Evaluation of Constructed Wetlands and Pretreatment Options For the Treatment of Flow-through Trout Farm Effluent

Horizontal subsurface flow (HSSF) constructed wetlands were evaluated for the treatment of flow-through trout farm effluent, phosphorus sorption affinity of gravel-bed media, and influence on Rhodamine WT (RWT) transport. HSSF wetlands coupled with mechanical pretreatment demonstrated significant (p <0.05) removal of total ammonia-nitrogen (TAN), total phosphorus (TP), total organic carbon (TOC), total suspended solids (TSS), five-day biochemical oxygen demand (BOD5), and turbidity. Treatment occurred predominantly within the wetland cells, with minimal removal of studied water quality parameters by means of sedimentation or microscreen filtration (80 ?m mesh). HSSF wetlands removed 69% of influent TSS, 24% of influent TP, and reduced turbidity by 66%. The removal of organic matter within the wetlands, as measured by BOD5, COD, and TOC was 62%, 50%, and 55%, respectively. After receiving effluent from a flow-through trout farm for about one year, the gravel media exhibited moderate removals of soluble phosphorus in batch and column sorption experiments. Partition coefficients (Kd) from batch sorption tests ranged from 45-90 mL/g. Low (60 mL/min) and high (165 mL/min) flow column experiments removed about 50 and 40% of influent PO4-P, respectively. The conservative nature of RWT in subsurface media has been called into question by many authors. Tracer response curves from tests conducted in pilot-scale HSSF wetlands exhibited elongated tails and dual peaks, in addition to mean tracer retention times far exceeding the theoretical value. Laboratory column testing of RWT and the more conservative NaCl tracer supported field data, indicating that RWT was more reactive within the wetland media.