ANAMMOX PROCESS AT WASTE WATER PLANT

Abstract

A process for converting ammonium (NH.sub.4.sup.+) of a mainstream of a wastewater plant to dinitrogen gas (N.sub.2), including the consecutive steps of i.) removing biodegradable carbon compounds in the mainstream, ii.) converting ammonium (NH.sub.4.sup.+) in the mainstream to nitrite (NO.sub.2.sup.−) in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitration vessel (133a-133d); 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 (133a-133d) to water suppressing growth of nitrite oxidizing bacteria (NOB).

Claims

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

2. The process of claim 1, wherein step i.) comprises aerated biological treatment of the mainstream of the wastewater, such that the biodegradable carbon compounds react with oxygen (O.sub.2) to form carbon dioxide (CO.sub.2) and water (H.sub.2O).

3. The process of claim 1, wherein step i.) comprises an anaerobic treatment of the wastewater, such that methane (CH.sub.4) is formed by the biodegradable organic compounds.

4. The process of claim 1, wherein step i.) comprises micro filtering of the biodegradable organic compounds.

5. The process of claim 1, wherein the water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) is water having a concentration of ammonium (NH.sub.4.sup.+) higher than that of the mainstream of a wastewater plant.

6. The process of claim 5, wherein the water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) has an ammonium (NH.sub.4.sup.+) concentration of 300 mg(N)/l to 1200 mg(N)/l.

7. The process of claim 1, wherein water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) contains concentrations of nitrite (NO.sub.2.sup.−) higher than that of the mainstream of a wastewater plant.

8. The process of claim 7, wherein the concentration of nitrite (NO.sub.2.sup.−) lies in the range from 100 mg(N)/l to 1200 mg(N)/l.

9. The process of claim 1, wherein the temperature of the water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) is 20 to 35 degrees centigrade.

10. The process of claim 1, wherein the time between subjecting the bacteria to water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) is between 0.5 and 10 weeks.

11. The process according to claim 1, wherein the water suppressing growth of nitrite (NO.sub.2.sup.−) oxidizing bacteria (NOB) is reject water (RW) from a sludge digester.

12. The process according to claim 1, wherein the denitritation step is performed by adding an electron donor to the water after step ii.) and allowing bacteria to convert the nitrite (NO.sub.2.sup.−) to dinitrogen gas (N.sub.2).

13. The process according to claim 1, wherein the mainstream in which ammonium (NH.sub.4.sup.+) has been converted to nitrite (NO.sub.2.sup.−) is transferred to the anammox vessel and said stream is denitrified by adding water containing ammonium (NH.sub.4.sup.+) to the water after step ii.) and allowing bacteria to convert the nitrite (NO.sub.2.sup.−) to dinitrogen gas (N.sub.2) using ammonium (NH.sub.4.sup.+) as the electron donor.

14. The process according to claim 1, wherein the mainstream in which NH.sub.4.sup.+ has been converted to NO.sub.2.sup.−is transferred to an anammox vessel in which nitrite (NO.sub.2.sup.−) formed in the nitritation vessel is allowed to react with residual ammonium (NH.sub.4.sup.+) in the wastewater to form dinitrogen gas (N.sub.2).

15. The process according to claim 1, wherein the anammox vessel is operated under non-aerated or anoxic conditions.

16. The process according to claim 15, wherein anoxic conditions refers to a concentration of dissolved oxygen of 0.5 milligrams per liter or less.

17. The process according to claim 1, wherein the bacteria are grown on free-flowing carrier elements.

18. The process according to claim 17, wherein the free-flowing carrier elements comprise protected surfaces for growth of a bacterial film thereon such that a thickness of a film in which the bacteria grow is limited to 500 μm, 300 μm or 200 μm.

Description

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.sub.3.sup.−. 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.sub.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.sub.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.sub.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.sub.4.sup.+, in the reject water is oxidised to form NO.sub.2.sup.−, 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.sub.3.sup.−, and promote bacteria forming nitrite, i.e. NO.sub.2.sup.−. 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.sub.2.sup.−, 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.sub.2.sup.− rather than NO.sub.3.sup.− only works with high NH.sub.4.sup.+ concentrations; if the NH.sub.4.sup.+ concentrations are too low, growth of bacteria forming NO.sub.3.sup.− from NO.sub.2.sup.− will be promoted, and the NO.sub.2.sup.−=>NO.sub.3.sup.− process is much faster than the NH.sub.4.sup.+=>NO.sub.2.sup.− process, meaning that once bacteria performing the NO.sub.2.sup.−=>NO.sub.3.sup.− reaction has been formed, virtually all NH.sub.4.sup.+ will end up as NO.sub.3.sup.−, which makes a subsequent anammox process impossible.

[0051] According to one embodiment of the invention, growth of bacteria forming NO.sub.3.sup.− from NO.sub.2.sup.− is inhibited by sequentially subjecting biofilm grown on biofilm carriers to a water having high concentration of NH.sub.4.sup.+. 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.sub.2.sup.− rather than NO.sub.3.sup.− is a necessity for the conversion NH.sub.4.sup.++NO.sub.2.sup.−=>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.sub.4.sup.+ 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.sub.2.sup.− or NO.sub.3.sup.−, 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.sub.4 to NO.sub.2.sup.−, 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.sub.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.sub.2 rather than bacteria converting NH.sub.4.sup.+ to NO.sub.2.sup.−. Please note that there are several different parallel nitritation vessels 133a-133d 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.sub.4.sup.+. Due to the high concentration of NH.sub.4.sup.+ in the reject water, and/or the high concentrations of NO.sub.2.sup.− 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.sub.4.sup.+ to NO.sub.2.sup.− is promoted, rather than bacteria converting NO.sub.2.sup.− to NO.sub.3.sup.−, i.e. NOB.

[0056] Surprisingly, it has been found that the NH.sub.4.sup.+ to NO.sub.2.sup.− 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.sub.4.sup.+ to NO.sub.2.sup.− rather than NO.sub.3 even if the nitrification vessel thereafter is connected to “normal” wastewater, i.e. wastewater containing NH.sub.4.sup.+ 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.sub.4.sup.+ to NO.sub.2.sup.− can be maintained for “normal”, i.e. low, concentrations of NH.sub.4.sup.+ 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.sub.4 concentration reject water outputs mostly NO.sub.2.sup.− rather than NO.sub.3.sup.−, 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 133a-133d in regular or irregular cycles, or though intermittent feeding of reject water RW to the different nitritation vessels 133a-133d.

[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.sub.2.sup.− to NO.sub.3.sup.− 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.sub.4.sup.+ or NO.sub.2.sup.− concentrations in order to regain the ability to convert NH.sub.4.sup.+ to NO.sub.2.sup.− rather than NO.sub.3.sup.−, 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 133a-133d, 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.sub.4.sup.+=>NO.sub.2.sup.− 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.sub.2.sup.− from the nitritation processes to form N.sub.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.sub.2.sup.−) and the ammonium (NH.sub.4.sup.+) will react to form water (H.sub.2O) and dinitrogen gas (N.sub.2), in an anoxic deammonification reaction.

[0062] According to the invention, the bacteria in the nitritation vessels 133a-133d are intermittently treated to promote bacteria forming NO.sub.2.sup.− from NH.sub.4.sup.+ rather than NO.sub.3.sup.− from NH.sub.4.sup.+. This could be done in several ways:

[0063] 1. Increase the amount (i.e. concentration) of NH.sub.4.sup.+ to which the bacteria are subjected;

[0064] 2. Increase the amount of NO.sub.2.sup.− 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.sub.2.sup.− 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.sub.4.sup.+; will be converted to NO.sub.2.sup.−; in one embodiment of the invention, the stream of water to and from the vessel containing the bacteria to be adapted to promote NO.sub.2.sup.− 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.sub.4.sup.+, as been oxidised to NO.sub.2.sup.−. If desired, the water may even stay in the vessel for a significant time after all of the ammonium has been oxidised to NO.sub.2.sup.−. In one specific embodiment, the mainstream wastewater is treated in the nitritation vessel 133a-133d in such a way that only part of the ammonium, NH.sub.4.sup.+ is converted to NO.sub.2.sup.−, 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 133a-133d during the nitration process, or by leading the mainstream wastewater from the nitritation vessel 133a-133d to an anammox vessel 200 before the nitrification process has oxidized all ammonium, NH.sub.4.sup.+ to NO.sub.2.sup.−. The mainstream wastewater is treated in the nitritation vessel 133a-133d under aerobic conditions wherein part of the ammonium in the wastewater is oxidized to nitrite. After the nitritation vessel 133a-133d, 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.sub.4.sup.+ to NO.sub.2.sup.− 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.sub.4.sup.+ to NO.sub.2.sup.− is unbalanced (i.e. not 1:1), additional NO.sub.2.sup.− may be added to the anammox vessel from the nitritation process in the vessels 133a-133d, or additional NH.sub.4.sup.+ 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 133a-133d to be treated such that growth of bacteria converting NO.sub.2.sup.−=>NO.sub.3.sup.− 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.sub.2.sup.−=>NO.sub.3.sup.− 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.sub.2.sup.−=>NO.sub.3.sup.− oxidising bacteria at an optimum temperature.

[0068] Dimensions

[0069] Typical NH.sub.4.sup.+ concentrations in the wastewater having the low concentration of NH.sub.4.sup.+ may be from 10-50 mg/l, measured in nitrogen equivalent, i.e. 10-50 mg(N)/l.

[0070] The NH.sub.4.sup.+ concentration of the reject water may be 300-1200 mg(N)/l

[0071] A typical hydraulic retention time in the nitritation vessels (133a-133f) may be 0.5-3 hours for the water having the low concentration of NH.sub.4.sup.+

[0072] The hydraulic retention time for the water suppressing growth of NOB, i.e. bacteria converting NO.sub.2.sup.− to NO.sub.3.sup.− 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.