Based on the sequence of anaerobic reaction, microbial species involved can be roughly divided into the following three categories: (a) bacteria responsible for hydrolysis; (b) acid-producing bacteria; and (c) methane-producing bacteria. In general, the acid-producing bacteria tolerate a low pH with an optimal pH of 5.0 to 6.0; however, most methane-producing bacteria can only function optimally in a very narrow pH range of 6.7-7.4 (Bitton, 1999). This explains why pH is more inhibitory to methane-producing bacteria than to acidogenic bacteria in UASB reactors. Once the reactor pH falls outside the range of 6.0-8.0, the activity of methane-producing bacteria is adversely affected which poses serious operational problem leading to reactor failure. Under normal operating conditions, the pH reduction caused by acid-producing bacteria can be buffered by bicarbonate produced by the methane-producing bacteria.
Teo et al. (2000) studied the effects of the environmental pH on anaerobic granulation process. They found that from pH 8.5 to 11.0, the strength of anaerobic granules in term of turbidity change decreased with the pH increase, indicating that high pH conditions weakened the granular structure; from pH 5.5 to 8.0, the strength of granules was unchanged, showing that the granular structure was relatively stable at this pH range; from pH 3.0 to 5.0, the increase in the strength of granule was very sharp. These results showed that the relatively low pH conditions would facilitate the maintenance of anaerobic granular structure, and can be satisfactorily explained by the proton translocation-dehydration theory. Consequently, in situ operation engineers need to regularly monitor the reactor pH and its changes.
Characteristics of the feed are considered a key factor influencing the formation, composition, and structure of anaerobic granules. The complexity of substrate may exert a selection pressure on microbial diversity in anaerobic granules which influences the formation and microstructure of granules. Based on their free energy of oxidation, organic substrates can be roughly classified into high-energy and low-energy feeds. During the UASB start-up period, high-energy carbohydrate feeding can sustain the acidogens and facilitate the formation of extracellular polymers. The more readily the acidogens take up and metabolize the substrate, the more rapidly the proton pumps will be activated, and sooner the methanogens will obtain the substrate (Tay et al., 2000). Thus, the rapid growth of acidogens due to the presence of high-energy substrate in the influent would facilitate the overall process of sludge granulation in the UASB reactors.
The granules grown on volatile fatty acid mixture (acetate, propionate, and butyrate) under mesophilic conditions can be classified into three distinct types according to the predominant acetate utilizing methanogens present: (1) rod-type granules, which are mainly composed of rod-shaped bacteria in fragments of about four to five cells resembling Methanothrix; (2) filament-type granules, which consist predominantly of long multicel-lular rod-shaped bacteria; and (3) sarcina-type granules, which develop when a high concentration of acetic acid is maintained in the reactor (Hulshoff Pol et al., 1983; de Zeeuw, 1984).
A trend has been observed towards increasing diversity of methanogenic subpopulations with an increasing complexity of the waste composition. At least four distinct microcolonies have been observed in granules treating brewery wastewater (Wu, 1991). One of these microcolonies was composed of Methanothrix-like rods only, while the other microcolonies consisted of hydrogen-carbon dioxide utilizing Methanobacterium-like rods juxtapositioned with three different rod-shaped syntrophs (Hickey, 1991).
Full-scale UASB experience confirms that anaerobic sludge granulation occurs in many different types of wastewaters. Because of the extremely low growth rate of anaerobic bacteria, the energy content of the substrate are important for anaerobic granulation; however, the complexity of substrate also exerts a selection pressure on the microbial diversity in anaerobic granules. This selection pressure may in turn influence the formation and microstructure of granules through its effect on the food chain and community signaling communications.
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