Nutrient retention in surface flow constructed wetland
The research site consists of a pilot-scale surface flow constructed wetland at the farm Mezaciruli located in Zalenieki County, Jelgava Region, in the middle part of Latvia. The constructed wetland was installed in June 2014 to improve water quality in agricultural catchment and examine nutrient retention at the constructed wetland receiving surface and drainage runoff. The constructed wetland’s surface area of 0.37 ha corresponds to 0.5 % of the total catchment area. During the observation period of 32 months (2014-2017) water quality parameters such as total suspended solids (TSS), nitrate-nitrogen (NO3-N), ammonium-nitrogen (NH4-N), total nitrogen (TN), orthophosphate-phosphorus (PO4-P), and total phosphorus (TP) were monitored twice a month using a grab sample approach.
Retention efficiency for monitored water quality parameters was calculated based on their concentrations at the inlet and out let. The monitoring results obtained during this study showed a reduction within the constructed wetland for all examined parameters.
Solution to which problem
Agricultural lands can cause a diffuse pollution load on the environment due to release of nutrients from cultivated areas. Constructed wetlands are implemented to improve water quality, to offer a habitat for biological diversity support in simplified, uniform areas, and also as water storage for water reuse in dry summers.
The width of the surface flow constructed wetland is in range between 6 and 42 m. The wetland is functionally divided into two parts with a maximum water depth of 2.10 m at the inflow part to facilitate sedimentation processes and a shallower part at the outflow part with a maximum water depth of 1.40 m for nutrient transformation processes. The surface area of the deep section at the inflow and more shallow section at the outflow part is 0.29 ha and 0.08 ha, respectively.
The constructed wetland was built by transforming a 240 m long part of an existing open drainage ditch within a distance of 100 m before inflow into the Eglone River. Disposition of the constructed wetland on the existing ditch is related to the position of a water level control and measuring structure, which ensure appropriate water level and prevent downstream areas from flooding.
The concentrations of NO3-N, NH4-N and TN were reduced on average by 21 %, 35 % and 20 %, respectively for the surface flow constructed wetland. PO4-P and TP concentrations were reduced on average by 31 % and 45 %, respectively for the surface flow constructed wetland. Total suspended solids were reduced by 17% at the outlet of the surface flow constructed wetland. Water from the wetland is reused to irrigate greenhouse plants of 0.5 ha, taking 7- 15 m3 water daily throughout the growing season.
However, in some cases, an increase in the amount of nutrients leaving the constructed wetland was observed. Higher removal efficiency for the studied wetland was observed for phosphorus compounds as phosphorus is mainly retained through physical processes such as sedimentation in the water column. Absence of plants within the wetland could be the main reason for relatively lower reduction rates for nitrogen compounds. The results of standard deviations indicate that nutrient concentrations at the inflow and outflow vary greatly.
Groundwater recharge can be considered as one of the benefits of the practice. As the water treatment plant is designed with an open connection to groundwater, some water amount infiltrates into the upper ground levels depending on the site-specific conditions. The water quality is monitored in terms of the ongoing scientific activities and is acceptable for this initiative.