Development of a recitculating plastic media Biological Aerated Filter (REBAF)

True biological aerated filters (BAFs) combine two unit operations in one reactor: aerobic biological treatment and subsequent biomass separation from the effluent (Stephenson et al., 1993). The submerged media used as support for the microbial biofilms are usually granular in nature and are therefore able to also act as depth filtration media. Therefore BAFs should provide a small footprint alternative to traditional aerobic processes. Indeed, Dillon and Thomas (1990) noted that a "good quality effluent" was possible up to a loading of 4.1 kgBOD/m3/d (9.1 kgCOD/m3/d); with an automated backwash programme influent BOD5 was reduced by between 90.3 % and 97.6 %. Pujol et al. (1992) found an effluent standard of 90 mgCOD/I difficult to achieve for loading rates above 6.0 kgCOD/m3/d. Stensel et al., 1988) achieved 88 % removal of ammonia at a loading rate of 1.6 kgBOD/m3/d and modern BAFs can achieve simultaneous carbonaceous BOD and ammonia removal at loading rates of 2.5 kgBOD/m3/d (Rogalla and Payraudeau, 1987). Despite the impressive loadings rates possible, BAFs require large volumes of water for backwashing on a regular basis to prevent blockage of the media. Therefore these processes require large volume tanks to hold effluent for backwashing and mudwells for collection of the backwash water. In addition, large pumps and pipework systems are needed to achieve the required backwash flowrates. In depth filters, this problem can be overcome by using continuous filtration systems, e.g. Dynasand (Kramer and Wouters, 1993). These incorporate a media recycle and wash system which allows uninterrupted treatment to occur. In Dynasand, the liquid flow is upwards and the media flow downward. The sand is removed from the base of the reactors, cleaned and then transported to the top of the bed. Thus the water exiting the filter always see a clean polishing layer of sand. The sand is removed by means of an airlift pump at the base of the reactor. Moving bed systems have been tested on sewage as an alternative to sedimentation (Mixon, 1973). It is possible that the advantages of the moving bed principle could be combined with BAF technology to reduce or eliminate the need for expensive backwashing facilities. Such a system would not be the same as moving bed biofilm reactors described by Rusten (1984) and Hem et al. (1994) which require separate secondary sedimentation tanks. The current paper reports on an investigation into operating a plastic media BAF with media recirculation in order to eliminate backwashing.