Oxygen transport through aquatic macrophytes: The role in wastewater treatment

Laboratory experiments were conducted to determine the effectiveness of three floating and six emergent aquatic macrophytes in improving domestic wastewater quality, based on their capacities for O₂ transport into the effluent. Oxygen transport into the rooting zone of the plants created an oxidized microenvironment, thereby stimulating C and M transformations critical to wastewater treatment. Plants were cultured in flasks containing deoxygenated primary and secondary sewage effluent for an 8-d period. Oxygen transport by the plants was measured in terms of both O₂ consumed by the effluent (biological O₂ demand reduction-BOD₅) and increased effluent dissolved O₂. Two floating plants, pennywort (Hydrocotyle umbellata L.) and waterhyacinth [Eichhornia crassipes (Mart.) Solms], and the emergent plats pickerelweed (Pontederia cordata L.) and common arowhead (Sagittaria latifolia L.) were superior in improving primary sewage effluent quality, by reducing BOD₅ up to 88%, NH₄-N up to 77%, and increasing dissolved O₂ up to 6.1 mg L^-1. Nitrification rates in pennywort- and water hyacinth-based water treatment systems were calculated to be in the range of 12 to 47 kg NH₄-N ha^-1 d^-1. Oxygen transport through plants accounted for up to 90% of the total O₂ transported into the effluent. In separate batch experiments, the effectiveness of diffuse mechanical aeration (5 and 50 mL air min^-1)and biological aeration (O₂ transport by selected plants including pennywort, waterhyacinth, pickerelweed, and common arrowhead) on the rate of contaminant removal from deoxygenated primary sewage effluent were compared for a 26-d period. Biological and mechanica aeration effected similar BOD₅ removal. First-order reaction rate constants for BOD₅ removal were from 0.0066 to 0.0079 h^-1 and from 0.0041 to 0.0051 h^-1 for biological and mechanical aeration, respectively. Rate constants for NH₄-N removal were from 0.0024 to 0.0107 h^-1 for the plant treatments. Virtually complete BOD₅ removal occurred in biological and mechanical aeration treatments within 20 d. Complete nitrification of NH₄-N had occurred within 12 d after mechanical aeration was initiated, but subsequent N-loss by denitrification was inhibited. In the biological aeration treatments, negligible effuent (NO₃ + NO₂)-N levels were measured, but 65 to 100% NH₄-N loss occurred both by plant assimilation and by sequential nitrification-denitrification reactions.