Oxygen Transport through Aquatic Macrophytes: The Role in Wastewater Treatment

Abstract

AbstractLaboratory 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 N 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 plants pickerelweed (Pontederia cordata L.) and common arrowhead (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 of 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 mechanical 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 effluent (NO3 + NO2)‐N levels were measured, but 65 to 100% NH4‐N loss occurred both by plant assimilation and by sequential nitrification‐denitrification reactions.