| China’s surface water pollutants, nitrogen and phosphorus discharged from farming respectively account for 59% and 38% of agricultural source effluent, and the most amounts are discharged from paddy fields. Therefore, nitrogen and phosphorus pollution resulted from agricultural runoff, which is consisted of paddy fields’ percolation and overland flow, should not be ignored. In recent years, this environmental issue has aroused widespread concern. Ecological drainage ditch is a type of constructed wetland playing the role for nitrogen and phosphorus removal in agricultural runoff, and substrate as well as hydrophyte is key component of ecological drainage ditch. Those two elements are used for intercepting nitrogen and phosphorus, performing in-situ remediation, and they are able to reduce secondary pollution to receiving water. Consequently, it is significant to research on the role of substrate and hydrophyte in ecological drainage ditch for nitrogen and total phosphorus removal from agricultural runoff.In study, we used gravel and broken bricks as substrate materials to research their adsorptive characteristics for ammonium nitrogen (NH4+-N) and total phosphorus(TP). Next, we simulated hydrophyte-substrate as integrated mode through Ipomoea aquatic with gravel, and then we researched on effect of removing nitrogen and phosphorus through the simulated drainage ditch, as well as the effect difference in different HRT purifying. In order to screen a kind of hydrophyte which is able to metabolize as usual living in non-frigid zone of Sichuan province, we choose Oenanthe javanica which can metabolize under 3～15℃ as plant material for nitrogen and total removal according to literature and on-the-spot investigation.Based on the process of paddy planting and fertilizing, the study simulated agricultural runoff quality in terms of the concentration of NH4+-N、total nitrogen(TN) and TP in runoff to research on the removal of nitrogen and phosphorus by substrate and hydrophyte. Firstly, we researched on substrates’ adsorptive capacity to pollutants in agricultural runoff substrate according to thermodynamics of adsorption and sorption kinetics. And this is helpful for us applying substrates to NHU+-N and TP removal in agricultural runoff:Linear, Langmuir, and Freundlich are three adsorption isotherm models in thermodynamic study. And for kinetic study, the experimental adsorption data are evaluated by the means of first-order, pseudo-second-order and Elovich models. Secondly, using Ipomoea aquatic+gravel+ simulated agricultural runoff composition as simulated hydroponics system to research on the system’s removal of nitrogen and phosphorus. Lastly, using Oenanthe javanica+simulated agricultural runoff composition as another simulated hydroponics system to study on the system’s removal of nitrogen and phosphorus.Main findings are summarized as follows:(1) For certain concentrations of TP and NH4+-N adsorption, we added gravel and broken bricks, which were made by the same quantity (3 pieces) and proximate ratio liquid-volume/solid-volume (ratio for isotherm adsorption study:Group of gravel-2.67, Group of broken bricks-2.96; ratio for sorption kinetics study:Group of gravel-3.33, Group of broken bricks-3.70):TP equilibrium adsorbate capacity of broken bricks (213.35mg/kg-227 mg/kg) is 18～19 times the quantity of gravel’s (11.79mg/kg～11.36 mg/kg). NH4+-N equilibrium adsorbate capacity of broken bricks is 210.48mg/kg, while equilibrium adsorbate capacity of gravel is 16.62 mg/kg～18.36mg/kg. Consequently, the ability of TP and NH4+-N adsorption by broken bricks is better than gravel’s.(2) In simulated system consisted by Oenanthe javanica+simulated agricultural runoff, with average planted density about 688 plants/m2, decrement of nitrogen and phosphorus concentration in the 3rd day and the 5th day are respectively as follows:NH4+-N concentration reduce 85.38%,95.52%, TN concentration reduce 47.67%.63.70%, TP concentration 84.94%,96.23%; with average planted density about 162 plants/m2, decrement of nitrogen and phosphorus concentration in the 3rd day and the 5th day are respectively as follows:NH4+-N concentration reduce 20.30%、48.82%, TN concentration reduce 16.55%、 30.0%, TP concentration 60.75%、89.04%.(3) In the range of NH4+-N and TP influent concentration designed by experiment, there are positive correlation between decrement rate of pollutants’ concentration and planted density. NH4+-N influent concentration are respectively in the range of 4.2mg/L-5.7mg/L、 7.8mg/L～10.4mg/L, and TP influent concentration are respectively in the range of 0.24-0.46mg/L, the higher planted density leads to larger decrement rate of NH4+-N and TP concentration; at 3rd day, with average planted density about 688 plants/m2, NH4+-N concentration decrement rate is over 80%, while the concentration decrement rate is 20.3% with average planted density about 162 plants/m2. It shows that planted density has significant impact on NH4+ -N concentration decrement rate. During the same HRT purifying process, in the experiment group with average planted density about 688 plants/m2, TP concentration decrement on the 2nd day is 67.17%, while TP concentration decrement on the 2nd day is 26.86% in the experiment group with average planted density about 688 plants/m2.(4) In simulated system consisted by Ipomoea aquatic+gravel+simulated agricultural runoff, the decrement of NH4+-N and TP concentration are respectively 50% and 70% within HRT for 3 days, while the decrement of TN concentration is 28.12%～38.64%. After 5 days’HRT, NH4+-N and TP concentration respectively reduce 80% and 90%, and TN concentration reduces 44.17%～46.75%. |
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