Ionic composition and presence of polymer in the anaerobic system are believed to have significant roles in the forming granules through various mechanisms and models. The effects of various cations and polymers are discussed in the following sections.
Divalent and trivalent cations have positive effects on flocculation of dispersed sludge. Commonly used divalent cations are calcium and magnesium while iron can be used as both a divalent or trivalent cation depending on its oxidation state. Evidence shows that the presence of divalent and trivalent cations, such as Ca2+, Mg2+, Fe2+, and Fe3+, helps bind negatively charged cells together to form microbial nuclei that promote further granulation (Mahoney et al., 1987; Schmidt and Ahring, 1993; Teo et al., 2000; Yu et al., 2001).
The use of divalent or trivalent cation to assist in granulation lies in their ability to condense the diffusive double-layers resulting in relatively stronger effect of van der Waals attractive forces. Calcium was also found to form calcium bridge between its ion and extracellular polymers (ECP) (Forester and Lewin, 1972; Rudd et al., 1984).
According to McCarty et al. (1986), calcium stimulates granulation at concentration of 100-200 mg/l and becomes inhibitory at >2500 mg/l. Similarly, de Zeeuw (1984); Mahoney et al. (1987) reported that the rate of sludge granulation was significantly enhanced in a calcium concentration range of 100-200 mg/l. Verrier and Albagnac (1985) suggested the possibility that divalent calcium indirectly promote bacterial adhesion by increasing surface hydrophobicity.
Grotenhuis et al. (1988) found that contacting granular sludge with calcium chelating agent (EGTA) will result in granules disintegrating and becoming weaker. Based on this observation, it was then concluded that calcium plays an important role in granulation in 2 ways:
1. Inorganic calcium precipitates serve as surface for adhesion of anaerobic bacteria;
2. Calcium may be a constituent of extracellular polysaccharides and/or proteins that are not present as sticking material.
Research by Teo et al. (2000) showed that an increase in Ca2+ concentration from 0 to 80 mg/l substantially improved the strength of anaerobic granules, as indicated by a 60% decrease in turbidity. A study by Batstone and Keller (2001) using granules from UASB reactor was conducted to investigate the influence of calcium on granular sludge in a full scale UASB treating paper mill wastewater. It was found that the granules were small (1.0 mm) with a narrow size distribution. The core of the granules which was 200-400 micron in diameter consisted mainly of calcium precipitates. The rest of the granules were biologically active. With the observation that the core varied in consistency rather than size, it was concluded that it may have formed in the bulk liquid as amorphous calcium carbonate and subsequently acted as a nucleus for granule formation. As the granule increased in size, the calcium probably continued to precipitate in the core until saturation, after which scaling and granule deactivation occurred. At high calcium concentrations, problems such as the precipitation of calcium on the surface of granules and accumulation of calcium inside anaerobic granules with consequent reduced microbial activity have also been reported (Yu et al., 2001).
The role of cations in anaerobic granulation processes is still uncertain. Despite the positive effects reported, there were studies indicating that calcium ions did not induce sludge granulation at all (Guiot et al., 1988) and high concentration of magnesium ion (used as a divalent cation) caused granules to fall apart (Schmidt and Ahring, 1993). This may be due to the notion that, at high cation concentrations, bacteria could change their surface charge from negative to positive resulting in repulsion which deters the granulation process.
It is generally accepted that ECP plays an important role in the formation of a supporting matrix for the microorganisms. Production of ECP is believed to be affected by the nutritional balance and/or the diversity of the granules microflora.
According to Dolfing (1985), ECP contributes to about 1-2% on a dry weight basis. Ross (1984) found that ECP accumulation plays an important role in the "clumping" of bacteria that is comparable to the role of microbiological agglutination in the flocculation of aerobic sludge. Harada et al. (1988) found that biopolymer production on acetate is limited and therefore ECP is not a prerequisite for granulation. de Zeeuw (1984) however observed high growth yield factors in batch fed reactors and UASB reactors using acetate as a single substrate. He explained this by presuming that most of the growth took place in the form of ECP production. This conclusion was supported by the observation that extra ammonia fixation could not be detected.
Synthetic and natural polymers have been widely used in coagulation/ flocculation processes. These polymers are known to promote particle agglomeration and have been used to enhance the formation of anaerobic granules. The influence of synthetic polymers (Percol 763) and natural chitosan polymers on the granulation rate of suspended anaerobic sludge was studied in laboratory-scale UASB reactor (El-Mamouni et al., 1998). Results showed that reactor supplemented with either natural or synthetic polymers achieved better granulation. A greater granulation was obtained with chitosan compared to Precol 763. The superior granulation performance of chitosan may be related to its polysaccharidic structure which is similar to ECP that helps in aggregating anaerobic granules. The polymer enhanced granules had about the same specific activity of methane production as the granules formed without the polymer. Polymeric chains enhance flocculation by bridging microbial cells. Such initial microbial nuclei are the first step in microbial granulation. In short, the results showed that polymers play a critical role in enhancing anaerobic granulation in UASB-like reactors.
Kalogo et al. (2001) used a water extract of Moringa oleifera seeds (WEMOS) to assist in the start-up of UASB reactor. The ability of WEMOS to adsorb on the surface of dispersed bacteria which eventually lead to neutralization of their surface charge assist in the granulation process.
In cationic polymer-assisted anaerobic granulation processes, it has been observed that the start-up period required for the development of granular sludge blanket can be shortened significantly compared to when no polymers are used (Uyanik et al., 2002). Two mechanisms appear to be involved in polymer enhancement of anaerobic granulation. The addition of polymers to anaerobic systems likely changes the surface properties of bacteria to promote association of individual cells. Polymer may also form a relatively solid and stable three-dimensional matrix within which bacteria multiply and daughter cells are then confined. The polymer additives appear to play a similar role as do the naturally secreted extracellular polymeric substances (EPS) in aggregating anaerobic sludge.
Show et al. (2004) investigated the influence of a coagulant polymer on start-up, sludge granulation, and the associated reactor performance in laboratory-scale UASB reactors. The experimental results demonstrated that adding the polymer at an appropriate dosage markedly accelerated the start-up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4g COD/l.d was reduced by approximately 50% as compared with the control reactor, while other reactors also recorded varying degree of shortening. Monitoring on granule development showed that the granule formation was accelerated by 30% from the use of the appropriate dosage of polymer. Subsequent granules characterization indicated that granules developed in the polymer-assisted reactor exhibited the best settleability, strength, and methanogenic activity at all OLRs. The organic loading capacities of reactors were also increased by the polymer addition to as high as 40 g COD/l.d. The laboratory results obtained demonstrated that adding the cationic polymer could result in shortening of start-up time and enhancement of granulation, which in turn lead to improvement in organics removal efficiency and loading capacity of the UASB system. The authors hypothesized that positively charged polymer form bridges among the negatively charged bacterial cells through electrostatic charge attraction. The bridging effect would enable greater interaction between biosolids resulting in preferential development and enhancement of biogranulation in UASB reactors.
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