Synthetic polymers have been widely used in water coagulation and floc-culation processes, and can significantly promote particle agglomeration. Similarly, the synthetic polymers can also be applied to expedite development of anaerobic granules. It was found that the supplementation of polymer Chitosan, which has a similar structure to polysaccharides, significantly enhanced the formation of anaerobic granules in the UASB-like reactors. Granulation rate in the Chitosan-containing reactor was 2.5-fold higher than that in the control reactor without addition of the polymer, while the specific activities of methane production were comparable in both reactors (El-Mamouni et al., 1998). In fact, it is not surprising to obtain such results since freely moving polymeric chains may form a bridge between cells, and this would facilitate the formation of initial microbial nuclei, which is the initial step towards granulation.
Kalogo et al. (2001) used water extract of Moringa oleifera seeds (WEMOS) to enhance the start-up of a UASB reactor treating domestic wastewater, and they found that the dosage of WEMOS in the feed favored the aggregation of coccoid bacteria and growth of microbial nuclei, which are precursors of anaerobic granulation. WEMOS, as a kind of natural polymers, is known to be effective in flocculating organic matter. Adsorption of WEMOS on the surface of the dispersed bacteria and neutralization of their surface charges would be a principal mechanism to promote anaerobic granulation.
Recently, Show et al. (2004); Wang 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. A control reactor R1 was operated without added polymer, while the other three reactors designated R2, R3, and R4 were operated with polymer concentrations of 5mg l-1, 10 mg l-1, and 20 mg l-1, respectively. The experimental results indicated that adding the polymer at a concentration of 20 mg l-1 markedly reduced the start-up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4.8 g COD l-1 d-1 was reduced by more than 36% (R4) as compared with both R1 and R3, and by 46% as compared with R2. R4 was able to handle an OLR of 16 g COD l-1 d-1 after 93 days of operation, while R1, R2, and R3 achieved the same loading rate only after 116, 116, and 109 days, respectively. Compared with the control reactor, the start-up time of R4
was shortened by about 20% at this OLR. Granule characterization indicated that the granules developed in R4 with 20 mg l-1 polymer exhibited the best settleability and methanogenic activity at all OLRs. The organic loading capacities of the reactors were also increased by the polymer addition. The maximum organic loading of the control reactor (R1) without added polymer was 19.2 g CODl-1d-1, while the three polymerassisted reactors attained a marked increase in organic loading of 25.6 g COD l-1d-1.
The findings by Show et al. (2004); Wang et al. (2004) demonstrated that adding the cationic polymer could result in shortening of start-up time and enhancement of granulation, which may 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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