A 2-yr study compared the performance of seasonally and continuously loaded constructed wetlands treating dairy farm wastewater. One wetland was loaded during the growing season (GS) periods only, while the other was continuously loaded. Weekly samples were analyzed for 5-d biochemical oxygen demand (BOD5), total suspended solids (TSS), total Kjeldahl N (TKN), total ammoniacal N (TAN), total P (TP), and Escherichia coli. Annual average daily mass removal rates (kg ha-1) were similar for both wetlands in both years; however, seasonal differences were observed. With the exception of BOD5 in Year 2, average daily GS areal mass removal rates were higher for the seasonal wetland. However, GS mass exports from the seasonal wetland were higher by 28 to 94%, with the exception of BOD5 in Year 1. Annual mass reductions (MRs; %) for nutrients were higher for the continuous wetland in both years. Annual MRs were similar for E. coli in both years and for TSS in Year 2. Annual mass exports from the seasonal wetland were higher for nutrients and E. coli by 14 to 77% in both years. Pollutant MRs generally decreased during the nongrowing season (NGS) for the continuous wetland; however, in Year 2 when lower loading rates were used, the wetland still removed 84 to 99% of the pollutant masses. The continuous wetland also performed better during periods of high flow that occurred during the GS. Although there were minimal differences in annual treatment performance, continuously loaded systems require less additional infrastructure and should require less maintenance and may, therefore, be more attractive for agricultural applications.
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