Fluctuation of organic loading often diminishes dissolved oxygen (DO) concentration especially at peak loading. Insufficient blowing capacity is a common culprit, but it can be also attributed to the low oxygen transfer efficiency (OTE) at high organic loading rate as discussed here. Lagging air flow adjustment can also cause temporarily low DO condition even if aeration capacity is sufficient. Though membrane fouling tendency increases with organic loading fluctuation, it is hard to know whether the fluctuating organic loading is a culprit or the low DO is a culprit.
An experiment was performed with two bench scale MBRs equipped with flat sheet Kubota membranes under a well controlled lab condition. One MBR was run at a constant organic loading (control train) and the other MBR was run at variable organic loading (treatment train). As shown in Fig. 1, feed TOC for the treatment train was doubled for an hour twice a day. By reducing feed TOC in later part of the day, average daily TOC loadings were maintained identical for the two MBRs. Due to the sufficient aeration, DO stayed above 2 mg/L even at peak loading conditions.
As shown in Fig. 2, variable organic loading caused high TOC and polysaccharide concentrations in supernatant in the first 80 days (Phase 1), but both supernatant TOC and polysaccharides concentrations decreased sharply after that (Phase 2). On the contrary, the two concentrations were fluctuating within a range in constant organic loading case. Overall, higher supernatant TOC and polysaccharides concentrations were observed with variable organic loading in Phase 1, but the trend was reversed in Phase 2. Average particle size data was also in line with other observations, where it was smaller with variable organic loading than with constant organic loading in Phase 1, but it the trend reversed in Phase 2.
In the same study, the trend of TMP increase was well matched with the trend of supernatant TOC, polysaccharide, and particle size. At a constant flux of 20 LMH, TMP increased faster with variable TOC loadings than with constant TOC loadings in Phase 1 as shown in Fig. 3. But the trend was reversed in Phase 2 as shown in Fig. 4, where TMP increased faster with constant organic loadings.
The above observations suggest that shock organic loading at 2x peaking factor does not permanently increase membrane fouling rate, if dissolved oxygen is maintained high enough, e.g. 1-2 mg/L or higher. Although this is a very plausible postulation, there are no enough evidences obtained from rigorous lab- and full-scale experiments.
Fig. 1. The hourly variations in TOC of feed and DO in aeration tank in one day (Zhang, 2010).
Fig. 2. Polysaccharide in supernatant at constant and variable loading (Zhang, 2010).
Fig. 3. Time curves of TMP with constant and variable organic loading rates in Stage 1 (Zhang, 2010).
Fig. 4. Time curves of TMP with constant and variable organic loading rates in the Stage 2(Zhang, 2010).
© Seong Hoon Yoon