Phosphorus dynamics in the marshland upwelling system

The Marshland Upwelling System (MUS) is an alternative onsite wastewater treatment technology designed to utilize the natural ecology of saltwater marshes to remove human-borne contaminants. Previous research has assessed the ability of MUS to remove both total and orthophosphate. Studies have also indicated a clear zone of phosphorus (P) saturation occurring in MUS soils. Laboratory column study was performed to accomplish the objectives of this study which were to: 1) determine the fate and fractionation of phosphorus in the soil matrix, 2) understand sorption kinetics and determine phosphorus sorption potential of wetland soils in retaining phosphorus in the MUS, 3) determine the service life of the MUS for phosphorus retention. Column study was performed under saltwater and freshwater conditions, wherein artificial wastewater was injected in the columns at a flow rate of 0.7 mL/min, every alternate day. At the end of the study, soil in all columns receiving different salinity treatments was analysed for different phosphorus fractions. Inorganic-P was found to be dominating in sub-surface layers. Organic-P fractions were found in considerable amount in surface layers, which were potentially released by the soil microbial activity. Inorganic-P fractions were likely precipitated by high concentrations of Fe, Al, Ca and, Mg cations present in the soil, under low redox and near neutral to alkaline pH conditions. According to the P-sorption studies conducted, Langmuir one-site isotherm proved best to predict phosphorus sorption mechanism for the MUS soils. It showed a minimum of 361 mg P/kg soil and maximum of 646 mg-P/kg-soil of maximum adsorption capacity at different ionic strengths under anaerobic conditions. Significant differences (p\u3c0.0001) were found between soil and salinity (ionic strength) interactions for the sorption phenomena for aerobic and anaerobic conditions. Longevity parameter (LT) was developed to predict the service life of MUS based on the saturation of phosphorus observed. Service life of minimum and maximum of 15 and 26.9 years, respectively were predicted by the LT under anaerobic conditions for a representative filter volume of 125 m2 at a constant depth of 4m and hydraulic loading rate of 2016 L/d at an influent phosphorus concentration of 15 mg/L