Modelling biofilm nitrogen transformations in constructed wetland mesocosms with fluctuating water levels

• Published in: Ecological engineering Vol. 14; no. 1; pp. 93 - 106
• Main Authors: McBride, Graham B., Tanner, Chris C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2000
• Subjects: Artificial wetlands; Biofilms; Denitrification; Modeling; Nitrification; Wastewater treatment; Denitrification; Biofilms; Modeling; Wastewater treatment; Artificial wetlands; Nitrification
• ISSN: 0925-8574; 1872-6992
• DOI: 10.1016/S0925-8574(99)00022-1

A mathematical model has been developed that attempts to reproduce patterns of nitrogen removal observed in experiments investigating constructed wetland treatment of ammonium-rich wastewaters under a range of frequencies of water level fluctuation. The experiments were carried out using batch-fed gravel-filled mesocosms, with and without plants, subjected to fluctuating oxygen input through a drain-and-refill regime. Experimental data showed that removal of ammoniacal-nitrogen (NH 4–N) and chemical oxygen demand (COD) increased markedly with fluctuation frequency. Plants also tended to enhance the removal of NH 4-N and COD. For the highest fluctuation frequency (16 cycles per day, plants absent), accumulation of oxidised nitrogen (NO x –N) was observed to continue even when the wastewater NH 4–N had disappeared from solution. A process-based numerical model was developed to elucidate the strength of competing nitrogen transformation processes, which were postulated to be strongly influenced by biofilms and adsorption/desorption associated with gravel surfaces and organic matter, particularly when the mesocosm was empty and liquid on the biofilms was exposed to the atmosphere. A combination of thin-biofilm theory, the microbiological kinetics in the IAWQ activated sludge model No. 2, reversible sorption kinetics and mixing equations was used to demonstrate that very rapid initial decreases in NH 4–N were likely to be caused by adsorption onto the gravel and that during the latter part of the batch periods nitrification was likely to be controlled by the rate of desorption of this NH 4–N. Nitrification could therefore continue when NH 4–N was almost absent from the bulk water. At moderate-to-high fluctuation frequencies (≥6 cycles per day) the presence of plants enhanced NH 4–N removal and NO x –N accumulation, through a combination of direct uptake of NH 4–N and increased root-zone reaeration.