 1 and 2 . In  FIG. 2 , a first embodiment of the present invention is shown. The wastewater first enters the previously disclosed sieve  110  and sediment arrangements  130  for collection of particulate matter, which sieves and sediment arrangements do not form part of the invention, and hence will not be disclosed further, and continues to a biological process, which will be described below. The sludge from the sieves and sedimentary steps is conveyed to the digester  120 , where it is digested; so far, the process is identical to the prior art process shown in  FIG. 1 . 
         [0053]    After the sedimentation step  130 , the wastewater is treated to oxidise biodegradable carbon compounds at the process step  135 . By keeping the load on this step high, i.e. maintain a high flux of biodegradable carbon compounds into the process step, growth of bacteria oxidising ammonium to either NO 2   −  or NO 3   − , i.e. AOB and NOB, can be kept low, and large amounts of excess biological sludge can be formed, which increases the production of biogas in the digester. 
         [0054]    After the step  135 , the wastewater stream is divided into first  131  and second  132  streams. The second stream  132  is conveyed to several parallel nitritation processes (i.e. nitrite converting processes) in which the bacteria are adapted (in a way to be described later) to oxidise NH 4  to NO 2   − , i.e. nitrite, while the first stream bypasses these vessels. In this process, which takes place under aerobic conditions, any remaining biodegradable carbon compounds in the wastewater are oxidised to CO 2 . It is, however, crucial that the concentration of biodegradable carbon compounds is low; otherwise, the bacteria in the nitritation vessels will be dominated by bacteria converting biodegradable carbon compounds to CO 2  rather than bacteria converting NH 4   +  to NO 2   − . Please note that there are several different parallel nitritation vessels  133   a - 133   d  for performing the nitritation process. 
         [0055]    According to the invention, not all of the nitritation vessels are connected to the second stream  132 ; one of the nitritation vessels is instead connected to the reject water outlet, RW, which, as mentioned, contains high concentrations of NH 4   + . Due to the high concentration of NH 4   +  in the reject water, and/or the high concentrations of NO 2   −  formed in the vessel, the bacteria in the vessel connected to the reject water outlet will grow such that conversion (i.e. oxidation) of NH 4   +  to NO 2   −  is promoted, rather than bacteria converting NO 2   −  to NO 3   − , i.e. NOB. 
         [0056]    Surprisingly, it has been found that the NH 4   +  to NO 2   −  converting bacteria, AOB, that have been promoted due to the connection of the nitritation vessel to the reject water outlet will continue to convert NH 4   +  to NO 2   −  rather than NO 3  even if the nitrification vessel thereafter is connected to “normal” wastewater, i.e. wastewater containing NH 4   +  in significantly lower concentrations. Hence, by sequencing the connection of the nitrification vessels between the reject water outlet and the main stream wastewater, conversion of NH 4   +  to NO 2   −  can be maintained for “normal”, i.e. low, concentrations of NH 4   +  during the periods between the vessel being connected to the reject water outlet. 
         [0057]    In the shown embodiment, the sequencing between running the vessel with reject water and wastewater is achieved by controlling a valve assembly VA in a way well known by persons skilled in the art, such that wastewater during some periods are conveyed to some vessels and reject water to some of the vessels. After a certain time, for example when it can be shown that the vessel fed with high NH 4  concentration reject water outputs mostly NO 2   −  rather than NO 3   − , another vessel will be fed reject water RW. Thus, the bacteria can be periodically subjected to water suppressing growth of nitrite oxidizing bacteria (NOB), such as through consecutive feeding of reject water RW to the parallel nitritation vessels  133   a - 133   d  in regular or irregular cycles, or though intermittent feeding of reject water RW to the different nitritation vessels  133   a - 133   d.    
         [0058]    It should be noted that the nitrification vessels preferably are, at least to a certain degree, filled with carrier elements provided with protected surfaces for growth of a bacterial film thereon. In one preferred embodiment of the invention, the carrier elements are designed such that a thickness of the bacterial film will not exceed a certain value, e.g. 500, 300 or 200 μm. This is beneficial since the bacterial film will have the same properties in its entire thickness and conditions that may favour the oxidation of NO 2   −  to NO 3   −  deep in a thick biofilm can be avoided. 
         [0059]    In another embodiment of the invention, the wastewater and the reject water are always connected to the same vessels, and the carrier elements are moved from vessel to vessel once the bacteria on the carrier elements of a vessel connected to wastewater need a time at high NH 4   +  or NO 2   −  concentrations in order to regain the ability to convert NH 4   +  to NO 2   −  rather than NO 3   − , i.e. such that nitrite rather than nitrate is formed for the water having low concentration of ammonium. 
         [0060]    After the nitritation process in the vessels  133   a - 133   d,  the water of the second stream and the reject water RW that has been treated in the nitritation vessel in which the bacteria required treatment in order to regain the NH 4   + =&gt;NO 2   −  conversion efficiency will be transferred to an anammox vessel  200 , where it will be mixed with water from the first stream  131 , which contains ammonium, that may react biologically with the NO 2   −  from the nitritation processes to form N 2 . 
         [0061]    The anammox vessel ( 200 ) is preferably operated under non-aerated or anoxic conditions, where anoxic conditions refers to water that has a very low concentration of dissolved oxygen (such as less than 0.5 milligrams per liter). In the anoxic process performed by so-called anammox bacteria, the nitrite (NO 2   − ) and the ammonium (NH 4   + ) will react to form water (H 2 O) and dinitrogen gas (N 2 ), in an anoxic deammonification reaction. 
         [0062]    According to the invention, the bacteria in the nitritation vessels  133   a - 133   d  are intermittently treated to promote bacteria forming NO 2   −  from NH 4   +  rather than NO 3   −  from NH 4   + . This could be done in several ways: 
         [0063]    1. Increase the amount (i.e. concentration) of NH 4   +  to which the bacteria are subjected; 
         [0064]    2. Increase the amount of NO 2   −  to which the bacteria are subjected; 
         [0065]    3. Increase the temperature (in a “normal” main stream of a water purification plant, the temperature is often 12-15 degrees centigrade, whereas the optimum temperature for promoting bacteria for NO 2   −  formation is in the order of 30 degrees centigrade). 
         [0066]    It should be noted that the above ways may be freely combined. It should also be noted that item 1 above leads to item 2, since the ammonium, NH 4   + ; will be converted to NO 2   − ; in one embodiment of the invention, the stream of water to and from the vessel containing the bacteria to be adapted to promote NO 2   −  production is simply shut off, after which a sufficient dose of highly concentrated ammonium or nitrite is added to the water in the vessel. Then, the water is left in the vessel until all, or the majority, of the ammonium, NH 4   + , as been oxidised to NO 2   − . If desired, the water may even stay in the vessel for a significant time after all of the ammonium has been oxidised to NO 2   − . In one specific embodiment, the mainstream wastewater is treated in the nitritation vessel  133   a - 133   d  in such a way that only part of the ammonium, NH 4   +  is converted to NO 2   − , leaving residual ammonium in the wastewater. This can be achieved in several ways, for instance by managing the addition of ammonium to the nitritation vessel  133   a - 133   d  during the nitration process, or by leading the mainstream wastewater from the nitritation vessel  133   a - 133   d  to an anammox vessel  200  before the nitrification process has oxidized all ammonium, NH 4   +  to NO 2   − . The mainstream wastewater is treated in the nitritation vessel  133   a - 133   d  under aerobic conditions wherein part of the ammonium in the wastewater is oxidized to nitrite. After the nitritation vessel  133   a - 133   d,  the mainstream wastewater is led to an anammox vessel  200  operated under anoxic conditions in which nitrite formed in the nitritation vessel is allowed to react with residual ammonium in the wastewater to dinitrogen gas according to the anammox reaction. If the ratio of NH 4   +  to NO 2   −  is balanced (i.e. 1:1), the anommox reaction may proceed close to completion without additional addition of an electron donor. If the ratio of NH 4   +  to NO 2   −  is unbalanced (i.e. not 1:1), additional NO 2   −  may be added to the anammox vessel from the nitritation process in the vessels  133   a - 133   d,  or additional NH 4   +  may be added from the first stream  131 , until the ratio is balanced (i.e. 1:1) allowing the anommox reaction to proceed close to completion. Such a strategy is suitable in the case where the flow or the ammonium content of the mainstream wastewater is highly variable. 
         [0067]    In still another embodiment of the invention, the water in a nitritation vessel  133   a - 133   d  to be treated such that growth of bacteria converting NO 2   − =&gt;NO 3   −  is suppressed is pumped out, after which the water suppressing growth of such bacteria is filled into the vessel. After the bacteria in the vessel have been treated, the water suppressing growth of NO 2   − =&gt;NO 3   −  oxidising bacteria can be used to treat the bacteria of another vessel. This embodiment is beneficial in that it is economically possible to maintain the water suppressing growth of NO 2   − =&gt;NO 3   −  oxidising bacteria at an optimum temperature. 
         [0068]    Dimensions 
         [0069]    Typical NH 4   +  concentrations in the wastewater having the low concentration of NH 4   +  may be from 10-50 mg/l, measured in nitrogen equivalent, i.e. 10-50 mg(N)/l. 
         [0070]    The NH 4   +  concentration of the reject water may be 300-1200 mg(N)/l 
         [0071]    A typical hydraulic retention time in the nitritation vessels ( 133   a - 133   f ) may be 0.5-3 hours for the water having the low concentration of NH 4   +   
         [0072]    The hydraulic retention time for the water suppressing growth of NOB, i.e. bacteria converting NO 2   −  to NO 3   −  may be 6-100 hours. 
         [0073]    The time between subjecting the bacteria to water suppressing growth of NOB may be 0.5 to 10 weeks.