| The key operating parameter of hydraulic loading of the extremely shallow subsurface flow constructed wetlands was studied for simultaneous treating wastewater and amelioration of aeolian sandy soil; the characteristics of nutrient enrichment in aeolian sandy soil substrate of contructed wetland was also discussed. The experiment system was built in a wastewater treatment plant in Xi’an, the substrate depth of the extremely shallow subsurface flow constructed wetlands was 0.1 m, the substrate was aeolian sandy soil and cultivated plant were Festuca arundinacea, Lolium perenne, Cynodon dactylon, Phragmites australis and Typha orientalis Presl. The controls were a respective slow rate land treatment system with substrate of 0.1 m depth without plants cultivating and two subsurface flow wetlands with 0.6 m depth and a mix cultivated plant Festuca arundinacea, Lolium perenne and Cynodon dactylon, as well as another of mix cultivated Phragmites australis and Typha orientalis Presl. The raw water of experimental system was from the effluent of primary sedimentation tank.The main contents include:(1) Optimization of Hydraulic loading of the extremely shallow subsurface flow constructed wetlands(0.1 m depth wetlands) in summer and winter;(2) The removal efficiency of pollutants in water and the characteristics change of organic matter, total nitrogen and total phosphorus in aeolian sandy soil during a long-time operation of the constructed wetlands;(3) Material balance computation of carbon, nitrogen and phosphorus in system.The results showed:(1) In order to keep 0.1 m depth wetlands in normal operation in summer, the optimal hydraulic loading should be controled in a proper range with the upper limit of 15mm/d and the lower limit of 7.5mm/d. While in winter, the hydraulic loading can be regulated with the upper limit of 7.5mm/d, which can ensure that the effluent COD and TN reach the level of B, TP to reach the discharge standard of two grade.(2) The effluent concentrations of COD, TN and TP of 0.1 m depth wetlands were significantly lower than raw water(P=0.000<0.01), and the average removal efficiency of COD, TN and TP were higher than 74.39%, 68.60% and 64.78%. The effluent quality of 0.1 m depth land treatment system was low.This result was partly caused by clogging problem of the substrate, and it should be regularly dredged. For 0.6 m depth wetlands, they were blockaded after eight-month operation, which led the operating modefrom the subsurface flow into surface flow.(3) There was significant difference between the content of organic matter, total nitrogen and total phosphorus of aeolian sandy soil in wetlands compared with original soil(P=0.000<0.01), which indicated that the content of organic matter, total nitrogen and total phosphorus were significantly increased. In a word, the wetlands can enrich nutrients rapidly and ameliorate aeolian sandy soil rapidly.(4) The 0.1 m depth wetlands, which can maintain a long-time stable operation state, and also had high removal efficiency for COD, TN and TP. Furthermore, they also had a high rate of nutrient enrichment. In other words, 0.1 m depth wetlands could achieve synchronization amelioration of aeolian sandy soil and sewage purification.(5) Festuca arundinacea and Lolium perennewere choosed to be the cultived plant, because of their strong adaptability and strong growth ability in winter. Meanwhile, they both can form a dense root system in the surface of substrate, which was conducive to the fixation of aeolian sandy soil. Therefore, Festuca arundinacea and Lolium perennecan can be used as the preferred plant of amelioration of aeolian sandy soil.(6) Based on the material balance analysis of carbon, nitrogen and phosphorus in 0.1 m depth wetlands, we can get conclusions as follows: the removal of organic matter in water mainly relied on microbial degradation. The nitrogen removal was mainly due to the biological nitrification and denitrification, secondly due to the substrate enrichment. And plant absorption accounted for only 2.05%~11.35%. Phosphorus removal mainly resulted in the substrate enrichment. In addition, plant absorption and microbial assimilation can also remove a spot of P. |
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