Research On Water Quality Evolution Of Landscape Lakes Considering The Implementation Of Ecological Measures

In recent years,the urban landscape lake,which is a landscape place with functions of flood control/drainage,beautifying city appearance,adjusting regional climate,improving air quality and edifying sentiment,has become a hot spot in urban planning and construction around the world.However,under the dual influence of natural environment and human activities,the urban landscape lake is suffering from some problems,such as deterioration of water quality,frequent algal outbreaks,decrease of lake ecosystem stability,deterioration of service function,etc.Therefore,the management and maintenance of water environment of urban landscape lake has become a hot topic in the current research field.The application of ecological measures is a common method to improve water quality in landscape lake.The paper studied the water quality improvement by artificial substrates(ASs)made in carbon fiber and floating treatment wetlands(FTWs)made in composite fiber in the landscape lake through mesocosm experiments,in-suit experiments and numerical simulations.Firstly,the effectiveness in improving water quality and the main removal path of nitrogen and phosphorus by ASs and FTWs were analyzed in the mesocosm experiments.Secondly,the in-situ experiments were constructed to analyze the water quality evolution of landscape lake considering the implementation of ASs and FTWs.And the difference of water quality improvement and growth characteristics during growth and senescent phases of two plant species were compared.Finally,a hydrodynamics and water quality coupling model for landscape lake was developed and calibrated using experimental and literature data.The model was applied to compare the characteristics on hydrodynamics and water quality improvement.The main study contents are as follows:(1)The mesocosm experiments for the evaluation of the water quality improvement of ecological measures showed that the ASs showed better performance than the FTWs for TP removal,whereas the FTWs had a higher removal efficiency than the AS treatments for TN.Sedimentation was the main pathway for N and P removal in the AS treatments,accounting for 82.1% and 65.7% of the nitrogen and phosphorus removal.Periphyton on plant roots and floating rafts likely plays an important part for N removal that was not accounted for in the mass balance which contributed for 47.1%of the nitrogen removal,while plant uptake was the main removal pathway for P removal in the FTW treatments which contributed for 53.0% of the phosphorus removal.(2)The in-suit experiments in landscape lake for the water quality evolution showed that FTWs had a higher removal efficiency than the AS treatments for TN,while the TP removal performance has no significant difference between the ASs and the FTWs.The combined ecological measures of ASs and FTWs not only has the highest TN and TP removal performance,but also inhibited the nutrients release from periphyton and plants under low temperature conditions.In the experiments,Iris pseudacorus performed better than Lythrum salicaria in TN and TP removal when planted in the combined ecological measures.The scale effects resulted in some agreements and differents between the mesocosm experiments and the in-suit experiments.In the AS treatments,the DO and p H both increased in different scale of experiments.In the FTW treatment,the DO and p H both decreased in different scale of experiments.And the FTWs could impede heat exchange between the water and the atmosphere.However,the nitrogen and phosphorus removal effect of ASs and FTWs in the in-suit experiments was higher than that in the mesocosm experiments.(3)A hydrodynamics and water quality coupling model for landscape lake was developed.The calibrate results using the mosocosm experiment data,in-suit enclosure experiment data and literature data showed that the coupling model performed well on modeling the hydrodynamics and water quality evaluation in landscape lake considering the implementation of ecological measures.The evaluation results showed that Nitrogen removal efficiency increases with HRT,while plant uptake contributes most when HRT = 11.2 day.Specifically,N:P in plant tissues was found to be an indicator of nutrients limitation in the water column.When N:P in water is less than that in plant tissues,nitrogen will be the limiting factor for plant growth,otherwise phosphorus will be the limiting factor for plant growth.(4)The coupling model was used to analysis the evolution of hydrodynamics and water quality in the 7 arrangement schemes of ecological measures(take FTWs as an example).The modelling results showed that the main stream would turn to the bottom and side of the FTWs because of the block of the FTWs.The difference of hydrodynamic characteristics between arrangement schemes leads to the difference of water quality improvement effect.When the two FTWs were deployed in series,the TN and TP removal performance increased with the spacing between the two FTWs.