Mechanism of nitrogen removal by adsorption and bio-transformation in constructed wetland systems

Key words: 
• constructed wetland /
• vermiculite /
• ammonianitrogen /
• adsoption /
• bio-tranformation

Abstract: Three constructed wetland systems, a droppingflow system(Ⅰ), a biofilm bed system(Ⅱ) and a nonbiofilm bed system(Ⅲ) (the control system), were established using natural vermiculite as adsorbent filler. The experiment，conducted at a NH+4-N concentration level of 20 mg/L，was designed to evaluate the contribution of NH+4-N removal from wastewaters between pure adsorption and adsorption plus bio-transformation. The results showed that the natural vermiculite minerals could well be used as a wetland filler to remove NH+4-N from municipal sewage. The NH+4-N removal rate in systemsⅠand Ⅱ was not only 20% higher but also much more stable than that in system Ⅲ. Oxygen supply in the droppingflow process further enhanced the treatment efficiency. In the later experiment period the NH+4-N removal rate was 24% higher in system I than that in system Ⅱ. Filler adsorption and bio-transformation were found to be the main mechanisms for NH+4-N removal in the treatment system without plants. In comparison, bio-transformation played an even more important role in NH+4-N removal and its highest contribution rate amounted to 87.4%. In addition to nitrification and denitrification, an anaerobic ammonium oxidation process might also be involved in nitrogen bio-transformation. Results from correlation analyses indicated that the main factors affecting the intensity of nitrification and denitrification in the tested systems are DO, the number of involved bacteria and the bio-mass of formed bio-films.