Biological Nutrient Removal

Biological nutrient removal (BNR) processes are modifications of the activated sludge process that incorporate anoxic and/or anaerobic zones to provide nitrogen and/or phosphorus removal. Many BNR variants have been developed, representing a wide range of nutrient removal capabilities. This chapter presents the basic design and operational principles for several of them. It builds upon the theoretical concepts presented in Chapters 2, 3, 6, and 7.

11.1 PROCESS DESCRIPTION 11.1.1 Historical Overview

The system components (aerobic and anoxic zones) that form the basis for biological nitrogen removal were developed in the 1960s, resulting in a number of approaches.*4'"'''* One used a series of separate suspended growth systems to accomplish removal of organic matter and nitrogen in a step-wise fashion. Organic matter was removed in the first step, nitrification was accomplished in the second, and denitrification was achieved in a third. These three-stage nitrogen removal systems were discussed extensively in the literature, but received little full-scale use due to high capital and operating costs/7 Another approach, referred to as single-sludge nitrogen removal, incorporated both aerobic zones for nitrification and anoxic zones for denitrification in a single system, with carbon oxidation occurring in both zones. Among the concepts incorporated into the single-sludge approach were the recirculation of nitrate-N to an initial anoxic zone to allow use of the readily biodegradable substrate for denitrification, as discussed in Section 7.5, and the use of a second anoxic zone for additional denitrification using slowly biodegradable substrate and biomass decay, as discussed in Section 7.6. These concepts are now widely used in biological nitrogen removal.

Concurrently with development of nitrogen removal systems, enhanced phosphorus removal was observed in certain full-scale activated sludge systems.*4'* * These systems generally used plug-flow bioreactors with uniform aeration along their length, resulting in very low dissolved oxygen (DO) concentrations in the initial sections. We now know that these inadequately aerated sections provided the anaerobic zone necessary for selection of the phosphorus accumulating organisms (PAOs) required for biological phosphorus removal, as discussed in Sections 2.4.6 and 3.7. Nevertheless, controversy existed for more than a decade concerning the phosphorus removal mechanisms operating in these plants.41* While some chemical phosphorus removal will occur in systems with anaerobic and anoxic zones," it is now recognized that biological mechanisms are responsible for most of the phosphorus removal. In spite of the controversy concerning the removal mechanisms, research during the 1960s resulted in the first commercial biological phosphorus removal (BPR) process, the Phostrip® process.4"41

With this background, the stage was set for the integration and refinement of the basic concepts into the single sludge biological nitrogen and phosphorus removal processes that we know today. The initial major strides were provided by Barnard," '" who made two significant conceptual leaps forward. The first was the integration of aerobic and anoxic zones, along with nitrate recirculation, to create the effective and cost competitive single-sludge nitrogen removal system now known as the four-stage Bardenpho process. The second was the observation that biological phosphorus removal would occur in these systems if nitrate was sufficiently depleted in the initial anoxic zone. Comparing this observation with the operating conditions in the full-scale plants that achieved enhanced phosphorus removal, Barnard added an initial anaerobic zone to his nitrogen removal system to obtain the five-stage Bardenpho process, which removes both nitrogen and phosphorus. Since that time a great deal has been discovered about the mechanisms, microbiology, stoichiometry, and kinetics of BNR systems, and many process variants have been developed. Consequently, our understanding is sufficient to allow the design and operation of facilities that achieve reliable and predictable results.

11.1.2 General Description

BNR systems are modifications of the basic activated sludge process described in Chapter 10 and incorporate the four features common to them: (1) a flocculent slurry of microorganisms, (2) quiescent sedimentation, (3) settled solids recycle, and (4) solids retention time (SRT) control. In addition, the bioreactor of a BNR system is divided into anaerobic (ANA), anoxic (ANX), and aerobic zones (AER), with provision for mixed liquor recirculation (MLR), as illustrated in Figure 11.1. These zones are distinguished by the terminal electron acceptor utilized. In aerobic zones, oxygen is the electron acceptor; in anoxic zones, nitrate-N is the electron acceptor; and in anaerobic zones, neither oxygen nor nitrate-N is present. The division of the bioreactor to provide these alternative biochemical environments is the distinguishing

ANA - Anaerobic AER - Aerobic

ANX - Anoxic MLR - Mixed Liquor Recirculation

Figure 11.1 Single-sludge biological nutrient removal process.

ANA - Anaerobic AER - Aerobic

ANX - Anoxic MLR - Mixed Liquor Recirculation

Figure 11.1 Single-sludge biological nutrient removal process.

feature of a BNR system. The aerobic zone is a necessary component of all BNR systems, while the anaerobic zone is necessary to accomplish phosphorus removal, and the anoxic zone is necessary for nitrogen removal.

Nitrogen removal occurs through the processes of nitrification and denitrifica-tion, as discussed in Chapters 2, 3, 6, and 7. Nitrification is an aerobic process and, consequently, will occur only in aerobic zones. Denitrification is the conversion of nitrate-N to nitrogen gas by heterotrophic bacteria that utilize nitrate-N as their terminal electron acceptor as they oxidize organic matter in the absence of dissolved oxygen, and thus it occurs in anoxic zones. The denitrification rate in the first anoxic zone is relatively rapid because the bacteria use readily biodegradable substrate added by the influent wastewater as the electron donor. Denitrification in the second anoxic zone is much slower because exogenous substrate concentrations are normally low due to their oxidation in the upstream anoxic and aerobic zones. Consequently, endogenous substrates must be used as electron donors, although some slowly biodegradable substrate may be available. The primary function of the final aerobic zone is stripping of nitrogen gas generated in the preceding anoxic zone and the addition of oxygen prior to passage of the mixed liquor suspended solids (MLSS) to the clarifier.

The incorporation of anoxic zones impacts the microbial ecology of the biomass. In nitrogen removal systems, the initial anoxic zone functions as an anoxic selector to minimize the growth of filamentous bacteria through metabolic selection, as mentioned in Section 10.2.1. Most filamentous bacteria are not capable of utilizing nitrate-N as an electron acceptor, whereas many floc-forming bacteria can.1 Addition of the wastewater to an anoxic zone allows denitrifying bacteria to metabolize a portion of the readily biodegradable substrate and reduce the amount that passes into the aerobic zone, where it could be used by the filamentous bacteria. This limits the size of the filament population. In fact, anoxic zones have been incorporated into some nitrifying activated sludge systems simply to control filament growth. Denitrification also results in alkalinity production, which can partially offset the alkalinity consumed in nitrification.57

Biological phosphorus removal is accomplished by creating conditions favorable for the growth of PAOs, causing the activated sludge community to become enriched in them. As discussed in detail in Sections 2.4.6 and 3.7, and illustrated in Figure 11.2, the anaerobic zone provides the selective advantage for the PAOs by allowing them to grow at the expense of other heterotrophic bacteria. Because oxygen and nitrate-N are absent, oxidation of organic matter cannot occur in the time provided, making it impossible for most species of heterotrophic bacteria to transport and store or metabolize organic matter. Rather, they only carry out fermentation reactions, resulting in the formation of volatile fatty acids (VFAs). Phosphorus accumulating organisms are able to transport VFAs into the cell and store them as polyhydroxyalkanoic acids (PHAs) and other carbon storage polymers, using energy from the cleavage of intracellular polyphosphate, releasing inorganic phosphate. The VFAs are then unavailable to the other heterotrophic bacteria when the mixed liquor flows into the aerobic zone. Rather, the stored substrate is used exclusively by the PAOs for growth and to provide energy for reforming polyphosphate from inorganic phosphate in the wastewater. Only the slowly biodegradable substrate is available to the other heterotrophs. As a consequence, PAOs become a significant part of the community. Because of its role in microbial selection, the anaerobic zone is referred

Metabolism Paos
Figure 11.2 Relationship between phosphorus and organic matter metabolism in the anaerobic and aerobic zones of a BPR process.

to as an anaerobic selector.' Since the PAOs generally grow in a flocculent rather than a filamentous form, anaerobic selectors have also been used to control filamentous sludge bulking, providing another method of metabolic selection.

The enrichment of the biomass with PAOs, which contain a high concentration of polyphosphate at the end of the aerobic zone, provides the mechanism by which phosphorus is removed from the wastewater. The phosphorus content of a typical activated sludge is on the order of 1.5 to 2% (expressed on the basis of phosphorus to volatile suspended solids in the mixed liquor, or P/VSS), whereas when PAOs are present the P/VSS ratio will typically be increased to the 5 to 7% range, with values as high as 12 to 15% sometimes observed. Referring to Figure 11.1, phosphorus inputs to and outputs from the process include the influent, the treated effluent, and the waste activated sludge. By increasing the mass of phosphorus in the waste activated sludge (WAS), the mass in the treated effluent must be decreased in order to maintain a phosphorus mass balance.

The composition of the wastewater influences the reactions in the anaerobic zone, and hence its design. The readily biodegradable substrate in domestic wastewaters is a mixture of VFAs and other small biogenic organic compounds, with the proportions depending on the amount of fermentation that has occurred in the sewer. Generally, PAOs can only transport and store the short chain VFAs, acetic and propionic acid, in the anaerobic zone. However, the facultative bacteria will ferment the other readily biodegradable organic matter to produce VFAs, which can subsequently be utilized by the PAOs. Uptake of VFAs by the PAOs is relatively rapid, while fermentation is slower. Consequently, fermentation will be the rate limiting reaction in the anaerobic zone if only a small proportion of the readily biodegradable organic matter in the influent wastewater is present as VFAs. Thus, the size of the anaerobic-zone in a phosphorus removal system will be influenced by the wastewater composition.

As demonstrated by the simulations of Section 7.7, nitrification adversely impacts biological phosphorus removal if nitrate-N is recycled to the anaerobic zone. There are two reasons for this: (l) denitrifying bacteria compete directly with the PAOs for readily biodegradable substrate, and (2) less formation of VFAs occurs by fermentation. Both reduce the selective advantage for PAOs. As a consequence, fewer PAOs are grown, the phosphorus content of the mixed liquor is reduced, and phosphorus removal is diminished.

Table 11.1 summarizes the biochemical transformations occurring in the various zones of a BNR process. It also presents the functions that these zones provide, as well as which zones are required to remove each nutrient. A key point is that an aerobic zone is required in all BNR systems. It is required for nitrogen removal because nitrifying bacteria are obligate aerobes. It is required for phosphorus removal because the stored and exogenous organic matter must be oxidized by the PAOs in an aerobic environment to generate the energy required for growth. This table may be used to understand the relative roles and the interactions between the various zones in BNR processes.

Because BNR processes are variations of the activated sludge process, they are constructed using the same materials and equipment components. The major differences are: division of the bioreactor into anaerobic, anoxic, and aerobic zones; provision of mixed liquor recirculation pumping facilities; and provision of mixing equipment in the anaerobic and anoxic zones to maintain solids in suspension while minimizing oxygen transfer. Figure 11.3 illustrates two types of mixers often used. Further discussion of the physical facilities is provided elsewhere/4 7

11.1.3 Process Options

Biological nutrient removal systems may be categorized according to their nutrient removal capabilities as nitrogen removal processes, phosphorus removal processes, and systems that remove both nitrogen and phosphorus.

Table 11.1 Summary of Biological Nutrient Removal Process Zones

Zone

Biochemical transformations

Functions

Zone required for

Anaerobic

• Uptake and storage of VFAs by PAOs

• Fermentation of readily biodegradable organic matter by heterotrophic bacteria

• Phosphorus release

■ Selection of PAOs

• Phosphorus removal

Anoxic

• Denilriticalion

• Conversion of NOs-N

• Nitrogen removal

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    How to measure the oksigen requirement in biological nutrient removal?
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