The effect of media size on biological aerated filter (BAF) performance

BAFs are used for the secondary treatment of wastewater and combine biological treatment with suspended solids capture, negating the need for a separate solids removal stage. The costs associated with BAFs (apart from construction costs) are mainly due to aeration and backwashing. Further research is needed to optimise the process and decrease energy requirements and associated costs. One area which may be important in this respect is the size of media employed. Research was carried out to compare the performance of four different sizes of the same media type at different hydraulic/organic loading rates. A number of process variables (including effluent quality, headloss, oxygen transfer) were monitored to gain an understanding of how the process as a whole was affected by changing the size of media employed. Results showed that effluent quality in terms of BOD, SS and NH4-N generally improved with decreasing media size. The smallest media size (2-4mm) had a decrease in nitrification above loads of 0.6kg NH4-N/m3/d whilst 2.8-5.6mm media nitrified up to 1kg NH4-N/m3/d, this suggests that there may be an optimum size for combined carbonaceous treatment and nitrification. Oxygen transfer rates were similar for three of the media sizes with the small media having high transfer rates, this was reflected in the low denitrification rates seen in small media. The small media sizes accumulate more solids than large media, this leads to shorter run times and more frequent backwashing than in large media. Large media sizes are therefore useful for decreasing high BOD/SS loads (roughing filtration), whilst small media can be used for tertiary nitrification processes where there are strict NH4-N consents