Influence of the redox condition dynamics on the removal efficiency of a laboratory-scale constructed wetland.

A laboratory reactor planted with Juncus effusus treating an artificial wastewater was used to investigate the short-term and long-term variations and interactions in the redox conditions as well as the removal efficiency of C and the N turnover. The permanent circulation of the process water enabled the micro-gradient processes to be evaluated for an operating period of 20 months. Steady-state conditions were achieved throughout the operating period with high mean removal efficiencies of 92.7% total organic carbon, 82.0% ammonia and 97.6% nitrate. Daily variations in the redox state of the rhizosphere of a few hundred mV were observed, ranging from about -200 to oxidized conditions of about +200 mV and driven by daylight. Variations in pH associated with changes in light and redox were linked to the dynamics of the fates of organic and inorganic carbon species. The ammonia removal processes were found to be firmly established, including for moderately reduced redox conditions with high efficiencies for E(h)>-50 mV. The enrichment of ammonia (up to 13 mg l(-1)) closely linked to the light, particularly during summertime, indicates the existence of hitherto unconsidered additional N turnover pathways in the rhizoplane involving N(2) produced by microbes or released by plants. C turnover was strongly related to the seasonal variation in illumination with minimum efficiencies during the dark season. In addition, it was characterized by oscillation with periods of approximately 1 month. The relationships found are dominant for biofilms on the rhizoplane and decisive for the removal efficiency of especially simple constructed and natural wetlands. The results highlight the importance of helophytes and their physiological specifics for removal processes.