AWWA WQTC59009

Growing concerns over the presence of perchlorate in drinking water sources has promoted research on treatment methods to remove this compound. Biological treatment using packed bed reactors, such as biological activated carbon (BAC), has been proven to be a viable option. Perchlorate is removed by perchlorate reducing bacteria (PRB) growing as biofilms in BAC. PRB and other heterotrophs constitute the biofilm. PRB can utilize perchlorate as electron acceptor. However, PRB can also use oxygen when available. Therefore, maintaining low oxygen concentrations is important to accomplish removal of perchlorate. Backwashing in biological filters serves two purposes. First, regular backwashing is necessary to prevent clogging and excessive pressure drops across the filter and, second, backwashing is necessary to maintain an active biofilm and ensure good external mass transfer of the contaminant to the biofilm (Hozalski and Bouwer, 1998). A previous study has shown that biomass in BAC filters is retained both in the form of thin biofilm attached to the surface of the carbon fibers and also in the form of large microbial aggregates (up to several hundred micrometers) that accumulate in the inter-particle space between the fibers (Choi et al., 2003). These large aggregates can be viewed as thick biofilms that are diffusion limited in contrast to the thin biofilms on the carbon fibers that are likely to be fully penetrated. Backwashing can be expected to washout a large fraction of these loosely attached aggregates (Delahaye et al., 1999). Biofilm directly attached to the carbon fibers is more resistant to shear and only a smaller fraction of this directly attached biomass is expected to be lost during backwashing. Thus, backwashing should not only reduce the total biomass in the system, but also is expected to change the relative amount of thick biofilms (i.e., large microbial aggregates) to thin biofilms (directly attached to the surface of the carbon fibers). This paper demonstrates that large aggregates are important to maintain perchlorate removal in cases of increased influent oxygen concentrations. Mass transfer limitations in the large aggregates cause anaerobic zones to develop in the center of these aggregates, allowing for perchlorate removal to take place. With increased oxygen concentrations in the bulk phase, the thin biofilms on the surface of the carbon fibers will be fully penetrated and will not allow for perchlorate removal to occur. Thus, the hypothesis tested in this paper is that vigorous backwashing of a BAC filter has only a limited influence on perchlorate removal as long as oxygen concentrations in the bulk phase are low. However, after backwashing, BAC filters are more vulnerable to increased bulk phase oxygen concentrations due to the loss of the majority of the large microbial aggregates. To test this hypothesis, a lab-scale BAC filter was operated and perchlorate removal was monitored before and after intensive backwashing. Includes 6 references, figures.