Performance of a Constructed Wetland Wastewater Treatment System Used to Treat Beet Sugar Wastewater

摘要

In 1993, a 63 ha.(157 acre) constructed wetland wastewater treatment system was completed by American Crystal Sugar Company(ACSC) to meet National Pollutant Discharge Elimination System(NPDES) limits. This has proven to be an effective means of treating wastewater. Significant discharging began in 1995. Removal rates observed in 1995 for biochemical oxygen demand (BOD5), total kjeldahl nitrogen (TKN), and total phosphorous (TP) were 89%, 90% and 47% respectively. However, questions remained regarding the fate of phosphorous compounds, an apparent softening of the wastewater as it moved through the wetland system in 1995, changes in operational trends over time, and how to most efficiently operate the system. In order to answer these questions, biweekly water quality analyses were performed on water from each cell, and wetland influent and effluent records were compared to those of the previous year. Results show that the trent toward wastewater softening observed in 1995 was again present in 1996. Total kjeldahl nitrogen concentration trends remained unchanged from 1993 to 1996. pH of the wastewater consistently rises as the water moves through the wetland. Because another Red River Valley flow-through wetland with similar sedimentary composition behaved similarly, it is though that the effect may be natural. Results show that phosphorous concentration trends have changed since 1995. The amount of phosphorous recored in the effluent was greater than that observed in the influent for a period of time in 1996, and a phosphorous peak can be detected moving through the system. It appears as though the wetland functioned as a phosphorous sink in 1995 and a source in 1996. Denitrification is complete by the time the wastewater has moved through cell 3 or cell 4 in the wetland.The treated water storage cell(cell 8) was found to be of great interest. A substantial rise in pH can be seen in cell 8, as well as a substantial decrease in the phosphorous concentration and hardness of the water . The constructed wetland was also designed with wildlife values in mind. There are nesting islands built into five of the cells and the area has become a habitat for many thriving populations of various species. It has become apparent that wetlands constructed for wastewater treatment work similarly to natural wetlands when treating wastewater. This behavior is evident in the concentrations of phosphorous, removal of nitrate and pH levels observed in both natural and constructed wetlands.