ENVIRONMENTAL ENGINEERING SCIENCE Volume 16, Number 3, 1999 Mary Ann Liebert, Inc. A Review of Biological Aerated Filters (BAFs) for Wastewater Treatment LEOPOLDO MENDOZA-ESPINOSA and TOM STEPHENSON School of Water Sciences Cranfield University Cranfield, Bedford, MK43 OAL United Kingdom ABSTRACT Biological aerated filters (BAFs) can combine ammonia, carbonaceous matter, and solids removal in a single-unit process. Biological nutrient removal (N and P) can also be accomplished. Removal rates based on reactor volume for carbonaceous BOD, ammonia, and nitrates of up to 4.1 kg BOD m- 3 day-I, 1.27 kg NH3-N m- 3 day-I and 5 kg N0 3 -N m- 3 day-I, respectively, are normally re- ported. The small footprint and adaptability of BAFs allows them to be used in upgrading estab- lished works, especially those in built up areas where space is at a premium. Although many con- figurations are available, including upflow and downflow, sunken, and floating media, the process still requires some optimization, especially with regard to media type, backwashing rates, and aer- ation control to decrease power consumption. Key words: biological aerated filter; wastewater treatment; submerged media bioreactor; carbonaceous matter. removal; nitrification INTRODUCTION S UBMERGED AERATED FILTERS are biofilm systems used for secondary and tertiary biological treatment of wastewater (Stensel and Reiber, 1983). They consist of three phases: a solids phase that acts as the support me- dia for microbial growth, a liquid phase in which the solid material is submerged, and a gas phase created by the in- put of air into the reactor. The media in submerged aer- ated filters can be structured or granular. When granular media is used, the system is capable of removing organic matter and suspended solids from wastewater at the same time; thus, there is no need for a subsequent solids sep- aration stage, i.e., sedimentation tanks. Periodic back- washing is necessary to avoid blocking of the system due to solids entrapment and biofilm growth. This review fo- 201 cuses on granular media-submerged biological aerated filters, also known as biological aerated filters (BAFs), which hav the unique capability of achieving solids, car- bonaceous organir. matter, and nutrient removal in a sin- gle reactor. History and Origins Although true BAFs, capable of removing solids and organic matter without requiring additional sedimenta- tion, did not appear until the end of the 1980s (Pujol et aI., 1994), their origins can be traced back to contact aer- ators, which have been used for more than 50 years (Rusten, 1984). Probably the first report of these reactor types dates from 1913, where aerated tanks containing layers of slate support.ing a biofilm ".mployed for