Treatment of Presettled Municipal Wastewater Using a Passively Aerated Vertical Bed

Abstract

A novel pilot-scale passively aerated vertical bed (PAVB) for the treatment of municipal wastewater was investigated. Two modes of PAVB operation were compared. In the first mode, "continuous feeding," an uninterrupted flow of wastewater was applied to the bed until clogging, followed by a "rest period" to regenerate pore space. During the rest period only passive aeration was provided by recirculation of clean effluent through the bed. In the second mode, "intermittent feeding," a daily schedule of 8 h of wastewater flow to the bed followed by a rest period of 16 h was employed. The results indicate that continuous operation is advantageous over intermittent operation. A high COD removal rate of about 800 g/m2/day was obtained when the system operated in the "continuous feeding" mode and the COD removal efficiency was about 50%. Clogging of the bed with apparent ponding occurred at a free pore space of 30 to 35% from its initial value. During both continuous and intermittent modes of operation, an irreversible clogging of 40% was observed. Higher organic removal efficiency was achieved by operating three vertical bed units in a series. The overall average COD and BOD reductions in the vertical bed system were 83 and 88%, respectively, with effluent CODt concentrations between 50 and 128 mg/L and BOD concentrations were between almost zero and 45 mg/l. In contrast to PAVB for tertiary treatment, no significant advantage was shown in using passive aeration when the PAVB was used for secondary treatment with a relatively large particle size (15 mm).