Download - Majid Hashemi_ Aerobic Granulation
Aerobic Granulation
TechnologyBy: Majid Hashemi(PhD Candidate of Environmental Health Engineering)
Isfahan University of Medical Sciences
INTRODUCTION
Microbial granulation is a process of cell-to-cell self-
immobilization involving biological, physical, and
chemical actions. Granules formed through self-
immobilization of the microorganisms are dense consortia
packed with different bacterial species that typically
contain millions of organisms per gram biomass.
INTRODUCTION
As compared to conventional activated sludge flocs,
granular sludge has regular, denser and stronger
microbial structure and good settling ability. These
characteristics result in high biomass retention and
withstand high-strength wastewater and shock
loadings.
INTRODUCTION
Granulation occurs in both aerobic and anaerobic
wastewater treatment systems. Formation of
anaerobic granules has been studied for decades,
and is probably best recognized in the upflow
anaerobic sludge blanket (UASB) reactor.
CONTINUE…
However, application of anaerobic granulation technology is
greatly limited by drawbacks such as the long start-up period
required (normally 2 to 8 months), a relatively high operation
temperature, and unsuitability for low-strength organic
wastewater. To overcome these weaknesses, recent research to
developing aerobic granulation technology for the removal of
organic wastes.
For instance, in an aerobic system, the outer part of the granule,
where oxygen is available, nitrifiers can grow, while in the inner
part, denitrifiers, anammox bacteria or phosphate accumulating
organisms (PAOs) can develop themselves under anaerobic and
anoxic conditions. Figure 1 shows the differences in the structure of
a floc and an aerobic granule.
Relation between substrate uptake rate and substrate concentration for
filamentous and granule formers according to the kinetic selection theory
Most of aerobic granules have been cultured in sequencing batch
reactors (SBRs) only.
Aerobic granules matured in both reactors after operation in SBR
for 3 weeks.At this stage, both glucose-fed and acetate-fed
granules had a very regular round-shaped outer surface.
Compared to acetate-fed granules, filamentous bacteria dominant
in glucose-fed granules made a fluffy outer surface of granules
Macrostructures of glucose-fed (a) and
acetate-fed (b) aerobic granules
Microstructures of glucose-fed (a) and
acetate-fed (b) aerobic granules
FACTORS AFFECTING AEROBIC GRANULATION
A) Substrate Composition
aerobic granulation seems to be insensitive to the nature of
substrate carbon source; for example, aerobic granules had been
successfully cultivated with a wide variety of substrates,
including glucose, acetate, ethanol, phenol, and synthetic
wastewater.
However, granule microstructure and species diversity appears to
depend on the type of carbon source.
B) Organic Loading Rate
The essential role of organic loading rate in the formation of
anaerobic granules has been widely recognized. A relatively high
organic loading rate facilitated the formation of anaerobic granules
in UASB systems. In contrast to anaerobic granulation, the
accumulated evidence suggests that aerobic granules can form
across a wide range of organic loading rates, from 2.5 to 15kg
COD/m3. day, i.e., aerobic granulation is less dependent upon the
organic loading rate applied .
C) Hydrodynamic Shear Force
A high shear force results in biofilms with a strong and
compact microbial structure, whereas a weak shear force
produces biofilms with a heterogeneous and porous
structure. The formation of aerobic granules and granule
stability was improved at a high shear force.
More regular, rounder, and compact aerobic granules were developed at
high hydrodynamic shear force. the production of extracellular
polysaccharides was closely associated with the shear force and the
stability of aerobic granules was found to be related to the production of
extracellular polysaccharides. Since extracellular polymeric substances
(EPS) are a major component of cell flocs and biofilms, they are
hypothesized to play a dominant role in all types of biofilm formations,
including flocculation and granulation.
EPS can be polysaccharides, proteins, nucleic acids,
phospholipids or humic substances. They participate in the
stability of granules through London forces (hydrophobic
character of proteins), electrostatic interactions (Ca 2+ ions) and
hydrogen bonds (hydroxyl groups -hydrophilic polysaccharides-
and water).
D) Presence of Calcium Ion in Feed
the addition of Ca 2+ accelerated the aerobic granulation process. With the
addition of 100mg Ca 2+ /L, the formation of aerobic granules took 16 days
compared to 32 days in the culture without the Ca 2+ addition. The Ca 2+
augmented aerobic granules also showed better settling and strength
characteristics, and had higher polysaccharide content. It had been
proposed that that Ca 2+ could bind to negatively charged groups present
on bacterial surfaces and extracellular polysaccharide molecules,
and act as a bridge to interconnect these components and promote
bacterial aggregation. Polysaccharides play an important role in
maintaining the structural integrity of biofilms and microbial
aggregates such as aerobic granules, as they are known to form a
strong and sticky nondeformable polymeric gellike matrix, and can
contribute to cell-to-cell adhesion through interactions between
secondary functional groups such as hydroxyl and calcium ions.
e) Reactor Configuration
Column-type upflow reactors and completely mixed tank reactors
(CMTR) have very different hydrodynamic behaviors in terms of
interactive patterns between flow and microbial aggregates. The air
or liquid upflow pattern in column reactors can create a relatively
homogenous circular flow along the reactor height, and microbial
aggregates are constantly subject to such a circular hydraulic
attrition.
The circular flow could force microbial aggregates to be
shaped as regular granules that have a minimum surface free
energy. In a column-type upflow reactor, a higher ratio of
reactor height to diameter (H/D) can ensure a longer circular
flowing trajectory, which in turn creates a more effective
hydraulic attrition to microbial aggregates.
However, in CMTR, microbial aggregates stochastically move with
dispersed flow in all directions. Thus, microbial aggregates are
subject to varying localized hydrodynamic shear force, flowing
trajectory and random collision. Under such circumstances, only
flocs of irregular shape and size instead of regular granules
occasionally form. Therefore, the column-type reactor with high
ratio of reactor height to diameter, which can provide an optimal
interactive pattern between flow and microbial aggregates, is
favorable for the formation of aerobic granules.
f) Dissolved Oxygen
Dissolved oxygen (DO) concentration is an important parameter in
the operation of aerobic wastewater treatment systems. Aerobic
granules formed at the DO concentration as low as 0.7 to 1.0mg/L in
a SBR, whereas they were also successfully developed at high DO
concentrations up to 5mg/L. It appears that DO concentration would
not be a decisive parameter in the formation of aerobic granules
Characteristics of Aerobic Granule
Morphology: Compared to conventional bioflocs, aerobic
granules have a defined spatial shape. The mean diameter of
mature aerobic granules varies, and depends on the substrate
composition, organic loading rate, shear force, etc.
Characteristics of Aerobic Granule
Settleability: The settling property of aerobic granules is a key
operation factor that determines the efficiency of solid–liquid
separation, and it is essential for the proper functioning of
wastewater treatment systems. The SVI of aerobic granules is
much lower than that of conventional bioflocs. The settling
velocity of aerobic granules is usually higher than 30m/h, which
is comparable with that of the UASB granules, and is at least
three times higher than that of activated sludge flocs, which have
a typical settling velocity of around 8 to 10m/h.
Granule density and strength: The specific gravity of aerobic
granules falls into a range of 1.004 to 1.065. The granules with
high physical strength would have a strong ability to withstand
high abrasion and shear. The physical strength, expressed as
integrity coefficient (%), which is defined as “the ratio of residual
granules to the total weight of the granular sludge after 5 minutes
of shaking at 200rpm on a platform shaker”, is higher than 95%
for the aerobic granules grown on glucose and acetate.
Cell surface hydrophobicity: The cell surface hydrophobicity
was 68% for glucose-fed aerobic granules and 73% for acetate-
fed granules. These values are two times higher than that of the
conventional bioflocs.
According to thermodynamic theory, increasing the
hydrophobicity of cell surfaces would cause a corresponding
decrease in the excess Gibbs energy of the surface, which in turn
would promote cell-to-cell interaction and further serve as a
driving force for bacteria to self aggregate out of the liquid phase
(hydrophilic phase).
MECHANISM OF AEROBIC GRANULATION
Step 1: Physical movement to initiate bacterium-to-bacterium
contact. The forces involved in this step are:
Hydrodynamic force.
Diffusion force.
Gravity force.
Thermodynamic forces, e.g., Brownian movement.
Cell mobility. Cells can move by means of flagella, cilia, and
pseudopods.
Step 2: Initial attractive forces to keep stable multicellular contacts. Those attractive
forces are:
Physical forces:
Van der Waals forces
Opposite charge attraction
Thermodynamic forces including free energy of surface; surface tension
Hydrophobicity
Filamentous bacteria that can serve bridge to link or grasp individual cells
together
Chemical forces:
Hydrogen liaison
Formation of ionic pairs
Formation of ionic triplet
Interparticulate bridge and so on
Biochemical forces:
Cellular surface dehydration
Cellular membrane fusion
Step 3: Microbial forces to make cell aggregation mature:
Production of extracellular polymer by bacteria, such as
exopolysaccharides,
Growth of cellular cluster
Metabolic change and genetic competence induced by
environment, which facilitate the cell–cell interaction, and results
in a highly organized microbial structure
Step 4: Steady state three-dimensional structure of microbial
aggregate shaped by hydrodynamic shear forces.
APPLICATIONS OF AEROBIC GRANULATION TECHNOLOGY
High-Strength Organic Wastewater Treatment
Aerobic granules were able to sustain the maximum
organic loading rate of 15.0kg COD/m3 .day employed,
and attained COD removal efficiencies greater than 92%.
Phenolic Wastewater Treatment
Phenol-containing wastewater is difficult to treat because of
substrate inhibition. Microbial growth on phenol substrate and
concomitant phenol biodegradation are hindered by the toxicity
exerted by high concentrations of the substrate itself. However, the
selfimmobilization or aggregation of microbial cells into compact
granules could serve as an effective protection against high phenol
concentrations.
For an influent phenol concentration of 500mg/L, a stable effluent
phenol concentration of less than 0.2mg/L was achieved in the
aerobic granular sludge reactor
The phenol-degrading aerobic granules had a specific phenol
degradation rate as high as 1.4g phenol/g MLVSS day, which was
two times higher than that of acclimated seed sludge
Biosorption of Heavy Metals by Aerobic Granules
aerobic granules are ideal for removing heavy metals in
wastewater because of their strong microbial structure with large
surface area and high porosity. the aerobic granule-based
biosorption process is an efficient and cost-effective technology
for the removal of heavy metals from industrial wastewater.
Biogranules versus suspended sludge
Dense and strong microbial structure
Excellent settling ability
High biomass retention
Mixed and diverse microbial community
Good solid-liquid separation
Alleviate the impact of fluctuated loading rate
Reduce land area requirement for sludge settling
Simultaneous carbon and nutrient removal
Advantages of Aerobic Granules
• a strong and compact structure
• excellent settling ability
• rapid self-immobilization
• high resilience to shock loadings
• high endurance to chemical toxicity
• low sludge growth yield
• high organic loading rate
• reduce the reactor volume