Fate and Effect of Dissolved Silicon within Wastewater Treatment Effluent

In large rivers, the ratios of silicon (Si)/nitrogen (N)/phosphorus (P) have changed dramatically as anthropogenic additions of N or P are not matched by Si. Wastewater effluent is a recognized source of N and P to coastal environments. Few previous studies, however, have examined the Si load of a large wastewater plant's effluent or the molar ratios of Si/N and Si/P in effluent. We examine the annual flux of dissolved silicon (DSi) carried by effluent from the second largest treatment plant by flow in the United States (Deer Island Treatment Plant, DITP, Boston, MA). We compare treatment plant nutrient fluxes to local urban river nutrient fluxes and trace the impact of the DITP DSi loading on receiving waters. Estimates (±95% confidence interval) of treated effluent (67 800 ± 1500 kmol DSi year^-1) compared to untreated (69 500 kmol DSi year^-1) indicate that the process of sewage treatment at DITP likely does not remove DSi. DITP effluent was Si-limited and this Si-limitation is reflected in the receiving waters (Massachusetts Bay). However, Si-limitation appears only in the area immediately surrounding the effluent discharge. We use these results to explain phytoplankton patterns in Massachusetts Bay and to provide the first estimate of DSi loading (3.6 Gmol SiO₂ year^-1) from wastewater effluent across the US.