| At present, constructed wetlands have been widely used in the treatment of sewage treatment plant effluent. However, the removal of the excessive nutrients from polluted water remains a challenge in high-latitude areas or during winter in low- to middle-latitude areas with average temperatures of lower than 10 ℃ due to the significant relationship between temperature and the activity of both macrophytes and microorganisms. Macrophyte species selection has been regarded as one technology to mitigate the decrease in wetland purification functions during winter However, few studies have been focused on the purification capabilities of constructed floating wetlands with cool-season aquatic macrophytes or on the importance of macrophytes in removing nutrients from polluted water in winter. The secondary effluent quantity from wastewater treatment plants is generally relatively constant between seasons, and consequently a relatively constant purification ability is required in the wetlands to treat the effluent, the possible improvement in removal efficiency in winter has special significance.In this study, Oenanthe javanica as floating plant, three types of systems were established:unplanted control without floating mats (CK), unplanted control with floating mats (FW) and planted floating mats (FW-M). Research the growth dynamics and physiological characteristics of winter Oenanthe javanica in sewage treatment plant secondary effluent treatment to change, winter artificial floating wetland system of sewage treatment plant secondary effluent pollutant removal and nitrogen removal pathway. One-way and two-way analysis of variance (ANOVA) were employed to test the significance of difference in purification ability among treatments or seasons, the main results of the research was shown as follows:(1)During the experimental period, the local atmospheric daily temperature fluctuated between -6.2 and 19.1℃, with an average of 6.2℃, and the water temperature (50 cm below the water surface) fluctuated between 3.3 and 14.5℃, with an average of 6.8℃. The average air temperatures were 11.9,9.45,11.1,3.25,2.30, 3.85,3.60 and 4.70℃ for the 1st,2nd,3rd,4th,5th,6th 7th and 8th batches, respectively. Based on air temperature, the overall experimental period can be divided into two sub-periods:the first three batches (the late autumn batches) and latter five batches (the winter batches).(2)The difference of conductivity and TDS was significant between different systems. The average pH values were 8.0,8.1 and 8.4 for the CK, FW and FW-M systems, respectively, and the values exhibited no significant differences between treatments (p=0.236). The FW-M treatment had a significantly lower conductivity value (0.79 mS/cm) than that of the CK (1.22 mS/cm,p=0.003) and FW (1.25 mS/cm, p=0.014) systems. Significant different TDS values were observed (p=0.006), i.e., 0.78,0.79 and 0.51 g/L for the CK, FW and FW-M systems, respectively.(3)In the winter, Oenanthe javanica maintained a relatively high activity in the floating wetland. In the FW-M system, the roots of Oenanthe clecumbens lengthened rapidly in the first two batches, from 7.8 cm on September 31 to 18.09 cm on November 20, and then lengthened slowly over the course of the next six batches. The aboveground plant length also increased rapidly in the first two batches, from 20.02 cm to 25.15 cm, and then slowly decreased due to the withering of old leaves. The leaf chlorophyll concentration gradually decreased over the entire experimental period, whereas root activity initially decreased gradually before December 30 and then increased slightly. The root porosity and oxygen excretion increased in autumn experiment batch, after entering the winter began to decline. Although some leaves withered, the roots retained relatively high levels of activity during the winter, which had an average air temperature of 3.63 ℃.(4)Ecological floating wetland effectively removed nitrogen and phosphorus from the secondary effluent, but the removal rate of COD was lower. During the experimental period, with average values of 8.13%,14.59% and 18.53% for the CK, FW and FW-M systems, respectively, No significant differences were observed in the COD removal efficiency between treatments (p=0.437). For TN, NO3--N, NH4+-N, TDN and TP, the average removal efficiency values were 26.48%,36.18%,39.30%, 26.23% and 24.64% in the FW-M system,4.38%,2.91%,7.76%,3.96% and 2.73% in the CK system, and 3.76%,14.77%,5.74%,5.55% and 4.77% in the FW system, respectively. FW-M generally had significant higher removal efficiencies for TN, NO3--N, NH4+-N, TDN and TP than CK and FW (p values ranged from 0.002 to 0.016).(5)The decrease in the N uptake rate of the plants was the most important cause of the decrease in N removal efficiency in winter relative to late autumn. For the FW-M system, the plant’s N uptake rate dramatically decreased from 67.13 mg N-m-2-d-1 in the late autumn batch to an average of 20.09 mg N-m-2-d-1 in the winter batches. The plant’s N uptake rate decreased by an average of 70.07% between late autumn and winter batches. Similarly, the proportion of total removed N represented by the plant uptake pathway decreased from 33.39% in the late autumn batch to an average of 15.01% in the winter batches. The microbial N removal rate decreased from 127.66 mg N·m-2·d-1 in the late autumn batch to an average of 96.34 mg N·m-2·d-1 in the winter batches, a decrease of 24.53%. The microbial removal pathway proportion of the total N removal increased from 63.50% in the late autumn batch to 71.57% in the winter batches. |
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