Study On Selection Of Adsorbent Fillers And Treatment Efficiency Of Aerated Adsorption Bed In Constructed Wetland System

Constructed wetland is a resource-conserving and environment-friendly method frequently applied in wastewater treatment. Selection of adsorbent fillers and study on adsorption mechanisms are important topics in related fields. The main problem associated with the adsorption technology is its high cost in regeneration of the adsorption capacity of the adsorbent. In the present study equilibrium adsorption experiments were conducted to determine the adsorption capacity of the adsorbents for selection of proper adsorbent fillers. Aerobic adsorption systems were established using the selected adsorbents for treatment of sewage mixed with piggery wastewater. Investigation on the feasibility of regenerating the system adsorption capacity by means of aerobic digestion of the organic pollutants adsorbed on the adsorbent surface was further carried out. The purpose of the study was to provide both theoretical and experimental supports for improvement of constructed wetland technology. The results obtained from the study are summarized as follows:1) Activated carbon made of fruit shells was found to have the highest adsorption capacity among the five tested adsorbents with respect to the removal of organic pollutants from the mixed wastewater and its capacity to adsorb COD was determined as 24.45 mg/g. Natural zeolite was shown to be effective to remove NH4+ while hydrotalcite and red soil were effective to remove PO43- ions.2) The curve of the change in chemical potential of the adsorption reaction was shown to be useful for determination of the adsorption capacity of the adsorbent (p). The p value for the natural zeolite sample to adsorb NH4+ and that for the red soil to adsorb PO43- ions were found to be 2.75 and 2.20 mg/g, respectively.3) The removal rates of COD in aerobic adsorption systems followed the order: system A (active carbon)>system C (red soil)>system B (natural zeolite); that of TN followed the order:system B (natural zeolite)> system A (active carbon)> system C (red soil) and that of TP:system C (red soil)>system B (natural zeolite)> system A (active carbon).4) In the whole tested treatment period, the removal rates of COD, TN and TP were all consistently higher in aerobic adsorption systems than in anaerobic adsorption systems, but the difference in removal rates between the two systems decreased gradually with increase in the amount of treated wastewater. The present results indicated that the system adsorption capacity could be maintained by aerobic digestion of the adsorbed organic pollutants and the function of aerobic digestion in regeneration of the system adsorption capacity was related to both the COD concentration and the amount of treated wastewater.5) The contribution of aerobic bio-degradation rate in aerobic adsorption systems presented a cycle mode of decrease-increase-decrease. The experimental results showed that the optimal aerobic bio-degradation rate in the active carbon system should be designed in the range from 30% to 45% in order to maintain systems treatment efficiency at stable levels.6) The absolute value of the change in the Gibbs free energy of the anaerobic adsorption systems decreased and finally approached to zero with increase in the amount of treated wastewater, indicating that the adsorbent in the anaerobic adsorption system was gradually saturated. In contrast, the change in the Gibbs free energy of the aerobic adsorption system was irregular in the tested period, showing that the treatment efficiency of the system was comparably stable.The main contributions made in the present study are1) Proposed a method to determinate the adsorption capacity of the adsorbent using the curve of the change in chemical potential of the adsorption reaction.2) Determined the proper range of aerobic bio-degradation contribution rate for design of aerobic adsorption systems.The results obtained from the present study are of certain theoretical and practical values for development of aerobic adsorption technologies.