As bacteria have negatively charged surfaces under normal pH conditions, a basic idea to expedite anaerobic granulation is to reduce the electrostatic repulsion between negatively charged bacteria by introducing multi-valence positive ion, such as calcium, ferric, aluminum, or magnesium ions into the seed sludge. It had been reported that reduced electrostatic repulsion between bacteria would promote anaerobic granulation (Mahoney et al., 1987; Schmidt and Ahring, 1993; Yu et al., 2001a). Addition of Ca2+ in the range of 80-200 mg/l, Mg2+ of 12-120 mg/l, or Al3+ of 300 ml/l increased the rate of anaerobic granulation in UASB reactors (Schmidt and Ahring, 1993; Teo et al., 2000; Yu et al., 2001b). However, high calcium concentration of above 500 mg/l (Guiot et al., 1988; Thiele et al., 1990) or 600 mg/l (Yu et al., 2001a) was found detrimental to anaerobic granulation. High calcium concentrations also cause serious problems, e.g. precipitation and accumulation of calcium in anaerobic granules, as well as reduced microbial activity of granules.
The multivalence positive ion-bonding model is based on a simple electrostatic interaction between negatively charged bacteria and positive ion. The DLVO1 theory postulates that when two surfaces carry a charge of the same sign, there is a free energy barrier between them, which acts as a repulsive force. This force could seriously prevent approach of one cell to another. A positive ion added to sludge would partially neutralize the negative charges on bacterial surfaces by adsorption, causing a significant reduction in the electrical repulsion between bacteria. The positive ion hence initiates cell-to-cell interaction which is a crucial initiation towards granulation.
In addition, the multivalence positive ion could also compress the double layer to promote cell aggregation (Zita and Hermansson, 1994). Moreover, the multivalence positive ion may promote sludge granulation by bonding with extracellular polymers (ECPs), and high affinity between ECPs and calcium ion had been reported (Forster and Lewin, 1972; Rudd et al., 1984). This implies that calcium ion may bridge ECPs to ECPs and/or link cells to ECPs to form an initial three-dimension structure of microbial community, in which bacteria could grow further.
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