Mathematical modeling of aerobic granular sludge: A review

Aerobic granulation may play an important role in the field of wastewater treatment due to the advantages of aerobic granules compared to the conventional sludge flocs, such as denser structure, better settleability and ensured solid-effluent separation, higher biomass concentration, and greater ability to withstand shock loadings, which is promising for a full-scale implementation. As an aid for this implementation, mathematical modeling would be an invaluable tool. In this paper, the existing mathematical models available in literature concerning aerobic granule systems are reviewed, including the modeling of the dynamic facets of the aerobic granulation process, the mass transfer and detachment in aerobic granules, the granule-based sequencing batch reactor, the fate of microbial products in granules, and the multi-scale modeling of aerobic granular sludge. An overview of the parameters used in the aerobic granular modeling approaches is also presented. Our growing knowledge on mathematical modeling of aerobic granule might facilitate the engineering and optimization of aerobic granular sludge technology as one of the most promising techniques in the biological wastewater treatment.     

Biomass and porosity profiles in microbial granules used for aerobic wastewater treatment

AIMS: To obtain biomass and porosity profiles for aerobically grown granules of different diameters and to determine a suitable range of granule diameters for application in wastewater treatment. METHODS AND RESULTS: Microbial granules were cultivated in an aerobic granulated sludge reactor with model wastewaters containing acetate, or ethanol plus acetate, or glucose as the main carbon source. Granules were formed by retaining microbial aggregates using a settling time of 2 min. Sampled granules had diameters ranging from 0.45 to 3 mm. Microbial biomass in the granules was detected with the nucleic acid stain SYTO 9 and confocal laser scanning microscopy. The thickness of the microbial biomass layer was proportional to the granule diameter, and had a maximum value of 0.8 mm. The thickness of the microbial biomass layer correlated with the penetration depth of 0.1 microm fluorescent beads into the granule. CONCLUSIONS: The microbial biomass and porosity studies suggest that aerobically grown microbial granules should have diameters less than a critical diameter of 0.5 mm, if deployed for wastewater treatment applications. This critical diameter is based on the assumption that whole granules should have a porous biomass-filled matrix. SIGNIFICANCE AND IMPACT OF THE STUDY: This work could contribute to the development of aerobic granulation technology for effective biological wastewater treatment.     

The effect of hydraulic retention time on the stability of aerobically grown microbial granules

AIMS: The aim of this study is to evaluate the effect of hydraulic retention time (HRT) on the development of aerobically grown microbial granules. METHODS AND RESULTS: Five column-shaped sequential aerobic sludge blanket reactors (SASBRs) were seeded with aerobically grown microbial granules and operated in a cyclic mode at different HRTs. At the shortest HRT of 1 h, the strong hydraulic pressure triggered biomass washout and led to reactor failure. At the longest HRT of 24 h, which represented the weakest hydraulic selection in this study, aerobic granules were gradually substituted by bioflocs because of the lower frequency of volumetric exchange. Within the optimum range of HRTs from 2 to 12 h, however, aerobic granules became stabilized in the presence of adequate hydraulic selection in the reactors, with good mixed liquor volatile suspended solids (MLVSS) retention, high volumetric chemical oxygen demand (COD) removal, low sludge volume index (SVI) values, good effluent quality, low sludge production rate, stronger and more compact structures, high cell hydrophobicity and high ratios of extracellular polysaccharides (PS) to extracellular proteins (PN). CONCLUSIONS: HRTs between 2 and 12 h provided the hydraulic selection pressures favourable for the formation and maintenance of stable aerobic granules with good settleability and activity. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first systematic study on the effect of HRT on heterotrophic aerobic granules. The results of the investigation are useful in understanding how aerobic granules can be applied for wastewater treatment.     

菌藻共生系统对序批式反应器处理养猪废水脱氮除磷效果及微生物群落结构的影响

【背景】养猪废水作为高浓度有机废水,是导致我国农业面源污染的主要因素之一。目前采用菌藻共生系统处理养猪废水越来越受到关注,与传统序批式反应器(Sequencing Batch Reactor,SBR)相比,藻辅助SBR具有提高脱氮除磷效果、增加污泥活性和降低能源消耗的特点。【目的】针对SBR中菌藻共生系统对养猪废水脱氮除磷效能的影响,比较分析菌藻共生系统与常规SBR系统中污泥特性及微生物群落结构特征差异。【方法】在室温条件下分别平行运行SBR+微藻(R1)和作为对照系统不添加微藻的SBR(R2)。监测R1和R2系统废水处理效果,污泥的粒径、沉降性和代谢产物等污泥特性。利用变性梯度凝胶电泳(Denaturing Gradient Gel Electrophoresis,DGGE)技术分析R1和R2系统中的微生物种类和分布。【结果】与对照R2反应器相比,R1的化学需氧量(Chemical Oxygen Demand,COD)去除率提高了5.1%,NH4+-N提高了20.3%,总氮(Total Nitrogen,TN)提高了19.4%,总磷(Total Phosphorus,TP)提高了23.9%。进一步对反应器中的污泥特性进行分析发现,与R2相比,R1的胞外聚合物(ExtracellularPolymericSubstances,EPS)平均含量提高3.7%,可溶性微生物产物(Soluble MicrobialProduct,SMP)平均增加了38.5%。同时R1的污泥粒径较R2提高了14.8%,污泥体积指数(Sludge Volume Index,SVI)值较R2降低了11.7%,污泥的好氧呼吸速率(Specific Oxygen Uptake Rate,SOUR)较R2提高了64.8%,而且稳定的菌藻共生系统的形成进一步减少反应器出水中的悬浮固体浓度,表明藻类的添加对R1污泥特性具有改良作用【结论】R1反应器形成的菌藻共生体系可进一步优化微生物群落结构,其中放线菌纲(Actinobacteria)、α-变形菌纲(Alphaproteobacteria)和γ-变形菌纲(Gammaproteobacteria)为R1反应器的主要菌群,对养猪废水的处理起到重要作用。R1反应器中的藻类主要为链带藻属(Desmodesmus)和尖带藻属(Acutodesmus),对养猪废水的脱氮除磷起到重要作用。     

电场刺激技术在微生物工程中的应用

电场刺激是利用电解法将惰性电极插入电解液中,形成一种电解池系统,电解液对细胞培养物会产生不同影响,并导致生物过程发生改变。电场刺激对微生物会产生促进和杀灭两种不同作用,并在微生物工程中已有所应用,如促进微生物生长和代谢、强化废水处理、进行生物修复和用于杀菌消毒等。研究表明电场刺激在微生物工程和环境生物工程等方面有广泛的应用前景。     

保藏温度对厌氧氨氧化颗粒污泥特性的影响

为考察保藏温度对厌氧氨氧化污泥颗粒特性的影响,同时优化保藏厌氧氨氧化颗粒污泥温度参数,本试验首先通过HRT调控进水基质负荷培养厌氧氨氧化颗粒污泥,并采用KHCO3和Na HCO3交替提供无机碳源。然后分别在–40℃、4℃、(27±4)℃室温和35℃条件下避光保藏。结果表明,Na HCO3可代替KHCO3作为厌氧氨氧化菌生长的无机碳源。相比于其他保藏温度,4℃保藏能够较好地维持生物量和生物活性,同时能较好地维持颗粒污泥的沉降性能、颗粒污泥和细胞结构完整性。在保藏过程中,一阶衰减指数模型可拟合厌氧氨氧化颗粒污泥生物量及活性的衰减过程,衰减指数与胞溶程度正相关,而且生物量的衰减比活性的衰减更快。同时,颗粒污泥胞外聚合物中蛋白质与多糖的比值(PN/PS)和血红素不能有效指示保藏过程中颗粒污泥沉降性能和活性的变化,而生物活性与胞溶程度呈负相关。     

Unmasking photogranulation in decreasing glacial albedo and net autotrophic wastewater treatment

In both natural and built environments, microbes on occasions manifest in spherical aggregates instead of substratum-affixed biofilms. These microbial aggregates are conventionally referred to as granules. Cryoconites are mineral rich granules that appear on glacier surfaces and are linked with expanding surface darkening, thus decreasing albedo, and enhanced melt. The oxygenic photogranules (OPGs) are organic rich granules that grow in wastewater, which enables wastewater treatment with photosynthetically produced oxygen and which presents potential for net autotrophic wastewater treatment in a compact system. Despite obvious differences inherent in the two, cryoconite and OPG pose striking resemblance. In both, the order Oscillatoriales in Cyanobacteria envelope inner materials and develop dense spheroidal aggregates. We explore the mechanism of photogranulation on account of high similarity between cryoconites and OPGs. We contend that there is no universal external cause for photogranulation. However, cryoconites and OPGs, as well as their intravariations, which are all under different stress fields, are the outcome of universal physiological processes of the Oscillatoriales interfacing with goldilocks interactions of stresses. Finding the rules of photogranulation may enhance engineering of glacier and wastewater systems to manipulate their ecosystem impacts.     

Starter culture of <Emphasis Type="Italic">Pseudomonas veronii</Emphasis> strain B for aerobic granulation

Aggregation of bacterial cells is used in formation of microbial granules. Aerobically grown microbial granules can be used as the bio-agents in the treatment of wastewater. However, there are problems with start up of microbial granulation and biosafety of this process. Aim of this research was selection and testing of safe microbial strain with high cell aggregation ability to shorten period of microbial granules formation. Five bacterial strains with cell aggregation index higher than 50% have been isolated from the granules. Strain of Pseudomonas veronii species was considered as most probably safe starter culture for granulation because other strains belonged to the species known as human pathogens. The microbial granules were formed after 3 days of cultivation in case when P. veronii strain B was applied to start-up aerobic granulation process using model wastewater. The granules were produced from activated sludge after 9 days of cultivation. Microbial aggregates produced from starter culture of P. veronii strain B were more compact (sludge volume index was 70 ml/g) than those produced from activated sludge (sludge volume index was 106 ml/g). It is a first proof that application of selected safe starter pure culture with high cell aggregation ability can accelerate and enhance formation of microbial granules.     

Specific layers in aerobically grown microbial granules

AIMS: To determine the optimal size of aerobically grown granules for wastewater treatment by measuring specific layers within the granules. METHODS AND RESULTS: A variety of biological layers were detected by oligonucleotide probes, specific fluorochromes, and fluorescent microspheres. The channels in the granule matrix penetrated to depths of 900 microm. A layer of obligate anaerobic bacteria was detected at a depth of 800 microm below the granule surface. Dead cells were also observed in the granule interior. CONCLUSIONS: Aerobically grown granules contained layers of aerobic and anaerobic micro-organisms. SIGNIFICANCE AND IMPACT OF THE STUDY: The optimal diameter of the aerobic granule is less than 1600 microm. This is twice the distance from the granule surface to the anaerobic layer. This approach can be used to optimize the thickness of other microbial aggregates such as flocs, colonies and biofilms.     

Bioremediation of wastewaters with recalcitrant organic compounds and metals by aerobic granules

Compared to activated sludge flocs, aerobic granules have a regular shape, and a compact and dense structure which enhances settleability, higher biomass retention, multi-microbial functions, higher tolerance to toxicity, greater tolerance to shock loading, and relatively low excess sludge production. The potential for improved process efficiency and cost-effectiveness can be attractive when it is applied to both municipal and industrial wastewaters. This review discusses potential applications of aerobic granulation technology in wastewater treatment while drawing attention to relevant findings such as diffusion gradients existing in aerobic granules which help the biomass cope with inhibitory compounds and the ability of granules to continue degradation of inhibitory compounds at extreme acid and alkaline pHs.     

Characteristics of aerobic granular sludge in a sequencing batch reactor with variable aeration

Aerobic granules can be used for the treatment of industrial or municipal wastewater, but high aeration rate is required for the stable operation of the granular sludge system. Therefore, the aim of this research was to reduce aeration rate greatly to decrease the energy consumption for the technology of aerobic granules. Based on the characteristics of sequencing batch reactor with distinct feast and famine periods, aeration rate was reduced from 1.66 to 0.55 cm s⁻¹ in the famine period after granules were formed. It was found that the settleability of aerobic granules in reactor R1 with reduced aeration was the same as that of aerobic granules in reactor R2 with constant aeration rate of 1.66 cm s⁻¹. However, the outer morphology of aerobic granules gradually changed from round shape to long shape, and minor population showed certain shift after aeration rate was reduced in the famine period. Since good settleability is the most essential feature of aerobic granules, it can be said that reducing aeration rate in famine period did not influence the stable operation of aerobic granular sludge system. Furthermore, the experimental results indicated that aeration rate in feast period was much more important to the stable operation of aerobic granules than that in famine period.     

Understanding the properties of aerobic sludge granules as hydrogels

Aerobic sludge granules are larger, denser microbial aggregates than activated sludge flocs with a smoother and more regular surface, which facilitates greater wastewater treatment intensity. Factors important in their growth are still poorly understood, which is an impediment to the construction and operation of full-scale aerobic sludge granule processes. Data in this article obtained with granules treating an abattoir wastewater provide evidence that aerobic sludge granules are hydrogels. The results also demonstrate a method for characterizing macromolecular associations. The rheological profile of these granules was found to be analogous with that of typical polymer gels. Water uptake or swelling reflects an equilibrium between granule elastic modulus and osmotic pressure, whereby uptake is increased by reducing solute concentration or the elastic modulus. A weakening of the extracellular polymeric substance (EPS) matrix as demonstrated with mechanical spectroscopy was induced by several environmental factors including temperature, pH and ionic strength. Uniform and elastic deformation was observed at low strain. Enzymatic degradation studies indicate that proteins and alpha-polysaccharides were the major granule structural materials. The aerobic sludge granules in the current study were therefore protein-polysaccharide composite physical hydrogels. While aerobic sludge granules treating an abattoir wastewater are used as a case study, many of the fundamental principles detailed here are relevant to other granulation processes. The paradigm established in this study can potentially be applied to better understand the formation of aerobic sludge granules and thus overcome a hurdle in the acceptance of aerobic sludge granulation as an alternative to more traditional wastewater treatment processes.     

Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review

A review concerning the definition, extraction, characterization, production and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment reactors is given in this paper. EPS are a complex high-molecular-weight mixture of polymers excreted by microorganisms, produced from cell lysis and adsorbed organic matter from wastewater. They are a major component in microbial aggregates for keeping them together in a three-dimensional matrix. Their characteristics (e.g., adsorption abilities, biodegradability and hydrophilicity/hydrophobicity) and the contents of the main components (e.g., carbohydrates, proteins, humic substances and nucleic acids) in EPS are found to crucially affect the properties of microbial aggregates, such as mass transfer, surface characteristics, adsorption ability, stability, the formation of microbial aggregates etc. However, as EPS are very complex, the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.     

The influence of short-term starvation on aerobic granules

Evidence shows that almost all aerobic granules can only be cultivated in sequencing batch reactor (SBR). Compared to continuous process, the unique feature of SBR is its cycle operation, which results in a periodical starvation in the reactor. So far, the effect of such a periodical starvation on aerobic granulation process remains unknown. Thus, this study investigated the responses of aerobic granules to the respective carbon-, nitrogen-, phosphorus-, potassium-starvation and also their collective effects in terms of cell surface hydrophobicity, surface zeta potential, extracelluar polysaccharides content, specific oxygen utilization rate and biomass growth. Results showed that short-term C-, N-, P- and K- starvations would pose negative effects on aerobic granules, e.g. reduce EPS content, inhibit microbial activity, weaken structural integrity and worsen settleability of aerobic granules. This study likely provides primary evidence that the substrate and nutrients starvation would not contribute to the stability of aerobic granules in a significant way.     

群体感应在生物强化功能菌定殖及降解能力增强中的作用研究进展

由于难降解有机污染物和外界环境对水处理系统的冲击干扰,污水水质常出现不达标现象。引入外源含有相关功能基因并且具有基因水平转移能力的工程菌株进行生物强化处理是提高污水处理效能的有效措施。污水处理系统中存在能够分泌信号分子的菌体,菌间具有群体感应现象,当种群密度达到感应阈值时,菌体会通过释放信号分子来触发一些群体行为,从而激活相关基因的表达(如生物膜形成、生物发光、抗生素合成和毒力因子表达等)。早期的群体感应技术研究主要集中在信号传递学、微生物社会行为学和医学微生物领域,近年来,在水处理领域也开始有相继报道,研究表明群体感应在污水生物处理中发挥重要作用,并且影响生物强化菌株的定殖和污染物降解,因此群体感应行为调控是生物强化技术成效显著与否的关键因素。本文综述了群体感应及信号分子的作用机制、信号分子释放及存在的影响因素以及群体感应对菌株定殖、微生物群落结构和污染物去除的影响,并对从群体感应角度出发研究生物强化过程进行了展望,旨在为生物强化技术的有效实施及提升污水处理效能提供一种新思路,为深入理解生物强化过程中群体感应调控行为提供理论参考。     

The physical characteristics of anaerobic granular sludges in relation to their internal architecture

A range of granular sludges was taken from industrial anaerobic sludge blanket reactors treating a wide variety of wastewaters and a comparison was made between the polymers which were extractable from the granules and their internal structures. The study of the internal structure, using sequential staining of ultra-thin sections, showed the complexity of granular sludges. Much of the area was occupied by Gram-negative cells and the area which stained positive for protein was found to increase nearer the centre of the granules. This was accompanied by a decrease in the carbohydrate positive areas. Positive areas for lipid were widespread throughout the granules. Changes in the internal structure were observed when the type of wastewater treated by the granules was changed and a comparison between sludges treating the same type of wastewater showed that factors other than the nature of the substrate must be considered as parameters which will affect the structure of the granules. Although an appreciable variation in the granule strengths was noted, it was not possible to relate these differences, on an overall basis, to either the internal structure or the chemical composition of the extracted polymers. However, an examination of data for granules produced during the treatment of nominally similar wastes did suggest that there would be a relationship between polymer composition and granule strength in these cases.     

Microbial Community Profiles in Wastewaters from Onsite Wastewater Treatment Systems Technology

The aim of the study was to determine the potential of community-level physiological profiles (CLPPs) methodology as an assay for characterization of the metabolic diversity of wastewater samples and to link the metabolic diversity patterns to efficiency of select onsite biological wastewater facilities. Metabolic fingerprints obtained from the selected samples were used to understand functional diversity implied by the carbon substrate shifts. Three different biological facilities of onsite wastewater treatment were evaluated: fixed bed reactor (technology A), trickling filter/biofilter system (technology B), and aerated filter system (the fluidized bed reactor, technology C). High similarities of the microbial community functional structures were found among the samples from the three onsite wastewater treatment plants (WWTPs), as shown by the diversity indices. Principal components analysis (PCA) showed that the diversity and CLPPs of microbial communities depended on the working efficiency of the wastewater treatment technologies. This study provided an overall picture of microbial community functional structures of investigated samples in WWTPs and discerned the linkages between microbial communities and technologies of onsite WWTPs used. The results obtained confirmed that metabolic profiles could be used to monitor treatment processes as valuable biological indicators of onsite wastewater treatment technologies efficiency. This is the first step toward understanding relations of technology types with microbial community patterns in raw and treated wastewaters.     

Stability of aerobic granules during long-term bioreactor operation

Aerobic granular sludge technology has been extensively studied over the past 20 years and is regarded as the upcoming new standard for biological treatment of domestic and industrial wastewaters. Aerobic granules (AG) are dense, compact, self-immobilized microbial aggregates that allow better sludge-water separation and thereby higher biomass concentrations in the bioreactor than conventional activated sludge aggregates. This brings potential practical advantages in terms of investment cost, energy consumption and footprint. Yet, despite the relevant advances regarding the process of AG formation, instability of AG during long-term operation is still seen as a major barrier for a broad practical application of this technology. This paper presents an up-to-date review of the literature focusing on AG stability, aiming to contribute to the identification of key factors for promoting long-term stability of AG and to a better understanding of the underlying mechanisms. Operational conditions leading to AG disintegration are described, including high organic loads, particulate substrates in the influent, toxic feed components, aerobic feeding and too short famine periods. These operational and influent wastewater composition conditions were shown to influence the micro-environment of AG, consequently affecting their stability. Granule stability is generally favored by the presence of a dense core, with microbial growth throughout the AG depth being a crucial intrinsic factor determining its structural integrity. Accordingly, possible practical solutions to improve granule long-term stability are described, namely through the promotion of minimal substrate concentration gradients and control of microbial growth rates within AG, including anaerobic, plug-flow feeding and specific sludge removal strategies.     

Long-term storage and subsequent reactivation of aerobic granules

This study investigated a seven month storage and the subsequent reactivation of aerobic granules. The granule size and structure integrity were remained during storage, whereas some cavities and pleats appeared on the surface and further deteriorated the settleability. Along with the reactivation, the physical characteristics and microbial activities of aerobic granules were gradually improved. Activities of heterotrophs and nitrifiers can be fully recovered within 16days and 11days, respectively. Nitrifiers decayed slower during storage and reinstated rapider during reactivation than heterotrophs. In fresh aerobic granules, the dominated ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were Nitrosomonas and Nitrospira, respectively. During storage, the initially dominated populations decayed rapider than the initially less dominated ones. Extracellular polymeric substances (EPS) significantly decreased within the first month, and then gradually accumulated during the last six months storage. Accumulation of EPS was an effective strategy for maintaining structural integrity of aerobic granules during long-term storage.     

Growth and storage processes in aerobic granules grown on soybean wastewater

Aerobic granules in a sequencing batch reactor (SBR) are subjected to alternative feast and famine conditions, and are able to take up carbon substrate in wastewater rapidly and to store it as intracellular storage products when the substrate is in excess. This phenomenon could not be described by the widely used activated sludge model No.3 (ASM3). In this work, taking adsorption process, microbial maintenance, and substrate diffusion into account, the simultaneous growth and storage processes occurring in an aerobic-granule-based SBR are investigated with experimental and modeling approaches. A new model is established and successfully validated with the experimental results of an SBR fed with soybean-processing wastewater. Simulation results show that our approach is appropriate for elucidating the fates of major model components. Comparison between ASM3 and the model established in this work demonstrates that the latter is better to describe the substrate removal mechanisms and simultaneous growth and storage processes in aerobic granules.