Effect of hydraulic shock loading on membrane fouling

   Hydraulic shock can occur if rain water is mixed into raw wastewater. Since additional COD/BOD loading to MBR is not much, there should not be too much of change in biological conditions except the change of ionic strength depending on the differences of ionic contents in raw wastewater and rain water.

   In one experiment (Syed, 2009), the experimental setup shown in Fig. 1 was used to test the effect of hydraulic shock. Tank volumes of aeration tank and membrane tank were 380 L and 120 L, respectively, and membrane area of Z10 module was ~2.7 m2.  A drum screen with mesh size of 0.5 mm was used to filter municipal wastewater. Before the test, the experimental setup was run for around three months to obtain a steady-state condition. Hydraulic loading shock (at 2Q) was simulated by diluting influent with an equal amount of city water. Experimental conditions are summarized in Table 1.

   No significant changes of DO, MLSS, and pH were observed during the high hydraulic loading periods that lasted 24 hours each. But, TMP increased in order to obtain 100% more flow through the same membrane surface area as shown in Fig. 2. However, the large portion of the increased TMP was recovered after the flow went back to the normal.  

   One explanation for the marginal impact of hydraulic shock on membrane performance is the lowered SMP during the peaking event as shown in Fig. 3. According to this figure, SMP decreases during the hydraulic shock (first 24 hours) perhaps due to the expedited loss of SMP through permeate. On the contrary, EPS, which is attached on microorganisms, stayed at almost same level during the peaking event.

   Other change observed was particle sizes became slightly smaller during the peaking event, which might be affected by the lowered ionic strength due to the dilution water that contains less salts than the law wastewater. 

Shocki10Fig. 1. Experimental setup (Syed, 2009).

                        Table. 1. Experimental condition of hydraulic shock loading test  (Syed, 2009)             

Parameters Unit Steady state

(before peaking)

Hydraulic peaking condition
F/M ratio gCOD/gVSS/d 0.26 0.26
DO mg/L 2.5-3.5 2.5-3.0
SRT days 15 15
HRT hours 8 4
MLSS g/L 8 ~8
Test duration hours ~2,100 24

Shocki12Fig. 2. Effect of hydraulic peaking on TMP (Syed, 2009).

Shocki14Fig. 3. Effect of hydraulic peaking on soluble microbial products (SMP)

 

© Seong Hoon Yoon