Kaseamchochoung et al. (2006) also studied the effect of molecular weight of chitosan on flocculation. They controlled the deacetylation of chitosan samples at 83 ± 2% and studied two levels of molecular weight (3.5x10s and 1.4×106 dalton; Da). They found that the low molecular weight chitosan had a higher flocculation efficiency than the high molecular weight chitosan. Following Gregory (1993), they suggested that a possible explanation is that the longer polymers make more surface contacts per molecule and possibly saturate the cell surfaces, leaving no space for other polymers from different cell particles to initiate bridging.

2.1 Effect of environmental pH and ionic strength

Kaseamchochoung et al. (2006) found that the progression of anaerobic digestion in a UASB may cause pH to drop to 6 or even lower. At pH 6 and 7, approximately 90% flocculation was obtained by adding 2 mg chitosan/ g o. d. sludge of chitosan M70 and M85. However, at pH 5, approximately 95% flocculation was obtained at the same chitosan concentration (Fig. 3).

Similar results were obtained by Roussy et al. (2004). They studied chitosan efficiency at three different pH values (pH 5, 6.3, and 9). They found that a lower chitosan dosage (87% DD) was required at pH 5, while a significantly higher dosage of chitosan was required at pH 9 to obtain a residual turbidity below a fixed limit of 5 formalin turbidity units. Their explanation was that two possible mechanisms were possible at pH 5 —(a) coagulation by charge neutralization and (b) flocculation by entrapment in the polymer network. However, at pH 9 only the latter mechanism is possible, but its effect can only be significant at a high chitosan concentration.

Kaseamchochoung et al. (2006) found that both chitosan M70 and M85 were able to flocculate anaerobic sludge even when the system pH dropped to 5. A small degree of restabilization was observed after the charge neutralization point (CPN). That is, the percentage of flocculation dropped only slightly after the CPN, whereas zeta potential values became positive. A possible explanation given in Kaseamchochoung et al. (2006) is that the charge density of chitosan is greatly influenced by pH (Strand et al., 2001). Because the intrinsic pKa of chitosan is close to 6.5, most amine groups are protonated at pH 5, but become significantly less protonated when the pH increases. The polymer is therefore more highly positively charged at pH 5 than at pH 7. At pH 7, chitosan with 70%DD contains a lower charge density than chitosan with 85%DD, and the performance of chitosan (70%DD) would be noticeably lower at a low chitosan dosage (Fig. 2). Kaseamchochoung et al. (2006) suggested that charge density may play an important role in the flocculation mechanism and that this is not surprising because electrostatic forces are typically the main cause of polyelectrolyte adsorption on an oppositely charged surface. They concluded that chitosan has the potential to be used as an effective cationic bioflocculant, which is able to function either in acidic or neutral conditions, and that only relatively small amounts of chitosan (less than 4 mg/g dried sludge) are required.

chitosan concentration (mg/g o. d, studge}

Fig. 4. Percent flocculation as a function of chitosan M70 concentration in sludge suspension at different pH values and ionic strengths (from Kasemchochoung et al., 2006. Reprinted with permission from Water Environment Research. Volume 78, No. 11, pp. 2211 to 2214, Copyright © 2006 Water Environment Federation, Alexandria, Virginia.)

In addition to pH, ionic strength of a medium is also a major factor affecting flocculation. Kaseamchochoung et al. (2006) investigated the effect of ionic strength on flocculation by chitosan of high (0.1 M) and low (0.01 M) ionic strength. At pH 7, ionic strength did not signficantly influence the pattern of flocculation by chitosan M70 and the flocculation remained at approximately 95%. In contrast, at pH 5, chitosan M70 performed significantly better in the high-ionic-strength medium. Under the low ionic strength condition, the flocculation dropped from approximately 95% to 45% (Fig. 4). A possible explanation for the effect of salt was obtained from classical theories of colloidal stability (Strand et al., 2001). The extension of the double layer, which causes electrostatic repulsion between charged colloids and the range of repulsion forces, decreases with increasing ionic strength in the surrounding medium. Therefore, bacterial cells should be able to come closer and thus flocculate better in a high ionic strength medium.