Effect Of Long-term Operation On Performance And VOSCs Transformation In Subsurface Flow Constructed Wetland

As a biological wastewater treatment technology, constructed wetlands promise low investment, ease of management, the advantages of high efficiency and low consumption. The removal of pollutants in constructed wetlands is primarily through filtration, adsorption, precipitation, ion exchange, plant uptake and microbial decomposition. At the same time, the parameters of artificial wetland varied with the environment conditions and wastewater quality in long-running process, hence, great attention has been paid to its long-term operating efficiency. To better understand the mechanism of artificial wetland to purify polluted wastewater, it is necessary to perform the long-term monitoring of the artificial wetlands.In the long-running process, constructed wetlands accumulated organic pollutants, which provided the condition for the release of ordor gas. Volatile sulfides including dimethyl sulfide (DMS), dimethyl disulfide (DMDS), methyl mercaptan (MT), and hydrogen sulfide (H2S), are very important contribution to the formation of acid rain and the greenhouse effect. This paper discusses the effect of long-running artificial wetlands on pH, alkalinity and acidity, and also studied the release of volatile organic sulfides (VOSCs) in constructed wetlands. The main contents and results are as follows:1. Variation of pH, acidity and alkalinity in long term operation of the subsurface flow constructed wetland treating domestic wastewater. Investigation on the the variation of pH, alkalinity and acidity in subsurface flow constructed wetland treating domestic sewage demonstrated that pH values of the effluents fluctuated around pH7. The effluents of all wetland units stand at higher pH at the beginning run of the system. Furthermore, pH value of the effluent with cinder as substrate is higher than that of the effluent with gravel as substrate. The recycling of effluent could decrease the effluent pH in the cinder-based wetland, and enhance phosphorous removal. However, no significant difference was found for phosphorous removal in the gravel-based wetland, which was probably related to the existing form of metal element affected by the recycling of effluent. The acidity of the effluent is lower than that of influent; however, the alkalinity of the effluent is higher than that of influent. The alkalinity of the effluent is much higher than acidity of the effluent, suggesting the anaerobic conditions in wetland system. The organic loading rate (OLR) and temperature showed significant effects on alkalinity of the effluent.2. The effects of filler matrix from the clogging and the normal operation of constructed wetlands on the release of volatile organic sulfides (VOSCs) through culture experiments were studied. The impact of the constructed wetland depth on the VOSCs release was also explored. The results showed that:1), the release of dimethyl sulfide (DMS) from the normally running constructed wetlands was less, and filler matrix from8-10cm showed the major DMS contribution;2), clogging due to the long-running subsurface flow constructed wetland may lead to an increase in the release of the DMS, which was mainly from the surface of the rich organic matter filler matrix;3), sterilization experiments show the microbial degradation on VOSCs. DMS release increase after sterilization, which is probably from the adsorbed DMS by the filler matrix in the heating conditions. Sulfide could promote the non-biological production of DMS, DMDS and MT, suggesting the possible mechanism of biological sulfur particles.3. The effect of linear alkyl benzene sulfonate (LAS) on release of VOSCs in the wetland filler matrix was studied. The results indicated that gravel-based artificial wetlands showed almost no removal for LAS. The influent LAS concentration can affect the effluent sulfate concentration, possibly leading to increase of VOSCs emissions. A low concentration of LAS is helpful to decrease the release of the DMS. However, the DMS emission increased with increasing concentration of LAS, which is probably due to inhibition of the activity of methanogens.