Process Description

Since its inception by Arden and Lockett in 1914,4 the activated sludge process has grown in popularity until today it is the most widely used biological wastewater treatment process. Much experimentation has occurred since its initial development, and many variations are known. Most of the variations can be grouped into the eight named operations listed under activated sludge in Table 1.2. Theoretical simulations of the performance of many of those variations are presented in Chapters 5, 6, and 7. The purpose here is to describe and compare them as they appear in practice.

10.1.1 General Description

Four factors are common to all activated sludge systems: (1) a flocculent slurry of microorganisms (mixed liquor suspended solids [MLSS]) is utilized to remove soluble and particulate organic matter from the influent waste stream; (2) quiescent settling is used to remove the MLSS from the process flow stream, producing an effluent that is low in suspended solids; (3) settled solids are recycled as a concentrated slurry from the clarifier back to the bioreactor; and (4) excess solids are wasted to control the SRT to a desired value. Figure 10.1 illustrates how those four functions

WAS (Garrett Method)

Influent

WAS (Garrett Method)

Influent

WAS (Conventional)

Figure 10.1 Typical activated sludge process.

WAS (Conventional)

Figure 10.1 Typical activated sludge process.

are typically provided and is shown to allow the definition of some terms commonly used in practice. The basic similarity of Figure 10.1 to Figure 5.2 and the schematics of the systems used to simulate the various options in Chapter 7 should be noted. The bioreactor containing the MLSS is commonly called an aeration basin, and it is aerobic throughout, as indicated by AER in Figure 10.1. Sufficient mixing energy must be provided in the bioreactor to maintain the solids in suspension, as mentioned in Section 9.3.4. The MLSS concentration will depend on the characteristics of the influent wastewater, bioreactor hydraulic residence time (HRT), and process SRT, as discussed in Section 9.3.3. The stream of concentrated solids being recycled from the clarifier to the bioreactor is commonly called return activated sludge (RAS). The RAS suspended solids concentration will depend on the clarifier operating conditions, including the MLSS concentration and the influent and RAS flow rates. As discussed in Section 5.3, solids can be removed from the process at various points to maintain the desired SRT and the stream through which that is done is referred to as waste activated sludge (WAS). Figure 10.1 illustrates the two common solids removal points, from the secondary clarifier underflow (conventional method) and from the aeration basin (Garrettmethod). The relative merits of the two alternatives are discussed in Section 5.3. As its name suggests, the conventional method is more common.

Aeration basins are typically open tanks containing equipment to transfer oxygen into solution and to provide mixing energy to keep the MLSS in suspension. The depth is determined largely by the characteristics of the oxygen transfer/mixing system and typically ranges from 3 to 7.5 m. In special cases, depths as low as 2 m or as great as 20 m may be used. The bioreactor may be constructed of concrete, steel, or as an earthen basin lined with clay or an impermeable membrane. Vertical sidewalls are typically used with concrete and steel structures, while sloping side-walls are used with earthen structures. A wide variety of bioreactor configurations, i.e., length-to-width ratios, can be used. Bioreactor configuration, the characteristics of the oxygen transfer/mixing equipment, and the distribution of the RAS flow will affect the mixed liquor flow pattern within the bioreactor, which will affect process performance, as demonstrated in Chapter 7.

In many cases a single device is used both to transfer oxygen and to keep the MLSS in suspension. Typical devices include diffused air (both coarse and fine bub ble), floating or fixed mechanical surface aerators (both high speed and low speed), jet aerators, and submerged turbine aerators. Auxiliary mechanical mixers are used when the aeration device does not provide sufficient mixing energy to keep the MLSS in suspension.

The secondary clarifier provides two functions. One, called the clarification function, is removal of the MLSS to produce a clarified effluent that meets the effluent suspended solids goal. The other, called the thickening function, is the concentration of the settled solids for return to the bioreactor. A wide range of secondary clarifier configurations can be used, although circular and rectangular are the most common. Clarifier configuration has less impact on process performance than bioreactor configuration, as long as the clarifier is sized properly. Clarifiers contain an effluent collection device (typically an overflow weir and effluent collection launder) and a settled solids collection device. Most clarifiers are also provided with equipment to collect solids which float to the surface. The secondary clarifier can provide temporary storage of MLSS transferred from the bioreactor during periods of high flow. This occurs when suspended solids are transferred to the clarifier at a rate greater than they can be removed in the RAS and WAS streams. The solids storage capacity of a secondary clarifier is largely a function of its depth and the settling characteristics of the MLSS, but routine use of the secondary clarifier for solids storage is generally not recommended.

This book focuses on biological process design rather than on facility design. Readers interested in further information about the design of oxygen transfer devices, mixing systems, and final clarifiers are referred elsewhere."'1 "s:

10.1.2 Process Options

Due to the long history and varied use of the activated sludge process, a great number of named process options exist. In this section eight that represent the range and capabilities of activated sludge are described. Six were introduced in Chapters 5 and 7 where their theoretical performance was investigated through simulation.

Conventional Activated Sludge. The first continuous flow activated sludge processes used long narrow aeration basins with influent and RAS being added at one end and flowing in a plug-flowa fashion to the opposite end. Diffused air with a uniform aeration pattern along the length of the basin was initially utilized. Later it was recognized that the oxygen requirement varies along the length of the bioreactor, as illustrated in Section 7.2.4. This led to the use of tapered aeration, which is the variation of oxygen input along the length of the basin in response to the oxygen demand/'

Conventional activated sludge (CAS) is the modern embodiment of those early processes. The bioreactor is generally rectangular in shape with influent and RAS

It is apparent that the aeration and mixing in an activated sludge aeration basin prevent it from having the residence time distribution of a perfect plug-flow reactor (PFR) as defined in Chapter 4. Consequently, the use of the term "plug-flow" in the context of activated sludge and other biochemical operations means that the bioreactor is sufficiently long relative to its width to contain concentration gradients of soluble constituents as the fluid moves through the basin.

being added at one end and mixed liquor exiting at the opposite end. The flow pattern is quasi-plug-flow, with the residence time distribution depending on the length-to-width ratio of the basin, the mixing produced by the oxygen transfer equipment, and the details of the inlet and outlet."171 As discussed in Section 4.4.1, flow patterns such as these can be modeled as a series of continuous stirred tank reactors (CSTRs). For example, the flow pattern within a bioreactor with a length-to-width ratio of 5:1 can be approximated as three tanks in series. Increasing the bioreactor length-to-width ratio will increase the degree of plug—flow as represented by an increase in the number of equivalent tanks in series. The HRT typically ranges from 4 to 8 hours, while the SRT ranges from 3 to 8 days, but seldom exceeds 15 days. As discussed in Section 7.2.4, the MLSS concentration and composition vary little through the bioreactor because the SRT is long relative to the HRT and the mixed liquor is recycled many times before it is wasted.

Step-Feed Activated Sludge. As covered in Section 7.3.1 and illustrated in Figure 7.10, step-feed activated sludge (SFAS) differs from CAS by having the influent wastewater distributed at several points. Figure 10.2 illustrates ways in which this is typically accomplished in practice. Figure 10.2a depicts a single narrow basin with influent added at various points along its length, while Figure 10.2b shows a series of such basins (each often referred to as a pass), with influent added to each. Many variations of the SFAS process exist," with the fraction of the influent added at each feed point depending on the design objectives. Today SFAS is used primarily to redistribute the activated sludge inventory within the bioreactor, as discussed in Section 7.3.4. Compared to a CAS bioreactor of the same volume, this results in an increased activated sludge inventory, i.e, a longer SRT, and/or a lower concentration of MLSS entering the clarifier. SRTs are generally similar to those used with the CAS process.

Contact Stabilization Activated Sludge. The contact stabilization activated sludge (CSAS) process, discussed in Section 7.4 and illustrated in Figure 7.19, divides the bioreactor into two zones: the contact zone where removal of organic matter contained in the influent wastewater occurs, and the stabilization zone where RAS from the clarifier is aerated to allow stabilization of organic matter. Because of the relatively high MLSS concentration in the stabilization basin (equal to the RAS concentration), a smaller total bioreactor volume (contact plus stabilization) may be used relative to the CAS process, while maintaining the same SRT. As a consequence, CSAS can be used either to reduce the required bioreactor volume or to increase the capacity of an existing CAS facility. Contact zone HRTs are typically 0.5 to 2 hours, while stabilization zone HRTs are typically 4 to 6 hours based on the RAS flow. SRTs are similar to those used with the CAS process.

Completely Mixed Activated Sludge. Completely mixed activated sludge (CMAS) systems, described in Chapters 5 and 6, were developed in the late 1950s to treat high strength industrial wastewaters, particularly those containing inhibitory organic matter.1044 Treatment of such wastewaters in CAS systems is difficult due to the high concentrations of organic matter near the inlet, which can inhibit the biomass, causing poor performance. In contrast, in the CMAS process, influent wastewater is distributed uniformly throughout the bioreactor, maintaining a low concentration of biodegradable organic matter at all points. Thus, even if the organic matter is inhibitory at high concentration, inhibition is avoided, allowing biodégradation to

Influent

RAS (from Clarifier)

ML (to Clarifier)

RAS (from Clarifier)

Influent"

ML (to Clarifier)

Influent

Figure 10.2 Step-feed activated sludge (SFAS) process.

proceed. It was also thought that process stability would be enhanced by maintaining the microorganisms under nearly constant conditions. The HRTs and SRTs for the CMAS process are similar to those used with CAS.

Figure 10.3 illustrates two bioreactor configurations commonly used to achieve completely mixed conditions. The first, in Figure 10.3a, has been used with diffused aeration systems; complete mixing is achieved by distributing the influent along one side of a long, narrow bioreactor, with effluent being taken from the opposite side. In other instances (Figure 10.3b) an essentially square shaped bioreactor is used with influent and effluent positioned to achieve completely mixed conditions. Mechanical surface aeration is typically used with the latter because it provides good overall circulation of basin contents.

Extended Aeration Activated Sludge. Extended aeration activated sludge (EAAS) processes utilize long SRTs to stabilize the biosolids resulting from the

Influent

Influent

Activated Sludge Description

Figure 10.3 Completely mixed activated sludge (CMAS) process.

Figure 10.3 Completely mixed activated sludge (CMAS) process.

removal of biodegradable organic matter. SRTs of 20 to 30 days are typical, which means that HRTs around 24 hours are required to maintain reasonable MLSS concentrations. Long SRTs offer two benefits: reduced quantities of solids to be disposed of. and greater process stability. These benefits are obtained at the expense of the large bioreactors required to achieve the long SRTs, but for many small installations the benefits outweigh the drawbacks.

One of the primary drawbacks of large bioreactors is that they are typically mixing limited. In other words, as discussed in Section 9.3.4, the energy required to meet the oxygen requirement is less than the energy necessary to keep the MLSS in suspension. This limitation is minimized with the closed looped bioreactor, or oxidation ditch, illustrated in Figure 10.4. In this configuration an aeration device is used both to transfer oxygen and to provide motive velocity to the mixed liquor. Solids are maintained in suspension as the mixed liquor circulates around the ditch, typically at a velocity of 0.3 m/sec. Because the mixing energy is conserved in the form of fluid velocity, suspension of solids can be achieved at lower mixing energy inputs than typically required with other basin configurations. A variety of aeration devices can be used in oxidation ditches, including vertical and horizontal mechanical aerators, draft tube aerators, and jet aerators. EAAS designs using more conventional basin configurations are also possible. However, in those cases mixing generally controls the sizing of the oxygen transfer/mixing system, not oxygen requirements.

Aerator (TYP)

Aerator (TYP)

Oxidation Ditch
Figure 10.4 Oxidation ditch activated sludge system.

High-Purity Oxygen Activated Sludge. The high-purity oxygen activated sludge (HPOAS) process was developed in the late 1960s and was widely used during the 1970s and early 1980s.4" As illustrated in Figure 10.5, the bioreactor is staged, enclosed, and provides for cocurrent flow of an oxygen enriched gas phase with the mixed liquor. Influent wastewater and RAS are added only to the first stage, along with oxygen (typically 98% pure). Each stage is a completely mixed cell and three or four are generally used, although as many as six have been utilized. Mixing for solids suspension and oxygen dissolution is provided in each stage. A variety of mechanical devices have been used for this purpose, including slow speed mechanical surface aerators and submerged turbines. The use of high-purity oxygen increases the oxygen partial pressure in the gas of each stage, thereby allowing higher volumetric oxygen transfer rates than are possible with systems using air. Theoretically, the volumetric oxygen transfer rate could be as much as five times higher than the rate with air at atmospheric pressure, but the practical increase is around two to three times. Higher volumetric oxygen transfer rates allow smaller bioreactor volumes to be used, so HRTs are generally in the 2 to 4 hour range. SRTs as low as 1 to 2 days are often used to treat municipal wastewaters, with somewhat longer values being used to treat industrial wastewaters.

Cocurrent flow of mixed liquor and gas results in a gradient in oxygen transfer rates which corresponds to the gradient in process oxygen requirements typical of staged bioreactors, as illustrated in Section 7.2.4. Consequently, oxygen requirements can be effectively met while maintaining high dissolved oxygen concentrations (6

Oxygen Transfer Cocurrent Bioreactors

mg/L or greater) throughout the bioreactor. Cocurrent gas flow also results in efficient use of the applied oxygen; typically over 90% can be transferred into solution. Offgas from the last bioreactor contains unused oxygen, impurities present in the influent oxygen, and carbon dioxide produced by the biological reactions. Retention of carbon dioxide within the system results in depression of the pH; values in the 6.0 to 6.5 range are not unusual unless pH control is practiced. Reduced pH values adversely impact the growth of nitrifying bacteria, as shown in Figure 3.4.

Selector Activated Sludge. The selector activated sludge (SAS) process is a recent development used to control excessive growths of filamentous bacteria,1" '1"'s which can be a nuisance, as discussed in Section 2.3.1. A selector is a portion of an activated sludge system that precedes the main bioreactor, receives the influent wastewater and RAS, and has a high process loading factor. It provides environmental conditions that favor growth of fiocculent microorganisms at the expense of filamentous microorganisms, resulting in improved sludge settleability. Selectors use two mechanisms to accomplish microbial selection: kinetic and metabolic. " s Kinetic selection is achieved by imposing a high process loading factor on the biomass, thereby providing a selective advantage for those microorganisms with the ability to take up readily biodegradable substrate at high rates. Metabolic selection is accomplished by controlling the terminal electron acceptor available within the selector. Aerated selectors, which are considered in this chapter, utilize kinetic selection. Anoxic and anaerobic selectors, which are considered in Chapter 11, use metabolic selection, and may or may not also use kinetic selection mechanisms. A selector is generally a small fraction of the entire bioreactor volume and is often staged, as illustrated in Figure 10.6. It may be constructed as a separate structure, or it may simply be a portion of the bioreactor baffled to provide the necessary process loading factor. The bioreactor downstream of the selector may be either completely mixed or plug-flow, although experience suggests better overall performance with plug-flow.

Sequencing Batch Reactor Activated Sludge. Sequencing batch reactor activated sludge (SBRAS) is another recent development.17 The process, along with its typical operating cycle, is described in Section 7.8.1 and Figure 7.42. Each bioreactor in an SBRAS system is equipped with aeration and mixing equipment and an effluent decanting system." Oxygen transfer may be provided by diffused air (both coarse and fine bubble), jet aerators, mechanical surface aerators (both high and low speed),

or combined diffused air and mechanical mixing. Decanters of various designs have been used.6 Microprocessors control the influent flow, aeration, mixing, and effluent decanting functions. Solids are usually wasted during the idle time to take advantage of the increased solids concentration resulting from the settle period. Multiple bio-reactors are generally provided to allow at least one to always be in the fill mode. This allows treatment to occur on a continuous basis despite the periodic nature of any individual bioreactor. Most systems also make provision for a continuous operational mode to improve flexibility. In this mode, the idle period is eliminated and influent is added to the reactor on a continuous basis throughout the entire cycle, including settle and decant. Although some deterioration in effluent quality will occur, a significant degree of wastewater treatment can still be achieved even though only one bioreactor may be operational.

10.1.3 Comparison of Process Options

Table 10.1 summarizes the benefits and drawbacks of the eight activated sludge process options described above. Conventional activated sludge and CMAS require similarly sized bioreactors and clarifiers, resulting in similar capital and operating costs. However, sludge settleability is generally better for CAS than for CMAS, due to greater growth of filamentous microorganisms in CMAS systems. Completely mixed activated sludge offers the benefit of greater resistance to inhibitory shock loads.

In comparison to CAS and CMAS, SFAS and CSAS offer the advantage of reduced bioreactor volumes and correspondingly lower capital cost. However, the reduced volumes and costs are accompanied by diminished treatment efficiency, particularly for nitrification. Although listed as a drawback, the ability of SFAS and CSAS systems to maintain stable but elevated effluent ammonia-N concentrations, i.e. to partially nitrify, can be a benefit when only partial ammonia-N oxidation is needed.'4

Extended aeration activated sludge systems are simple to design and operate, produce a high quality effluent, and produce reduced quantities of more stabilized solids than other, comparable activated sludge systems. This is achieved at the expense of larger and more expensive bioreactors, increased oxygen requirements, and, in some instances, poor solids settling characteristics.

High-purity oxygen activated sludge offers the benefits of small bioreactor volumes, good resistance to excessive decreases in dissolved oxygen (DO) concentration caused by shock loads of biodegradable organic matter, and minimal air emissions. In contrast, they are mechanically complex and incompatible with situations requiring low process loading factors because then the bioreactor would be mixing, rather than oxygen transfer, limited. In such cases high-purity oxygen becomes unnecessary to meet oxygen requirements. Historically, the use of high-purity oxygen in activated sludge systems was claimed to result in an altered biological process with reduced sludge production rates, reduced oxygen requirements, and improved sludge settle-ability.^ However, more thorough analysis demonstrated that the biological characteristics of HPOAS systems are the same as other activated sludge systems.15*1

Selector activated sludge systems were developed to improve activated sludge settleability, and they have proven capable of doing so."'1"7* Selector activated sludge is a relatively new activated sludge option and experience with and knowledge about

Table 10.1 Comparison of Activated Sludge Process Options

Process Benefits Drawbacks

Table 10.1 Comparison of Activated Sludge Process Options

Process Benefits Drawbacks

Conventional activated

• Performance well characterized and predictable

• Moderate capital and operating costs

sludge (CAS)

• Process and facility design well known

• Moderate sludge settleability

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