Feasibility of Pulsed Air Operation in a Fixed Film Biological Reactor

An investigation was made into pulsed air sparging in a downflow, fixed film biological reactor receiving domestic wastewater feed (primary effluent). The pulsed mode of aeration was compared to continuous flow or non-pulsed aeration based on BOD5 and TSS removals. The reactor utilized 0.19 to 0.20 in. (ES) gravel media with a 5.5 ft. depth. Automatic backwashing was initiated by a high loss of head condition through a programmable controller. Average backwash water-volume as a percent of product water was, under stable operation, 16 to 18%. The pilot plan was operated for a period of three and one-half months. A six-week period of stable operation provided data for comparison of pulsed to non-pulsed operation. The pulsed mode of operation placed both cells in a sequence of process air on for one minute and air off for one minute. This mode was continued for one day followed by one or more days of continuous air flow at the same air supply rate for each cell. The organic loading rate during the final six-week period was 6.9 to 156 lb BOD5/1000 ft3-day. The hydraulic loading rate was limited by the organic loading rate to 0.3 to0.7 gpm/ft2. The low organic loading was necessitated by the large, gravel media. The influent BOD5 and TSS concentrations ranged from 70 to 130 mg/1 and 60 to 125 mg/1 respectively. The mean effluent BOD5 and TSS levels were approximately the same for pulsed aeration (16 mg/1 each) as for non-pulsed aeration (14 mg/1). Thus, the research showed no significant deterioration in effluent quality when pulsing the air supply. The mean oxygen supply rate for the pulsed and non-pulsed aeration was 10.65 and 22.94 lb O2/lb BOD5 removed respectively. The mean oxygen usage rate for the pulsed and non-pulsed aeration was 0.58 and 1.03 lb O2/lb BOD5 removed respectively. Thus, significant improvement in oxygen transfer was demonstrated with the pulsed mode of operation. The effluent quality, when pulsing, indicates that oxygen transfer occurs during the air off periods. The air bubbles are trapped or held up in the media during this quiescent period, increasing the air bubble residence time in the media. During this air off interval, the oxygen mass transfer into the biofilm occurs from the bulk liquid and from direct bubble contact with the biofilm. This latter pathway of oxygen transfer is termed interfacial oxygen transfer (ITF). Prior studies have shown this method of transfer may account for 30 to 70% of total oxygen transfer. Thus pulsing provides an ideal environment for the enhancement of ITF and realization of reduced energy requirements for oxygen supply in aerobic biological treatment systems