Study On Removal Efficiency Of Two Typical POPs Substances From Source Water In The Constructed Wetland

This study relying on the "Eleventh Five-Year" national scientific and technological support key project (2006BADO1B09-1) issues and topics of Jiangsu province education department (06KJD560214), carried out the experimental of removing PCBs and PAEs in micro-polluted source water by constructed wetland.Constructed wetland is low cost wastewater treatment method with minimum energy consumption, easy maintenance and ecological benefits. Its application is continuously widening, mostly in sewage and industrial wastewater treatment, urban storm runoff, agricultural wastewater treatment and lake pollution prevention. The constructed wetland has achieved satisfactory results for conventional pollutants treatment. However,less effort has been put into the pretreatment of slightly-polluted water source by constructed wetland, especially the POPs substances, which are usually in low concentration but extremely harmful to human being and ecological environment.The research mainly concentrates on the major factors that affect the removal of POPs substance by a pilot scale contacted wetland, feeded with polluted water source. The constructed wetland operated continuously for 10 month (from June 2007 to April 2008) before it is considered to be stabilized. The major experiments were carried out from April 2008 to May 2009 to study the removal of two typical POPs substance (PAE and PCB) by subsurface flow constructed wetland under different loading, flow pattern and seasonal variations. The main study includes the following aspects: (1) The removal efficiency of PAEs and PCBs in slightly polluted water source by the subsurface flow constructed wetland; (2) The effect of constructed wetland configurations on POPs removal efficiency, the major operational parameters include TOC, hydraulic load, DO, ORP and the pH variations in direction of water flow; (3) The basic mechamims the PAEs and PCBs are removed by the subsurface flow constructed wetland; (4) The spatial and temporal distribution of microbial population, biomass amount and its biological activity in wetland; (5) A model was proposed to evaluate the PCBs and PAEs removal by constructed wetland. The model was calibrated by experimental data to ensure its validity.The main conclusions:1. To a certain degree, the PAEs and PCBs substances in the slightly polluted water source can be removed by he Horizontal subsurface flow constructed wetland. Under the hydraulic retention time of two days, for Canna constructed wetland system during the trial period, the average removal rate of the PAEs are 32.82%(DMP),32.11%(DBP),20.20%(DEHP),18.72%(DOP) respectively, the average removal rate of the PCBs are 15.74%(PCB28),17.21%(PCB52),20.85%(PCB101),23.30%(PCB118),25.52%(PCB153),24.59%(PCB138) and 24.37%(PCB180) respectively. With the increasing residence time, The pollutant removal rate increased but the trend is less pronounced under higher residence time. Therefore, when using artificial wetland for the removal of PAE and PCB contaminants in Micro-polluted source water, the purification purposes can be achieved at the condition of test influent concentration, residence time for two days. Therefore, a two day residence time could be used in the PAEs and PCBs removal by the constructed wetland under the experiment pollutant concentration.2. In the constructed wetlands , PAEs and PCBs have different removal mechanism.The main mechanisms of PAEs removal by the constructed wetland include biological degradation, particle adhesion and plant absorption. The microbial degradation plays a major role in the PAEs removal, Depletion of organic carbon and the removal of PAEs significantly correlated; The PCBs is removed by particle adhesion , especially the PCBs monomer whose octanol - water coefficient (logKow) is larger, the plant absorption contribution is very limited.3. The effect of seasonal variations for PAEs removal are ranked in the order of Summer, Autumn, Spring and Winter, the removal effect was slightly better in the summer than in the autumn and wetland transpiration effect is the main reason. Seasonal changes have little effect on the removal of PCBs. This experiment, the wave subsurface flow wetland has higher removal efficiency than horizontal subsurface flow wetland.4. The biomass amount in the wave subsurface flow wetland is higher than horizontal subsurface flow wetland. A positive correlation was found between the bacteria amount and PAEs removal efficiency and no such correlation was found for PCB removal. In the wetland system, the dehydrogenase activity in the back was found to be stronger than that in the front, dehydrogenase activity in the upper part is generally stronger than the middle, dehydrogenase activity in the Wave subsurface flow wetland samples is stronger than that in the horizontal subsurface flow wetland, but the strength difference is less than 1%. Organic pollutants in water do not have significant impacts on microbial activity.5. When the average removal rate of PAEs and PCBs was 24.16% and 19.14%,by calculation, 27.64% of PAEs removal was contributed by plant absorption, 70.61% by particle adhesion and 1.75% by biological degradation. For the PCBs removal, the contribution by plant absorption, particle adhesion and by biological degradation are 7.75%, 90.91% and 1.34% respectively.6. The validity of the proposed kinetic model of PAEs and PCBs removal by subsurface constructed wetland was proved by comparing the experimental data with modeling results. The error between the modeled and experimental results was between 6.8%~30.6%, show that ecological dynamics model can better predict PAEs, PCBs effluent concentration in Canna artificial subsurface flow wetland system.