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Aerobic Granular Sludge: Formation, microbial communities, and impact on nutrient reduction
Traditional wastewater treatment techniques attempt to exploit natural processes to treat water more rapidly. The purpose for treating wastewater is to limit the impact on the natural environment, most commonly lakes or rivers. Treatment efficiency is quantified using certain parameters that refer to the strength of the waste (BOD, COD, TOC), turbidity caused by solids (TSS), and nutrient build-up (N, P). These pollutants can cause oxygen depletion, limited light penetration, increased algae growth and eutrophication, which negatively impact the environment. While regulations have been in place for BOD, TSS, and ammonia reduction, recent policy changes in many states are pushing wastewater treatment plants (WWTPs) to address a common shortcoming of discharging total nitrogen (TN) and total phosphorus (TP).
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Aerobic granular sludge (AGS) or granular activated sludge is categorized as a ‘self-immobilized microbial consortium’. First reported in 1991, this technology has improved significantly to focus on current biological nutrient reduction (BNR) limitations. The most commonly research and developed aerobic granular sludge has been used for aerobic degradation of organics and nitrogen removal (Liu et al., 2004). Another well-researched aerobic granular sludge was developed in aerobic conditions but consists of aerobic and anoxic zones. The granules are most efficiently developed using a sequencing batch reactor (SBR) (Liu et al., 2004). AGS has shown the capability to treat not only traditional pollutants, but also toxic pollutants from high loading rates (Nancharaiah and Reddy, 2017). Current research includes treatment efficiency, cultivation conditions, granulation factors, and identifying microbial communities present within the granule (Gao et al., 2010, Nancharaiah and Reddy, 2017).
Fig 1. Source: Wilen et al., 2018
To better understand AGS, a comparison to activated sludge is often used because traditionally activated sludge has been the most widely adopted technique implemented for wastewater treatment. Activated sludge is a defined as a flocculated microbial community that floats freely. Similarly, AGS is also defined by these constraints but includes granulation and therefore the microbial populations vary. Two physically separated tanks are used for treatment with AS, an aerated basin, responsible for biological removal of organic carbon and nitrification, and a settling tank where the AS separates from the treated water by flocculation. Disadvantages of implementing AS include low biomass concentrations in the aeration basin and a large footprint requirement for two tanks (aeration and settling). Additionally, enhanced biological nutrient reduction (BNR) is used to remove TP and TN. Supplementary tanks are required to efficiently cultivate the necessary microbial communities further increasing the footprint. Furthermore, the poor settling ability of activated sludge is termed ‘sludge bulking’ and can deteriorate the quality of the final effluent by losing excess sludge with the effluent. Moreover, activated sludge age is difficult to define and can rapidly, without warning, lose efficiency.
Aerobic granular sludge treatment provides a combination of a decreased mandatory footprint, shorter settling time, and on-site cultivation. These factors overcome the common limitations of a conventional activated sludge treatment in terms of a more sustainability and efficient treatment option.
There are defined critical characteristics of aerobic granular sludge to ensure successful wastewater treatment. Firstly, the rapid settling rate depends on the high density and large size of the granules. Next, the formation and preservation of aerobic, anoxic, and anaerobic redox microenvironments. These zones are mandatory for effective organic matter degradation, nitrification, denitrification, and phosphorus removal all in one tank. Additionally, the dependency of biomass retention and control of metabolic reactions on the regulation of dissolved oxygen is a defined critical condition developed during cultivation. Lastly, the metabolic cooperation between autotrophic and heterotrophic microorganisms is pertinent in developing an efficient microenvironment.
Fig 2. Source: Younmei et al., 2014.
To cultivate granules, column reactors are inoculated with activated sludge. Granular formation is achieved by selection-based techniques. Hydrodynamic shear force or up-flow aeration velocities and specific feeding regimes have the largest impact on the end product AGS. The high shear force of up-flow velocity is considered important in forming a dense and stable granule (Tay et al., 2001). Similarly, research has shown an increase production of exopolysacchrides (EPS), cell surface hydrophobicity and specific gravity of AGS with high air velocities (Lochmatter and Holliger, 2014). The shear hydrodynamic force also compacts the surface of the aggregate and aids in shaping the outer surface of the granule by detaching loosely attached microorganisms. A high shear force is believed to promote the formation of slow-growing microorganisms, which is a crucial characteristic of granule formation. However, lower up-flow velocities have been shown to cultivated granulation in combination with a feeding regime of feast-famine (Devlin et al. 2017). This leads to an assumption that AGS formation is a multi-parameter cultivation effort and as seen in Figure 2., has a dramatically different phylogenetic makeup than traditional activated sludge.
Cultivation, Importance of EPS
Extracellular polymeric substances (EPS) are credited with playing a central part in the aggregation of the microorganisms. The EPS matrix also provides stability to the structure of the granule. Using ex situ chemical analysis, In situ visualization using specific fluorophores, and confocal laser scanning microscopy (CLSM), the makeup of the granule was determined to be microbial cells and polysaccharides. The EPS producers comprise one of the most abundant functional groups, approximately 40% on average, while it’s cumulative relative read abundance added up only to approximately 13% in the seed sludge (Szabo et al., 2017). This difference is indicative of the importance of EPS in AGS function and formation.
Cultivation, Formation mechanisms
The method of formation is predominantly unknown but there are several proposed mechanisms. Initially, the aggregation of the microbial communities is significantly important for the success of the treatment. Sludge granulation occurs in a set of four steps: cell-to-cell contact, attractive forces between cells, development of the microbial aggregates with formation of EPS matrix, and granule formation (Wilen et al., 2018). Cellular mechanisms and their characteristics control the cell-to-cell contact. The ratio of protein to polysaccharide ratio is caused by the changes in EPS production and microbial community composition (Guo et al., 2011). This change in ratio is a phenomenon that leads to increased cell hydrophobicity.
Cellular interactions play an important role in granule formation. Quorum sensing has been proposed as one the main types of microbial community interaction. Pseduomas, Aeromonas, and Acinetobacter, known quorum sensing species, were all present in an AGS microbial community investigated by Tan et al., 2014. Moreover, there was a positive correlation between the relative abundance of known quorum sensing molecules (N-acyl-honoserine lactones) and species and the production of EPS.
Fig. 3. Source: Wilen et al, 2018
As shown in Figure 3, there are three microenvironments that provide the ability for effective comprehensive wastewater treatment. The outer layer of the granule has the largest interaction with the outside of the granule and the highest concentration of oxygen. Labeled the aerobic layer in Figure 2, it offers an environment fitting for the microbial communities that perform oxidation of organic matter and nitrification. This outer layer is capable of adequately oxidizing organic matter in the form of chemical oxygen demand (COD) removal by 94.46 ± 3.59% (He et al., 2016). To the interior of the aerobic zone is the anoxic zone with correlating oxygen concentration. The anoxic zone is the microenvironment responsible for denitrification and biological phosphorus removal. Lastly, the core of the granule is termed the anaerobic zone made of EPS and completely devoid of microbial cell density (Nancharaiah and Reddy, 2017). Due to the structural composition of the granule and the stability of the three microenvironments there is a large diffusion gradient of electron donors and acceptors.
Fig. 4. Source: Nancharaiah and Reddy, 2017
Nitrogen removal requires both nitrification of ammonium and denitrification of either nitrate or nitrite. These two microbial metabolisms are done in two different environments; ammonium nitrification is an aerobic process while denitrification is an anoxic process. The presence of both of these environments simultaneously is imperative for effective nitrogen removal. As seen in Figure 4, the granule maintains three microenvironments attributable to microbial diffusion and respiration in the outer region (Nancharaiah and Reddy, 2017). Incorporation of periods with high oxygen and low oxygen flux is vital to accomplishing complete nitrogen removal. Additionally, biomass concentration and retention time play important roles in microbial community growth. There are two ways nitrification and denitrification can occur, simultaneous nitrification denitrification (SND) or alternating nitrification denitrification (AND). SND represents a system where both nitrification and denitrification are executed in the same reactor, while AND was developed to introduce an anoxic phase to encourage more complete TN removal (Nancharaiah and Reddy, 2017). Alternating aeration is additionally credited for promoting functional redundancy within the microbial community. Common nitrifying microorganisms include ammonium-oxidizing bacteria (AOB) such as Nitrosamonas. Nitrite oxidizing bacteria (NOB) were also present, most commonly Nitrospira and Nitrosbacter. (Figure 5d) However, their relative abundance varied depending on the granulation conditions. The most abundant in the granule with the largest diversity was the denitrifying microorganisms in the anoxic zone. With a group relative abundance of over 50% the higher abundancies were from Denitromonas, Meganema, Thauera, Devosia, and Stenotrophomonas, which are interestingly all EPA, producing microorganisms (Szabo et al., 2017).
Lastly, phosphorus removal is represented by phosphate accumulating organisms (PAOs) on the intersection between the oxic/anoxic zone (Figure 4a). Selection for PAOs in important during granulation because they are slow growing and are responsible for the bulk of the phosphorus removal. These bacteria accumulate poly-phosphate intracellularly by using the energy from oxidizing PHAs. The cycle continues as PAOs use their stored poly-P and glycogen as energy (Henriet et al., 2016). The major competition is from glycogen accumulating organisms since they reduce the concentration of glycogen. This competition can be limited by cycling of aeration and a strict feeding-fasting regime.
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AGS is a great solution to an up and coming problem of nutrient reduction for retrofitting current plants with limited footprint. With removal efficiencies in the high 90%, AGS could be the wastewater treatment of the future. However, there is still substantial research that can be done. The microbial mechanisms specifically regarding cell-cell interactions are still mainly unknown. The reliability of efficient cultivation is a main concern and is continuing to be studied. Additionally, excess sludge removal and AGS functionality in low strength wastewater is a concern. Overall, AGS is a promising new wastewater treatment technique for the future.
Fig. 5. Source: Szabo et al., 2017.
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