Patent Application
20140238931
The present invention relates to a system and process for removing ammonium from a wastewater stream, and more particularly to a deammonification process that entails the use of aerobic ammonium oxidizing bacteria (AOB) and anaerobic ammonium oxidizing (anammox) bacteria.
Typically, wastewater influent includes ammonium nitrogen, NH4—N. Conventionally, to remove ammonium nitrogen, a two step process is called for, nitrification and denitrification. In this conventional approach to removing ammonium nitrogen, the process entails a first step which is referred to as a nitrification step and which entails converting the ammonium nitrogen to nitrate and a very small amount of nitrite, both commonly referred to as NOX. Many conventional activated sludge wastewater treatment processes accomplish nitrification in an aerobic treatment zone. In the aerobic treatment zone, the wastewater containing the ammonium nitrogen is subjected to aeration and this gives rise to a microorganism culture that effectively converts the ammonium nitrogen to NOX. Once the ammonium nitrogen has been converted to NOX, then the NOX-containing wastewater is typically transferred to an anoxic zone for the purpose of denitrification. In the denitrification treatment zone, the NOX-containing wastewater is held in a basin where there is no supplied air and this is conventionally referred to as an anoxic treatment zone. Here a different culture of microorganisms operate to use the NOX as an oxidation agent and thereby reduces the NOX to free nitrogen which escapes to the atmosphere. For a more detailed understanding and appreciation of conventional biological nitrification and denitrification one is referred to the disclosures found in U.S. Pat. Nos. 3,964,998; 4,056,465; 5,650,069; 5,137,636; and 4,874,519.
Conventional nitrification and denitrification processes have a number of drawbacks. First, conventional nitrification and denitrification processes require substantial energy in the form of oxygen generation that is required during the nitrification phase. Further, conventional nitrification and denitrification require a substantial supply of external carbon source.
In recent years it has been discovered that the concentration of ammonium in certain waste stream can be reduced by utilizing different bacteria from those normally associated with conventional nitrification-denitrification. In this case, a typical process combines aerobic nitritation and anoxic ammonium oxidation (anammox). In the nitritation step, aerobic ammonium oxidizing bacteria oxidize a substantial portion of the ammonium in the waste stream to nitrite (NO2−). Then in the second step, the anammox bacteria or biomass converts the remaining ammonium and the nitrite to nitrogen gas (N2) and in many cases a small amount of nitrate (NO3−). One particular application of this process is a sidestream process where the waste stream includes a relatively high concentration of ammonium, a relatively low concentration of carbon and a relatively high temperature. The principal drawback to the sidestream processes is that they only address the ammonium concentration in the sidestream and not an associated mainstream. Mainstream in a wastewater treatment plant typically refers to the forward wastewater treatment process where the waste streams include a relatively low ammonia, a relatively high carbon and a relatively low temperature.
There have been attempts at implementing the anammox process in mainstreams to reduce the nitrogen concentration therein. But again, these mainstream applications have focused on wastewater treatment plants that have primary clarifiers and anaerobic digesters. These processes typically include a separate BOD removal stage to remove BOD only from the mainstream and this increases the sludge production to the anaerobic digesters. Following the BOD stage, the mainstream treatment processes typically include nitritation and the anammox process. Since the mainstreams of these processes are not generally conducive to efficient anammox processes, the systems and processes employed have generally relied on a bio-augmentation step where anammox bacteria is transferred from the sidestream to the mainstream. In the end, these mainstream processes that have attempted to employ the anammox process have generally been restricted for a plant with anaerobic digesters and with sidestream anammox applications.
A mainstream wastewater treatment process is disclosed that utilizes anammox bacteria to reduce the nitrogen concentration of the wastewater. The process includes a pretreatment step that aims to condition the wastewater in the mainstream such that it is suitable for subsequent anammox treatment.
In another embodiment of the present invention, the process entails a mainstream process that converts a substantial portion of ammonium in the wastewater to nitrite such that the wastewater includes both ammonium and nitrite. Thereafter, the wastewater including the ammonium and nitrite is subjected to treatment by anammox bacteria which converts the ammonium and nitrite to nitrogen gas.
In yet another embodiment of the present invention, the process entails at least two stages with each stage including a sequence batch biofilm reactor (SBBR) having biomass supported on carriers. In the first stage, the process entails accumulating nitrite by converting a substantial portion of the ammonium in the wastewater to nitrite. In a second stage, conditions are provided or maintained that favor the growth of anammox bacteria on the carriers. The anammox bacteria carry out an anammox process which converts the nitrite and ammonium to nitrogen gas. Thus the process is a main stream process that effectively reduces the ammonium concentration in the wastewater flowing through the mainstream.
In another embodiment of the present invention, it is recognized that by controlling the conversion of ammonium to nitrite that a particular nitrite-to-ammonium stoichiometric ratio range can be achieved and this stoichiometric ratio range is effective to provide a wastewater stream that is conducive to being efficiently treated by anammox bacteria such that the ammonium concentration in the wastewater can be reduced to relatively low levels.
In another embodiment, two SBBRs and a deammonification process is employed to remove ammonium from a wastewater stream having a relatively low carbon-to-nitrogen ratio. Deammonification, however, is not totally relied upon to remove ammonium to relatively low levels. Instead, a pre-denitrification process to remove BOD is integrated into the deammonification process (partial nitritation and anammox process) and is utilized in the first stage SBBR. Additionally in some cases, the anammox process carried out in the second stage SBBR can be supplemented by converting additional ammonium to nitrite under oxic conditions or adding external carbon under anoxic conditions to reduce the concentration of nitrate. Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention.
The system and process described herein entails a two stage sequence batch biofilm (SBBR) reactor for removing BOD and ammonium. The first SBBR reactor receives wastewater, typically municipal wastewater, and functions to pretreat the wastewater such that when it is transferred to the second SBBR reactor it is conditioned such that anaerobic ammonium oxidation (anammox) bacteria is effective to remove ammonium and nitrite from the wastewater. There are two basic processes involved. In the first SBBR reactor, the aim is to conduct partial nitritation. This process entails utilizing ammonium oxidizing bacteria (AOB) in the first SBBR to perform nitritation which converts a substantial portion of the ammonium to nitrite. Once nitritation has been performed in the first SBBR, the wastewaters transfer to the second SBBR where a different species of bacteria is utilized to oxidize the ammonium and nitrite. The second species of bacteria utilized in the second SBBR reactor is anammox bacteria. Anammox bacteria present in the second SBBR is effective to convert the ammonium and nitrite to elemental nitrogen. The process of nitritation coupled with reducing the concentration of ammonium and nitrite with anammox bacteria is referred to as deammonification.
Performing a mainstream deammonification process is challenging because for the anammox process to be effective, it is desirable for the nitrite-ammonium stoichiometric ratio be in the range of about 1.1 to 1.5. Ideally, the ratio should be about 1.3. For many wastewaters, simply employing conventional nitritation will not result in an ideal or desirable nitrite-ammonium ratio. This means that some additional processes may be required in the second SBBR reactor so that the wastewater reaching the second SBBR will have a nitrite-ammonium ratio within the target range or close to the target range. Similarly, in many situations, the anammox process may not reduce ammonium and nitrite levels down to acceptable levels. Thus, even in the second SBBR reactor, some fine tuning, beyond the anammox process, may be appropriate to reduce the concentrations of ammonium and nitrite to acceptable levels. For example, if the nitrite concentration is too low, oxic conditions can be employed in the second stage SBBR to convert additional ammonium to nitrite. Also, if the nitrite concentration in the second SBBR is too high, then external carbon can be added under anoxic conditions to reduce the concentration of nitrite (which will also reduce the concentration of nitrate) in the wastewater.
In order to improve the efficiency of a mainstream deammonification process that relies on ammonium oxidizing bacteria and anammox bacteria, in some embodiments of the present invention, a heterotropic denitrification process is integrated with nitritation and the anammox process. A two stage SBBR system can be configured in a number of ways and utilized to carry out the processes discussed herein.
The two stage SBBR system disclosed herein can also be configured as moving bed biofilm reactors (MBBR). Such a system is schematically shown in
Furthermore, IFAS and MBBR systems can be integrated. In
The system and process of the present invention is particularly suited for wastewater having some BOD but including a relatively low carbon-to-nitrogen ratio. The term “relatively low carbon-to-nitrogen ratio” means a BOD to TKN (total concentration of organic nitrogen and ammonia) ratio of 4 or less. In some embodiments, the first SBBR utilizes BOD in the wastewater to carry out a pre-denitrification process utilizing heterotropic bacteria. Pre-denitrification will significantly reduce the concentration of nitrite in the wastewater and will, to a lesser degree, reduce the concentration of nitrate in the wastewater. This pre-denitrification will be carried out under anoxic conditions in the first SBBR for a selected time period. Thereafter, the first SBBR will employ oxic conditions which result in a partial deammonification process, sometimes referred to as nitritation, using AOB to convert some of the existing ammonium in the wastewater to nitrite. There may be one or multiple anoxic-oxic phases carried out in the first SBBR. In some embodiments, the process envisions providing multiple intermittent aeration cycles in the first SBBR. This will repress the growth of aerobic nitrite oxidizers. The objective of the first SBBR is to produce an effluent that includes ammonium and nitrite that can be efficiently removed through the anammox process that forms the second part of the deammonification process. To achieve this, the process aims for the effluent from the first SBBR to include a nitrite-to-ammonium ratio at a target stoichiometric ratio range of 1.1 to 1.5. As stated above, ideally the nitrite-to-ammonium ratio should be approximately 1.3.
Table 1 appearing above shows typical phases and performance for the first SBBR when utilized in an IFAS version. Note that in this example the feed is assumed to be 4 MGD, reactor volume is 0.5 million gallons, flow treated per day, QINF=1 MGD, SRT is two days, the wastewater temperature is 20° C. and the influent hydraulic retention time is 12 hours. In this example, note that in the first anoxic phase that pre-denitrification removed 5 mg/L of NOX—N with 20 mg/L of sBOD. This did not impact the ammonium concentration of 25.6 mg/L. It effectively reduced the nitrite (NO2—N) by 4.2 mg/L and the nitrate (NO3—N) by 0.8 mg/L. Note that during the anoxic first phase that the dissolved oxygen (DO) concentration was 0 mg/L. This first anoxic phase is followed by an oxic phase of 60 minutes. Even though this was an oxic phase, the dissolved oxygen concentration was maintained relatively low, at less than 1.0 mg/L and preferably less than 0.5 mg/L. During this oxic phase, partial nitritation occurred. AOB is used to convert portions of the ammonium (NH4—N) to nitrite (NO2—N). Note that during this oxic phase, the ammonium concentration was reduced from 25.6 mg/L to 17 mg/L and that the converted ammonium resulted in a slight increase of nitrate and a significant increase of nitrite to 17 mg/L.
Similar results are shown for the second anoxic and oxic phases for the first SBBR. Note that the final effluent produced by the first SBBR includes 17 mg/L of ammonium, 17 mg/L of nitrite and 1 mg/L of nitrate. Thus, the ratio of the nitrite to ammonium in the effluent is 1.1, which is within an acceptable stoichiometric ratio range for the efficient removal of ammonium and nitrite in the second SBBR via the anammox process. Also note that the soluble BOD in the wastewater has been removed and that after decanting, the first SBBR is ⅔ full.
It is desirable to utilize multiple mechanisms in the first SBBR to select the appropriate bacteria for the process. Dissolved oxygen concentration should be controlled. During the anoxic phases, the objective is to maintain dissolved oxygen concentration at 0 mg/L or near 0 mg/L. During the oxic phase, dissolved oxygen concentration should be maintained relatively low. In the first SBBR, the dissolved oxygen should be high enough to support nitritation but low enough to suppress NOB growth. In the examples illustrated herein and in a preferred embodiment, the dissolved oxygen concentration during oxic phases is maintained at 0.5 mg/L or less. When an IFAS system is used, it is desirable to maintain a relatively short SRT. In systems utilizing suspended growth, such as with an IFAS system, the aerobic SRT should be long enough to support AOB growth and short enough to wash out nitrite oxidizing bacteria. In one example, the SRT of the first SBBR is maintained at two days or less. Suspended biomass will help switch between oxic to anoxic conditions quickly by consuming oxygen with available influent BOD. The quick and frequent transition between oxic to anoxic phases in batch operation will help select AOB as well.
The effluent from the first SBBR is directed to the second SBBR and the objective is to reduce to relatively low levels the ammonium, nitrite and nitrate concentrations in the effluent. With reference to
It is sometimes difficult to achieve low levels of nitrogen species by relying solely on the anammox process. This is because it is sometimes difficult to produce an effluent from the first SBBR that includes a nitrite-to-ammonium stoichiometric ratio that is within the targeted range. Furthermore, the anammox process itself produces some nitrate. That is, approximately 11% of the ammonium present in an anammox process ends up in the form of nitrate. Thus, it is desirable for the second SBBR to perform additional processes in conjunction with the anammox process in order to achieve relatively low levels of ammonium, nitrite and nitrate. Thus, the process of the present invention entails a second SBBR that is capable of performing additional processes. First, the second SBBR is provided with aeration and mixing capabilities. There are cases when the nitrite-to-ammonium ratio in the feed to the second SBBR is less than the targeted ratio range. In this case, operating at a low DO or in an oxic phase will convert, through nitritation, some ammonium to nitrite. Oxic conditions can be employed until the stoichiometric ratio of nitrite-to-ammonium is within the target range which, in a preferred embodiment, is a range of 1.1 to 1.5. If the ratio of nitrite-to-ammonium is greater than the targeted range, then in a final anoxic phase external carbon is added to remove the excess nitrite and nitrate produced by anammox process to meet the final effluent requirements. Adding extra carbon in an anoxic environment results in heterotropic denitrifying bacteria reducing the concentration of both nitrite and nitrate.
Table 2 shows exemplary data that one would expect from the operation of the second SBBR in the IFAS mode. When feeding wastewater into the second SBBR, the freshly fed wastewater will be diluted by existing mixed liquor wastewater or biomass already in the second SBBR. Therefore, in the case of the example of Table 2, after the wastewater has been fed into the second SBBR, the ammonium concentration is 6.3 mg/L, the nitrite concentration is 6.3 mg/L and the nitrate concentration is 1.0 mg/L. Then, in the first or initial oxic phase with respect to this new feeding, the nitritation process converts part of the ammonium to nitrite and these result in an ammonium concentration of 5.4 mg/L and a nitrite concentration of 7.2 mg/L. Then the anammox process follows under anoxic conditions and it is seen where the ammonium concentration is reduced from 5.4 mg/L to 1.0 mg/L and the nitrite concentration is reduced from 7.2 mg/L to 1.0 mg/L. Because the anammox process produces nitrate, it is seen that the nitrate concentration increases from 1.0 to 2.4 mg/L. Because of the presence of nitrate in this example, at the end of the anammox process, external carbon is added. In this case, 5.6 mg/L of sBOD5 was added. Once the external carbon is added, heterotrophic denitrifiers act under anoxic conditions to reduce the nitrate concentration from 2.4 mg/L to 1.0 mg/L. Thus, it is seen that in the second SBBR, the first oxic phase is utilized to bring the nitrite-to-ammonium stoichiometric ratio within the targeted range and the external carbon is added to be used in a post-denitrification process, again using heterotrophic denitrifiers, to, in this case, reduce the concentration of nitrate.
Table 3 is exemplary data and shows typical phases and performances of the first SBBR in the MBBR version of
The “air off” phase is used primarily to grow anammox bacteria and to carry out the anammox process where biomass on biofilm carriers contained in the second SBBR are effective to convert both ammonium and nitrite to elemental nitrogen. However, there may be cases where the nitrite-to-ammonium stoichiometric ratio is outside of the targeted range. As discussed above, various means can be employed to bring the nitrate-to-ammonium stoichiometric target ratio within the selected range of 1.1 to 1.5. If an initial “air on” phase or oxic phase is employed, then some of the ammonium will be converted to nitrite in the second SBBR. If the concentration of the nitrite is too high, with respect to the targeted nitrite-ammonium target ratio range, then by adding external carbon this will reduce the concentration of both the nitrite and the nitrate through a post-denitrification process. Thus, the oxic conditions employed, as well as the post-denitrification process, are mechanisms for fine-tuning the nitrite-to-ammonium ratio so as to enhance the efficiency of the anammox process.
Table 4 is exemplary data showing the typical phases and performance of the second SBBR in
The processes shown in
The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
