Time series relationships between chlorophyll-a, dissolved oxygen, and pH in three facultative wastewater stabilization ponds
This study involved the evaluation of time-series relationships between chlorophyll-a (chl-a), dissolved oxygen (DO), and pH in three facultative wastewater stabilization ponds (WSPs) operated in series at a wastewater treatment plant in eastern Ontario, Canada. Since 2003, the WSP system has experienced high pH levels and excessive algae growth during the summer months. During the summer of 2013, chl-a, DO, pH, temperature, and daily solar radiation were monitored on an hourly basis for each WSP over a one-week period, with an additional two weeks conducted in WSP#2 to provide a more robust data set. Time-series analyses, consisting of cross-correlation evaluations, were performed on the dataset to investigate the time-dependent dynamics between chl-a, DO, and pH. Strong statistical dependence between chl-a, DO, and pH was identified during periods of high photosynthetic activity and algae growth. WSP#2 had the highest algae growth and the statistical relationships between chl-a and DO and chl-a and pH were the strongest, with maximum cross-correlation coefficients of 0.54 and 0.56 observed at lags of 20 and 16 hours for DO and pH, respectively. Strong diurnal patterns for DO and pH were observed in WSP#1 and WSP#4, and in WSP#2 during periods of reduced algae growth. During periods of excessive algae growth in WSP#2, no clear DO and pH diurnal patterns were noted. In general, across all WSPs, daily DO concentration peaks led those of chl-a by an average of 43 minutes, which subsequently led those of pH by an average of 2 hours and 15 minutes.