Abstract
In conventional activated sludge (CAS) involving aerobic biological processes, the retention of silica nanoparticles (SiO2 NPs) has no detrimental effect on chemical oxygen demand (COD) and ammonia nitrogen (NH3–N) removal. However, for the membrane bioreactor (MBR) system, which is also based on the activated sludge process in addition to the membrane separation process, it has implications not only on the process performance but also on membrane fouling. To investigate these two implications in lab-scale experiments, we continuously operated a control MBR and two experimental MBRs, in which the 28 nm SiO2 NPs and 144 nm SiO2 NPs were added separately to the influent at a final concentration of 100 mg/L. Although the retention of SiO2 NPs in the MBR, as confirmed by dynamic light scattering (DLS) analysis, did not compromise the COD and NH3–N removal, it resulted in substantial increases in the transmembrane pressure (TMP) suggesting the onset of membrane fouling. Analyses by batch-dead end filtration revealed the same fouling trend as observed during the continuous MBR experiments; membrane fouling is aggravated in the presence of SiO2 NPs. This was evident from permeate flux decline of between 30% and 74% at very low TMP (5 kPa) and the further increases in the total resistance.
Highlights
The presence of SiO2 NPs in wastewater discharged from domestic and industrial sources raises two concerns for the biological wastewater treatment systems
The evolution of mixed liquor suspended solids (MLSS) following the addition of SiO2 NPs in the two experimental membrane bioreactor (MBR) setups (Figure 2a) indicates retention of SiO2 NPs in the mixed liquor
At the end of the experiment, the final MLSS concentration in the experimental MBR setups relative to the control MBR setup (4.2 g MLSS/L) increased by 28% and 64%
Summary
The presence of SiO2 NPs in wastewater discharged from domestic and industrial sources raises two concerns for the biological wastewater treatment systems. One is their substantial stability in wastewater [1] that leads to the release of some of them into the effluent stream [2]. Another one is the potential impact of their consequent retention in activated sludge [3,4], which is the core of biological wastewater treatment processes. The greater concern for developing strategies is to improve their aggregation with, incorporation into and settling along with activated sludge [1,5] instead
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