The effects of media size on the performance of biological aerated filters

Biological aerated filters (BAFs) are an attractive process option, particularly when low land usage is required. They can combine BOD, solids and ammoniacal nitrogen removal and can be utilised at both secondary and tertiary stages of wastewater treatment. Media selection is critical in the design and operation of BAFs to achieve effluent quality requirements. Two size ranges, 1.5–3.5 and 2.5–4.5 mm, of a foamed clay called StarLight C were used in pilot-scale reactors. Both performed well as BAF media, with reactor loads up to 12 kg COD m−3 d and 4 kg suspended solids m−3 d (based on working volumes). The most consistent effluent was obtained using the smaller medium since, at flow rates above 0.4 l min−1, the BAF using the larger medium produced an effluent containing more than 20 mg l−1 of suspended solids for over 30 min after backwashing. Up to 70% longer run times, as determined by reaching a set head loss, were recorded for the BAF containing the larger rather than the smaller medium. Additionally, the development of pressure above the smaller medium filter bed tended to be logarithmic rather than linear. Reactor profiles indicated that suspended solids removal did not occur over the full 2.3 m depth of the columns. The BAF containing the smaller medium utilised a mean depth of 1.7±0.3 m, whereas a mean depth of 2.1±0.3 m was used by the larger medium BAF. Both the head loss development data and the suspended solids removal profiles indicated that the smaller medium BAF was underperforming as a filter.