Design of a Constructed Wetland for Treatment of Facultative Lagoon Effluent in Rural Alaska

Abstract

Many rural communities in Alaska rely on large constructed lagoons to treat their wastewater. The quality of effluent released from these lagoons and the ability of the receiving bodies to dilute the effluent varies as the thawed season progresses. The receiving bodies tend to have capacity before the effluent has reached levels acceptable for discharge and algae growth degrades effluent quality as capacity in receiving bodies is decreasing, leaving only a short window to discharge effluent with minimal impact. The expansion of an existing facultative lagoon and the addition of a constructed wetland in Galena, Alaska is estimated to increase BOD and TSS removal to levels that will consistently exceed permit levels. The lagoon will be drawn down by discharging to the constructed wetland over a 120 day discharge period. The addition of the constructed wetland will keep TSS within permit levels even when algae drives TSS values over permit levels in the facultative lagoon. While the constructed wetland is expected to reduce fecal coliform concentration in the wastewater treatment facility effluent it may not bring fecal coliform levels down to below permit levels. Additional dilution or disinfection may be required. Unlike BOD, TSS, and fecal coliform, which are expected to improve through the addition of the constructed wetland, dissolved oxygen levels are expected to decrease as a result of treating the wastewater in the constructed wetland. While the dissolved oxygen concentration of the constructed wetland effluent will be low, the decreased BOD concentration will result in an effluent that is more readily able to reaerate over an effluent with a higher BOD concentration. Overall the results of this project suggest that adding constructed wetlands treatment to facultative lagoons prior to discharge to receiving bodies has the potential to create effluent of consistent quality that will meet or exceed ADEC permit requirements.