ANALYSIS ON THE ADSORPTIVE CHARACTERISTICS FOR AMMONIA NITROGEN AND PHOSPHORUS OF DIFFERENT SUBSTRATES IN CONSTRUCTED WETLANDS

Nitrogen and phosphorus adsorptive characteristics of four substrates, namely, lytag, gravel, zeolite and limestone, were studied through basic experiments. Substrate is an important component of constructed wetlands, playing a main role in the removal of nitrogen and phosphorus. The results revealed that nitrogen and phosphorus adsorption rates of zeolite were the highest among the four substrates which were all greatly affected by pH. The phosphorus adsorbance of zeolite and limestone increased as pH rose, and the opposite was true for lytag and gravel; neutral and acidic conditions were conducive to zeolite absorption of ammonia nitrogen, while for the other three substrates, the ammonia nitrogen absorbance increased with the rise of pH. Freundlich and Langmuir models were used to fit adsorption isotherm equation experimental results, revealing that in terms of Go in ammonia nitrogen adsorption, zeolite > lytag > gravel > limestone; and in terms of Go in phosphorus adsorption, lytag > limestone > zeolite > gravel. The liberation experiments carried out after substrate adsorption saturation revealed that the ammonia nitrogen adsorbance of zeolite was large, its adsorption efficiency was high and the liberation quantity after saturation was low; while the phosphorus adsorbance of lytag was large and percentage of liberation was low. The results demonstrated that the liberation of nitrogen and phosphorus, occurring in the four substrates, may lead to the secondary pollution of water bodies.