Sequencing Batch Reactor Proc he sequencing batch reactor(SBR) 如100265 reactor with complete mixing during the batch reaction step(after filling) and where the subsequent steps of aeration and clarification occur in the tank All SBr systems have five steps in common, SETTLE which are carded out in sequence as follows: (1)fill, (2)react(aeration), 1313515 (sedimentation/clarification), (4) draw(decant ), and(5)idle. Each of these steps is illustrated on Fig 7-11 and described in Table 7-5 For continuous-flow applications, at least two sbr tanks must be Fig. 7-11 SBR activated sludge process:(a)schematic diagram;(b)view provided so that one tank receives of a opical SBR; (c)wiew of movable weir used to deant contents of SBR. flow while the other completes its Weir is located on the far side of the second dividing wall shown in(b) treatment cycle. Several process modifications have been made in the times associated with each step to achieve nitrogen Sludge wasting in is anothe rtant step in the sbr operation that greatl affects performance as one o ive basic process steps because there is no set time period within the cycle dedicated to wasting. The amount and frequency of sludge wasting is determined by performance requirements, as with a conventional continuous-flow system. In an SBR operation, sludge wasting usually occurs during the react phase so that a uniform discharge of solids (including fine material and large floc particles)occurs. A unique feature of the SBr system is that there is no need for a return activated-sludge(RAS)system Tab. 7-5 Description of operational steps for the sequencing batch reactor Operational Because both aeration and settling occur in the same chamber no sludge is During the fill operation, volume and substrate(raw wastewater or primer lost in the vent) are added to the re rise from 75 a tor may be 50% of the full oyde firm bwo n Fu cal reachon moy be mixed ab react step and none has to be returned by lat the end of the idl to 100%. when Nw to maintain the solids content in the mixed and aerated to promote bide with the infuse aeration chamber. The SBr process can also be modified to operate in a During the react period, the biomass consumes the substrate under ed environmental conditions continuous-flow mode as discussed 地bwdw卿 barate from the liquid under quiescent conditions. later in this chapter. Because of the substrate concentration Decant Clarified effluent is removed during the decont period. Many types of changes with time, the substrate utilization and oxygen demand rates 可购付地闻切邮 vels. The aeration system should be designed to reflect the changing its in Process Design of SBRs. Because of the many design variables involved in an SBR design, an iterative approach is necessary in which key reactor design conditions are first assumed. A set of different design conditions can be evaluated by use of a spreadsheet analysis to determine the most optimal choice. The (1)the fraction of the tank ts removed during decanting and (2) the settle, decant, and aeration times. Because the fill volume equals the decant volume, the fraction of decant volume equals the fraction of the sBr tank volume used for the fill volume per cycle. The design procedure for the SBr system is presented in Table 7-6 Tab. 7-6 Computation approach foe the design ofa sBr 7-16
7-16 Sequencing Batch Reactor Process The sequencing batch reactor (SBR) process utilizes a fill-and-draw reactor with complete mixing during the batch reaction step (after filling) and where the subsequent steps of aeration and clarification occur in the same tank. All SBR systems have five steps in common, which are carded out in sequence as follows: (1) fill, (2) react (aeration), (3) settle (sedimentation/clarification), (4) draw (decant), and (5) idle. Each of these steps is illustrated on Fig. 7-11 and described in Table 7-5. For continuous-flow applications, at least two SBR tanks must be provided so that one tank receives flow while the other completes its treatment cycle. Several process modifications have been made in the times associated with each step to achieve nitrogen and phosphorus removal. Sludge Wasting in SBRs. Sludge wasting is another important step in the SBR operation that greatly affects performance. Wasting is not included as one of the five basic process steps because there is no set time period within the cycle dedicated to wasting. The amount and frequency of sludge wasting is determined by performance requirements, as with a conventional continuous-flow system. In an SBR operation, sludge wasting usually occurs during the react phase so that a uniform discharge of solids (including fine material and large floc particles) occurs. A unique feature of the SBR system is that there is no need for a return activated-sludge (RAS) system. Tab. 7-5 Description of operational steps for the sequencing batch reactor Because both aeration and settling occur in the same chamber, no sludge is lost in the react step and none has to be returned to maintain the solids content in the aeration chamber. The SBR process can also be modified to operate in a continuous-flow mode as discussed later in this chapter. Because of the substrate concentration changes with time, the substrate utilization and oxygen demand rates change, progressing from high to low levels. The aeration system should be designed to reflect the changing requirements in oxygen demand. Process Design of SBRs. Because of the many design variables involved in an SBR design, an iterative approach is necessary in which key reactor design conditions are first assumed. A set of different design conditions can be evaluated by use of a spreadsheet analysis to determine the most optimal choice. The key design conditions selected are (1) the fraction of the tank contents removed during decanting and (2) the settle, decant, and aeration times. Because the fill volume equals the decant volume, the fraction of decant volume equals the fraction of the SBR tank volume used for the fill volume per cycle. The design procedure for the SBR system is presented in Table 7-6. Tab. 7-6 Computation approach foe the design of a SBR Fig. 7-11 SBR activated sludge process: (a)schematic diagram; (b)view of a typical SBR;(c)view of movable weir used to deant contents of SBR. Weir is located on the far side of the second dividing wall shown in (b)
1. Obiain infuent wastewater characterization dat, define effluent requirements, and Staged Activated-Sludge Process 2. Select the number of sbr tank In the conventional plug-flow 3. Select the react/oerofion, setting, and decant times. Determine the fll time and totol ime activated-sludge system, the hydraulics and mixing regime may result 4. From the lotal number of cydes per day, determine the fil volume per cycle in two to four effective stages from the 5. Selec the MLSS concentrotion and determine the fill volume fraction relative ho the lotl. standpoint of biological kinetics tank volume. Determine the decant depth Using the computed depths, determine the se Activated-sludge processes can be 6. Determine the srT for the SBR designed with baffle walls to intentionally 7. Determine the amount of tkn added that is nitrified create a number of complete-mix 8. Calculate the nitrifier biomass concentrotion and determine if the aeration time selected is activated-sludge zones operating in series 9. Adjust the design as needed-additionol iterations may be don For the same reactor volume reactors in o. Dehurm ine the decant pumping rate series can provide greater treatment 1, Determine the oxygen required and average transfer rate efficiency than a single complete-mix reactor, or provide a greater treatment 3. Calculate the F/M and BOD volumetric looy 4. Evaluate alkalinity needs activated-sludge process configurations 5. Prepare design summary are used at several full-scale installations The oxygen demand varies in staged complete-mix reactor designs and can be high enough in the first stage to challenge the volumetric oxyger Fig.7-12 With Nitrification high-density fine bubble aeration diffusers, activated such as membrane aeration panels oxygen Nadily biodegradable COD(soluble transfer rates of 100 to 150 mg/Lh are possible, with some manufacturers claiming higher rates. The changes in oxygen uptake Slowly biodegradable COD (particulate) (OURs) in each stage of a fo ge rbCOD removal, particulate degradable COD, and endogenous respiration) are depicted on Most of the rbcod wil ed day de iaf madema zero-order e te for nne nrst the deg rare oagis due to higer naats concentrations in the early stages. Oxygen demand for endogenous respiration will be relatively constant he oxygen demand distribution may be estimated to determine the aeration design for staged processes The percent of the total oxygen consumption may range from 40 and 10 percent, respectively, fo e four-stage system. One design approach that can be used to obtain an estimate of t ove s and by providing an air supply system approach outlined above is satisfactory because during the life of the process, the oxygen demand'wili Use of Simulation Models. The other approach involves the use of simulation models, in which kinetics and changes in constituent concentrations in each stage are taken into consideration. This activated-slud approach involves solving a set of equations in each includes COD. NH4-N. endo concentration. Models and the effect of biological phosphorus removal on design and performance Alternative processes for bod removal and Nitrification Over the last 30 years numerous activated-sludge processes have been developed for the removal of organic material(BOD) and for nitrification 7-17
7-17 Staged Activated-Sludge Process In the conventional plug-flow activated-sludge system, the tank hydraulics and mixing regime may result in two to four effective stages from the standpoint of biological kinetics. Activated-sludge processes can be designed with baffle walls to intentionally create a number of complete-mix activated-sludge zones operating in series. For the same reactor volume, reactors in series can provide greater treatment efficiency than a single complete-mix reactor, or provide a greater treatment capacity. As a consequence, staged activated-sludge process configurations are used at several full-scale installations. Oxygen Demand in Staged Designs. The oxygen demand varies in staged complete-mix reactor designs and can be high enough in the first stage to challenge the volumetric oxygen transfer capability of aeration equipment. With high-density fine bubble aeration diffusers, such as membrane aeration panels oxygen transfer rates of 100 to 150 mg/L.h are possible, with some manufacturers claiming higher rates. The changes in oxygen uptake rates (OURs) in each stage of a four-stage activated-sludge process (defined as a function of oxygen needed for nitrification, rbCOD removal, particulate degradable COD, and endogenous respiration) are depicted on Fig. 7-12. Most of the rbCOD will be consumed in the first stage, and the OUR for pCOD degradation will decrease from stage to stage as a function of the degradation kinetics. Nitrification rates may be at a maximum zero-order kinetic rate for the first one to three stages due to higher NH4-N concentrations in the early stages. Oxygen demand for endogenous respiration will be relatively constant from stage to stage. The oxygen demand distribution may be estimated to determine the aeration design for staged processes. The percent of the total oxygen consumption may range from 40, 30, 20, and 10 percent, respectively, for a four-stage system. One design approach that can be used to obtain an estimate of the oxygen demand in a staged system is to calculate the total oxygen demand as would be done for a CMAS process, and then estimate the oxygen demand distribution with consideration to the various components described above. With proper selection of the type and placement of the diffusers and by providing an air supply system with DO control in each portion of the system, the air can be provided where needed. Generally, the approach outlined above is satisfactory because during the life of the process, the oxygen demand will vary across the tank as the load changes. Use of Simulation Models. The other approach involves the use of simulation models, in which the kinetics and changes in constituent concentrations in each stage are taken into consideration. This approach will typically result in a more optimal design and can be used to assess the real capacity of a given activated-sludge design. The simulation approach involves solving a set of equations in each stage for each constituent, which includes rbCOD, pCOD, NH4-N, endogenous respiration, and biomass concentration. Models also include phosphorus and the effect of biological phosphorus removal on design and performance. Alternative Processes for BOD Removal and Nitrification Over the last 30 years numerous activated-sludge processes have been developed for the removal of organic material (BOD) and for nitrification. Fig. 7-12 Changes in oxygen uptake rates for staged activated sludge process