WO2011134011A1

WO2011134011A1 - Production of nitrite

Info

Publication number: WO2011134011A1
Application number: PCT/AU2011/000482
Authority: WO
Inventors: Zhiguo Yuan
Original assignee: The University Of Queensland
Priority date: 2010-04-28
Filing date: 2011-04-27
Publication date: 2011-11-03

Abstract

A method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria, and controlling the reactor to convert ammonia to nitrite at a yield in excess of 60%. The pH in the reactor is maintained to (a) ensure that there is adequate alkalinity to take part in the oxidation reaction of ammonium to nitrite and (b) obtain a level of free nitric acid in the reactor that allows ammonium oxidising bacteria to grow but inhibits the growth of nitrite oxidising bacteria. The pH may fall within the range of from 4 to 7.5, preferably 5 to 7, more preferable 6.0 to 6.5.

Description

PRODUCTION OF NITRITE

FIELD OF THE INVENTION

The present invention relates to a method for producing nitrite. In another aspect, the present invention relates to a method for producing free nitrous acid.

BACKGROUND TO THE INVENTION

Microbial processes play a central role in wastewater management. In particular, they underpin biological treatment of wastewater, the most cost-effective and environmentally friendly method for wastewater treatment.

A typical advanced wastewater treatment plant receives wastewater from sewage mains. The wastewater is first treated to remove large particulates (by screening, or passing through a primary settler, or both). The liquor then passes to bioreactors, where bacteria mineralise organic carbon (often referred to as biological oxygen demand or BOD) to C0₂ and convert ammonia to nitrate, and in some cases further to nitrogen gas. Some bioreactors also achieve biological phosphorus removal. This process results in the growth of biomass. The biomass is then separated from the liquor, typically in a secondary settler.

The sludge from the secondary settler (which includes most of the separated biomass) is then treated in an anaerobic digester, sometimes together with primary sludge resulting from the settling process in the primary settler. In the anaerobic digester, the BOD of the sludge is converted to methane. Products from the anaerobic digester also include solids that may be disposed of and a liquid stream.

The liquid stream from the anaerobic digester typically contains ammonium at concentrations of 1 .0 to 1.5 gN/L. Aerobic digestion liquors typically have limited alkalinity. For example, bicarbonate is typically present with a molar ratio with ammonium of approximately 1 : 1. This water stream is presently treated either by returning it to the mainstream aerobic bioreactor or by a sidestream treatment process. Processes that have been used to treat the liquid stream from an anaerobic digester using sidestream treatment include:

1. Nitrogen is removed through nitrification and denitrification with nitrite as the intermediate. This technology involves oxidising approximately 50% of the ammonium to nitrite with an aerobic process, which is followed by denitrification where nitrite is reduced to nitrogen gas with externally supplied carbon sources. The denitrification process re-generates alkalinity to support further ammonium oxidation to nitrite. The aerobic and anoxic cycle is repeated for a few times, after which most of the ammonium is converted to nitrogen gas;

2. Nitrogen is removed via nitrification followed by the ANAMMOX process.

Similar to the above process, approximately 50% of the ammonium is firstly converted to nitrite. The resulting liquor that contains ammonium and nitrite with a molar ratio of approximately 1 : 1 will be transferred to an ANAMMOX reactor, where the ANAMMOX bacteria will oxidise ammonia with nitrite as the electron acceptor. The ANAMMOX reaction produces nitrogen gas and also some nitrate as the final products.

Both of the above processes require additional capital costs and increased operational costs.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

In a first aspect, the present invention provides a method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria, and controlling the reactor to convert ammonia to nitrite at a yield in excess of 60%.

In some embodiments of the present invention, the reactor is controlled such that the yield of conversion of ammonium to nitrite is in excess of 70%, more suitably in excess of 80%,^'more suitably in excess of 90%, even more suitably within the range of 90% to 95%. In one embodiment, the method further comprises removing a nitrite containing liquid from the reactor.

In some embodiments, the reactor is controlled by maintaining the pH above 4, or above 5.5. In other embodiments, the pH is maintained at a level above 6.0. In other embodiments, the pH is maintained at a level of 6.2 or, above, or even a level of 6.5 or above.

In some embodiments, the pH of the reactor is controlled by monitoring the pH of the liquid in the reactor and adding an alkaline material to the reactor if the pH drops below a predetermined level. In some embodiments, the predetermined level falls within the range of 6.0 to 6.5.

In some embodiments, the pH of the reactor is controlled by adding a bicarbonate containing material to the reactor. Bicarbonate materials are alkaline in nature and adding the bicarbonate materials to the reactor results in an increase in the pH of the liquid in the reactor. Adding bicarbonate also secures supply of inorganic carbon that is required for autotrophic growth by ammonia oxidising bacteria, which could reduce to low levels due ^'to C0₂ stripping.

Suitably, the reactor is operated such that minimum formation or accumulation of nitrate takes place. Suitably, the reactor is operated such that negligible conversion of nitrogen containing compounds to N₂ occurs.

Aqueous streams that contain nitrite (N0₂ ^") will also contain free nitrous acid (HN0₂) with the ratio between N0₂ ^" and HN0₂ determined by pH and temperature, among other factors. A lower pH favours the formation of HN0₂. In some embodiments of the present invention, the liquid stream leaving the reactor (which is an aqueous stream containing nitrite) will also contain free nitrous acid.

The method of the present invention may further comprise the steps of removing a liquid stream from the reactor and reducing the pH of the liquid stream to form more free nitrous acid. This aqueous stream containing free nitrous acid may be used in other parts of the water treatment process. In a second aspect, the present invention provides a method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria, and controlling pH of a liquid phase in the reactor to fall in the range of from 4.0 to 7.5 to thereby convert ammonium to nitrite.

In a third aspect, the present invention provides a method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria, and controlling pH of a liquid phase in the reactor so that the pH is maintained above a predetermined value, said predetermined value falling within the range of from 4,0 to 7.5, to thereby convert ammonium to nitrite.

In the second and third aspects of the present invention the pH may fall within the range of from 5 to 7, or from 6.0 to 6.5.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows (A) the profiles of influent NH₄ ⁺ and effluent NFLf, N0₂ ^" and N0₃ ^' concentrations during 200 day operation and (B) the mixed liquor suspended solids (MLSS) and volatile suspended solids (VSS) concentrations during the operation of the Example. Synthetic wastewater was used until Day 1 70, after which real anaerobic sludge digestion liquor was used; and

Figure 2 shows profiles of dissolved oxygen, pH (A) and ammonium, nitrite and nitrate concentrations (B) during the cycle study implemented on Day 133 of the Example. DETAILED DESCRIPTIO OF THE INVENTIO

Example

In this embodiment, it was an objective to convert greater than 90% of the ammonium in the water stream taken from an anaerobic digester (hereinafter referred to as "digestion liquor") to nitrite with minimum accumulation of nitrate or negligible conversion to N₂. The digestor liquor contains ammonium and alkalinity. The ammonium and alkalinity are present in the digestion liquor at a molar ratio of about 1 : 1 . In order to obtain a high yield of conversion of ammonium to nitrite, conditions in the nitritation reactor are controlled by:

1 . Adding additional alkalinity to satisfy the demand of alkalinity by ammonium oxidation; and

2. providing appropriate conditions that allow ammonia-oxidising bacteria to proliferate while ^'the growth of nitrite-oxidising bacteria is minimised.

Adding additional alkalinity to the reactor was achieved by adding bicarbonate. For example, sodium bicarbonate could be added to -the reactor. In one embodiment, bicarbonate was added in two steps:

1 . In the first step, an additional amount of bicarbonate that is approximately 50% of the bicarbonate already present in the anaerobic digestion liquor is directly added to the liquor. This additional amount, along with the bicarbonate already present, allows for the oxidation of approximately 75% of the ammonium to nitrite. The amount of this addition of bicarbonate can vary between 0- 100% (of the additional amount required to achieve complete conversion of ammonium to nitrite) with 50% being one example. As the alkalinity to ammonium ratio in the anaerobic digestion liquor may vary, adding too much alkalinity in this step may result in overdosing, incurring unnecessary chemical costs. On the other hand, adding too little could slow down the biological reaction.

2. In the second step, bicarbonate is added through a pH controller, for example, during an SBR (sequencing batch reactor) cycle. When ammonium oxidisation proceeds, alkalinity is consumed and pH decreases. When the pH drops to a preset or predetermined value, the pH controller is initiated, and this adds bicarbonate to the reactor to keep pH at or above this preset or predetermined pH level. This ensures that alkalinity is replenished while being consumed until (almost) full oxidation of ammonium is achieved. In this period, the ammonium oxidation rate is lower, likely limited by alkalinity. Hence, adding an amount of bicarbonate in Step 1 is beneficial in terms of increasing the reaction rate and hence reducing the cycle time.

A combination of the above two steps ensures that an adequate amount of alkalinity is made available for the full oxidation of ammonium without overdosing. However, in some embodiments, alkalinity addition may occur by either step 1 alone or step 2 alone.

The pH set-point -in Step 2 at which additional alkalinity is added is suitably in the range of 4.0 to 7.5. such as from 6.0 to 6.5. It will be understood that the production of nitrite under acidic conditions also results in the formation of free nitrous acid. Free nitrous acid (FN A) has been found to be an inhibitor of microorganisms and it has a stronger inhibitory effect on nitrite oxidising bacteria than on ammonia oxidising bacteria. It is important that FNA is maintained in a range that allows the growth of ammonia-oxidising bacteria (AOB) but inhibits the growth of nitrite-oxidising bacteria (NOB). It is believed that an optimal pH should be in the range of 6.0 to 6.5. A pH of 6.2 was selected for this example.

Experimental results

Reactor operation

An 8L reactor was seeded using sludge taken from the Luggage Point wastewater treatment plant (Brisbane, Australia), which contained ammonia-oxidising bacteria, nitrite-oxidising bacteria as well as many other microorganisms, judged from the function of the plant.

Synthetic wastewater that mimics anaerobic digester liquor was initially used as a feed to the reactor. In the start-up phase (first 41 days), the ammonium concentration in the feed was 500 mg NH ⁺-N/L. The bicarbonate to ammonium ratio was 1.5: 1 (mole/mole). Based on the satisfactory reactor performance, the ammonium concentration in the feed was increased to 750 mg NH₄ ⁺-N/L between Day 42 and 48, and l g NH₄ ⁺-N/L from Day 49 onwards, which is similar to the ammonium concentration in anaerobic digestion liquor. The bicarbonate concentration was also increased proportionally.

From Day 1 71 , real anaerobic digestion liquor taken from the Luggage Point wastewater treatment plant was used as the feed.

An SBR cycle of 8 hours, consisting of 7.5 hours aerobic period followed by 0.5 hour wasting, settling and decanting periods, \vas used. During the aeration period, the dissolved oxygen concentration in the reactor was maintained between 2.5 and 3.5 mg/L through the use of an ON/OFF controller of air sparging. Wastewater was pumped into the reactor as two pulses at 0 and 3.7 hours, respectively. In each feeding period, 1 L of feed was added in 2.8 minutes. To maintain a desirable volatile suspended solid (VSS) concentration, certain amount of mixed liquor was removed at the end of the aeration period. From Day 126 onwards, 500 mL mixed liquor was wasted in each cycle, resulting in a sludge retention time of 16 days. In the decanting period, 2 L of effluent was discharged.

A one-way pl controller was implemented, which adds bicarbonate when the measured pH drops below a pre-selected pH set-point. This value was set at 6.2 to keep the concentration of FNA at a desirable level.

Long-term performance

Figure 1 A shows the influent ammonium and effluent ammonium, nitrite and nitrate concentrations over the 200 day experimental period. In the first 10 days, the ammonium oxidation rate was close to zero. During Days 10-30, the ammonium oxidation rate gradually increased and nitrite was the main final product of the conversion. With the satisfactory reactor performance, the ammonium concentration in the feed was increased to 750 mg NFI₄ ⁺-N/L between Day 42 and 48, and l g NH₄ ⁺- N/L from Day 49 onwards. At steady operation conditions, more than 90% of the ammonium feed was converted to nitrite, while less than 5% of the loaded ammonium was oxidized to nitrate. On Day 171 , the feed was switched to real digestion liquor with 822 mg NH -N/L in the influent. After 5 day operation, the rector achieved steady performance with more than 92% ammonium conversion to nitrite. The effluent nitrite concentration was steadily above 760 mg NCV-N/L.

The volatile suspended solid concentration (VSS) increased gradually, reaching 1.5 g/L on Day 133 and remained- around 1.3g/L during the real wastewater test period (see Figure I B).

Cycle study

Figure 2 shows the profiles of ammonium, nitrite and nitrate concentrations measured during a cycle study. The liquid volume was 6 L at the start of cycle. The 1 L feed, containing lg NH₄ ⁺-N, was pumped into the reactor as the first feed. The same feed was added again at 3.6 hours. Over the cycle, nitrite varied between 800- 1000 mg N/L, while nitrate varied between 20-30 mg N/L and dissolved oxygen level between 2.5 and 3.5 mg/L. With the 50% extra bicarbonate supplied, most of the ammonium feed was consumed within I hour. More bicarbonate was dosed to remove residual ammonium, and control the pH at about 6.2 at the same time.

The present invention allows for the conversion of ammonium in an anaerobic digestion liquor to nitrite at high yields. The nitritation reactor is controlled by controlling pH to (a) ensure that there is adequate alkalinity to take part in the oxidation reaction of ammonium to nitrite and (b) obtain a level of free nitric acid in the reactor that allows ammonium oxidising bacteria to grow but inhibits the growth of nitrite oxidising bacteria. The present inventors believe that if the level of free nitric acid is too low (resulting from a pH that is too high), adequate inhibition of nitrite oxidising bacteria will not occur, thereby resulting in excessive oxidation of nitrite to nitrate. If the level of free nitric acid is too high (resulting from a pH that is too low), growth of ammonium oxidising bacteria will also be inhibited, thereby reducing the rate of production of nitrite.

An aqueous liquid stream containing nitrite and free nitrous acid is withdrawn from the reactor. This stream may be used in other parts of the wastewater treatment process. If it is required to increase the concentration of free nitrous acid in this stream, it is a simple matter to add additional acid to this stream to ^' enhance the amount of free nitrous acid in the stream. As the product stream from the reactor contains little or no alkalinity (and thus has little buffering capacity), addition of only a small amount of acid can be used to cause significant reductions in the pH of the stream, thereby resulting in a significant increase in the free nitrous acid concentration in this stream. The additional acid that is added may be an inorganic acid or an organic acid. HC1 may typically be used.

Recent laboratory studies by the present inventors have demonstrated that almost all the ammonium contained in a digestion liquor can be converted to nitrite at a pH of approximately 6.0. This readily gives rise to an FNA concentration of 2.0-4.0 ppm in the liquor leaving the nitritation reactor, which is already strongly biocidal. The nitrified liquor has a very low level of alkalinity and hence its pH can be easily reduced to 2-4 (or even lower if required) through moderate acid dosage (experimentally determined as 0.008 and 0.08 mol/L to reach pH 4 and 2, respectively), giving rise to FNA concentrations of hundreds to over a thousand ppm depending on the need. The maximum daily production of nitrite can be up to 10-20% of the total nitrogen loading to the wastewater treatment plant, representing an almost unlimited supply of FNA. The production is biological, with little additional costs in comparison to the current methods used for the treatment of this stream. FNA thus produced is "green", renewable and plentiful.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be appreciated that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Throughout this specification, the term "comprising" and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise.

Claims

1. A method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria, and controlling the reactor to convert ammonia to nitrite at a yield in excess of 60%.

2. A method as claimed in claim 1 wherein the reactor is controlled such that the yield of conversion of ammonium to nitrite is in excess of 70%, more suitably in excess of 80%, more suitably in excess of 90%, even more suitably within the range of 90% to 95%.

3. A method as claimed in claim 1 or claim 2 further comprising removing a nitrite containing liquid from the reactor.

4. A method as claimed in any one of the preceding claims wherein the reactor is controlled by maintaining the pH above 4, or the pH is maintained at a level above 5.5, or the pH is maintained at a level above 6.0, or the pH is maintained at a level of 6.2 or above, or even a level of 6.5 or above.

5. A method as claimed in claim 4 wherein the pH is maintained to fall within the range of 6.0 to 6.5.

6. A method as claimed in any one of the preceding claims wherein the pH of the reactor is controlled by monitoring the pH of the liquid in the reactor and adding an alkaline material to the reactor if the pH drops below a predetermined level.

7. A method as claimed in claim 6 wherein the predetermined level falls within the range of 6.0 to 6.5.

8. A method has claimed in any one of claims 5 to 7 wherein the pH of the reactor is controlled by adding a bicarbonate containing material to the reactor.

9. A method as claimed in any one of the preceding claims wherein the pH in the reactor is maintained to (a) ensure that there is adequate alkalinity to take part in the oxidation reaction of ammonium to nitrite and (b) obtain a level of free nitric acid in the reactor that allows ammonium oxidising bacteria to grow but inhibits the growth of nitrite oxidising bacteria.

10. A method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria and controlling pH of a liquid phase in the reactor to fall in a range of 4.0 to 7.5 to thereby convert ammonium to nitrite.

1 1 . A method for producing nitrite comprising the steps of providing a liquid stream containing ammonia to a nitritation reactor, the reactor containing ammonium oxidising bacteria and controlling pH of a liquid phase in the reactor so that the pH is maintained above a predetermined value, said predetermined value falling within the range of 4.0 to 7.5, to thereby convert ammonium to nitrite.

12. A method as claimed in claim 10 or claim 1 1 wherein the pH falls within the range of from 5 to 7.

13. A method as claimed in claim 12 wherein the pH falls within the range of from 6.0 to 6.5.

14. A method as claimed in any one of the preceding claims wherein the liquid stream containing ammonia is an anaerobic digestion liquor.