Effects of open water on the performance of a constructed wetland for nonpoint source pollution control

1 to 1000 Policy: Controlling Phosphorous Pollution from Tea Farms with Bioretention Cells

When implementing nonpoint source pollution control plans, the size or number of required controlling facilities is a very practical issue. However, quantifying nonpoint source pollution is difficult because it is generated by variable and random rainfall events. This study develops a two-stage optimization process to demonstrate the determination of the optimal bioretention cell size for tea farm pollution control. The optimization process was based on a verified watershed-scale model and a verified site-scale model. The verified watershed model was used to obtain total phosphorous (TP) reduction loads. Once the goal of watershed management was decided, the reduction loads were then allocated and the unit reduction loads were determined. Using the unit reduction loads, the verified tea farm model was used to assess the optimal bioretention cell size for tea farms. A case study using the Jinggualiao stream in the Feitsui Reservoir watershed, Taipei, Taiwan was presented. The results showed that the unit tea farm TP reduction loads were 270 g/ha-year and 326 g/ha-year to reach two water quality goals, and a total of 350 m2 and 600 m2 of bioretention cells were needed, respectively. A 1 to 1000 ratio of the standard bioretention cell area to the tea farm area is recommended as a general control rule.

Treatment of a small stream impacted by agricultural drainage in a semi-constructed wetland

During the period 2014-2016, removal of nutrients, organics and suspended solids from a small rural stream impacted by agricultural drainage in a natural and semi-constructed wetlands was evaluated. In 2014, the wetland was irregularly flooded during high flows in the stream, in 2015, the stream bed was cleaned and as a result the water level dropped down by about 0.5 m, thus no flooding occurred. In 2016, the stream was blocked and water was permanently diverted into a wetland dominated by Phalaris arundinacea, Carex nigra and Scirpus sylvaticus. The results clearly revealed a positive role of permanent flooding resulting in the highest removal of all monitored parameters, i.e. nitrogen (38%), phosphorus (40%), BOD₅ (42%) and suspended solids (67%). The permanent flooding increased aboveground biomass of P. arundinacea and C. nigra while S. sylvaticus was eliminated by permanent flooding. The repeated harvest yielded higher aboveground biomass for P. australis and S. sylvaticus, while for C. nigra one harvest provided more biomass. In 2016, the removal expressed as the loading averaged 1102 kg TN ha^-1 yr^-1 and 894 kg NO₃-N ha^-1 yr^-1 and the plant uptake was responsible for removal of 36.1% TN. The average removal of total phosphorus amounted to 164 kg P ha^-1 yr^-1 with plant uptake being responsible for 18.7%. Average removal of BOD₅, COD and TSS amounted to 1108, 5953 and 9220 kg ha^-1 yr^-1, respectively. While the removal of BOD₅ and COD increased exponentially with increasing water temperature, removal of TSS was not affected by water temperature. The results proved that slightly modified natural wetlands with negligible investment costs can be as effective as fully constructed wetlands to treat agricultural drainage.

A topography analysis incorporated optimization method for the selection and placement of best management practices

Best Management Practices (BMPs) are one of the most effective methods to control nonpoint source (NPS) pollution at a watershed scale. In this paper, the use of a topography analysis incorporated optimization method (TAIOM) was proposed, which integrates topography analysis with cost-effective optimization. The surface status, slope and the type of land use were evaluated as inputs for the optimization engine. A genetic algorithm program was coded to obtain the final optimization. The TAIOM was validated in conjunction with the Soil and Water Assessment Tool (SWAT) in the Yulin watershed in Southwestern China. The results showed that the TAIOM was more cost-effective than traditional optimization methods. The distribution of selected BMPs throughout landscapes comprising relatively flat plains and gentle slopes, suggests the need for a more operationally effective scheme, such as the TAIOM, to determine the practicability of BMPs before widespread adoption. The TAIOM developed in this study can easily be extended to other watersheds to help decision makers control NPS pollution.