POSSIBLE BIOGEOCHEMICAL WEATHERING IN A TIDAL WETLAND WASTEWATER TREATMENT PLANT

Constructed wetland wastewater treatment plants are a relatively common alternative to large centralized wastewater treatment plants, and are equally as effective in transforming and removing nitrogen. Our study of the chemical composition of wastewater in a tidal wetland wastewater treatment system, however, suggests that a number of other chemical reactions occur during the treatment process that may affect the long-term effectiveness of the biofilm-based system.

We analyzed the chemical composition of wastewater as it moved from the settling chamber through the first and second recirculation chambers and finally into the effluent chamber. Several trends emerged. Both pH and conductivity decrease and dissolved oxygen increases through the treatment process. The proportion of calcium relative to sodium plus potassium increases, and the proportion of sulfate plus chloride relative to bicarbonate increases dramatically. The proportion of dissolved silicon also increases slightly. This corresponds to statistically significant increases in the concentrations of calcium, sulfate, dissolved silicon, iron, and dissolved carbon dioxide, and statistically significant decreases in the concentrations of bicarbonate and DOC. The concentrations of sodium, magnesium, potassium, and chloride did not change significantly. Visual MINTEQ speciation results showed that 100% of the Fe in the system is organically bound and about 11% and 12% of the calcium and magnesium is also organically bound, respectively.

The change in chemical composition can be explained by a complex set of reactions occurring within the tidal wetland system. The increase in dissolved carbon dioxide and decrease in pH and DOC indicates the importance of microbial respiration. The increase in calcium, dissolved silicon, iron, and perhaps sulfate, suggests weathering of either the expanded shale substrate and/or reaction with the concrete walls of the treatment cells is an important process. Weathering of the substrate would imply that a constructed wetland has a fixed lifespan and the aggregate would need to be replaced when clay formation decreases surface area of the substrate. This lifespan would be important to know when calculating cost and convenience factors for constructed wetlands in comparison to other treatment options.