Nitrogen and phosphorus removal in biological aerated filters (BAFs)
Three downflow pilot-scale biological aerated filters (BAFs) using different media, gravel, lava rock and plastic rings were operated to study their effectiveness as an add-on system or polishing process for aerated lagoons. Of the three media tested, gravel with a diameter of 5 mm was found to provide more than 96% nitrification at hydraulic retention time (HRT) as low as 0.5 hour with 200% recirculation. Experimental results showed that polishing BAF with gravel can be operated at 1 hour HRT without recirculation with nitrification as high as 83%;At a HRT of 2 hours and a wastewater temperature of 6.5°C, more than 98% of ammonia was nitrified in a gravel BAF. For a HRT of 1 hour at 6.5°C, ammonia removals (%) can be improved from 54.3 +/- 2.7% without recirculation to 76 +/- 3.8% with 100% recirculation. With 200% recirculation, ammonia removal improved further to 92 +/- 1.5%. C/N ratio of influent wastewater had a negative impact on nitrification with less than 5% ammonia removal for a C/N ratio of 8 at 6.5°C. Ammonia loading reached a maximum of 0.36 kg NH3-N/m3-day at 6.5°C while the maximum ammonia loading was 0.63 kg NH3-N/m3-day at 24°C;A pilot-scale partially aerated BAF was operated with an anaerobic, anoxic and oxic zone at a temperature of 24°C for the removal of nitrogen at various hydraulic loading rates and recirculation rates. With 300% recirculation at a HRT greater than 3 hours, the total nitrogen removed was approximately 80%. sCOD removals and nitrification at HRT of 3 hours with recirculation rates of 200% and 300% was more than 96%;By adding two alternating attached growth columns (diameter = 75 mm) after the partially aerated BAF, the phosphorus removal of more than 96% were obtained for influent phosphorus concentrations of 8 mg P/L and 16 mg P/L. The experimental results demonstrated that by using two alternating biofilm filters in conjunction with a partially aerated BAF, phosphorus removal can be achieved.