METHODS FOR ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL

Abstract

The present invention relates to methods for enhanced biological phosphorus removal from water. In particular, the present invention provides methods for enhanced biological phosphorus removal from water by a biofilm in which the amount of oxygen supplied in the aerated step of the method is dependent on the amount of nitrite and/or nitrate detected. The invention also provides water treatment systems for enhanced biological phosphorus removal water.

Claims

1. A method for enhanced biological phosphorus removal from water by a biofilm, said method comprising: (i) an anaerobic step in which said biofilm is subjected to anaerobic conditions; (ii) an aerated step in which said biofilm is subjected to aerated conditions by supplying oxygen to the water; (iii) detecting the level of nitrite and/or nitrate in the water in or after aerated step (ii); wherein the amount of oxygen supplied in aerated step (ii) is dependent on the level of nitrite and/or nitrate detected.

2. The method of claim 1, wherein said biofilm is present on free flowing biofilm carriers.

3. The method of claim 1, wherein said water is wastewater.

4. The method of claim 1, wherein said method is performed in a reactor, preferably a Moving Bed Biofilm Reactor (MBBR).

5. The method of claim 4, said biofilm is present on biofilm carriers and wherein the filling ratio of biofilm carriers is between 1% and 100%, preferably between 30% to 75%, of the wet volume of the reactor.

6. The method of claim 1, wherein said method is a continuous method.

7. The method of claim 1, wherein said anaerobic step (i) is carried out in an anaerobic zone of a reactor and said aerated step (ii) is carried out in an aerated zone of a reactor.

8. The method of claim 1, wherein said anaerobic zone and/or said aerated zone is sub-divided into a plurality of sub-chambers.

9. The method of claim 1, wherein the biofilm is present on biofilm carriers and at the end of the aerated step said biofilm carriers are transferred from the aerated chamber to the anaerobic chamber without significant transfer of water.

10. The method of claim 9, wherein said transfer is performed by a mechanical device, preferably said mechanical device is an elevator, transport screw or conveyer belt.

11. The method of claim 1, wherein said oxygen is supplied in the form of air, preferably by a bubble diffuser or blower.

12. The method of claim 1, wherein the level of nitrite (NO.sub.2) is detected.

13. The method of claim 1, wherein the level of nitrate (NO.sub.3) is detected.

14. The method of claim 1, wherein the level of nitrite (NO.sub.2) and nitrate (NO.sub.3) is detected.

15. The method of claim 1, wherein the level of nitrite and/or nitrate is detected by one or more in-line sensors.

16. The method of claim 1, wherein the amount of oxygen supplied in aerated step (ii) is decreased in response to the detection of an increased, or increasing, level of nitrite, preferably said amount of oxygen supplied in aerated step (ii) is decreased by decreasing a DO-setpoint.

17. The method of claim 1, wherein the amount of oxygen supplied in aerated step (ii) is increased in response to the detection of a decreased, or decreasing, level of nitrite and/or nitrate, preferably said amount of oxygen supplied in aerated step (ii) is increased by increasing a DO-setpoint.

18. The method of claim 16, wherein said increased or increasing or decreased or decreasing level of nitrite and/or nitrate is an increase or decrease relative to a nitrite and/or nitrate setpoint level.

19. The method of claim 18, wherein said setpoint level of nitrite and/or nitrate is 0.5-5 mg/l, preferably 1-2 mg/l, more preferably 1.5 mg/l.

20. The method of claim 1, wherein in aerated step (ii) there is a continuous supply of oxygen to the water.

21. The method of claim 1, wherein said method achieves biological phosphorus removal from water such that there is a concentration of PO.sub.4P of less than 0.5 mg/l, preferably less than 0.2 mg/l or less than 0.1 mg/l.

22. The method of claim 1, wherein said method further comprises the removal of nitrogen in the form of ammonium from said water.

23. The method of claim 22, wherein said removal of nitrogen is by simultaneous nitrification-denitrification (SND) by microorganisms in the biofilm.

24. A water treatment system configured to perform the method of claim 1.

25. A water treatment system for enhanced biological phosphorus removal from water by a biofilm, said system comprising (i) a reactor (ii) a means for supplying oxygen to said reactor; (iii) one or more valves through which oxygen can enter said reactor; (iv) one or more means for determining the concentration of nitrite and/or nitrate in water; (v) one or more means for determining the concentration of dissolved oxygen (DO) in water; and (vi) one or more controllers configured to regulate the amount of oxygen entering the reactor, said regulation being dependent on the concentration of nitrite and/or nitrate determined by the means of (iv).

Description

[0266] The invention will now be further described in the following non-limiting Examples with reference to the following drawings:

[0267] FIG. 1: A schematic depiction of the MBBR-reactor setup used in Example 1 described herein. The influent is the influent wastewater (i.e. water that is to be treated). The effluent is the effluent (i.e. treated) wastewater, i.e. the water that has been treated. The open circles represent air (air bubbles). Thus, the sub-chambers of the MBBR containing these open circles together define the aerated chamber (i.e. the zone in which the aerated step of the method takes place). The sub-chambers lacking these open circles together define the anaerobic chamber (the zone in which the anaerobic step of the method takes place). The cartwheels represent biofilm carriers having biofilms thereon. The circles containing the term DO represent DO (dissolved oxygen) sensors. The circle containing the term NO.sub.2/NO.sub.3 represents a sensor for detecting nitrite and/or nitrate level. The bow-tie shapes represent valves. The arrow on the conveyor depicts the transfer of the biofilm carriers from the final aerated sub-chamber back into the first anaerobic sub-chamber. The stirrers in the anaerobic chamber are for the mechanical mixing of the biofilm carriers and the water. The controller labelled C1 receives the DO level measurements from the DO sensors and can adjust the DO-setpoint in response to a signal to do so from the controller labelled C2. The controller labelled C2 can receive the nitrite and/or nitrate level measurements from the sensor for detecting nitrite and/or nitrate level and compare this level to a nitrite and/or nitrate setpoint and communicates with controller C1 to cause controller C1 to adjust the DO-setpoint (increase or decrease) if the nitrite and/or nitrate level is different from the nitrite and/or nitrate setpoint. Adjustment of the DO-setpoint adjusts the amount of air being supplied into the MBBR.

[0268] FIG. 2: Graph showing a 24-hour period of operation of the method described in Experimental Example 1, which was operated with an MBBR setup as shown schematically in FIG. 1. The DO-setpoint, measured DO concentration and measured nitrite concentration are all in mg/l (the scale for which is on the left-hand y-axis). The measured air flow (measured air supply/amount) is m.sup.3/h (the scale for which is on the right-hand y-axis).

[0269] FIG. 3: (A) Graph showing measurements from the aerated chamber of the bioreactor during a 24-hour period of operation of the method described in Experimental Example 2. The measured DO.sub.2 concentration (DO.sub.2, also referred to simply as DO, dissolved oxygen), measured nitrite concentration (NO.sub.2N) and measured nitrate concentration (NO.sub.3N) are all in mg/l. The power of the blower (which supplied air and thus oxygen to the aerated chamber) is given in Hz. (B) Graph showing influent NH.sub.4N concentration, effluent NH.sub.4N concentration, influent PO.sub.4P concentration and effluent PO.sub.4P concentrations during a 24-hour period of operation of the method described in Experimental Example 2 (the same 24 hour period of operation to which FIG. 3A relates). All concentrations are given in mg/l. The dashed line depicts a concentration of 0.1 mg/l PO.sub.4P.

[0270] FIG. 4: Schematic depiction of structuring (layering) of a biofilm in aerated conditions in wastewater, with different DO-concentrations in different layers of the biofilm structure.

[0271] FIG. 5: A schematic depiction of an MBBR-reactor setup in accordance with the invention. The influent is the influent wastewater (i.e. water that is to be treated). The effluent is the effluent (i.e. treated) wastewater, i.e. the water that has been treated. The dots represent air (air bubbles). The cartwheels represent biofilm carriers having biofilms thereon. The circle containing the term DO represents a DO (dissolved oxygen) sensor. The circle containing the term NO.sub.2/NO.sub.3 represents nitrite and nitrate sensors. The bow-tie shape represents a valve.

EXPERIMENTAL EXAMPLES

Example 1

Method

[0272] Described below is experimental testing of a method for enhanced biological phosphorus removal from water by a biofilm in accordance with the present invention. This exemplified method is a continuous method. The water (i.e. influent water) was municipal wastewater at the Hias wastewater treatment plant in Norway, which is received from four municipalities in Hedmark county.

[0273] This method has been demonstrated in a full-scale municipal Moving Bed Biofilm Reactor, in which biofilm carriers colonized with biofilms were present in the water, and the biofilm carriers (AnoxKaldnes K3 (Veolia)) moved through the reactor from the anaerobic chamber to the aerated chamber. AnoxKaldnes K3 biofilm carriers are plastic carriers. The filling ratio used was 55%. The biofilm carriers were then transferred from the aerated chamber (final sub-chamber thereof) back to the anaerobic chamber (first sub-chamber thereof) via a conveyor belt. Thus, there is a continuous cycling of the biofilm carriers. When the biofilm carriers were transferred from the aerated chamber back into the anaerobic chamber there was no significant transfer of water from the aerated chamber back into the anaerobic chamber. The anaerobic chamber and the aerated chamber of the reactor were each sub-divided by walls into multiple sub-chambers, the walls of course having openings to allow the carriers to flow, entrained in the water, from one sub-chamber to the next. The anaerobic chamber contained stirrers to mechanically mix the carriers in the water. In the aerated chamber, air was continuously (but variably) supplied by a bubble diffuser (or blower) and the bubbles mixed the carriers in the water. The influent wastewater flowed into the anaerobic chamber (the first sub-chamber thereof). The effluent (treated) water flowed out of the aerated chamber (final sub-chamber thereof). The aerated chamber of the reactor was provided with an in-line sensor for detecting the concentration of nitrite (NO.sub.2N) and nitrate (NO.sub.3N), and multiple sensors for detecting the concentration of DO (dissolved oxygen) were provided (as shown on FIG. 1). For the avoidance of any doubt, the enhanced biological phosphorus removal method described in this experimental example is not an activated sludge method; there is no recycling of sludge (no sludge return). In this experiment, in samples taken from the effluent water there was an average of 250 mg/l of suspended solids (SS). To measure the SS a well-mixed sample of 50 ml of effluent water was filtered through a weighed standard glass-fiber filter and the residue left on the filter was dried to a constant weight at a temperature of 105 C. The increase in weight of the filter represents the total suspended solids of the sample, and was used to calculate the concentration of suspended solids in the water (in mg/l).

[0274] A schematic depiction of the MBBR-reactor setup used in this Example is set out in FIG. 1.

[0275] In this method, the amount of air (and thus oxygen) supplied to the aerated chamber was adjusted (i.e. increased or decreased), via controllers, depending on of the nitrite concentration detected in the aerated water (i.e. in the water in the aerated chamber). A nitrite-setpoint of 1.5 mg/l was set. The concentration of nitrite was constantly surveyed (measured) by the in-line nitrite sensor. (In this specific experiment the nitrate level was very low (much lower than the nitrite level) and was thus not used as a component of the setpoint concentration (but that is not always the case at other times). When the concentration of nitrite measured by the nitrite sensor fell below the nitrite setpoint, the DO-setpoint was increased and accordingly more air was supplied to the aerated chamber. When the concentration of nitrite measured by the nitrite sensor rose above the nitrite setpoint, the DO-setpoint was decreased and accordingly less air was supplied to the aerated chamber. Thus, the amount of air (and thus oxygen) supplied to the aerated chamber was adjusted depending on the concentration of nitrite detected. Put another way, the DO-setpoint, and accordingly the air supply to the aerated chamber, were varied during the performance of the method, with the variation being a function of the nitrite concentration detected/the fluctuation of the measured nitrite concentration from the nitrite setpoint.

[0276] As described elsewhere herein the DO-setpoint can be a DO-setpoint profile, in which there are different DO-setpoints in different aerated sub-chambers of the reactor. This was the case in the study of this experimental example, so with reference to the study of this experimental example the increasing or decreasing of the DO-setpoint means that the DO-setpoint profile was increased or decreased in response to changes in the measured nitrite level relative to the nitrite setpoint. The DO-setpoint profile used was characterised by there being a higher DO-setpoint for sub-chambers at the early part of the aerated chamber as compared to for sub-chambers later in the aerated chamber (i.e. closer to the outlet). In this regard, in the present Example, and with reference to FIG. 1, The DO-setpoint in the first aerated sub-chamber was higher than the DO-setpoint in the fourth aerated sub-chamber, which in turn was higher than the DO-setpoint in the sixth aerated sub-chamber. These different DO-setpoints in different sub-chambers together formed a DO-setpoint profile, which was adjusted (increased or decreased) based on the measured nitrite level relative to the nitrite setpoint.

[0277] Each of the DO-sensors in the reactor communicated (or signaled) the DO concentration of the water in the sub-chamber in which the sensor was located to a controller (a PID controller). Thus, in this study, and with reference to FIG. 1, the controller labelled C1 in fact is composed of three separate PID controllers, each operating in response to its respective DO sensor. However, for convenience, FIG. 1 depicts single controller receiving the DO concentrations from the three DO sensors. Each of the PID controllers that received the measured DO concentration from its respective DO sensor adjusted the amount of air supplied to the sub-chamber in which its respective DO sensor was located. Of course, not all of the sub-chambers in the reactor have DO sensors (and corresponding PID controllers), so those aerated sub-chambers that did not have a DO sensor were supplied with an amount of air based on the DO-setpoint for an adjacent sub-chamber that did have a DO-sensor. More specifically, and with reference to FIG. 1, the amount or air that was supplied to aerated sub-chamber 2 was based on the measured DO level and DO-setpoint for aerated sub-chamber 1 (which has a DO sensor). The amount or air that was supplied to aerated sub-chamber 3 was based on the measured DO level and DO-setpoint for aerated sub-chamber 4 (which has a DO sensor). The amount or air that was supplied to aerated sub-chambers 5 and 7 was based on the measured DO levels and DO-setpoint for aerated sub-chamber 6 (which has a DO sensor).

Results

[0278] FIG. 2 shows measurements in the aerated step during a 24-hour period of operation of a method as described above, operated with an MBBR setup as shown schematically in FIG. 1. The DO-setpoint and the Measured DO concentration lines on FIG. 2 shows the DO-setpoint and the Measured DO concentrations specifically for the first aerated sub-chamber of the reactor (which with reference to FIG. 1 is the fourth sub-chamber counting from the sub-chamber which received the influent water). This trend shown by these lines (curves) was also reflected in the DO setpoint and measured DO concentrations lines for subsequent aerated sub-chambers, although these lines were shifted down the y-axis due to the lower set DO-setpoints in these later sub-chambers, as described above (data not shown).

[0279] In FIG. 2 it can be seen that when the measured nitrite concentration fell below the nitrite setpoint (1.5 mg/l), the DO-setpoint was increased, the measured air supply (measured air flow) was increased and the measured DO concentration in the water increased. The measured nitrite concentration then increased, and when it rose above the nitrite setpoint, the DO-setpoint was decreased, the measured air supply (measured air flow) was decreased and the measured DO concentration in the water decreased.

[0280] Operating an analogous method of enhanced biological phosphorus removal at a constant DO-setpoint, in contrast to adjusting/varying the DO-setpoint as per the present method, would, in this example, have more than doubled the air supply demand, from about 200 Nm.sup.3/h to about 500 Nm.sup.3/h. To supply air to a biological water treatment plant is expensive (the energy costs are high), so a method which can reduce the amount of air supply required offers a distinct advantage.

[0281] Of course, it is important that a method of enhanced biological phosphorus removal that has an advantage in terms of the reduced amount of air that needs to be supplied (and thus an advantage in terms of energy use and cost saving) and/or an advantage in terms of providing a stable/more optimally operating biofilm still effectively removes phosphorus from the waste water. The presently exemplified method of enhanced biological phosphorus removal indeed effectively removed phosphorus from the wastewater and the method is at least as effective in terms of removing phosphorus from wastewater as analogous methods that employ a constant DO-setpoint (data not shown). The PO.sub.4P concentration in the treated water (i.e. effluent water) was in the range 0.1 mg/l to 0.2 mg/l. This is a very low concentration. This PO.sub.4P concentration in the effluent water was significantly lower than the PO.sub.4P concentration in the influent water. The PO.sub.4P concentration in the influent water was around 5 mg/l. The exemplified method also effectively removed (reduced the level of) nitrogen (in the form of ammonium) from the water by around 50% (data not shown).

Discussion

[0282] As described above, the inventors have developed a method for enhanced biological phosphorus removal that achieves excellent biological phosphorus removal from wastewater, and which involves adjusting the DO-setpoint during the aerated step such that the amount of air supplied is only the amount of air that is actually required by the relevant microorganisms in the biofilm. This is in contrast to other analogous methods of enhanced biological phosphorus removal that employ a constant DO-setpoint during the aerated step. This method represents a significant advantage in terms of reduced energy usage associated with the air supply, and thus reduced cost. As described above, central to this advantageous method is the use of measured nitrite and/or nitrate concentration as a parameter to control the amount of air being supplied to the aerated chamber. In this method, only the amount of air (and thus only the amount of oxygen) that is actually required for the relevant biological processes being carried out by the relevant microorganisms in the biofilms is supplied to the aerated chamber. Put another way, in this method, sufficient air (and thus oxygen) is supplied, but in this method only the amount of air that is actually necessary for the relevant biological processes is supplied (i.e. there is no significant superfluous air supplied).

[0283] Without wishing to be bound by theory, and as discussed elsewhere herein, the variation in the air supply during the aerated step in the present method controls the oxygen profile within the biofilm. In this regard, and again without wishing to be bound by theory, in a biofilm different types of bacteria live and grow based on what conditions they experience. Therefore, it is possible to obtain bacteria that use oxygen for respiration and bacteria that do not use oxygen in different layers of a biofilm. In biofilms present in wastewater heterotrophic bacteria (HET) are fast growing and take up carbon from the wastewater by using oxygen. In an anaerobic/aerobic process (such as enhanced biological phosphorus removal methods) Phosphate Accumulating Organisms (PAOs), that are slower growing, can use oxygen for phosphate uptake. Nitrifiers (nitrifying bacteria, N) are slow growing bacteria that compete for oxygen with the heterotrophic bacteria and PAOs. During aeration the competition for oxygen is an important parameter in the structuring of the biofilm. The fast growing organisms are situated in the outer part of the biofilm (i.e. the part (or layer) closest to the wastewater) and the slower growing bacteria are situated further into the biofilm. Even further into the biofilm there is no oxygen and the conditions can be anoxic (no (or essentially no) oxygen, but with NO.sub.2 and/or NO.sub.3) or anaerobic (no oxygen and no NO.sub.2 or NO.sub.3). An anoxic layer allows for bacteria that do not use oxygen, but which use NO.sub.2 or NO.sub.3, to live. These bacteria can for instance be denitrifiers (denitrifying bacteria, DN), such as denitrifying PAOs (DNPAOs). This structuring (layering) of a biofilm in aerated conditions in wastewater, with different DO-concentrations in different layers of the biofilm structure, is depicted schematically in FIG. 4.

[0284] The nitrifying bacteria (N) normally lose the competition for oxygen to both the heterotrophic bacteria (HET) and PAOs. Thus, if nitrite (NO.sub.2) and/or nitrate (NO.sub.3), which are produced by nitrifying bacteria (N), is detected in the water, this is indicative that both the HET and the PAOs have received enough oxygen, as oxygen has now reached even the slower growing nitrifying bacteria (N) in the biofilm. Thus, of particular relevance to a method of enhanced biological phosphorus removal, the detection of nitrite and/or nitrate in the aerated step (e.g. a concentration above a given setpoint) is indicative that PAOs in the biofilm have received enough oxygen to carry out the process of biological phosphorus removal from the water (i.e. to effect uptake of phosphorus from the water) and that as such the air supply (and thus the oxygen supply) can be decreased. Conversely, if no significant nitrite (NO.sub.2) and/or nitrate (NO.sub.3) is detected in the water (e.g. nitrite and/or nitrate is detected at a concentration that is below a given setpoint), this is indicative that both the HET and the PAOs may not have yet received enough oxygen, as oxygen has not reached the slower growing nitrifying bacteria (N) in the biofilm Thus, of particular relevance to a method of enhanced biological phosphorus removal, a lower (or decreasing) nitrite and/or nitrate (e.g. a concentration below a given setpoint) is indicative that PAOs in the biofilm have not received enough oxygen to carry out the process of biological phosphorus removal from the water and that as such the air supply (and thus the oxygen supply) can be increased.

[0285] By detecting (e.g. constantly detecting) the concentration of nitrite and/or nitrate and adjusting the DO-setpoint (if necessary) on the basis of the nitrite and/or nitrate concentration, the present method ensures that the oxygen requirements of the relevant bacteria (e.g. PAOs) are being met, whilst at the same time not supplying more oxygen than is actually needed.

[0286] It is also believed that by dynamically controlling the DO-setpoint (and thus as a result the oxygen profile in the biofilm), the biofilm is more stable and more optimal for the purpose of biological phosphate removal by PAOs. For example in this regard, operating with a constant DO-setpoint could mean that more oxygen than is actually required by the PAOs for phosphate uptake is supplied, which in turn would result mean that there could be a high concentration of superfluous oxygen (DO) that may be used by nitrifying bacteria to produce high concentrations of nitrites and nitrates. High concentrations of nitrites can inhibit phosphate uptake by PAOs, so it is typically desirable for there not to be high concentrations of nitrites during aeration in an enhanced biological phosphorus removal method.

[0287] Having a variable (as opposed to a constant or fixed) DO-setpoint that is dynamically adjusted based on actual oxygen requirements of PAOs, is clearly advantageous, and the present inventors have provided such a method herein.

[0288] The local environmental regulations at the Hias wastewater treatment plant do not require that nitrogen is removed from the water before the water is discharged to the environment. However, in addition to achieving good PO.sub.4P removal by EBPR as described above, the method exemplified in this experimental example also reduced nitrogen levels in the wastewater. In this regard, the sum of the NH.sub.4N concentration in the effluent (treated) water and the measured NOx-N concentration was significantly less than the NH.sub.4N concentration of the influent (untreated) water (data not shown). Lab analysis also showed NOx-N levels in the effluent water that were consistent with the NOx-N levels measured in the water in the reactor. These results indicate that a significant amount of ammonium derived nitrogen was completely removed from the system by simultaneous nitrification-denitrification (SND) of ammonium, i.e. indicate that the NOx generated by the nitrifying bacteria was converted to nitrogen gas (N.sub.2) or nitrous oxide (N.sub.2O) by denitrifying bacteria present in the anoxic part (or anoxic layer) of the biofilm. The method can thus operate as a combined EBPR/nitrification-denitrification (e.g. SND) method.

[0289] Particularly in the context of methods that effect EBPR and also further comprise the removal of nitrogen from water being treated (e.g. by simultaneous nitrification-denitrification), controlling the DO-profile in the biofilm is important in order to ensure that anoxic conditions are maintained in deeper layers of the biofilm (as anoxic conditions are important for denitrification by denitrifying bacteria); a high nitrite and/or nitrate level can indicate that oxygen is penetrating too far into the biofilm thus disturbing the anoxic conditions.

Example 2

[0290] A further experimental test was carried in a pilot wastewaster treatment plant at the Hias wastewater treatment plant in Norway. The method was analogous to the full scale plant method described in Example 1. However, in this Example, AnoxKaldnes K1 biofilm carriers were used. AnoxKaldnes K1 biofilm carriers are plastic carriers. The filling ratio used in this Example was 60%. For the avoidance of any doubt, the enhanced biological phosphorus removal method described in this experimental example is not an activated sludge method; there is no recycling of sludge (no sludge return).

[0291] In this experiment, in samples taken from the effluent water there was an average of 250 mg/l of suspended solids (SS). To measure the SS a well-mixed sample of 50 ml of effluent water was filtered through a weighed standard glass-fiber filter and the residue left on the filter was dried to a constant weight at a temperature of 105 C. The increase in weight of the filter represents the total suspended solids of the sample, and was used to calculate the concentration of suspended solids in the water (in mg/l).

[0292] The results for a 24 hour period of operation are depicted in FIGS. 3A and 3B. The NOx-setpoint was 1.5 mg/l. In this example, the NOx concentration value was the sum of the NO.sub.2N concentration and 1.5 times the NO.sub.3N concentration (i.e. in this example NOx=NO.sub.2N+1.5NO.sub.3N). As shown in FIG. 3A, as the measured NOx concentration falls below the 1.5 mg/l setpoint, the power of the blower (which supplies air and thus oxygen) is increased (as the DO-setpoint was increased in response to the NOx concentration falling below the 1.5 mg/ml setpoint), meaning more air (and thus more oxygen) is supplied. The dissolved oxygen (DO.sub.2, a.k.a. DO) concentration in the water increases accordingly, and the concentration of NO.sub.2N and NO.sub.3N ultimately starts to rise. Please note that 37.5 Hz was the minimum power supplied to the blower throughout the aerated step, which explains the constant blower flat line on FIG. 3A for the first 8 hours or so. The measurements depicted in FIG. 3A are of course measurements that were taken during an aerated step (in aerated zone of the reactor).

[0293] FIG. 3B, shows results for the same 24 hour period of operation as for FIG. 3A, with FIG. 3B showing the NH.sub.4N concentration in the influent (in) and effluent (out) and showing the PO.sub.4P concentration in the influent (in) and effluent (out). NH.sub.4N concentration was measured by on-line sensors. PO.sub.4P concentration was measured by filtering grab samples of the wastewater through a 1-micron filter, and then analysing the filtered wastewater with a Merck Millipore phosphate test kit and a Merck Spectroquant NOVA 60 Spectrophotometer.

[0294] The data in FIG. 3B clearly demonstrate that phosphorus (and nitrogen in the form of ammonium) was efficiently removed by this method. As shown in FIG. 3B, the effluent concentration of phosphorus (PO.sub.4P) was very low, 0.1 mg/l or less. This method is at least as effective as analogous methods that employ a constant DO-setpoint in terms of removing phosphorus from wastewater.

[0295] The discussion above in relation to Example 1 is also applicable to this Example.