Runoff characteristics and non-point source pollution analysis in the Taihu Lake Basin: a case study of the town of Xueyan, China.

Non-point source pollution is a significant environmental issue in small watersheds in China. To study the effects of rainfall on pollutants transported by runoff, rainfall was monitored in Xueyan town in the Taihu Lake Basin (TLB) for over 12 consecutive months. The concentrations of different forms of nitrogen (N) and phosphorus (P), and chemical oxygen demand, were monitored in runoff and river water across different land use types. The results indicated that pollutant loads were highly variable. Most N losses due to runoff were found around industrial areas (printing factories), while residential areas exhibited the lowest nitrogen losses through runoff. Nitrate nitrogen (NO3-N) and ammonia nitrogen (NH4-N) were the dominant forms of soluble N around printing factories and hotels, respectively. The levels of N in river water were stable prior to the generation of runoff from a rainfall event, after which they were positively correlated to rainfall intensity. In addition, three sites with different areas were selected for a case study to analyze trends in pollutant levels during two rainfall events, using the AnnAGNPS model. The modeled results generally agreed with the observed data, which suggests that AnnAGNPS can be used successfully for modeling runoff nutrient loading in this region. The conclusions of this study provide important information on controlling non-point source pollution in TLB.

Effects of plant roots on the hydraulic performance during the clogging process in mesocosm vertical flow constructed wetlands.

The aim of this study was to evaluate the effects of plant roots (Typha angustifolia roots) on the hydraulic performance during the clogging process from the perspective of time and space distributions in mesocosm vertical flow-constructed wetlands with coarse sand matrix. For this purpose, a pair of lab-scale experiments was conducted to compare planted and unplanted systems by measuring the effective porosity and hydraulic conductivity of the substrate within different operation periods. Furthermore, the flow pattern of the clogging process in the planted and unplanted wetland systems were evaluated by their hydraulic performance (e.g., mean residence time, short circuiting, volumetric efficiency, number of continuously stirred tank reactors, and hydraulic efficiency factor) in salt tracer experiments. The results showed that the flow conditions would change in different clogging stages, which indicated that plants played different roles related to time and space. In the early clogging stages, plant roots restricted the flow of water, while in the middle and later clogging stages, especially the later stage, growing roots opened new pore spaces in the substrate. The roots played an important role in affecting the hydraulic performance in the upper layer (0-30 cm) where the sand matrix had a larger root volume fraction. Finally, the causes of the controversy over plant roots' effects on clogging were discussed. The results helped further understand the effects of plant roots on hydraulic performance during the clogging process.

An integrated model of substrate clogging in vertical flow constructed wetlands.

This paper presents an integrated model of substrate clogging in a vertical flow constructed wetland (VFCW). The model simulates the reduction of pore space in the wetland substrate due to combined influences of various physical, biogeochemical and plant-related processes. A series of experiments based on laboratory-scale VFCWs were conducted to examine and measure key parameters related to clogging of the wetland substrate during operation under different conditions. The model was then validated using data collected from the experiments. The results showed that the model was able to replicate the clogging phenomenon as observed in the experiments, in particular, the characteristic clogging time. The model also predicted well individual contributions to clogging by accumulated inert suspended solids, microbial biomass and plant root materials during the wetland operation. Although the validation was based on the laboratory data, the results indicated that the model describes well the processes underlying the clogging and has the potential to become a tool for assessing the performance of prototype CWs in relation to clogging at both the design and operation stages.

A conceptual approach based on suspended solids to estimate clogging time in constructed wetlands.

A conceptual model has been developed by using the parameter of influent suspended solids (SS) concentration to estimate the clogging time in constructed wetlands (CWs). The depth of the clogging layer is estimated through the effective porosity vertically along the CWs. The basis of the model to predict the clogging time lies in the fact that, when clogging occurs, the pore spaces were reduced and thus the infiltration rate was reduced. A group of laboratory scale CWs was employed to develop the model and validate its utilization. Good agreement between the experimental data and the predicted clogging time was obtained. This indicates that the model can be used in different operation conditions for estimating clogging time in CWs.

Clogging pattern in vertical-flow constructed wetlands: insight from a laboratory study.

Substrate clogging caused by the accumulation of the particulate solids is the worst operational problem for vertical-flow constructed wetlands (VFCW). In this paper, the effects of particulate solids distribution and their accumulation in the substrate with different gravel sizes were investigated. The results demonstrated that the clogging layer can be considered as two parts: one is the blanket-like deposition layer, and the other is the upper substrate clogging layer. Furthermore, the clogging process shall be partitioned as three stages of puncture phase for the pollutants; the formation of the blanket-like deposition layer; and the formation and compaction phase to the whole clogging layer. With reference to the clogging mechanism, it is believed that the particulate solids (< 100 microm) were absorbed firstly by electrostatic forces and van der Waals' forces. This is followed by the "bridging" made by the accumulated solids which act as a "sieve", thus further restricting larger particulate solids to flow through.