Effects of umbrella palms and wastewater depth on wastewater treatment in a subsurface flow constructed wetland

On-site subsurface flow constructed wetlands are designed to provide secondary quality effluent. Plants and wetland volume are considered in their design. There have been no studies, however, comparing wastewater treatment at different wastewater depths, and plant effects in wetlands are not completely understood. Investigations were conducted on these variables using four wetland cells 228 m wide by 4.75 m long containing 1 to 5 cm diameter river rock. Cyperus alternifolius (umbrella palms) were planted in one cell, and side-by-side comparisons were made between the planted and a control cell. Side-by-side comparisons were also made between cells with equal surface areas and different depths. At best umbrella palms improved effluent 5-day biochemical oxygen demand (BOD5) by 8%, suspended solids by 6%, and did not improve fecal coliform or P wastewater quality in July. When ambient air temperatures were < or = 12 degrees C during December, plants did not improve most treatment parameters. They did, however, significantly improve NH4+ treatment even when ambient air temperatures were as low as 8 degrees C. Increasing wastewater depth enhanced fecal coliform die-off but did not reduce effluent considered when maximum NH4+ reduction is a BOD5, suspended solids, NH4+, or P in effluent. Umbrella palms should be treatment goal and it is not necessary to reduce other parameters. Surface area was more important to wastewater treatment than depth.