Combining submerged membrane technology with anaerobic and ...

Chapter 7cBPC concentrations. Regarding TEP, although it seemed to control the progressiveaccumulation observed until day 150, its concentration remained at extremely high levels,never reported before.The evolution of sludge filterability properties was also monitored before and after thePAC addition. On figure 7.9 can be observed the evolution of SRF and cake and colloidalfraction resistances. Although it was demonstrated that these parameters did not remainconstant (figures 7.6 and 7.7), it was necessary to calculate punctual values in order tofollow its evolution. In this sense, filterability data corresponding to the first phase offormation of the cake were discarded and only the data corresponding to the moment onwhich the cake was consistently formed were taken into account (from 1000s of filtrationonwards). Contrary to observed in other studies (Choo et al., 2000; Hu and Stuckey,2007; Akram and Stuckey, 2008), the addition of PAC to the AnMBR on day 152 did notexhibit a significant effect on membrane performance. Although SRF punctually decreasedafter the addition of PAC, cake and colloidal fraction resistances were not affected, andeven increased slightly (figure 7.9). As occurred with TEP and BPC concentration, SRFvalues obtained were much higher than those typically reported for anaerobic sludge(Metcalf & Eddy, 2003). For instance, Cho et al. (2000) observed a decrease on SRF afterthe addition of PAC reaching values of 9.9·10 15 m·kg -1 , which was twice smaller than thatof the cake without PAC. These values represented only the 15 % of typical SRF valuesobtained during the present study. Regarding cake resistances, the values obtained were10-fold higher than those reported by Robles et al. (2013) in a submerged AnMBR treatingmunicipal wastewater at MLTS concentrations up to 25 g·L -1 .Moreover, critical flux was also determined before and after the addition of PAC. Anysignificant improvement was observed with respect to critical flux, remaining between 3.0and 3.6 L·m -2·h -1 . This might indicate that the amount of activated carbon used in this studywas probably less than optimum, and thus the flux could probably be further improved byadding more PAC to the reactors as suggested by Park et al., 1999. Nevertheless, Akramand Stuckey (2008) found the worsening of membrane performance fluxes with anincrease in PAC addition. Others factors such as solution chemistry (Braghetta et al.,1997) and type and concentration of dissolved organic compounds (Chang and Lee, 1998)also influenced the role of PAC in flux improvement.It is recognized that the colloidal material is mainly responsible for fouling in anAnMBR (Choo and Lee, 1996, 1998). Lower diffusion rates of the colloidal particles resultinto a slower transport back into the bulk solution than coarser ones (Choo and Lee, 1998),which means they tend to accumulate at the membrane surface and form a dense cake188