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Statistically significant differences were found when comparing MLSS and VSS concentrations for activated sludge recirculating from each module and the MBR system (Table 4). According to these results, it can be concluded that in those modules where sonication was supplied at lower frequencies, the MLSS and VSS concentration increased in a more significant way. Discrepancies can be found about this issue. Authors such as Na et al. (2007) observed mineralization of organic matter by sonication with the consequent decrease of suspended solid concentration. Other authors such as Feng et al. (2009) also showed that ultrasonication strongly affected the amount of soluble matter due to deflocculation and solubilization of particulate material, in agreement with the result of PSD analysis. This phenomenon would imply a loss of biomass because smaller particles could pass through the membrane, as seen with COD for lower assayed US frequencies, which is favoured by the efficiency in removing cake layers with lower US frequencies because pores remain free. However, these results showed that when lower frequencies were applied, higher MLSS and VSS concentrations resulted.

Table 4

ANOVA and Student–Newman–Keuls test (p < 0.01) for activated sludge parameters

  40 Hz30 Hz25 Hz20 HzMBRp-value
MLSS g/L 4.71 ± 0.67a 5.32 ± 0.94a 6.16 ± 1.10b 6.41 ± 0.97b 5.13 ± 0.76a 0.0001 
VSS g/L 3.61 ± 0.50a 4.06 ± 0.74a 4.70 ± 0.86b 4.90 ± 0.74b 3.92 ± 0.58a 0.0001 
Viscosity cP 3.11 ± 0.43a 3.56 ± 0.56a 4.29 ± 0.68b 4.05 ± 0.61b 3.60 ± 0.40a 0.0001 
  40 Hz30 Hz25 Hz20 HzMBRp-value
MLSS g/L 4.71 ± 0.67a 5.32 ± 0.94a 6.16 ± 1.10b 6.41 ± 0.97b 5.13 ± 0.76a 0.0001 
VSS g/L 3.61 ± 0.50a 4.06 ± 0.74a 4.70 ± 0.86b 4.90 ± 0.74b 3.92 ± 0.58a 0.0001 
Viscosity cP 3.11 ± 0.43a 3.56 ± 0.56a 4.29 ± 0.68b 4.05 ± 0.61b 3.60 ± 0.40a 0.0001 

a,bDifferent letters mean significant statistical differences between groups.

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