In a conventional activated sludge (CAS) process under reasonable operating condition, effluent quality fluctuation occurs mainly by suspended solids not removed in clarifier as a consequence of sludge bulking. The fine organic particles contained in the effluent contribute not only to TSS, but also to BOD. On the other hand, all the fine particles are completely removed by membrane in MBR, which results in much more stable effluent quality.
MBR effluent is distinguished from CAS effluent by high COD/BOD ratio. Relative to CAS, readily biodegradable COD in mixed liquor is very scarce in MBR due to the low F/M ratio. As a result, BOD of MBR effluent is much lower than that of CAS, but COD is not as low as the low BOD suggests. For example, typical effluent COD is 15-30 mg/L in municipal MBR, but BOD is typically only 1-2 mg/L.
According to Hirani et al. (2013), 90% of MBRs produce effluent with equal or better quality than below based on the survey of 38 satellite MBR across the US.
- NH4-N < 0.4 mg/L-N
- TOC <8.1 mg/L
- Turbidty < 0.7 NTU
- Total coliform bacterial count < 100 CFU/100 ml
- MS-2 bacteriophase < 20 PFU/100 ml
Fig. 1 shows the cumulative distribution of effluent quality for seven different water quality parameters.
Fig. 1. Cumulative probability distributions for water quality parameters measured in effluents of 38 MBR facilities in the US (Hirani et al., 2013).