6/96 



9/96 12/96 



3/97 



6/97 



9/97 



12/97 



3/98 



Figure 4. Sulfate (as S) Concentration as a Function of Time 



system prior to October 1997. The straight line after October 1997 represents the 

 approximate total nitrogen inflow as a greater proportion of organic N was added 

 to the influent. Although all cells expressed some reduction in NH4 prior to 

 August 1996, only planted treatments have exhibited appreciable long-term 

 ammonia removal. 



Measured phosphate concentration data are presented as a nine point moving 

 average in Fig. 3. Overall removal efficiency clearly varies by treatment and 

 season. By August 1996, four months after the start of wastewater application, 

 the control treatment's removal efficiency had decreased from approximately 

 40% to virtually zero, and has remained there since. Both cattail and bulrush 

 were dormant between December 1996 and June 1997, and after October 1997. 

 During the 1997 growing season, cattail growth was more vigorous than bulrush. 

 During this active plant growth phase, phosphate removal efficiency increased, 

 especially for cattail. It is suspected that uptake by macrophytic plants exhibiting 

 vegetative growth is a significant mechanism of phosphate removal. 



Sulfate concentration data are shown as a nine point moving average in Fig. 

 4. Higher influent concentrations prior to August 1996 and after October 1997 

 correlate to lower effluent concentrations, especially for the planted treatments. 

 Note that influent COD concentrations vary in a manner similar to sulfate. An 

 odor of H2S from the wetlands during higher influent concentrations suggests the 

 reduction of SO4 by sulfur-reducing bacteria (SRB's). SRB activity implies the 

 existence of anoxic regions within the wetlands during this time. It is believed 

 that the reduced oxygen demand during the period August 1 996—October 1 997 led 

 to a slight increase in dissolved oxygen content (though this was not directly 

 measured), resulting in inhibition of SRB activity in planted cells. Continued 

 sulfate reduction by the control treatment suggests that implanted cells may have 

 lower dissolved oxygen contents. Thus, the sulfate data presents circumstantial 

 evidence of the positive influence of macrophytic plants on oxidation of the root 

 zone, at least for conditions of relatively low oxygen demand in the water. 



Stein et al 



