Phosphorus Removal in Passive Cold Region Waste Stabilization Ponds

Abstract

A majority of communities in the Canadian territory of Nunavut rely on passive waste stabilization ponds (WSP) for domestic wastewater treatment. Little research has been conducted on the treatment performance of these systems. In response to impending federal wastewater regulations, a research program was conducted in order to characterize contaminant removal. Due to its role in receiving water eutrophication, phosphorus is a contaminant of particular concern. Phosphorus is removed in WSPs both chemically (precipitation, adsorption) and biologically (microalgae and bacteria uptake). Due to extended photoperiods during the ice-free summer treatment season, it is hypothesized that microalgae play an important role in phosphorus removal. The objectives of this research were to (i) characterize phosphorus removal and identify promising removal mechanisms occurring in full-scale WSPs in Nunavut, Canada, (ii) quantify microalgae growth rates and phosphorus uptake, and identify microalgae uptake mechanisms under simulated cold region summer conditions, and (iii) develop a predictive stochastic ecological model for microalgae growth and effluent phosphorus concentration in WSPs in various geographical locations in Nunavut, Canada. Full-scale WSP evaluations occurred at four communities (Kugaaruk, Pond Inlet, Grise Fiord and Clyde River) over a period of four years (2011-2014). Phosphorus removal was highly variable. Most of the WSPs operated anoxically with minimal microalgae growth; however, the highest removal was observed during a microalgae bloom. A factorial laboratory experiment was then done to quantify microalgae growth rates and phosphorus uptake under varying climate conditions (temperature, photosynthetically active radiation) and phosphorus concentrations. Growth rates were similar to those found at temperate climates. Biomass phosphorus concentrations were found to be 45% greater than previously observed in studies at temperate climates. A stochastic ecological model with integrated equilibrium temperature model was then developed to predict microalgae growth and phosphorus removal in WSPs at various geographical locations in Nunavut. The model utilized a Monte Carlo Simulation to account for parameter uncertainty. The model showed that July temperature and summer treatment season temperature were the best predictors of microalgae growth. Modeled phosphorus removal was consistent with secondary treatment if WSP depth was less than 2 m and the WSP is operating facultatively.