Impacts of hydraulic retention time and inflow water quality on algal growth in a shallow lake supplied with reclaimed water

Landscape lakes supplied with reclaimed water often face the threats of algal blooms. To guarantee the water quality in an artificial shallow lake (Lake M), algal growth and water quality changes were predicted by Delft3D. When the concentrations of influent total nitrogen (TN) and total phosphorous (TP) were fixed at 10 mg N/L and 0.2 mg P/L, respectively, increases in the hydraulic retention time (HRT) which was induced by water depth increased algal growth. The average algal density in Lake M was 6.42 x 104 cells/mL when the HRT was 4 d, which increased to 2.14 x 105 cells/mL when the HRT was 8 d. These results showed that the shallowness of the lake contributed positively to algae control. The shallower water depth came with the shorter HRT while nutrient load and light intensity were increased subsequently, which usually favor algal growth but not the dominant factors of algal density in Lake M. When the influent TN concentration was between 2 and 15 mg N/L of which nitrate changed, the HRT was fixed at 8 d, and the influent TP concentration was 0.2 mg/L, the algal density was approximately 2 x 105 cells/mL. Algae were not sensitive to changes in the influent TN concentration, and TN should be considered as a secondary factor in algal growth. Increases in the TP concentration promoted algal growth because phosphorous is the dominant nutrient for algae to acquire from reclaimed water if there is sufficient nitrogen. When the HRT of the lake was 8 d and the influent TN concentration was 10 mg/L, the average algal density in the lake was on the order of 104 cells/mL as the TP concentration was 0.1 mg/L. Because the water depth determines the HRT, it should be carefully considered during design and construction of artificial lakes. Influent phosphorus should be monitored to control algal blooms during maintenance of artificial lakes.