System and Process for Removing Ammonium from a Wastewater Stream

Abstract

The present invention relates to a side stream deammonification process where deammonification is performed by a non-continuous flow integrated fixed film activated sludge sequencing batch reactor (IFAS SBR) without the need of employing an external clarifier. More particularly, the present invention entails a single reactor designed to operate as an IFAS SBR or a moving bed bioreactor (MBBR). With the design of the single tank, the two operation modes, MBBR and IFAS SBR, are interchangeable depending on the treatment needs.

Claims

1. A deammonification reactor for removing ammonium from wastewater and configured to operate in one mode as a moving bed biofilm reactor (MBBR) or operate in a second mode as an integrated fixed film activated sludge sequencing batch reactor (IFAS SBR), the deammonification reactor comprising: a single deammonification tank; an aeration grid disposed in a bottom portion of the tank; wherein the deammonification tank is configured to contain biofilm carriers having biomass supported thereon; the single tank including a retaining screen for discharging treated wastewater from the deammonification reactor and for retaining the biofilm carriers in the tank and preventing the biofilm carriers from being discharged with treated wastewater from the tank; the retaining screen having a bifurcated outlet extending therefrom through which treated wastewater flows after passing through the retaining screen; first and second flow control valves incorporated into the bifurcated outlet and configured to control whether the deammonification reactor is operating in an MBBR mode or an IFAS SBR mode; the first and second valves in the bifurcated outlet configured to function as follows: when the first valve is open and the second valve is closed, the deammonification reactor functions as a continuous flow MBBR; when the first valve is closed and the second valve is open, the deammonification reactor functions as a non-continuous IFAS SBR; wherein the retaining screen and the bifurcated outlet extending therefrom is fixed relative to the tank; and wherein when the first valve is closed and the second valve is open, the retaining screen and at least a portion of the bifurcated outlet are configured as a decanter for decanting treated wastewater from the deammonification reactor.

2. The deammonification reactor of claim 1 wherein the single tank deammonification reactor is configured to operate as an IFAS SBR in the absence of a downstream clarifier.

3. The deammonification reactor of claim 1 wherein the biofilm carriers have a specific gravity of 1 or less and are configured to float in the wastewater contained in the tank.

4. A method of treating wastewater in the deammonification reactor of claim 1 including: directing wastewater to a mainstream biological treatment process and subjecting the wastewater to biological treatment and producing sludge and a clarified effluent; treating the sludge in a side stream by: directing the sludge to an anaerobic digester in the side stream; digesting the sludge in the anaerobic digester to produce digested sludge; dewatering the digested sludge in the side stream to produce reject water; directing the reject water to the deammonification reactor of claim 1 which contains biomass supported on carriers and suspended biomass and removing ammonium from the reject water in the deammonification reactor through a batch process that includes the steps of: filling the deammonification reactor with the reject water; aerating the reject water in the deammonification reactor; settling the biomass in the deammonification reactor; and decanting treated reject water by directing the reject water through the retaining screen and at least the portion of the bifurcated outlet.

5. A method for treating wastewater containing ammonium and removing ammonium from produced reject water in a side stream process through deammonification in an integrated fixed film activated sludge sequencing batch reactor (IFAS SBR), the method comprising: directing the wastewater to a mainstream biological treatment process and subjecting the wastewater to biological treatment and producing sludge and a clarified effluent; treating the sludge in a side stream process by: directing the sludge to an anaerobic digester in the side stream; digesting the sludge in the anaerobic digester to produce digested sludge; dewatering the digested sludge in the side stream to produce the reject water; directing the reject water to the IFAS SBR which contains biomass supported on carriers and suspended biomass and removing ammonium from the reject water in the IFAS SBR through a batch process that includes a series of steps including: filling the IFAS SBR with the reject water; aerating the reject water in the IFAS SBR; settling the biomass in the IFAS SBR; and decanting treated reject water by directing the treated reject water through a fixed carrier retaining screen.

6. The method of claim 5 wherein the suspended biomass includes aerobic ammonium oxidizing bacteria (AOB) and wherein the biomass supported on the biofilm carriers includes anaerobic ammonium oxidizing bacteria (AnAOB); and wherein the AOB contacts the reject water in the IFAS SBR and performs nitritation and wherein the AnAOB contacts the reject water in the IFAS SBR and performs an anaerobic ammonium-oxidizing (anammox) process and wherein the combination of the AOB and AnAOB substantially reduces the ammonium concentration of the reject water in the IFAS SBR.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 illustrates the reactor of the present invention which is designed to be operated in either an MBBR mode or an IFAS SBR mode.

[0036] FIG. 2 illustrates the reactor being operated in an MBBR mode.

[0037] FIG. 3 illustrates the reactor being operated in the IFAS SBR mode during an initial phase of an IFAS SBR process.

[0038] FIG. 4 shows the reactor operating as an IFAS SBR process during a settling phase.

[0039] FIG. 5 shows the reactor operating in the IFAS SBR mode and illustrating the decanting phase.

[0040] FIG. 6 is a schematic illustration of a wastewater treatment process including a side stream deammonification process employing the reactor operating in an IFAS SBR mode.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0041] With further reference to the drawings, particularly FIG. 1, a single reactor is shown therein and indicated generally by the numeral 10. As discussed below, reactor 10 is designed as an MBBR or an IFAS SBR. Reactor 10 includes a tank 12 having an aeration grid 14 supported adjacent the bottom of the tank. A blower 16 is operatively connected to the aeration grid for supplying air to the aeration grid which in turn disperses air into the reactor 10. A scum trough 18 extends around at least a portion of the reactor 10 at a selected height. Scum trough 18 is designed to receive scum from an upper surface of the wastewater contained in the reactor 10 and to discharge collected scum from the scum trough.

[0042] Reactor 10 also includes biofilm carriers or media 20. Details of the biofilm carriers 20 are not dealt with herein because their construction and use is well known and appreciated by those skilled in the art. Biofilm carriers are typically made of plastic material with a density close to the density of water (1 g/cm.sup.3). In some cases, the density of the biofilm carriers is less than the density of water which means that they float in the wastewater contained in reactor 10. In many cases, floating carriers are preferable because their use generally increases the capacity of the reactor. In other cases, the density of the carriers 20 might be above the density of water, in which case these biofilm carriers sink when not being physically mixed with the wastewater in the reactor. It is appreciated that these biofilm carriers 20 support an active biomass or biofilm which is used to biologically treat wastewater in the reactor 10.

[0043] To mix the wastewater and the biofilm carriers 20, there is provided aeration grids 14 and a mixer or mixers 22. In some processes capable of being performed in the reactor 10, the mixer 22, along with air dispersed by the aeration grid 14, mixes the biofilm carriers 20 such that they are generally uniformly distributed throughout portions of the reactor 10. Generally, reactor mixing is mainly provided by the aeration. Mixers are used for special occasions such as startup and when anoxic phases are integrated into the SBR sequences.

[0044] To discharge treated water from the reactor 10, there is provided a fixed biofilm carrier retaining screen or screens (carrier screen), indicated generally by the numeral 24, for discharging treated wastewater from the reactor 10. Carrier screen 24 serves two functions. First, it serves to discharge treated water from the reactor 10. When reactor 10 is an MBBR, treated wastewater is continuously discharged via the carrier screen 24. When reactor 10 assumes an IFAS SBR, the carrier screen 24 is configured to be a decanter. Secondly, it acts as a retaining screen and prevents the biofilm carriers 20 from being discharged with the treated water from the reactor 10. One of the advantages of the present invention is that the carrier screen 24 serves to discharge treated water when the reactor 10 operates as an MBBR or as an IFAS SBR. Therefore, if there is occasion to convert the reactor 10 from an MBBR to an IFAS SBR, substantial modifications do not have to be made to the reactor 10.

[0045] Continuing to view the carrier screen 24, it is seen from FIG. 1 that the same comprises a biofilm carrier retaining screen 24A. Screen 24A includes openings that are sufficiently small to prohibit the biofilm carriers 20 from passing through when treated water is discharged from the reactor. Carrier screen 24 includes bifurcated outlets 24B and 24C. Outlet 24B includes a manual control valve 26A and is designed to discharge treated water when reactor 10 assumes an MBBR. An effluent tank 28 is communicatively connected to outlet 24B and in this example, forms a part of reactor 10. The effluent tank 28 effectively serves as an effluent weir box when reactor 10 assumes an MBBR that controls the water level in reactor 10. Outlet 24C includes an automatic control valve 26B and is designed to be used to discharge treated wastewater when reactor 10 assumes an IFAS SBR.

[0046] Reactor 10 is configured to be an MBBR or an IFAS SBR. The term “configured to” as used herein and in the claims means specifically designed to perform a recited function. Reactor 10 is designed to be employed in either an MBBR process or an IFAS SBR process.

[0047] FIG. 2 shows the reactor 10 operating as a continuous flow MBBR process. Here the reactor 10 includes the biofilm carriers 20. Although the specific gravity of the biofilm carriers 20 can vary, in many cases it is preferable that their specific gravity be less than water which enables them to float in the wastewater in the reactor 10. In this example, the position of the carrier screen 24 is disposed generally midway the height of the reactor 10. It is understood and appreciated that the particular position or height of the carrier screen 24 can vary.

[0048] Continuing to refer to FIG. 2, as wastewater flows through reactor 10, air can be supplied continuously or intermittently to the wastewater by the aeration grid 14. The combined effect of air dispersed by the aeration grid 14 and mixing by the mixer 22 causes the biofilm carriers 20 to be mixed with the wastewater and to generally be uniformly distributed through portions of the reactor 10. Although both aeration and mixer can be on at the same time, normally, mixer is turned on only during air off period.

[0049] As the wastewater flows through reactor 10, biofilm on the carriers 20 contacts the wastewater. Various biological treatments can be performed in this MBBR mode by the biofilm carriers 20. Generally the biomass supported on the carriers 20 consumes organic material. Specifically, an MBBR process can be employed for denitrification, nitrification, BOD/COD removal, and deammonification.

[0050] Reactor 10 can assume an IFAS SBR without significant alterations. As noted before, in an IFAS process, both suspended biomass (the suspended biomass is depicted at 21 in FIG. 3) and fixed film biomass (supported on carriers 20) are contained in the reactor 10 and employed to treat wastewater. When the IFAS process is phased or operated as a sequencing batch mode, various phases or steps take place over time in reactor 10 during the course of treating a batch of wastewater. These steps include: filling the reactor; aerating the wastewater in the reactor; reacting the suspended biomass and biofilm biomass with the wastewater; settling the wastewater; and decanting the treated wastewater. Some of these steps can be performed simultaneously. For example, in one phase the filling, aeration and reaction may occur simultaneously. In some cases, the settling phase can be substantially reduced or even eliminated if the decanting volume is relatively small.

[0051] FIGS. 3, 4 and 5 show the reactor 10 an IFAS SBR. In this example, there are three distinct phases in the process: (1) filling, aeration and reaction; (2) settling; and (3) decanting.

[0052] FIG. 3 shows the first phase. Wastewater to be treated is fed into the reactor 10. During this first phase, control valves 26A and 26B are closed, assuring that no wastewater is discharged from the reactor 10. During filling, the aeration grid 14 supplies air to the wastewater. Further, at appropriate times after starting to fill the reactor, the mixer 22 can be actuated to mix the wastewater, suspended biomass and biofilm carriers 20 if aeration is stopped. During this filling, aeration and reaction phase, biological reactions take place and the treatment of the wastewater ensues. As discussed in more detail below, the IFAS SBR can perform, for example, deammonification. During the reaction phase, AOB and AnAOB work together to convert ammonium to nitrogen gas. Once the reactor 10 is filled to a selected level, the feed to the reactor is shut off. The influent feeding in a preferred embodiment will generally last the entire length of the reaction phase. The advantage of a relatively long feeding time is that it reduces the requirement for influent equalization. Before proceeding to the second phase, settling, it may be appropriate in some cases to extend the reaction time beyond the conclusion of filling.

[0053] During the settling phase, as shown in FIG. 4, aeration is off, mixer 22 is not actuated and valves 26A and 26B remain closed, preventing any discharge of wastewater from the reactor 10. In the settling process, the suspended biomass 21 settles to a bottom portion of the reactor 10 and forms what is referred to as a sludge layer 40. See FIG. 4. In this example, the biofilm carriers 20 have a specific gravity less than water and hence float. As shown in FIG. 4, the biofilm carriers 20 will form what is referred to as an upper media layer 42. Between the media layer 42 and the sludge layer 40 is what is referred to as a clear zone 44. In this example as shown in FIG. 4, the carrier screen 24 is placed at the interface formed by the media layer 42 and the clear zone 44. During the settling phase, sludge can be entrained in the media layer 42. A sludge/media separation phase can be incorporated into the SBR sequence. That is, by operating the mixer 22 at a very low speed to stir the biofilm carriers 20, this will facilitate the release of solids contained in the media layer 42.

[0054] After the suspended biomass in the reactor 10 settles, decanting follows. Now valve 26B is open and valve 26A remains closed. Treated wastewater passes through the carrier screen 24 (which now acts as a decanter) into outlet 24C. Note that during decanting the media layer 42 containing the biofilm carriers 20 slowly drifts down and approaches the sludge layer 40. The decanting phase can be terminated at various times relative to the wastewater level in the reactor 10. Of course, when the upper surface of the wastewater moves below the carrier screen 24, it follows that decanting is complete. Once decanting is terminated or is complete, then another batch of wastewater is directed into the reactor 10 and the same process follows.

[0055] In a typical design, the sludge volume is determined by the process's capacity for nitritation. The clear water volume, which is proportional to the sludge volume, provides a safety factor to prevent the loss of sludge during the decanting phase. Thus, the location of the carrier screen 24 should, in one example, be located at the top of the clear water zone 44 (after settling). In other words, the carrier screen 24 is located at the interface of the media layer 42 and the clear water zone 44, as shown in FIG. 4. Carrier volume is determined by the process's capacity for anammox. Typically, carriers or media are added into the tank at 50% of the reactor volume. In FIG. 4, the media layer 42 should take about 50% of the height of the tank without considering the free board. Typically the decanting phase ends when the reactor water level reaches the carrier screen 24. There will generally be some portion of the carriers above the water level. This means that the media layer 42 will settle due to its own weight during the decanting phase. It should be noted that at the end of decanting, the bottom of the media layer 42 can reach the top of the sludge layer 40.

[0056] One of the purposes of settling, decanting and wasting sludge is to maintain an adequate solids retention time (SRT) to retain enough AOB and at the same time to repress nitrite oxidizing bacteria (NOB) in the suspended growth. Therefore, sludge wasting can be accomplished by either sludge wasting pumps, gravity blowdown or releasing biological solids with the treated effluent.

[0057] It is pointed out that the same carrier screen 24 employed to decant in the IFAS SBR mode is the same carrier screen used to continuously discharge treated wastewater from the reactor 10 when it operates as an MBBR. This means that conventional floating decanters are not required in the reactor 10.

[0058] FIG. 6 is a schematic illustration of a wastewater treatment process having a main stream and a side stream. The side stream includes an IFAS SBR similar to that shown in FIGS. 3-5 and discussed above. As explained below, the IFAS SBR in the side stream is designed to remove ammonium from the side stream through a deammonification process.

[0059] With further reference to FIG. 6 and the wastewater treatment system and process shown therein, wastewater to be treated is directed to a primary clarifier 50. Primary clarifier 50 treats the wastewater influent by producing a primary effluent and primary sludge. As discussed below, the primary sludge is directed to the side stream for treatment. Primary effluent from the primary clarifier 50 is directed to a biological reactor 52 for secondary treatment. The biological process employed in the main stream to treat primary effluent generally focuses mainly on biochemical oxygen demand (BOD) removal and nitrification. There are various BOD removal and nitrification systems that are well known and appreciated by those skilled in the art. For example, BOD can be removed by conventional activated sludge processes, MBBR processes, as well as IFAS processes. In any event, the biological reactor 52 produces a secondary effluent. The secondary effluent is directed to a secondary clarifier 54, which in turn produces a clarified treated effluent and secondary sludge. In some cases, the secondary sludge produced by the secondary clarifier 54 can be recycled as return activated sludge (RAS) to the biological treatment reactor 52. As another option, as shown in FIG. 6, the secondary sludge can be directed to a sludge thickener 56 which dewaters and thickens the sludge. As those skilled in the art will appreciate, the primary clarifier 50, biological reactor 52 and secondary clarifier 54 form part of the main stream system and process.

[0060] Turning to the side stream as depicted in FIG. 6, primary sludge from the primary clarifier 50 is directed to a sludge holding tank 58. As an option, the secondary sludge can be mixed with the primary sludge in the sludge holding tank 58. Sludge in the sludge holding tank 58 can be directed to a thermal hydrolysis unit 60. As FIG. 6 indicates, the thermal hydrolysis unit 60 is optional and may not be required or needed in some side stream processes. While thermal hydrolysis is optional, when it is used with anaerobic digestion, this typically increases the volatile solids reduction by as much as 50% compared with anaerobic digestion alone. It is postulated that the ammonium release from the anaerobic digester also increases substantially when the anaerobic digestion process is preceded with a thermal hydrolysis process. Therefore, it is likely that thermal hydrolysis increases the nitrogen load in the side stream.

[0061] In any event, if a thermal hydrolysis unit 60 is employed, then the effluent therefrom is directed to an anaerobic digester 62. In cases where the thermal hydrolysis unit is not employed, sludge from the sludge holding tank 58 is directed into the anaerobic digester 62. As people ordinarily skilled in the art appreciate, the anaerobic digester 62 anaerobically digests the sludge.

[0062] Effluent from the anaerobic digester 62 is directed to a dewatering unit 64. Dewatering unit treats the sludge by producing a sludge cake and reject water which is typically relatively high in ammonium. Reject water produced by an anaerobic digester typically has a high temperature, a relatively high ammonium concentration, and generally the ratio of ammonium to carbon is relatively high. When a thermal hydrolysis unit 60 is employed in the side stream, the thermal hydrolysis unit can produce warm dilution water that is mixed with the reject water.

[0063] Reject water is directed into reactor 10 which is operated in the IFAS SBR mode and performs a deammonification process that removes ammonium from the reject water. It may be beneficial to briefly review the fundamentals of a deammonification process in the context of an IFAS SBR. Deammonification involves two separate bacteria, aerobic ammonium oxidizing bacteria (AOB) and anaerobic ammonium oxidizing bacteria (AnAOB). These are discussed in the background of the invention. An IFAS SBR process seems particularly suited for performing a deammonification process because an IFAS SBR process employs biofilm biomass and suspended biomass. In an IFAS SBR deammonification process, the suspended biomass or bacteria includes the AOB and the AOB performs a nitritation process. This leaves the biofilm biomass (AnAOB) to perform an anaerobic ammonium oxidation process that converts the remaining ammonium and the nitrite to nitrogen gas (N.sub.2) and a small amount of nitrate (NO.sub.3.sup.−).

[0064] The side stream IFAS SBR unit as shown in the side stream of the process of FIG. 6 performs a deammonification process through a series of phases or steps as explained above and as shown in FIGS. 3-5. Once the decanting phase is reached, treated wastewater is directed from the reactor 10 via the carrier screen 24 and outlet 24C. Here the carrier screen 24 acts as a decanter for the IFAS SBR.

[0065] Treated effluent from the IFAS SBR, which is depleted in ammonium, is recycled to the main stream for further treatment. Noteworthy is the fact that there is no clarifier or solids-liquid separation unit downstream of the IFAS SBR. In this particular process, there is little concern for total suspended solids (TSS) in the effluent leaving the IFAS SBR. This is because the effluent from the IFAS SBR is recycled to the main stream for further treatment.

[0066] Thus in the FIG. 6 process, during various phases of the IFAS SBR process, the suspended AOB and the biofilm AnAOB remove ammonium via the deammonification process. In one example, the IFAS SBR process begins deammonification during filling and aeration. Even during settling and decanting, the AnAOB converts ammonium and nitrite to nitrogen gas. The flexibility of the reactor 10 means that an anoxic phase can be incorporated into the overall process to denitrify the relatively small amount of nitrate that might remain from the deammonification process. With the flexibility of an SBR operation, air-off periods and mixer 22 on can be incorporated into the operating sequence for heterotrophic denitrification to remove the nitrate produced by the anammox process.

[0067] There are numerous advantages to the reactor 10 and the side stream IFAS SBR deammonification process discussed herein. First, reactor 10 operates as a single tank deammonification reactor that can employ a continuous flow MBBR process or an IFAS SBR process without requiring substantial modifications to the reactor. Secondly, an IFAS SBR is compact. The suspended sludge retention time required for AOB growth can be very short (about 2.5 days for example) because the side stream temperature is usually relatively high, typically about 30° C. Because of the short suspended sludge retention time, the volume required for holding suspended biomass is relatively small. This is another factor that contributes to the compactness of an IFAS SBR. Thirdly, the reactor 10 includes a fixed carrier retaining screen that discharges treated water from either an MBBR or an IFAS SBR. Thus, in the case of an IFAS SBR, this design eliminates the need for conventional floating decanters. Fourthly, the present invention entails a highly efficient side stream deammonification process for removing ammonium from anaerobic digester reject water through a deammonification process carried out by an IFAS SBR.

[0068] The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects 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.