Microbial Granules for Phosphorus Removal

To eliminate phosphorus from wastewater, the phosphorus-accumulating granules (PAG) were developed at different substrate P/COD ratios in the range of 1/100 - 10/100 by weight in SBRs (Lin et al., 2003; Liu et al., 2005). Results showed that granules had typical phosphorus accumulating characteristics, with concomitant uptake of soluble organic carbon and the release of phosphate in the anaerobic stage, followed by rapid phosphate uptake in the aerobic stage.

Formation of PAGs

After a two-month operation, PAGs with a respective mean size of 1.65, 1.22, 1.03, 0.69, and 0.42 mm were formed and dominated in the SBRs run at the substrate P/COD ratios of 1/100-10/100. In contrast to the seed sludge with a very loose and irregular structure, the PAGs cultivated show a compact structure and clear spherical outer shape (Fig. 8.10).

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Fig. 8.10. Morphology of microbial granules developed at different substrate P/COD ratios. Bar: 2mm (Lin et al., 2003).

Characteristics of PAGs

Settleability

The SVI of PAGs decreased with the increase in the substrate P/COD ratio. The lowest SVI of 12ml/g was obtained at the highest substrate

P/COD ratio of 10/100 (Lin et al., 2003). This seems to imply that the substrate P/COD ratio has a profound effect on the structure of PAGs, i.e. the structural compactness of microbial granules are likely related with the feeding P/COD ratios. Compared to the seed sludge, with an SVI of 270 ml/g, the settleability of PAGs was improved distinctly. The reported SVI of non-P-accumulating aerobic granules fell in the range of 50-100 ml/g (Beun et al., 1999; Tay et al., 2001; Moy et al., 2002).

Specific Gravity

The specific gravity of PAGs appears to be proportionally related to the substrate P/COD ratio (Lin et al., 2003). This is consistent with the SVI trend obtained at different substrate P/COD ratios. In the field of environmental engineering, specific gravity has been commonly used to describe how compact or dense a microbial community is. The PAGs cultivated exhibit much denser and more compact structure as compared to the seed sludge (Lin et al., 2003).

Phosphorus Content

In EBPR processes, P in wastewater is essentially accumulated in the form of polyphosphate (poly-P) in microorganisms. The P content in PAGs was in the range of 1.9-9.3% by weight (Lin et al., 2003). If the seed sludge with a P content of 0.85% by weight is taken as reference, the P content of PAGs was 2-11 times higher than that in the seed sludge. It should be pointed out that the P content was typically in the range of 1.5-2% by dry weight in conventional activated sludge processes without the EBPR (Droste, 1997), and 4-15% in the EBPR processes (Crocetti et al., 2000; Panswad et al., 2003). These results seem to indicate PAGs developed under alternative anaerobic and aerobic conditions have a high P-accumulating ability which is essential and desired in EBPR process.

Profiles of Soluble COD and P in One Typical Cycle

Figure 8.11 shows a typical soluble COD and phosphorus profiles in one cycle operation of the microbial granular sludge SBR operated at the

Granular Sludge

Time (minutes)

Fig. 8.11. Cycle profiles of solution COD (o) and phosphorus (•) concentrations in microbial granular sludge SBR run at substrate P/COD ratio of 10/100 (Lin et al., 2003).

Time (minutes)

Fig. 8.11. Cycle profiles of solution COD (o) and phosphorus (•) concentrations in microbial granular sludge SBR run at substrate P/COD ratio of 10/100 (Lin et al., 2003).

substrate P/COD ratio of 10/100 (Lin et al., 2003). These results indicate that the PAGs would release phosphate with concomitant uptake of organic carbon in the anaerobic stage, and assimilate phosphate in the aerobic stage. Figure 8.11 seems to suggest that the microbial granules developed in this study have a great ability to accumulate phosphorus. In fact, Fig. 8.11 exhibits a typical biological phosphorus removal process by PAGs and the curves of COD removal and P release in anaerobic phase and uptake in aerobic phase are quite similar to those obtained in conventional EBPR processes using suspended activated sludge (Comeau et al., 1986; Wentzel et al., 1988; Hiraishi et al., 1989; Jeon and Park, 2000). Thus, it is likely that the mechanisms of the P accumulation in the aerobic granules are similar to those of P accumulation in suspended culture.

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