Artificial Undercurrent Wetland Purification Of Eutrophic Water Test

Constructed wetlands are engineered systems designed and constructed to utilize the natural processes involving wetlands vegetation, soils and their associated microbial assemblages to assist in treating wastewater. Due to their relatively small space requirements and good sanitary conditions, vertical subsurface flow(VSSF) constructed wetlands have recently become a common choice for the removal of organic matter, nitrogen and phosphorus and showed great potential in restoration of polluted landscape waters. The current efficiency with which VSSF removes most organic substances is satisfactory, but the removal of nitrogen and phosphorus is known to be somewhat problematic. In this dissertation, effect of wetlands components including substrate and plant on nutrient removal was evaluated and optimum candidate substrates and plants were selected to construct VSSF constructed wetlands treating eutrophic Jinhe River water. The role of intermittent artificial aeration and biofilm carrier (polyhedron hollow polypropylene ball (PHPB)) in nutrient removal was also explored. The main contributions to the current research of this topic are described below:1. Measurement of dry weight, hydraulic conductivity, particle size distribution, porosity and other selected physical properties was conducted to make the primary selection of candidate substrates. High porosity, uniformity of particle size distribution and low hydraulic conductivity, these good qualities showed shale had the potential to be packed as wetlands main filter media while close grained gravel had low water retention capacity, and should be used to distribute and collect water.2. Indoor column purification experiments were initiated to make further selection of wetlands substrates. Shale, coarse gravel, hornblende and iron stone showed significant difference in Jinhe River water treatment capacity, and shale performed best in COD, TN and TP removal. Moreover, combination between shale and coarse gravel treated COD, TN and TP well, while coarse gravel and ironstone contributed little to above nutrient variables removal.3. Phosphorus removal by VSSF constructed wetlands depended on substrates type and grain size, influent concentration and hydraulic residence time (HRT). The ranking order of the maximum phosphorus adsorption capacity for the substrates was as follows: shale > ironstone > hornblende > coarse gravel. After HRT values were increased from 2.2days to 3.1 days, phosphorus removal performance was more dependent on substrate phosphorus adsorption than on influent concentrations.4. Growth conditions, purification performance and biomass nutrient (nitrogen and phosphorus) accumulation capacity of selected candidate wetlands plants were investigated. All of the seven species including Phragmites communis, Typha angustifolia Linn, Acorus tatarinowii Schott, Lythrum salicaria Linn, Iris wilsonii C. H. Wright, Canna generalis and Scirpus validus Vahl grew well in the constructed wetlands, and showed excellent nitrogen and phosphorus removal performance. Aboveground and belowground biomass (root, stem and leaf) production and corresponding nitrogen and phosphorus content analysis suggested that Typha angustifolia Linn had high root biomass production, excellent nutrient uptake capacity and can be used potentially as constructed wetlands plant to remove nitrogen and phosphorus.5. Take the advantageous of aerated biofilter, intermittent artificial aeration was introduced to VSSF constructed wetlands to investigate the effect of oxygen availability on nutrient removal. In contrast with the non-aerated wetlands, intermittent artificial aeration greatly enhanced COD, NH₄⁺-N, TN, SRP and TP reduction but reduced NO₃^--N removal. The analysis of wetlands plant biomass indicated that intermittent aeration restrains the increase in biomass production, but stimulates assimilation of nitrogen and phosphorus into stems and leaves.6. Like bio-contact oxidation process, biofilm carrier (PHPB) was used to replace part of shale and packed as wetlands substrate to examine its impact on nutrient removal. In contrast to wetlands without biofilm carrier, polypropylene pellets improved the COD, NH₄⁺-N, TN, SRP and TP removal. Biofilm carrier, however, played no significant role on NO₃^--N removal. The analysis of wetlands plant biomass indicated that bioflim carrier restrains the increase in biomass production, but stimulates uptake of nitrogen and phosphorus into stems and leaves.7. Effect of intermittent artificial aeration on nutrient removal by VSSF constructed wetlands using biofilm carrier as part of substrate was evaluated. In contrast with the non-aerated wetlands, intermittent artificial aeration greatly enhanced COD, NH₄⁺-N, TN, SRP and TP reduction but reduced NO₃^--N removal. The analysis of wetlands plant biomass indicated that intermittent aeration stimulates the increase in biomass production and enhanced the accumulation of nitrogen and phosphorus into stems and leaves.8. Finally, further two-way ANOVA analysis results indicated that intermittent artificial aeration led to significant statistic difference in COD, NH₄⁺-N, NO₃^--N, TN, SRP and TP removal. The using of PHPB as part of wetlands substrate was significant for COD, NH₄⁺-N and TN removal. Except for NH₄⁺-N, no significant interaction effect between intermittent artificial aeration and PHPB on other nutrient variables removal was detected. If plant uptake and store nitrogen and phosphorus only occurred from water column, mass balance calculation showed that the enhancement in nitrogen and phosphorus accumulation by intermittent artificial aeration and PHPB was the major factor responsible for the total observed improvement in nitrogen and phosphorus removal.