The present invention relates to a system and process for treating wastewater, and more particularly to an integrated fixed film activated sludge sequencing batch reactor system and processes.
Sequencing batch reactors (SBRs) have been employed in wastewater treatment since the 1920s, and now are used throughout the world. SBRs are widely used in the United States, China and Europe to treat both municipal and industrial wastewaters. They are especially practical in applications having low or varying flow patterns. There are other characteristics of SBRs that make them a viable option in certain cases. For example, where there is a limited amount of space, an SBR offers the opportunity to treat wastewater in a single tank, instead of multiple tanks. This enables wastewater treatment systems to be constructed on a relatively small footprint. In addition, SBRs can be controlled to provide aerobic, anaerobic and anoxic conditions in order to achieve biological nutrient removal including nitrification/de-nitrification, nitrification only and, in some cases, phosphorus removal. Biochemical oxygen demand (BOD) can be removed to relatively low levels. SBRs are efficient in removing total nitrogen down to as low as 5 mg/L. This is achieved by employing aerobic conditions to convert ammonia to nitrate and nitrite (nitrification) and anoxic treatment of the nitrate and nitrite to yield nitrogen gas (de-nitrification). All of this can be achieved in the same tank. In some cases, SBRs can be employed to reduce phosphorus concentrations down to less than 2 mg/L by employing anaerobic treatment.
SBRs are a variation of the conventional activated sludge process. They differ from activated sludge plants in that SBRs combine the treatment steps and processes into typically a single basin or tank, whereas conventional activated sludge processes rely on multiple tanks. An SBR typically includes four different steps or phases: (1) fill, (2) react, (3) settle and (4) decant. During the fill phase, the tank receives influent wastewater. Influent delivers food to the microorganisms in the activated sludge and creates an environment for biochemical reactions to take place. During the fill operation, the wastewater can be mixed and/or aerated.
During the react or reaction phase, the biomass or bacteria in the wastewater consumes nutrients. In one example, the SBR in the react phase is operated under aerobic conditions. Here the biomass performs a nitrification process by converting ammonia to nitrite and nitrate. In this process, the wastewater is aerated and mixed. The addition of oxygen to the wastewater encourages the multiplication of aerobic bacteria.
The settling stage or phase follows the react phase. During this stage, the sludge formed by the bacteria is allowed to settle to the bottom of the tank. Generally, the aerobic bacteria continue to multiply until the dissolved oxygen is consumed. As the bacteria multiply and die, the sludge within the tank increases over time and a waste-activated sludge pump removes some of the sludge during the settling stage for further treatment. During the settling stage, activated sludge is allowed to settle under quiescent conditions. There is no influent entering the tank and no aeration or mixing takes place. The activated sludge tends to settle as a mass of flocs, forming an interface with the clear supernatant. Sometimes the sludge mass is referred to as a sludge blanket. The settling phase is an important part of the SBR cycle because if the solids do not settle rapidly, some sludge can be drawn off during the subsequent decant phase and this will degrade the quality of the effluent.
During the decanting phase, a decanter is used to remove the clear supernatant effluent. In some cases, a floating decanter is used. Floating decanters have an inlet orifice slightly below the water level to minimize the removal of solids in the effluent during the decant phase.
Many SBR processes rely on a single tank and activated sludge where the biomass is suspended in mixed liquor. These SBR designs have limited load capacity. In addition to the limited capacity of conventional SBR systems, there is a problem with many existing SBR systems in use today. Many existing SBR systems are operating at their designed capacity or near design capacity. There are few viable options for upgrading or expanding capacity without constructing additional reaction tanks or settling basins.
Therefore, there is a need to address the limited capacity of conventional SBR processes and, at the same time, provide a viable option to increase capacity of existing SBR units without requiring the construction of additional tanks.
The present invention provides an integrated fixed film activated sludge (IFAS) sequencing batch reactor (SBR) process where both suspended biomass and biomass supported on biofilm carriers are utilized to biologically treat wastewater received by the SBR. In one embodiment, the SBR includes two hydraulically connected tanks with suspended biomass being contained in one tank and biomass supported on biofilm carriers in the other tank. The process is carried out such that the suspended biomass and biomass supported biofilm carriers are efficiently utilized to increase the capacity of the SBR.
Various processes, such as nitrification—de-nitrification, nitrification only, phosphorus removal, and BOD removal, can be carried out in the IFAS SBR. In any one of these processes, filling, settling and decanting of the two tanks can be carried out simultaneously. Furthermore, both tanks can be subjected to the reaction phase at the same time.
In another embodiment, ballast is added to one or more of the tanks to facilitate the settling of sludge during the settling phase. Flocs comprising biomass and other solids agglomerate around or attach to the ballast forming ballasted flocs. These relatively heavy ballasted flocs substantially increase the settling rate of the flocs.
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.
With further reference to the drawings, the IFAS sequencing batch reactor of the present invention is shown therein and indicated generally by the numeral 10. SBR 10 includes two tanks or basins, a first tank 12 and a second tank 14. Tanks 12 and 14 are separated by a wall 18. An opening 20 is provided in the wall 18 that enables tanks 12 and 14 to be hydraulically connected. In the embodiment illustrated in
Second tank 14 includes biofilm carriers or media 16. The media 16 could be the moving type or the fixed type. As those skilled in the art appreciate, biofilm carriers 16 support biomass that are effective in biologically treating the wastewater in tank 14. Details of the biofilm carriers 16 are not dealt with herein in detail because such is not per se material to the present invention. For a more detailed and unified understanding of biofilm carriers and their role in biologically treating wastewater, reference is made to U.S. Pat. No. 7,189,323, the disclosure of which is expressly incorporated herein by reference. In one embodiment, the first tank 12 is not provided with biofilm carriers 16 or any significant amount of biofilm carriers. In this embodiment, the SBR process described herein relies on suspended biomass to biologically treat the wastewater in the first tank 12. It should be pointed out that while there are biofilm carriers 16 in the second tank 14, the second tank would also typically include suspended biomass. In cases where it is desirable to only provide biofilm carriers 16 in a single tank, it is appreciated that some means is employed to retain the biofilm carriers in the tank and prohibit them from moving or migrating to the second tank. In the case of the embodiment shown in
Both tanks 12 and 14 of the SBR 10 include the capability to aerate the wastewater therein. As seen in
In a typical application, wastewater to be treated by the SBR 10 is directed through an influent line 22 into the SBR 10. In the case of the embodiment illustrated herein, the influent line 22 is directed into the first tank 12.
In some processes, it is appropriate to recycle wastewater from one tank to another tank. In the
SBR 10 is also provided with a decanting device. The decanting device may be provided or designed so as to decant from both tanks 10 and 12 simultaneously. However, because of the opening 20 in wall 18 and the fact that the tanks 12 and 14 are hydraulically connected, a decanting device provided in one tank may be sufficient to decant treated wastewater from both tanks. In the example illustrated herein, there is a single decanting device provided in the second tank 14. This decanting device includes a floating inlet 34 that is coupled to an effluent line 36. During the decanting phase, treated wastewater enters the floating inlet 34 and is directed therefrom through the effluent line 36 and out the SBR 10.
Viewing the ballasted flocculation system shown in
Turning to
After the filling phase, the SBR 10 is operated in a react phase. See
After the react phase, the SBR 10 is operated in a settling phase. Mixing and aeration are off and the decant valve is closed. Biofilm carriers 16 are relatively heavy, that is they are of the sinking type and they settle along with the suspended biomass. As shown in
As discussed above, the SBR 10 can be used to perform various biological wastewater treatments such as nitrification/de-nitrification and nitrification only. It may be useful to review how the SBR 10 performs a nitrification/de-nitrification process. In this case, suspended biomass is maintained in tank 12. Both biofilm carriers 16 and suspended biomass are maintained in tank 14. Fixed biomass is supported on biofilm carriers 16. During the filling phase, wastewater is directed into tank 12 and from tank 12 the wastewater moves through opening 20 into tank 14. There is no supplied air in tank 12 while tank 14 is aerated. Generally, tank 12 is maintained under anoxic conditions while tank 14 is maintained under aerobic conditions. It should be pointed out that during the filling process the BOD or a substantial portion of the BOD in the wastewater being directed into tank 12 is removed during the filling process. Normally in some embodiments at least 30% of the BOD is removed from the wastewater prior to reaching the second tank 14. Typically approximately 30% to approximately 50% of the BOD is removed prior to reaching the second tank 14. Generally, all or substantially all of the readily biodegradable COD is removed in the first tank 12 prior to the wastewater reaching the second tank 14. The reduction of BOD concentration and the removal of readily biodegradable COD can be achieved with a relatively small volume. For example, in one embodiment, the volume of the first tank 12 can be 10-30% of the total volume of the reactor. The significance of this will be discussed later. Thus, even during the filling phase, tank 14 performs a nitrification process. That is, ammonia in the wastewater in tank 14 is converted to nitrate and nitrite by the biomass in tank 14. A substantial portion of the nitrification process is carried out by the biomass supported on barriers 16 as this biomass is particularly efficient in a nitrification process. Tank 12, which is operated under anoxic conditions even during the filling phase, performs denitrification; that is, the suspended biomass in tank 12 converts the nitrate and nitrite to nitrogen gas. This results because a portion of the wastewater nitrified in tank 14 is recycled via line 32 from the second tank 14 to the first tank 12. This basic nitrification and de-nitrification process using an integrated fixed film activated sludge process is extended into the react phase. As occurred during filling, in the react phase the biomass supported on the biofilm carriers 16 in the second tank 14 perform a nitrification process and a portion of the wastewater that has been nitrified in tank 14 is recycled to tank 12, which is operated under anoxic conditions and which de-nitrifies the wastewater therein.
After the react phase, the SBR 10 is operated in the settling and decanting mode as discussed above. In a nitrification/de-nitrification process, it may be advantageous to design the process such that all or a substantial portion of the BOD (meaning at least approximately 30% of the BOD in the influent) is removed from the wastewater before entry into the nitrification tank 14. This tends to enable the autotrophic microorganisms to proliferate and dominate and hence improve overall nitrification efficiency. Thus, in the exemplary nitrification/de-nitrification process described above, it is advantageous to remove the BOD in the wastewater in tank 12 before the wastewater reaches the nitrification tank 14.
Although the schematic illustrations suggest that tanks 12 and 14 include a generally equal volume, it should be pointed out that in some processes one tank may have a greater volume than the other. For example, in the nitrification/de-nitrification process just described, it is contemplated that in some cases it is advantageous to provide the nitrification tank 14 with a greater volume than the first tank 12. This is because a greater volume may be required to efficiently nitrify than may be required to de-nitrify.
The SBR 10 can also remove phosphorus from the wastewater influent. For example, in the nitrification/de-nitrification process described above, tank 12 generally operates under anoxic conditions, meaning that there is nitrate and nitrite available to the microorganisms in the tank. However, the total process can be controlled such that the nitrite and nitrate in tank 12 can be depleted. When this occurs, tank 12 then begins to operate under anaerobic conditions which is suitable for phosphorus removal.
What has been described above is a two stage IFAS SBR. There are numerous advantages to the two stage IFAS SBR compared to a one stage conventional SBR. The following is a comparison of these two SBR systems for a nitrification only process and a nitrification/de-nitrification process.
Nitrification Only
Conventional SBR and two-stage IFAS SBR are designed to treat typical domestic sewage with following characteristics:
The target effluent qualities are:
Conventional SBR for nitrification uses a single reactor with a single aerobic phase plus settling and decanting. The two-stage IFAS SBR uses two zones, both under aerobic conditions. The first zone is non-media, activated sludge only and the second zone is filled with biofilm carriers. The typical designs are summarized in Table I below to demonstrate the features of the two-stage IFAS system. The first zone of the IFAS SBR is sized to remove readily biodegradable organics in the influent, which will improve the ammonia surface removal rate on the biofilm carriers in the second zone.
Nitrification/De-Nitrification
Conventional SBR and the two-stage IFAS SBR are designed to treat typical domestic sewage with following characteristics:
The target effluent qualities are:
Conventional SBR for nitrification and de-nitrification uses a single reactor with anoxic and oxic (aerobic) phases plus settling and decanting. The two-stage IFAS SBR uses the first zone under anoxic conditions and the second zone under aerobic or oxic conditions. The first zone is non-media, activated sludge only and the second zone is filled with biofilm carriers. The typical designs are summarized in Table II below to demonstrate the features of the two-stage IFAS system. The first zone of the IFAS SBR is sized to achieve the de-nitrification to meet the effluent nitrate requirement, which will improve the ammonia removal rate on the media carriers in the second zone.
It should be pointed out that for a nitrification—de-nitrification application, a conventional SBR may have to increase total reactor volume to realize the recirculation rate required for de-nitrification (R=Vo/VF=3 or 4). This means that a conventional SBR has to be operated at short cycle time. The IFAS SBR of the present invention with recirculation will eliminate this requirement. For more stringent effluent requirements, both a conventional SBR and an IFAS SBR as disclosed herein will operate at a short cycle. For this case, the IFAS SBR of the present invention will be more desirable than a conventional SBR.
The present invention also relates to retrofitting existing SBRs. As discussed above, many existing SBRs are at or near design capacity. These SBRs may include only one tank or, in some cases, the SBR could include multiple tanks. The present invention envisions segmenting these tanks and providing for an integrated fixed film activated sludge process to be performed in two tanks or tank pairs. That is, existing tanks would be provided with a wall that includes an opening therein that would effectively form two tanks or basins from a single tank or basin. One of the segmented tanks that is formed is designed specifically to contain biofilm carriers 16 while the other tank is designed to perform processes relying on suspended biomass. This will increase the efficiency of biological treatment in these existing SBRs without requiring new tanks to be constructed and without increasing the footprint of the SBRs. This is because the IFAS SBR process described herein has greater capacity to biologically treat wastewater on a unit area basis than conventional SBR processes.
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.