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