Dear Muhammad,

The following text describes the differences between activated sludge flocs and granules and the methods used to characterize them:

Aerobic activated sludge granules are dense, spherical biofilms which can strongly improve purification efficiency and sludge settling in wastewater treatment processes. In this study, the structure and development of different granule types were analyzed. Biofilm samples originated from lab-scale sequencing batch reactors which were operated with malthouse, brewery, and artificial wastewater. Scanning electron microscopy, light microscopy, and confocal laser scanning microscopy together with fluorescence in situ hybridization (FISH) allowed insights into the structure of these biofilms. Microscopic observation revealed that granules consist of bacteria, extracellular polymeric substances (EPS), protozoa and, in some cases, fungi. The biofilm development, starting from an activated sludge floc up to a mature granule, follows three phases. During phase 1, stalked ciliated protozoa of the subclass Peritrichia, e.g., Epistylis spp., settle on activated sludge flocs and build tree-like colonies. The stalks are subsequently colonized by bacteria. During phase 2, the ciliates become completely overgrown by bacteria and die. Thereby, the cellular remnants of ciliates act like a backbone for granule formation. During phase 3, smooth, compact granules are formed which serve as a new substratum for unstalked ciliate swarmers settling on granule surfaces. These mature granules comprise a dense core zone containing bacterial cells and EPS and a loosely structured fringe zone consisting of either ciliates and bacteria or fungi and bacteria. Since granules can grow to a size of up to several millimeters in diameter, we developed and applied a modified FISH protocol for the study of cryosectioned biofilms. This protocol allows the simultaneous detection of bacteria, ciliates, and fungi in and on granules.

To view the full publication, please use the following link:

http://aem.asm.org/content/73/19/6233.full

You may also use the following links for more information regarding this topic:

Appl Microbiol Biotechnol. 2013 Aug;97(16):7447-58. doi: 10.1007/s00253-012-4472-7. Epub 2012 Oct 14.

Microbial diversity differences within aerobic granular sludge and activated sludge flocs.

Winkler MK1, Kleerebezem R, de Bruin LM, Verheijen PJ, Abbas B, Habermacher J, van Loosdrecht MC.

Author information

Abstract

In this study, we investigated during 400 days the microbial community variations as observed from 16S DNA gene DGGE banding patterns from an aerobic granular sludge pilot plant as well as the from a full-scale activated sludge treatment plant in Epe, the Netherlands. Both plants obtained the same wastewater and had the same relative hydraulic variations and run stable over time. For the total bacterial population, a similarity analysis was conducted showing that the community composition of both sludge types was very dissimilar. Despite this difference, general bacterial population of both systems had on average comparable species richness, entropy, and evenness, suggesting that different bacteria were sharing the same functionality. Moreover, multi-dimensional scaling analysis revealed that the microbial populations of the flocculent sludge system moved closely around the initial population, whereas the bacterial population in the aerobic granular sludge moved away from its initial population representing a permanent change. In addition, the ammonium-oxidizing community of both sludge systems was studied in detail showing more unevenness than the general bacterial community. Nitrosomonas was the dominant AOB in flocculent sludge, whereas in granular sludge, Nitrosomonas and Nitrosospira were present in equal amounts. A correlation analysis of process data and microbial data from DGGE gels showed that the microbial diversity shift in ammonium-oxidizing bacteria clearly correlated with fluctuations in temperature.

http://www.ncbi.nlm.nih.gov/pubmed/23064482

https://mediatum.ub.tum.de/doc/601101/601101.pdf

Composition and Distribution of Extracellular Polymeric Substances in Aerobic Flocs and Granular Sludge

B. S. McSwain1,2,*,

R. L. Irvine1,

M. Hausner2 and

P. A. Wilderer2

+Author Affiliations

1Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana

2Institute of Water Quality Control and Waste Management, Technical University of Munich, Garching, Germany

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ABSTRACT

Extracellular polymeric substances (EPS) were quantified in flocculent and aerobic granular sludge developed in two sequencing batch reactors with the same shear force but different settling times. Several EPS extraction methods were compared to investigate how different methods affect EPS chemical characterization, and fluorescent stains were used to visualize EPS in intact samples and 20-μm cryosections. Reactor 1 (operated with a 10-min settle) enriched predominantly flocculent sludge with a sludge volume index (SVI) of 120 ± 12 ml g−1, and reactor 2 (2-min settle time) formed compact aerobic granules with an SVI of 50 ± 2 ml g−1. EPS extraction by using a cation-exchange resin showed that proteins were more dominant than polysaccharides in all samples, and the protein content was 50% more in granular EPS than flocculent EPS. NaOH and heat extraction produced a higher protein and polysaccharide content from cell lysis. In situ EPS staining of granules showed that cells and polysaccharides were localized to the outer edge of granules, whereas the center was comprised mostly of proteins. These observations confirm the chemical extraction data and indicate that granule formation and stability are dependent on a noncellular, protein core. The comparison of EPS methods explains how significant cell lysis and contamination by dead biomass leads to different and opposing conclusions.

http://aem.asm.org/content/71/2/1051.full

Granule Formation Mechanisms within an Aerobic Wastewater System for Phosphorus Removal▿†

Jeremy J. Barr1,2,*,

Andrew E. Cook1 and

Phillip L. Bond1,2,*

+Author Affiliations

1University of Queensland, Advanced Water Management Centre (AWMC), QLD 4072, Australia

2Environmental Biotechnology Cooperative Research Centre, Sydney, New South Wales, Australia

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ABSTRACT

Granular sludge is a novel alternative for the treatment of wastewater and offers numerous operational and economic advantages over conventional floccular-sludge systems. The majority of research on granular sludge has focused on optimization of engineering aspects relating to reactor operation with little emphasis on the fundamental microbiology. In this study, we hypothesize two novel mechanisms for granule formation as observed in three laboratory scale sequencing batch reactors operating for biological phosphorus removal and treating two different types of wastewater. During the initial stages of granulation, two distinct granule types (white and yellow) were distinguished within the mixed microbial population. White granules appeared as compact, smooth, dense aggregates dominated by 97.5% “Candidatus Accumulibacter phosphatis,” and yellow granules appeared as loose, rough, irregular aggregates with a mixed microbial population of 12.3% “Candidatus Accumulibacter phosphatis” and 57.9% “Candidatus Competibacter phosphatis,” among other bacteria. Microscopy showed white granules as homogeneous microbial aggregates and yellow granules as segregated, microcolony-like aggregates, with phylogenetic analysis suggesting that the granule types are likely not a result of strain-associated differences. The microbial community composition and arrangement suggest different formation mechanisms occur for each granule type. White granules are hypothesized to form by outgrowth from a single microcolony into a granule dominated by one bacterial type, while yellow granules are hypothesized to form via multiple microcolony aggregation into a microcolony-segregated granule with a mixed microbial population. Further understanding and application of these mechanisms and the associated microbial ecology may provide conceptual information benefiting start-up procedures for full-scale granular-sludge reactors.

http://aem.asm.org/content/76/22/7588.full

Hoping this will be helpful,

Rafik

Thanks for reply. Mark Van Loosdrecht. But can you tell me what do you mean by SVI15 and SVI30. Either SVI15 means you are giving settling time of 15 mins to sludge while SVI30 means you are giving 30 minutes settling time to sludge or vice versa. 

One thing more, when we are giving more settling time then settled sludge value increases then how do you say that SVI15>> than SVI30. It should be SVI30>>SVI15. Isn't it?

Can you elaborate. Thanks

Dear all,

SVI is a sludge volume index and NOT settling time.

I suggest to read my first answer which contains two publications describing the mechanism and differences between the two.

Hoping this time my answer will find the appropriate attention.

Rafik

Yes, your answer is really very much appreciating. I like that and get insight depth knowledge regarding microbes and organic matter as well. One thing more, I want to know and clerify that as Sludge volume index is one of the main criteria in differentiating floc and granule. 

SVI means the Settled sludge volume/(sample volume x suspended soild concentration)

So, many articles discussed about SVI and settling time together.  Since, SVI is also function of settling time. Therefore, I was asking about SVI30 and SVI15. Can you share your knowledge regarding these SVI's also. Since, it is really very much helpful for me.

The standard SVI method is normally to measure the Volume of the sludge after 30 minutes settling - so yes there is a time dependency. When Mark writes that for Granules SVI5 and SVI30 are almost equal, he means that essentially all granules has already settled after 5 minutes and hence the volume will only change very little in the remaining 25 minutes were as for an activated sludge most of the sludge still has not settled after 5 minutes and hence there will be a large volume going from 5 minutes settling to 30 minutes settling. A part from this a definition of granules given is:

De Kreuk et al (2005) has defined Aerobic granules as follows: “Granules making up aerobic granular activated sludge are to be understood as aggregates of microbial origin, which do not coagulate under reduced hydrodynamic shear, and which settle significantly faster than activated sludge flocs”

de Kreuk M.K., McSwain B.S., Bathe S., Tay S.T.L., Schwarzenbeck and Wilderer P.A. (2005). Discussion outcomes. Ede. In: Aerobic Granular Sludge. Water and Environmental Management Series. IWA Publishing. Munich, pp.165-169)