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Microbial Granules: Promising Technology for Wastewater Treatment
Hiren Joshi
Biofouling and Biofilm Processes Section
Water and Steam Chemistry Laboratory
BARC Facilities, Kalpakkam 603102
Email: hirenjoshi@igcar.gov.in


Recent trends in the area of bioremediation mainly focus on pure culture techniques; in other words, people try to develop processes based on good degrading isolates. But when we discuss about in-field applications, results are not quite satisfactory. Most of the operational processes, starting from trickling filter to highly advanced pneumatic reactor involve mixed culture or microbial consortia. Considering the facts, process involving multi species consortia would be a better choice over mono culture concept. On this conceptual background, multi species microbial granules are emerging as most promising agents for state-of-theart wastewater treatment and bioremediation technologies. High satiability, high stability in variable organic load, effective mineralization and amenability for bioaugmentation are some of the superior parameters which make them one of the best choices for bioremediation and wastewater treatment. Here, an attempt is made to evaluate various merits and demerits of microbial granules based bioprocesses with special emphasis on aerobic granules.


Most of the real wastewaters are complex in nature and none of the individual bacterial species is capable of completely degrading complex wastes. Complete degradation of complex industrial wastes involves a complex suite of metabolic reactions
mediated by several closely related species. To address this difficult task we have a system where different microorganisms with different metabolic capability lie in close proximity of others. Biofilm is a classical example seen in the natural environment, where different species of microorganisms arrange themselves in specific spatial organization.

To make use of this natural phenomenon people have developed various bioreactors having biofilm on various inert substrata. Biofilm reactors are extensively used in environmental biotechnology because high biomass can be retained to treat large volumes of diluted aqueous solutions that are typical of industrial and municipal wastewaters. Here, specific surface area is a very important parameter and becomes crucial in case of high strength organic waste. The specific surface area will increase dramatically by growing biofilm in suspended forms. These can be achieved by two ways 1) Biofilm is allowed to grow on light weight spherical carrier particles (sand, plastic beads. etc.) and 2) Spherical biofilm without any substratum (aerobic and anaerobic granules). Therefore, these microbial aggregates satisfy two basic conditions of optimal treatment plant viz., broad metabolic capability of microbial community present in the system and efficiency of solid-liquid separation at the last stage of treatment procedure. Comparing suspended biofilm based reactors with conventional bioreactors, it has many distinguished characteristics such as a) excellent satiability (easy clarification) b) minimal effect of variable organic load c) simple design with minimum parameter control d) involves biodegradation, biotransformation and biosorption (complete biological process). Carrier bound biofilms have some drawbacks compared to carrier-less microbial granules as selection of proper carrier material is not an easy task. Similarly, it has less specific surface area compared to granular biomass. Anaerobic granulation in Up flow Anaerobic Sludge Blanket (UASB) reactors has been investigated and applied both at laboratory scale and as well as at large-scale. The formation of microbial granules under aerobic conditions has been invented very recently.

Present status

Formation of anaerobic microbial granules has been observed in case of upflow anaerobic sludge blanket (UASB) reactor in 1980’s. Recent review reports that nearly 6000 industrial wastewater treatment plants based on this technology are in operation world wide. However, anaerobic granulation technology has the following disadvantages: 1) requires long startup period (2 to 4 months) 2) relatively high operation temperature (30 to 35°C for mesophilic UASB reactors),3) unsuitable for low-strength organic wastewater 4) not suitable for nutrient (nitrogen and phosphorous) removal. In order to overcome these drawbacks, Mishima and Nakamura in 1991 developed microbial granules in aerobic upflow sludge blanket reactor. Later, Morgenroth et al., (1997) cultivated aerobic microbial granules in a sequencing batch reactor (SBR). Ever since, microbial granules have been successfully cultivated in sequencing batch reactors using synthetic wastewater or real wastewater as influent. The engineering parameters such as settling time, organic loading, dissolved oxygen etc., have been studied and optimum range determined for developing microbial granules. But the mechanism of microbial granulation is still not fully understood. Particularly, the biological factors such as aggregating abilities of individual bacterial strains which gives structural framework, the catabolic diversity of granules which gives the functional capability and the driving force for microbial cell aggregation require better elucidation. With the help of various advance technology like confocal laser scanning microscopy (CSLM) and 16Sr RNA based microbial identification along with fluorescence insitu hybridization (FISH), researchers have tried to explain the phenomenon associated with biogranulation.

Microbial granulation: A sequential developmental process

Aerobic granulation is a process involving the development from seed sludge (used as inoculum) to compact aggregates, further to granular sludge and then finally to mature nearly spherical granules. This self-immobilization process involves clearly distinguishable (based on size and morphology) multiple steps. Figure 1 displays the microstructure of granular biomass. Granulation may be initiated by self-adhesion of bacterial strains. This can be through autoaggregation or co-aggregation among individual bacterial strains. Auto-aggregation refers to the physical cell-to-cell interaction between genetically identical cells, while co-aggregation refers to the interaction between genetically distinct bacterial cells (Rickard et al., 2003). High shear and short settling time employed in reactor may select bacterial strains having aggregating ability and strains with poor aggregating ability may get washed out. Aerobic starvation in cyclic manneroperated in SBR may influence bacterial physiological activity and hence aggregating ability. At cellular level, these (autoaggregation and coaggregation) interactions are mediated by lectinsugar kind of interactions. Development of granules may share the similar events involved in multispecies biofilm development. Intercellular communication plays a role in organizing the three dimensional spatial distribution of bacteria in the granules. Cell surface hydrophobicity and extracellular polymeric substances (EPS) facilitate the aggregation of bacteria and maintenance of granular structure. Figure 2 indicates schematic representation of aerobic granules formation.

Application of aerobic granule technology

Compact biomass combined with diverse metabolic capability makes aerobic microbial granules primary choice for treatment of high organic load. Limited space availability to set up reactors can be overcome as aerobic granules provide high conversion rate along with efficient biomass separation. Treatment capacity can be easily altered by varying loading rate, wastewater composition and bioaugmentation. Following are some of the applications demonstrated at laboratory scale as well as at pilot scale using SBR and aerobic granules.

1.High strength organic waste water treatment

High biomass retention (up to 6-12 g/l) due to dense and compact structure enhances COD removal by aerobic granules. Tay (2004) have achieved 15 Kg COD/M3. A problem observed in case of very high organic loading is the development of loose and fluffy granules due to excessive growth of filamentous organisms. However, this can be controlled by intermittent loading and cycle variation.

2.Simultaneous removal of nitrogen and organics

Complete nitrogen removal by nitrification and denitrification. Nitrite and nitrate produced from nitrification are reduced to gaseous nitrogen by dinitrifiers. Yang et al. (2004) investigated the simultaneous removal of organics and nitrogen by aerobic granules. Heterotrophic, nitrifying and denitrifying populations were shown to successfully co-exist in the microbial granules. Increased substrate N/COD ratio led to significant shifts among these populations within the granules.

3.Phosphorus removal

Environmental regulations in many countries require reduction of phosphorus concentration in wastewater to levels of 0.5 to 2.0 mg/l before discharge. The well-known enhanced biological phosphorus removal (EBPR) process removes P without the use of chemical precipitation and is an economical and reliable option for P removal from wastewater. The EBPR process operates on the basis of alternating anaerobic and aerobic conditions with substrates feeding limited to the anaerobic stage. Most EBPR processes are based on suspended biomass cultures and require large reactor volumes. Although full scale experience shows a strong potential of the EBPR, difficulties in assuring stable and reliable operation have also been recognized. Often, the reasons for failure of biological phosphorus removal are not clear (Barnard et al., 1985; Bitton, 1999).

4.Phenolic waste water treatment

Phenol is a toxic and inhibitory substrate, but also a carbon source for the bacteria. The consequence of the presence of phenol in biological wastewater treatment is process instability, which can lead to the washout of the microorganisms (Allsop et al., 1993). In low concentrations, phenol is biodegradable, but high concentrations can kill phenol degrading bacteria. Industrial wastewaters from fossil fuel refining, pharmaceutical and pesticide processing are the major sources of phenolic pollution. Jiang et al., (2002, 2004) investigated the feasibility of treating phenol-containing wastewater with aerobic granular sludge. Granular sludge is less susceptible to toxicity of phenol because much of the biomass in the granules is not exposed to the same high concentration as present in the wastewater. The phenol-degrading aerobic granules displayed an excellent ability to degrade phenol (Jiang et al., 2002, 2004). For an in fluent phenol concentration of 500 mg/l, a stable effluent phenol concentration of less than 0.2 mg/l was achieved in the aerobic granular sludge reactor (Jiang et al., 2002, 2004). The high tolerance of aerobic granules to phenol can be exploited in developing compact high-rate treatment systems for wastewaters loaded with a high concentration of phenol. Aerobic granules may prove powerful bioagents for removing other inhibitory and toxic organic compounds from high strength industrial wastewaters.

Aerobic granules appear to be highly tolerant of toxic heavy metals (Xie, 2003).

5.Biosorption of heavy metals by aerobic granules

Heavy metals are often found in a wide variety of industrial wastewaters. More stringent metal concentration limits are being established in view of their relatively high toxicity. Many biomaterials have been tested as biosorbents for heavy metal removal. These include marine algae, fungi, waste activated sludge and digested sludge (Lodi et al., 1998; Taniguchi et al., 2000; Valdman and Leite, 2000). In view of the physical characteristics of aerobic granules as discussed earlier, these granules are ideal biosorbent for heavy metals. The granules are physically strong and have large surface area and high porosity for adsorption. In addition, the granules can be easily separated from the liquid phase after biosorption capacity is exhausted.

Future trends in aerobic granules

Aerobic granulation has been observed only in SBRs. The feasibility of attaining aerobic granulation in continuous culture systems needs to be investigated. Selection pressure is the main driving force for aerobic granulation; however details of the mechanism need to be worked out elaborately. As aerobic granules possess optimum characteristics required for waste water treatments, the technology has a great future in the treatment of wastewater.

Figure 1: Microstructure of granular biomass.

Figure 2: Schematic representation of aerobic granules formation.


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ENVIS CENTRE Newsletter Vol.6,No 3 September 2008 Back 
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