International Journal of Applied Environmental Sciences

ISSN 0973-6077 Volume 12, Number 8 (2017), pp. 1561-1573

© Research India Publications

http://www.ripublication.com

Influence of polymer addition on granulation in

upflow anaerobic sludge blanket reactor: A review

Vidya Singh, S. S. Narvi* and N. D. Pandey

Department of Chemistry, Motilal Nehru National Institute of Technology, Allahabad

211004, India.

*Department of Chemistry, Motilal Nehru National Institute of Technology, Allahabad

211004, India.

Corresponding Author (S.S.Narvi)

Abstract

The upflow anaerobic sludge blanket (UASB) reactor has been used

successfully treat a variety of wastewaters. The aim of this review is to

synthesize and analyze information on how the process of granulation is

affected by environmental and operational conditions in the reactor. The

factors reviewed are strength of substrate, nutrients, multivalent cations and

heavy metals, exocellular polymer, and addition of natural and synthetic

polymers. Nature and strength of substrate in conjunction with intra-granular

diffusion largely determines the microstructure of the granules. The addition

of external additives as natural cationic polymers Chitosan as well as cationic

fraction of Reetha (Sapindus trifoliata) extract, synthetic polymers "AA 184

H" may enhance granulation in the upflow anaerobic sludge blanket reactors.

Keywords: Cations, Chitosan, Granulation, Polymers, UASB reactor.

INTRODUCTION

The performance of the upflow anaerobic sludge bed (UASB) system is highly

dependent on the granulation process with particular organic wastewater. The UASB

1562 Vidya Singh, S.S. Narvi and N.D. Pandey

process is attractive because of its compactness, low operational cost, low sludge

production, and production of methane. The application of UASB reactors for the

treatment of high-strength industrial wastewater containing easily hydrolyzed

substrates such as sugar industry wastes, distillery wastes, and brewery wastes had

been successful1-4. Performances of UASB reactors treating difficult-to-hydrolyze and

complex substrates such as phenols, effluents from food and milk processing plants,

gelatine manufacturing plants, and animal slaughterhouses were less than

satisfactory5-7. UASB process for the treatment of domestic wastewater (low strength)

also suffers from a number of shortcomings, such as long start-up time, poor gas

production, susceptibility to shock loading, inability to form self-immobilized

bacterial granules, and necessity for post-treatment of the effluent8-9. The granulation

process is commonly believed to be sensitive to the sudden change of environmental

and operational conditions10. Factors governing granulation have been extensively

studied on a variety of wastewaters. Some of these factors are operating conditions11-

13, pH and alkalinity14-15, temperature16-17, strength and composition of wastewater18-

20, reactor hydrodynamics17,21, presence of metal ions and trace metals22-25, presence

of polymers26-29, microbial ecology30-31, and production of exo-cellular polymeric

substances by anaerobic bacteria32-33.mA number of reviews are available on

functioning of UASB reactor34-35, its applicability to sewage treatment9,36, treatment

of some of the toxic wastes37, nutrient requirements for UASB process15 and

mechanism of granulation38-40. The main purpose of this review is to synthesize and

take a critical look at the effect of different environmental factors on enhancement of

granulation. The divalent cations such as calcium and iron may enhance granulation

by ionic bridging and linking exo-cellular polymers. However, their presence in

excess may lead to cementation due to precipitation leading to increased ash content

and mass transfer limitation. The addition of external additives such as ionic

polymers may enhance granulation in the upflow anaerobic sludge blanket reactors.

2. THE UASB TECHNOLOGY

2.1. The UASB reactor

The UASB reactor is initially with inoculums such as digested, anaerobic, granular

and activated sludge. Sludge enters from the bottom of the reactor. Under appropriate

conditions, light and dispersed Particles will be washed out while heavier components

will retain, thus minimizing the growth of finely dispersed sludge whilst forming

granules or flocs consisting of the inert organic, inorganic matters and small bacterial

aggregates in the seed sludge38. The schematic diagram of a UASB reactor is shown

in Figure 1.

Influence of polymer addition on granulation in upflow anaerobic sludge… 1563

Fig. 1 Schematic diagram of a UASB reactor

The reactor consists of two parts: rectangular and cylindrical Column and a gas liquid

solid (GLS) separator. After a certain period (usually 2-8 months), depending on the

operating conditions and the characteristics of the wastewater and seed sludge a very

dense sludge bed which may be granular or flocculent in nature with high settling

properties develops. Above the dense sludge bed, there is a sludge blanket zone with

a much-diffused growth and lower particle setting velocities41. The biological

reactions take place throughout the highly active sludge bed and blanket zone. As the

flow passes upward, the soluble organic compounds in the influent are converted to

biogas consisting of mainly methane and carbon dioxide. The produced biogas and

the sludge buoyed by the entrapped gas bubbles are then separated from the effluent

by the immersed GLS separator, in which the baffles prevent as efficiently as possible

the washout of the viable bacterial matter or floating granular sludge by sliding the

settled solids back to the reaction zone38,42.

3. GRANULATION

The effectiveness and stability of a UASB reactor depends strongly on the initial

start-up, which in turn is main affected by numerous physical, chemical and

biological parameters19, such as the type of wastewaters, the operating conditions and

the characteristics, availability and growth of active microbial populations in the seed

sludge or inoculum. An acclimatization period is required before the full design

1564 Vidya Singh, S.S. Narvi and N.D. Pandey

organic loading rates can be applied to inoculate the seed sludge to the operating

conditions. This period which is typically 2-8 months2 is rather long and has been the

major hitch of the industrial applications of UASB reactors.

4. ENHANCING FACTORS OF GRANULATION

4.1 Substrate characteristics

No layer in the granules was reported to form with propionate, peptone, ethanol, and

glutamate as substrates6,18,43. The granules also do not show any layered structure in

the presence of an inhibitory substrate such as phenol or lindane43. Methanogenesis is

reported to be the rate-limiting step in the anaerobic degradation of the non-inhibitory

substrate whereas with the inhibitory substrate, acetogenesis, is identified as the rate-

limiting step11,44. Low substrate concentration in the feed may result in substrate

limitation at the core of the larger granules as majority of the substrate is utilized near

the surface40.

4.2 Multivalent cations and heavy metals

The cations may accelerate this process through bridging between negatively charged

groups on cell surfaces and linking exocellular polymers38,45,46. In addition,

multivalent cations condense the diffused double layers and facilitate flocculation due

to Vander Waals forces46. The predominant binding groups for metals on the surface

of bacteria are carboxyl and amino groups in proteins.

4.3 Inoculums

In order to select the best inoculums source for a specific type of wastewater, the

toxicity and biodegradability tests as reported in the literature can be used24. Although

a UASB reactor can perform efficiently without granules, granule formation during

start-up lends a decided advantage for its ability to provide high COD removal

efficiency within a much shorter period, allowing treatment for larger volumes of

wastewater. The importance of granules in the operation of UASB reactors has led to

an increasing number of studies on the relevant theories and mechanisms of anaerobic

granulation38,46. According to46 the initial development of granules can be divided

into four steps: (1) Transport of cells to a substratum (i.e. an uncolonized inert

material or other cells); (2) Initial reversible adsorption to the substratum by

physicochemical forces; (3) Irreversible adhesion of cells to the substratum by

microbial appendages or polymers and (4) Multiplication of the cells and

development of granules.

Influence of polymer addition on granulation in upflow anaerobic sludge… 1565

4.4 Different types of cations

Presence of divalent and trivalent cations exert positive impact on granulation process

by neutralizing negative charges on bacterial surfaces, serving as cationic bridges

between bacteria. Investigators24,47successively studied the roles of a few specific

multivalent cations on granulation using the same UASB reactor of 7.3 L in volume

for treating synthetic wastewater with influent COD concentration of 4000 mg/L.

They found that with an optimum concentration of iron (Fe2+) at 300 and 450 mg/L,

the granulation process was accelerated; higher Fe2+ concentration in the feed (more

than 450 mg/L) resulted in excessive deposition of minerals on the granules, therefore

deteriorating the bacterial specific activity24. As with the addition of calcium (Ca2+),

the optimum concentration of Ca2+ at 150-300 mg/L enhanced biomass accumulation

and granulation process by accelerating the adsorption, adhesion and multiplication of

the granulation process47. The addition of aluminium (Al3+) at 300 mg/L resulted in

the formation of large granules and shortened the granulation time by approximately

one month25,48 , examined the effect of adding aluminium chloride on the treatment of

low-strength (650-750 mg/L) synthetic wastewater. It is found that the addition of

Al3+ at 200-300 mg/L (either continuously throughout the operation or during the

start-up) adversely affected COD removal efficiency and growth of agglomerates. A

smaller concentration of Al3+ at 50 mg/L, did not affect the COD removal efficiency

but adversely affected the growth of agglomerates48. These studies25,48 have critically

suggested that the optimum concentration of multivalent cations is dependent on the

influent COD concentration. Consequently, more research studies are needed to

determine their relationships and roles in the granulation process. Magnesium (Mg2+)

ion is also influencing the properties of denitrifying sludge and granulation. Two

upflow anaerobic sludge bed (UASB) reactors were used, R1 was continuously

supplied with 50 mg L−1 of Mg2+ in the wastewater and R0 was used as a control

without magnesium. On the 258th day, the sludge properties in R0 and R1 were

analyzed when the nitrogen loading rates were both 36.0 kgN m−3 d−1 and the

nitrogen removal rates of R0 and R1 were 33.8 and 34.6 kgN m−3 d−1, respectively.

The diameters of R0 and R1 were 0.95 and 1.35 mm, settling velocity (74.5 ± 20.8

and102.0 ± 22.6 mh−1), extracellular polymeric substance (60.8 ± 2.1 and

76.0 ± 2.7 mg g−1VSS) and specific denitrifying activity (794.4 ± 9.6 and 825.6 ± 4.9

mgN d−1) were also compared. These results addressed that the addition of

magnesium to denitrifying reactor is an effective alternative to enhance sludge

properties49.

4.5 Different Polymers

In addition to the use of multivalent cations, the natural polymers, such as water

extract of the Moringa Oleifer seeds (WEMOS)26, Chitosan29,50, Reetha extract29 and

1566 Vidya Singh, S.S. Narvi and N.D. Pandey

powdered bamboo-charcoal51 as well as commercial and synthetic polymers, such as

the commercial cationic polymer “AA184H”27-28 and organic-inorganic hybrid

polymers52 also showed promising results in enhancing the start-up and granulation in

UASB reactors. A few cationic polymers reported to enhance granulation are natural

cationic polymers Chitosan29,53 and cationic fraction of Reetha (Sapindus trifoliata)

extract29, synthetic polymers AA 184 H and Percol 76353. The efficiency of Chitosan

was found to be better than the synthetic polymer Percol 76353 and natural polymer

Reetha extract29.

4.5.1 Moringa Oleifer seeds (WEMOS)

The enhancement of start-up of self-inoculation by adding a water extract of Moringa

oleifera seeds (WEMOS) by26. M. oleifera is a tropical plant belonging to the family

Moringaceae. WEMOS is effective in flocculating organic matter and removing

microorganisms from raw water54. It is currently used to treat drinking water, where it

has shown its efficiency as a coagulating agent. The active agents of WEMOS are

dimeric cationic proteins with a molecular weight of 13 kDa and an isoelectric point

between 10 and 11. The WEMOS solution contains several types of carbon sources,

e.g. carbohydrates, lipids and lignin, and also nutrients such as N, P and metal ions

which are capable of supporting microbial growth. It was therefore, hypothesised that

a continuous supply of WEMOS to a UASB reactor treating raw domestic wastewater

would result in an enhancement of its start-up period. Water extract of Moringa

oleifera seeds (WEMOS) was used to enhance the start-up of a self inoculated upflow

anaerobic sludge blanket (UASB) reactor treating raw domestic wastewater. Two

reactors labelled control (RC) and WEMOS addition (RM) were started without

special inoculum. Both reactors were fed continuously for 22 weeks with domestic

wastewater containing an average total chemical oxygen demand (COD) of 320 mg

O2/L and suspended solid (SS) of 165 mg/L. The reactors operated during the entire

experimental period at 29°C and at a hydraulic retention time (HRT) of 4 h. The RM

reactor received 2 ml WEMOS per litre of influent. WEMOS solution was prepared

on the basis of 2.5% (w/v) ground M. oleifera seeds in water. The results of 22

weeks’ operation showed an improvement in the performance of the RM compared to

that of the RC. The dosage of WEMOS in the feed (1) shortened the biological start-

up period by 20% (2) increased acidogenic and methanogenic activity by a factor of

2.4 and 2.2 respectively (3) increased the specific biogas production by a factor of

1.6,(4) favoured fast growth of the sludge bed and (5) allowed the aggregation of

coccoid bacteria and growth of microbial nuclei, which are precursors of anaerobic

granulation.

Influence of polymer addition on granulation in upflow anaerobic sludge… 1567

4.5.2 Chitosan and Reetha extract (bulk, cationic fraction, and anionic fraction)

Chitosan is a modified polysaccharide, mostly produced by alkaline deacetylation of

chitin. In a UASB reactor, granules start to form with the aggregation of acidogens,

and with methanogens enclosed inside, leading to the formation of an elastic

hydrophilic layer and a hydrophobic inner core, thus decreasing the washout of

methanogens.

This was attributed to the polysaccharide structure of the Chitosan, similar to

exocellular polymer found in aggregating anaerobic sludge and the water-absorbing

nature of the polymer55. The effectiveness of some water-absorbing polymers in the

enhancement of granulation56. Due to this nature, Chitosan has shown positive results

in enhancing sludge granulation and has exceeded Percol 763, as well as both the

cationic and anionic fraction of Reetha extract in treating synthetic wastewater29. In

the latter, large granules were formed and their stabilities were shown to be stable,

indicating the low possibility of substrate-diffusion limitations with low organic

loading. The effects of chitosan characteristics (i.e., degree of deacetylation [DD] and

molecular weight) and environmental conditions (i.e., ionic strength and pH) on the

flocculation of anaerobic sludge were investigated. The results showed that chitosan

enhanced the flocculation of sludge, and the flocculation efficiency depended on both

the degree of deacetylation and molecular weight. Chitosan with 85% DD was more

effective than that with 70% DD, as the former required a lower dose to obtain 90%

flocculation at all studied pH values. In addition, low molecular weight chitosan

enhanced the flocculation better than high molecular weight chitosan. The increase in

ionic strength (up to 0.1 M) of the suspension helped reduce destabilization that

occurred when chitosan was overdosed. In general, chitosan has potential to be used

as an effective cationic bioflocculant, which is able to function either in acidic or

neutral conditions, and very small amounts of chitosan (less than 4 mg/g dried sludge)

are required50. Concomitant early granulation with chitosan addition under a

syntroph-specific substrate and enhancement of extracellular polymeric substances

(EPS) production were aimed to build anaerobic granules with high syntrophic

activities in a short period. Two laboratory-scale upflow anaerobic sludge blanket

reactors were operated as control (R1) and chitosan addition (R2) reactors during

early granulation (phase 1). Chitosan decreased the negativity of microbial surface

charges (zeta potential) to −10.5 mV on day 58 which led to increases in average

diameter sizes, nuclei and granule ratio of approximately 115 µm, 55.1% and 8.2%,

respectively. While zeta potential in R1 slightly changed, this resulted in less

microbial aggregation. Although microbial aggregation in R2 was rapidly triggered

by chitosan addition during phase 1, its structure was clumpy with rough surface due

to lack of EPS. Substrate switching to glucose increased polysaccharides EPS during

phase 2 which was synergistically improved on the structural characteristics of

microbial aggregate in R2, that is, more spherical and compact, with a smoother

1568 Vidya Singh, S.S. Narvi and N.D. Pandey

surface. Rapid-growth microorganism was also boosted, which then dominated the

outer layer of the aggregate. The Archaea clumps were observed at a deeper layer and

were surrounded by Eubacteria, presumably acetogens, indicating a syntrophic

relationship due to substrate association between these microbial groups57.

4.5.3 Commercial cationic polymer “AA184H”

The impact of a commercial cationic polymer “AA184H” at concentrations 5-20

mg/L on granulation and organics removal efficiency in treating synthetic sludge with

COD 5000 mg/L with dosage of 20 mg/L27-28. The organic loading capacity of the

UASB reactor was increased from 19.2 to 25.6 g COD/L/d28 while with dosage of 80

mg/L, the start-up period was shortened and the granules in UASB reactors were

strengthened and exhibited the best settleability at all studied organic loading rates

(OLRs) (2-40 g COD/L/d leading to increased organic removal efficiency and loading

capacity of the UASB system27.

4.5.4 Organic-inorganic hybrid polymers

From Synthesized organic-inorganic hybrid polymers and added to the UASB

reactors52. It was found that granular sludge was formed within 5 min. The granules

were stable throughout the operation and the COD removal efficiency was as high as

90% at even up to 18 g COD/L/d of organic loading rate.

CONCLUSIONS

The optimum range of organic loading rate (OLR) and hydraulic retention time

(HRT) can only be decided after considering the influent characteristics and other

operating conditions. Layer formation in granules is due to interaction of

intragranular diffusion and degradation kinetics of the substrate. Thus, the granule

microstructure is governed by the substrate type and strength. The presence of

adequate nutrients and metals is essential for granulation. Calcium and iron may

enhance granulation but are also capable of causing mass transfer limitation when

present in large quantities. External addition of ionic natural and synthetic polymers

can be used to enhance granulation. They help in the initial stages of formation of

granules. Another challenge is to shorten the start-up time of the reactor by enhancing

granule formation. Various external additives have shown promising results in this

direction, however, most of these studies are limited to laboratory-scale reactor. The

effect of these additives should be investigated in pilot-scale reactors along with the

economics of the additives.

Influence of polymer addition on granulation in upflow anaerobic sludge… 1569

ACKNOWLEDGMENTS

Authors are grateful to MHRD, New Delhi MNNIT, Allahabad for their help in

compiling and preparing this manuscript.

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1574 Vidya Singh, S.S. Narvi and N.D. Pandey