The Research On Balance Between Zinc/Cadmium Medium Adsorption And Plant Absorption In The Constructed Wetland Sewage Treatment System

The renewable development of human world and economy was restricted increasingly due to shortage of the water resources and pollution of the water environment. In particular, along with the accelarated industrialization course, a mass of heavy metal containing waste waters was discharged into rivers and lakes, resulting in sever pollutions of heavy metals, in particular Zn,Pb,Cd,Cu and Cr, in water ecosystems. Not only the shortage of useable water resource was directly affected by heavy metal pollution, but also the security of drinking water, food and crop production, and the health of human being were endangered. Thus, an important orientation of manage environment is looking for a multiprocessing with waste water containing heavy metal.Constructed wetland, a new wastewater treatment technology, was developed in 1970s. On account of the merit of low investment, low energy consumtion, relatively high treatment efficiency, and easy operation and management, it has become a hotspot in the research field of sewage treatment. Recently, some studies indicated that the special and complex mechanism of constructed wetlands can play an important role in dispose of industry wastewater containing heavy metals. Effectively depression of the heavy metals in wastewater is the key to heavy metal sewage disposals. Physichemical adsorption and bio-absorption are uppermost approaches to wipe off the heavy metal hydronium. Thus, choose of proper adsorbents used as wetland fillers and determination of their adsorption characteristics, as well as selection of adequate plant species and study of their absorption natures are important topics for removale of heavy metals from wastewater using constructed wetland techniques. Heavy metal ions can affect the growth of plants in wetland and depress the treatment capacity of the whole system. On the other hand, there are limitations for introduction of "high-accumulative" plant species into constructed wetlands targeted at specific metal element. There are few sources of such plants available for selection and furthermore they are usually adopted to specific environmental conditions with low growth rates, It is therefore of significance to find altanative solutions with purpose to increase the treatment capacity and sustainability of constructed wetland system for disposal of heavy metal containing wastewaters.Aiming at these issues, we established an ion adsorption buffering system with vermiculite as adsorbent to accommodate the plant absorption system in constructed wetlands. Based on the established treatment systems experiments were carried out to investigate the essential factors that determine the treatment efficiency. The results obtained from the study indicated:1) Accounted for by its abundance in natural resource,, low price, high adsorption capability and re-generable nature, vermiculite can be used as wetland fillers for removal of heavy metals from wastewaters.2) The adsorption capacity of vermiculite samples was determined as X for zinc and Y for cadmium. The optimal adsorption time in terms of cost-efficiency was found to be 60 min within the proper temperature range 15～45℃and pH range 3.5-7.3) For the campus sewage containing 6.20 mg/L of Zn2+ and 0.8 mg/L of Cd2+,the designed load rate of vermiculite is 30 mg/kg for zinc and 11 mg/kg for cadmium.4) The four selected hydrophyte plants had different endurance to zine and cadminm concentration levels. The highest endurable concentration of zine was found to be 10 mg/L for Eichhornia crassipes,5 mg/L for Ipomoea aquatica Forsskal and Alternanthera philoxeroides, and 1 mg/L for Nymphoides peltatum. In comparison, The highest endurable concentration of cadmium was found to be 2 mg/L for Eichhornia crassipes,1 mg/L for Ipomoea aquatica Forsskal and 0.5 mg/L for Alternanthera philoxeroides and Nymphoides peltatum.5) Eichhornia crassipes and Ipomoea aquatica Forsskal had the stronger endurance capability and enrichment capability. The maxmum concentration in the enfluent for respective metal species that the selected plant species can not only stand for but also be able to fulfill the GB discharge standars was 1.84-10 mg/L(Zn) and 0.32～2mg/L(Cd) for Eichhornia crassipes;1.8～5mg/L(Zn) and 0.29～1 mg/L (Cd) for Ipomoea aquatica Forsskal. 6) Based on tank culture tests the designed parameter of harvest time for Eichhornia crassipes and Ipomoea aquatica Forsskal is 15-20 days. The proper range of zinc and cadmum concentration for Eichhornia crassipes is, respectively,1.84～4.00 mg/L and 0.32-0.50 mg/L, and that for Ipomoea aquatica Forsskal is 1.84～4.00 mg/L and 0.32～0.50 mg/L.7) The designed parameter for water load for Eichhornia crassipes is 0.05 g/kg.m3.d for Cd and 0.05g/kg.m3.d for Zn, and that for Ipomoea aquatica Forsskal is 0.16 g/kg.m3.d and 1.30 g/kg.m3.d for the respective metal species.8) Based on the tested results, the engineering parameters for the established vermiculite adsorption and plant absorption equilibrium model are:Vermiculite adsorption unit:HRT=2.1 h, exterior waterpower charge= 11.9 m3/m2.d, cubage waterpower charge=7.94 m3/m3.d, exterior charge of cadmium= 9.52 g/m2.d, exterior charge of zine= 73.78 g/m2.d, cubage charge of cadmium=6.35 g/m3.d, cubage charge of zine=49.21 g/m3.d.Plant absorption unit:HRT=12 h, surface hydraulic load=3.97 m3/m2.d, volume hydraulic load=1.98 m3/m3.d, surface cadmium load=1.59 g/m2.d, surface zinc load= 4.17 g/m2.d, volume cadmium load=0.79 g/m3.d, volume zinc load=2.08 g/m3.d.