The sewage treatment capability of non-backwash biological aerated filter systems for small communities.
Biological Aerated Filters (BAFs) are wastewater treatment systems which contain support
media for biofilm development and provide oxygen at the base of the reactor to sustain
aerobic microbial treatment processes. The aims of this study were to characterise and
quantify the treatment capability of non-backwash BAFs used in small sewage treatment
works, and therefore establish design recommendations. Three full scale trial BAFs, a field
operational small sewage treatment plant, and a full scale aeration testing facility were
studied. The trial BAFs, with simulated secondary settlement, provided carbonaceous
stabilisation under various hydraulic loadings and airflow rates. Nitrification showed no
relationship with airflow rate, but was inhibited at high hydraulic loadings. Sludge
production in the BAFs was less at higher airflow rates, due to improved stabilisation of
organic solids. Media specific surface area had little effect on treatment performance.
Downflow operation generally provided better treatment than upflow operation, with high
levels of suspended solids stabilisation, considered a function of longer residence times.
The mean BAF residence times determined empirically were considerably shorter than the
design residence times, and may have inhibited treatment potential. The oxygen transfer
efficiency (OTE) of coarse bubble diffusers was enhanced in a simulated BAF, due to the
effects of the support media. Fine bubble diffuser OTE was inhibited by the media. Coarse
diffusers may be more appropriate than fine diffusers for small non-backwash BAFs, a
function of performance and cost efficiency. A small packaged sewage treatment plant
incorporating BAFs produced a well nitrified effluent with reasonable organic stabilisation,
and showed little diurnal or seasonal variation in effluent quality. The plant compared well
to other small sewage treatment systems, providing treatment in a small footprint. Design
recommendations for non-backwash BAFs and small sewage treatment plants
incorporating BAFs have been established, based on the knowledge gained during this
study. It has been demonstrated that non-backwash BAFs are appropriate for use in small
sewage treatment works.