Enhanced aerobic/anoxic Downflow Hanging Sponge (DHS) bioreactors were assessed for carbon (C) and total nitrogen (TN) removal for decentralised domestic wastewater treatment applications. The initial design included upper aerobic and lower anoxic sponge layers, and effluent recirculation, and achieved >80% COD and >90% NH-N removal. However, effluent TN was higher. It was concluded the anoxic layer was C-limited for denitrification, therefore an influent bypass was added to the anoxic layer to provide supplemental C. Differed bypass ratios were compared, including 0%, 10%, 20% and 30% (% of total influent), and effluent TN declined with increasing bypass; i.e., 50.1±23.3mg-N/L, 49.9±27.8mg-N/L, 31.9±18.4mg-N/L and 10.7±5.8mg-N/L, respectively, and all reactors removed >80% COD. This design has potential because it uses limited energy, tolerates variable flows, and simultaneously removes C and TN; all key for effective decentralised treatment applications.
This paper evaluates the effect of variation in red light-emitting diodes (LEDs) irradiance on the growth rate and biomass productivity of a mixed culture of microalgae grown on synthetic municipal wastewater, with and without CO 2 addition. Red LEDs were used to illuminate microalgal culture from the centre of 21-L stirred-tank photobioreactors made of transparent Plexiglas, each reactor having a working volume of 16 L. The reactors were operated in batch mode with pH control, and under continuous illumination for 30 days at ambient temperature. Mixing was achieved through the use of overhead mechanical stirrers operated at 100±1 and 60±1 revolution per minute, before and after the addition of CO 2 , respectively. Three average values of irradiance of 429.9, 582.7 and 730 .8 µmol.s -1 .m -2 were used to illuminate the reactors, with a control reactor operated in the dark. CO 2 addition resulted in about two-fold increase in biomass productivity in all the experimental reactors. The bioreactor with medium irradiance yielded the highest biomass productivity and maximum specific growth rate of 0.034 g.L -1.d -1 and 0.109 d -1 , respectively. The findings in this study show that both microalgal growth rate and biomass productivity are not always directly proportional to irradiance, despite the influence of process and operational parameters. Furthermore, a medium amount of irradiance resulted in optimum growth and productivity of the mixed microalgal culture.
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