Fecal Bacteria Removal Within the Marshland Upwelling System Operated Under Near Freshwater Background Conditions

The effectiveness of the marshland upwelling system (MUS) installed in a low-background salinity area was investigated for the removal of fecal bacteria from domestic wastewater. It was hypothesized that travel distance required to meet the treatment objectives would be greater under low-salinity conditions. A suite of three injection regime (flow rate/frequency; 18.9 L/min-15 min/h; 0.95 L/min-15 min/h; and 1.9 L/min-15 min/h) studies was performed over a 1.25-year period to investigate the impact of injection flow rate on the removal of fecal coliforms and Escherichia coli from domestic wastewater injected into the MUS system. Each injection regime resulted in removal efficiencies greater than 99.9% for fecal coliforms and E. coli. The 18.9 L/min study (10 times design flow rate) tested the upper hydraulic loading limit of the system and resulted in eventual system channelization. First-order removal rate constants were estimated as 2.5–2.6 and 1.5–1.7 m^–1 for the 0.95 and 1.9 L/min studies, respectively. The removal rate constants calculated for the 1.9 L/min study were substantially lower than those obtained for MUS systems operated under high-salinity background conditions. While the 0.95 L/min study resulted in higher removal rate constants, both flow regimes met the treatment objectives. Additionally, a direct comparison is difficult due to variable field conditions (i.e., temperature, pH, loading rates, etc.). The predicted travel distance required to meet the 14 MPN/100-mL standard for shellfish harvesting waters was less than 5 m in all studies.