and for the conventional wastage approach:
which is equivalent to Eq. 5.71, except for the approximately equal to sign.
This approach offers significant potential for simplifying activated sludge operation and the concepts have been applied to systems with uniform MLSS concentrations." Use is made of a calibration chart to account for suspended solids in the process effluent and consideration is given to the constraints imposed by the thickening limitations of the clarifiers. The concepts can also be used with activated sludge systems with nonuniform MLSS concentrations.
Qualitative observations provide valuable information on the actual operating conditions in activated sludge systems, thereby helping operators to make decisions about alterations in process control parameters/"They are a necessary adjunct to long-term SRT control and application of the process fundamentals described above.
Bioreactor. Important information can be gained from observing the color and appearance of the biomass in an activated sludge bioreactor. The mixed liquor in a well operating bioreactor will be brown in color, and a small amount of fresh, crisp, white foam will be present on the liquid surface. An earthy, musty aroma will be present which represents normal biological degradation products. A black color, or the presence of the "rotten egg" odor of hydrogen sulfide, indicates that inadequate aeration is being provided. The presence of a voluminous, billowing white foam indicates that inadequate treatment is occurring. The foam is a result of incomplete degradation of surfactants contained in the influent wastewater and/or the production of surface active agents by rapidly growing bacteria. Such a condition indicates either that the SRT is too short or that a toxicant has entered the plant and inactivated a portion of the biomass. This condition can be remedied by increasing the SRT and/ or by locating and eliminating the source of the toxicant.
Under some conditions, a thick, viscous brown foam may accumulate on the surface of the bioreactor. This is typically caused by the growth of actinomycetes such as Nocardia, although other filamentous bacteria such as Microthrix parvicella can also cause it.1" In extreme cases, the foam can accumulate sufficiently to overflow the bioreactor, resulting in unsightly and unsafe conditions. Accumulations within the clarifier can result in carryover of solids to the effluent, degrading it. The factors that cause such foams are complex and are not yet fully understood. The causative organisms are relatively slow growing and can, in some instances, be controlled by lowering the SRT. Their growth and accumulation in activated sludge systems are facilitated by physical designs that trap accumulated foam in the bioreactor. Consequently, good design practice requires foam to flow easily out of the bioreactor to the clarifier, where it is collected and removed from the system.1" Once collected, the foam should be directed to the solids handling system in a way that minimizes recycle; direction to the plant headworks or to a gravity thickener from which the overflow is recycled should be avoided. Some experience suggests that selectors can provide control over the growth of the causative bacteria. Another approach is a classifying selector, where aeration is used to produce foam, which will be selectively enriched with the causative bacteria.' J" The foam is then removed, resulting in their selective removal. If all else fails, then a high concentration chlorine spray can be applied directly to the foam on the bioreactor.' A fine mist is used so that the chlorine will contact the foam without entering the mixed liquor itself. The chlorine oxidizes the foam and the microorganisms in it, thereby destroying them. Since the foam-causing bacteria will be enriched in the foam, this practice selectively removes them from the system.
Clarifier. Visual observations of the clarifier indicate its operational status and also provide clues about the settling characteristics of the activated sludge. Denitri-fication in the clarifier solids blanket causes large clumps of solids to rise to the liquid surface, break, and spread over it. This phenomena, known as clumping, is caused when nitrate-N is present, DO is depleted in the solids blanket, and the residence time of the solids in the blanket is long enough to allow the generation of sufficient nitrogen gas bubbles to cause the solids to float. It can be controlled by altering the SRT of the activated sludge and/or by increasing the RAS flow rate to reduce the size and residence time of the solids blanket. The SRT may be decreased to reduce the degree of nitrification, or it may be increased to reduce the respiration rate of the activated sludge, thereby reducing the production rate of nitrogen gas. Alternatively, an anoxic zone may be incorporated into the bioreactor to remove nitrate-N, thereby reducing its concentration in the clarifier. The design and operation of activated sludge systems with anoxic zones are discussed in Chapter 11.
Two problems are often associated with long SRTs and/or excessive DO concentrations. They are ashing and pin floe. Both result from an inadequate filament backbone in the activated sludge floe. Growth at long SRTs and high DO concentrations prevents the growth of filamentous microorganisms, which are necessary to produce a strong floe that is resistant to mechanical turbulence. Ashing is the term used when small dark brown to gray particles rise in the clarifier. Pin floe refers to dense, granular solids that flocculate poorly and result in particles that settle rapidly but leave behind a turbid supernatant. Such a situation can be alleviated by reducing the SRT and/or the DO concentration to encourage the growth of a moderate amount of filamentous bacteria.
Straggler floe and dispersed growth are often observed at short SRTs. Straggler floe are relatively large floe particles (0.25 to 0.5 mm) that appear to be light, fluffy, and almost buoyant within the clarifier. They are caused by excessive quantities of filamentous microorganisms, which expand the activated sludge floe, resulting in slowly settling floe particles that can be carried out of the clarifier by hydraulic-currents. As discussed in Section 10.2.1, dispersed growth is caused by inadequate flocculation and can often be corrected by increasing the SRT.
Observation of the clarifier solids blanket thickness, i.e., the vertical distance between the bottom of the clarifier and the top of the blanket, can provide an early indication of settleability problems. However, care must be exercised to properly interpret such observations because a blanket that is increasing in thickness may simply indicate improper system operating conditions. For example, an inadequate RAS flow rate will cause the blanket to increase in thickness as more solids are applied to it than can be removed by the RAS. An unintended increase in the acti vated sludge SRT can also cause the blanket to rise by increasing the MLSS concentration beyond that which can be handled by the clarifier. On the other hand, if an increase in the solids blanket thickness is coupled with an increase in the SVI, then a deterioration in solids settling and compaction characteristics is probably the cause. The term "bulking" should only be applied to that situation and not to the accumulation of solids within the clarifier. Bulking is generally caused by the growth of excessive quantities of filamentous bacteria, and its causes and cures are discussed in Section 10.2.1. If the corrective measures discussed there cannot be successfully applied or if an immediate remedy is required to prevent permit violations, then either oxidants such as chlorine or hydrogen peroxide may be applied to the activated sludge to reduce the population of filamentous bacteria, or polymers or other coagulants may be added to improve sludge settleability. The addition of oxidants to control the relative populations of filamentous and floc-forming bacteria is discussed in Section 10.4.3.
During Sludge Volume Index Measurement. Changes in activated sludge settling characteristics, referred to as "sludge quality" in operations manuals,*"*1 can often be observed in the vessel used to measure the SVI (settleometer) before they have an impact on the clarifier solids blanket thickness. Phenomena such as ashing, pin floe, straggler floe, and dispersed floe can be observed more precisely in the settleometer than in the clarifier. Denitrification will also often be observed first in the settleometer because its small hydraulic head allows the development of sufficient gas to float the solids before it occurs in the full-scale clarifier.'" An increase in the SVI value indicates an increase in the population of filamentous bacteria.
Some operators measure the settling velocity of the activated sludge in the settleometer during the SVI test and use it as a control parameter.*""' They then adjust the DO concentration and the SRT in the activated sludge system (sometimes called "oxidation pressure" by operators) until the relative populations of low DO filaments and Hoc-forming bacteria are balanced to attain the desired sludge settling velocity. The settling characteristics of the sludge can be adjusted relatively quickly using this technique. Longer times are required, however, to recover acceptable settleability for sludge with a high SVI because of the need to waste a large inventory of filamentous bacteria from the system. This emphasizes the need to carefully monitor solids settling characteristics and maintain them within acceptable ranges.
Microscopic Examination. Regular microscopic examination of an activated sludge can provide insight into the factors affecting system performance and greatly assist with process optimization.M One type of microscopic examination uses a relatively inexpensive microscope to determine the types of Eucarya present and to characterize the overall structure of the activated sludge floe. Such an examination can supplement observations of the bioreactor, the clarifier, and the characteristics of the sludge in the settleometer. The types of Eucarya present tend to correlate with the operating SRT and can provide rapid, visual confirmation that the proper value is being achieved. A rapid change in the types of Eucarya present can be an indication of an inadequate DO concentration or the presence of a toxicant. A second type of microscopic examination is used to characterize the types of filamentous bacteria present using the technique of Eikelbootrrl>: as modified by Jenkins et al.1" This examination requires a research grade microscope and a high degree of specialized training. The information obtained can be used to identify corrective actions required to alleviate filamentous bulking problems, as discussed in Section 10.2.1 and indicated in Table 10.5. Facilities that do not generally experience filamentous sludge bulking problems need not conduct this type of examination on a regular basis.
10.4.3 Activated Sludge Oxidation to Control Settleability
The application of oxidizing agents to activated sludge can be used to control the growth of filamentous bacteria. Because such chemicals oxidize filamentous bacteria faster than floc-forming ones, they reduce the relative population size of the filaments in the activated sludge, thereby influencing its settling characteristics."' Due to chlorine's low cost and ready availability, it is the oxidant used most often for this purpose, although others, such as hydrogen peroxide, can be used with equal effect. Three factors are important in the use of chemical oxidation to control activated sludge settling characteristics: (1) proper control of the oxidant dose, (2) selection of an appropriate dose point, and (3) mixing at the dose point.
Because the purpose of oxidant addition is to destroy part of the activated sludge, and because the oxidant should be added continuously, the dosing rate should be expressed as the mass of oxidant added per day per mass of activated sludge in the system. Typical units are g of oxidant/(kg MLVSS-day), with MLVSS being used to represent the activated sludge because only the organic fraction of the sludge will react with the oxidant. The dose required will depend on the severity of the filamentous bulking and the speed with which it is desirable to reduce the SVI, with dosing rates typically ranging from a low of about 2 g Cl:/(kg MLVSS-day) to a high of about 10. Prior to the initiation of chlorination, a target SVI is selected that will give the desired overall system performance, and a target SVI range is also chosen, often ± 20 raL/g. The dosing rate is then selected, chlorination is initiated, and the response of the system is monitored in terms of changes in the SVI and the abundance of filaments, as revealed by periodic microscopic examination. The dose is adjusted in accordance with the response until the SVI is within the target range. The dose is reduced if the SVI is within the target range but decreasing, while it is increased if the SVI is increasing. Chlorination is terminated when the SVI falls below the target range. If time is available to reduce the SVI slowly, then chlorination can begin at a relatively low dose of about 2 g CL/(kg MLVSS day) and be slowly increased until a downward trend in the SVI is established. If the SVI must be reduced rapidly, then a more aggressive dose, on the order of 6 g Cl:/(kg MLVSS-day), should be selected, with the dose being reduced when a downward SVI trend is established.
The dose point should be selected to avoid contact with the influent wastewater while achieving a desired dosing frequency. Convenience and adequate mixing should also be considered. Direct contact with the influent wastewater must be avoided because the oxidant will react with any organic matter present at the dose point, thereby reducing its effectiveness against the activated sludge. Dosing frequency is important because biomass circulates through activated sludge systems and the entire inventory must be exposed to the oxidant about three times per day. If the dosing frequency is less than this, significant filament growth can occur in (he fraction of the biomass that is not being dosed. For some systems, such as CAS, ("'MAS, CSAS, and SFAS, the RAS stream is an excellent location to add the oxidant. Little influent organic matter is present in the RAS stream and the bioreactor HRT is short enough that biomass will be circulated through the clarificr several times per day, thereby ensuring the needed dosing frequency. For other processes, such as EAAS, the bioreactor HRT is much longer and biomass circulates to the clarifier less than once per day. In such cases, the dosing frequency will be insufficient and the oxidant may not be fully effective in controlling filament growth. In this case, the oxidant should be added directly to the bioreactor to obtain the needed dosing frequency.
Mixing at the dose point is important because the reaction between oxidant and biomass is very fast. Thus, rapid mixing must be provided to avoid over-oxidation of a portion of the biomass and under-oxidation of the remainder. A diffuser is generally required to ensure good contact between the added oxidant and the process flow. In addition, mechanical or other mixing may also be needed to ensure adequate dispersion of the oxidant. When the oxidant is added directly to the bioreactor, adequate mixing is often provided by the aeration equipment itself. The reader is referred to the book by Jenkins et al."' for further discussion of the use of oxidants to control activated sludge settling characteristics.
Regulation of the SRT provides long-term control of an activated sludge system. Furthermore, visual observations provide feedback on the success of the SRT control strategy and allow fine-tuning of certain operating parameters, such as the DO concentration. While this approach will permit operational adjustments in response to seasonal variations in process operating conditions and long-term changes in process loadings, most activated sludge systems are also subject to short-term loading variations. Loading variations occur on a diurnal and a day-to-day basis, and process operation must be adjusted accordingly. Facilities may also be subject to shock organic loadings due to industrial discharges and shock hydraulic loadings due to the inflow and infiltration of precipitation into the wastewater collection system. Process operation must be adjusted in response to these variations as well. Activated sludge systems possess some ability to respond to short-term loading variations, particularly if they occur regularly and over a period of several hours. However, this capacity is limited by the capability of bacteria to rapidly adjust their enzyme levels and by the time required to grow significant quantities of additional biomass. Thus, some deterioration in performance should be expected in response to dynamic loadings. Dynamic simulation, as illustrated in Chapters 6 and 7, is one technique that can be used to define the deterioration in effluent quality that will result from variations in process loadings. It can be used to determine the treatment limits for a particular process configuration and the benefits to be gained from the use of equalization to dampen load variations. Dynamic simulation can also be used to identify alternative process design and operating conditions that will improve the dynamic response of the system.
Several factors must be considered to ensure that an activated sludge system has adequate dynamic response capability. One is selection of an appropriate SRT/: "4 The procedure used to select the design SRT for a nitrifying activated sludge system illustrates the principle; a safety factor for the design is selected with consideration of process loading variations. Equalization can be applied to smooth the organic and hydraulic loading variations. It will generally be a necessity for industrial waste-
waters in which significant short-term organic loading variations occur as a result of the operation of the production facility that generates the wastewater. On the other hand, experience indicates that the activated sludge process is generally quite resilient and can accept the dry weather variations in organic and hydraulic loadings typical of domestic wastewaters, while still providing acceptable performance. Even in that situation, however, flow equalization can improve the performance of the entire treatment train. Improved response to organic and hydraulic variations can also be achieved by providing alternative activated sludge process operating modes. For example, step feeding capability can be provided to allow peak hydraulic loadings to be processed without causing clarifier thickening failure.
While not yet in routine use, much research is being directed at the development and application of automatic control technology.' ^ Process instrumentation is available to monitor suspended solids concentrations, DO concentrations, oxygen uptake rates, and residual nutrient concentrations (ammonia, nitrate, and phosphate). The data collected by such sensors, supplemented with laboratory and flow rate data, can be interfaced with dynamic process models to allow investigation of alternative operating strategies. Such approaches have been used off-line to analyze the performance of existing activated sludge systems, " and may become available for on-line monitoring and process optimization. Expert systems are also being developed to assist with activated sludge process operation. All in all, these developments offer significant potential to optimize activated sludge operation, thereby improving performance and reliability, and reducing costs.
1. Four factors arc common to all activated sludge systems: (1) biomass grows as a flocculent slurry by oxidizing organic matter under aerobic conditions; (2) quiescent sedimentation removes the biomass, producing an effluent low in suspended solids; (3) settled biomass is recycled as a concentrated slurry back to the bioreactor; and (4) excess biomass is wasted to control the solids retention time (SRT).
2. Activated sludge bioreactors are typically open basins containing mechanical equipment to transfer oxygen and maintain the flocculent biomass in suspension. Several devices are used to do this, including coarse and fine bubble diffused air, mechanical surface aerators, jet aerators, and submerged turbines.
3. The clarifiers used in activated sludge systems provide two functions: (1) separation of the flocculent biomass to produce a clarified effluent (clarification), and (2) concentration of the biomass for recycle to the upstream bioreactor (thickening).
4. Eight major activated sludge process options exist: (1) conventional (CAS), (2) step feed (SFAS), (3) contact stabilization (CSAS), (4) completely mixed (CMAS), (5) extended aeration (EAAS), (6) high purity oxygen (HPOAS), (7) selector (SAS), and (8) sequencing batch reactor (SBRAS).
5. CAS and CMAS contain uniform mixed liquor suspended solids (MLSS) concentrations and typically have hydraulic retention times (HRTs) rang ing from 4 to 8 hours and SRTs ranging from 3 to 15 days. CAS uses a plug-flow bioreactor, while CMAS uses a single, completely mixed bioreactor. Sludge settleability is usually better for CAS facilities due to better control of the growth of filamentous bacteria. CMAS offers greater resistance to upset by inhibitory organic chemicals.
6. The return activated sludge (RAS) is sequentially diluted as it flows through SFAS and CSAS systems, resulting in an outlet MLSS concentration that is lower than the average concentration in the bioreactor. As a consequence, SFAS and CSAS biorcactors can be smaller than CAS or CMAS bioreactors with the same SRT. Effluent quality may be somewhat poorer from SFAS and CSAS systems, however.
7. EAAS systems use long SRTs (20 to 30 days) to partially stabilize the biomass produced. This requires large bioreactors (HRTs of 14 hours or more), but results in excellent process stability and the production of a high-quality effluent.
8. In HPOAS systems, biomass and oxygen enriched gas flow cocurrently through a staged, covered bioreactor. This allows use of high volumetric organic loading rates and short HRTs (generally 2 to 4 hours). Short SRTs are also generally used (1 to 5 days).
9. The SAS system uses a highly loaded section at the inlet end of the bioreactor (the selector) to create conditions favorable to the growth of floc-forming bacteria relative to filamentous bacteria. Selectors can be incorporated into the other activated sludge options.
10. SBRAS incorporates biological reaction and sedimentation into a single vessel. Microprocessors are used to automatically control the influent flow, aeration, mixing, and effluent decanting functions. They are used most often in smaller wastewater treatment plants.
11. Sludge settleability is typically quantified by the sludge volume index (SVI), which is the volume (in mL) occupied by one gram of settled solids. A value greater than 150 mL/g represents a poor settling, bulking sludge. Observation of settling rate and the clarity of the supernatant produced in the SVI test can provide valuable insights into the characteristics of the activated sludge.
12. Bioflocculation forms the microstructure of activated sludge floe by aggregating individual bacteria into dense, settleable particles. It occurs as a result of exocellular polymers produced by the biomass. A minimum SRT (typically 1 day for domestic wastewaters and 3 days for industrial wastewaters) is required to obtain adequate bioflocculation.
13. Filamentous bacteria provide the macrostructure for activated sludge floe, thereby forming the "backbone" that allows floe to resist mechanical shear. A proper balance between floc-forming and filamentous bacteria results in strong, compact floe that resists shear in the bioreactor, while settling quickly and compacting well in the clarifier.
14. Excessive quantities of filamentous bacteria cause activated sludge bulking. About 30 types of filamentous bacteria can be found in activated sludge systems and the type present provides valuable clues about the situation that needs to be corrected to eliminate the bulking problem.
15. The SRT required for many activated sludge systems is determined by the need for bioflocculation. In other cases, longer SRTs are needed to nitrify, to treat certain industrial wastes containing less biodegradable organic matter, and to stabilize the biomass produced. The reliability and capacity of the solids processing system must also be considered in the selection of the SRT.
16. Two factors limit the bioreactor MLSS concentration. One is solids thickening, which limits the maximum economical MLSS concentration to about 5,000 mg/L as total suspended solids (TSS). The other is bioflocculation, which typically requires a minimum MLSS concentration of 500 to 1,000 mg/L as TSS.
17. The primary effect of the dissolved oxygen (DO) concentration is on the growth of filamentous bacteria, although it will also affect the occurrence of nitrification. A DO concentration of 2 mg/L is a reasonable benchmark, but in some situations successful treatment can be obtained with lower values, whereas in others, higher values will be required.
18. Use of the oxygen transfer equipment both to transfer oxygen and to maintain solids in suspension places constraints on the size of the bioreactor. If it is too small, then the volumetric power input required to transfer the needed oxygen will cause floe shear. If it is too large, then mixing will control the volumetric power input, resulting in increased power requirements.
19. Adequate nutrients are required to allow balanced growth of biomass in activated sludge systems. Nutrient limitations can result in the growth of undesirable quantities of filamentous bacteria and/or the production of exocellular slime. Both interfere with activated sludge settling and compaction.
20. The temperature of an activated sludge system must be maintained in either the mesophilic (35° to 40°C) or the thermophilic (45° to 60°C) range; it should not fluctuate between the two. The oxidation of organic matter and ammonia-N results in the liberation of heat, whereas the physical configuration of the bioreactor and the nature of the oxygen transfer device influence the loss of heat.
21. The design of an activated sludge process generally consists of the following six steps:
a. Select the activated sludge option and SRT based on wastewater characteristics, effluent quality goals, facility capital and operating costs, and operational objectives.
b. Calculate the mass of MLSS in the system, (Xm i * V)s;vs,L,in, the quantity of waste sludge, WM ,, and the oxygen requirement, RO. by using the modified stoichiometric model of Chapter 5.
c. Distribute the oxygen requirement as required by the system configuration.
d. Calculate the upper and lower limits on the bioreactor volume based on mixing, floe shear, and oxygen transfer.
e. Using (XM , - VJs,,,,.,,, and the output from Step d, calculate the upper and lower limits on the MLSS concentration and choose a MLSS concentration within those limits based on consideration of final set-
tier design. Calculate the bioreactor volume associated with the chosen MLSS concentration, f. Optimize the system using tools such as activated sludge model (ASM) No. 1.
22. The mass of MLSS and the quantity of waste sludge for any activated sludge system can be calculated once the SRT is established. They are calculated for cold weather conditions because they produce the largest values, thereby determining the required size of the bioreactor.
23. The system oxygen requirement can also be calculated once the SRT is fixed. It will be greatest at the highest temperature, and thus summer conditions are used in design. This requirement must be apportioned among the different vessels in a multitank system. Transient requirements must also be considered when sizing the oxygen transfer system.
24. The size of the bioreactor is determined from (XM., ■ V)Sl„„m based on the design MLSS concentration. The choices of the MLSS concentration and the bioreactor volume must be consistent with the constraints given in Key Points 16 and 18.
25. Aerobic selectors prevent filamentous sludge bulking by allowing removal of the readily biodegradable organic matter in an environment with a sufficiently high specific growth rate for kinetic selection of floc-forming bacteria. To ensure that this occurs under a variety of loading conditions, selectors are staged and are sized to give a desired average process loading factor.
26. The RAS flow rate and the distribution of the influent wastewater must be considered when sizing the bioreactor for systems with nonuniform MLSS concentrations, such as SFAS and CSAS, because both affect the distribution of MLSS within the system.
27. In the absence of simulation, the spatial distribution of the oxygen requirement in multitank systems can be approximated by partitioning it into its component parts and assigning each to the appropriate portion of the bioreactor system. Convenient divisions are the oxidation of readily biodegradable substrate, the hydrolysis and subsequent oxidation of slowly biodegradable substrate, nitrification, and decay of heterotrophs.
28. Because of the analogy between continuous flow and batch activated sludge systems, SBRAS systems can be designed with the same basic procedures as the other activated sludge options. The primary difference is that the smallest allowable bioreactor size is governed by its role as the final settler.
29. Dynamic models such as ASM No. 1 can be used to refine designs developed using the procedures described above. They allow estimation of the impact of short-term loading variations on effluent quality, and provide better estimates of the spatial and temporal variations in oxygen requirements.
30. Three procedures are routinely used to determine the waste activated sludge (WAS) flow rate required to achieve a given SRT: MLSS analysis, centrifuge analysis, and hydraulic control. The WAS flow rate should not be changed by more than 20% each day, but solids wasting should be frequent enough so that MLSS concentrations do not change by more than 10%.
31. Observations of the color and appearance of the biomass in an activated sludge bioreactor provide information about the system operating conditions.
32. Phenomena such as denitrification in the final clarifier and an imbalance between floe-forming and filamentous bacteria can be detected by visual observations of the clarifier.
33. Microscopic examination of the activated sludge biomass should be performed routinely. Observation of the Eucarya present provides visual confirmation of the SRT value. Rapid changes in the Eucarya present indicate an inadequate DO concentration or the presence of toxic materials. Identification of the types of filamentous bacteria present can be used to determine the conditions causing excessive filament growth and the associated sludge bulking problems.
34. Oxidants such as chlorine and hydrogen peroxide can be used to oxidize excessive quantities of filamentous bacteria and control biomass settling characteristics. Oxidant doses are expressed as g oxidant/(kg MLVSS-day). The dose point should be selected to avoid contact with the influent wastewater while achieving a desired dosing frequency; convenience and adequate mixing should also be considered.
35. The dynamic response of an activated sludge system is constrained by the mass of biomass present in the system and by the ability of the microorganisms to synthesize additional enzymes. Longer SRTs generally provide greater capability to metabolize added organic matter. In some cases, equalization is necessary to limit the variations in activated sludge process loadings.
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