Journal of Environmental Engineering

This study investigates the efficacy of floating treatment wetlands (FTWs) for retrofitting waste-stabilization ponds (WSPs) operating in cold continental climates, such that in Alberta, Canada. The objective was to determine whether WSPs augmented with FTWs could achieve equal or superior treatment efficiency at shorter hydraulic retention times (HRTs of 25–45 days) than those required in the conventional WSPs (60–365 days). A field study was carried out on treatment of domestic wastewater in a two-stage pilot-scale FTW mesocosm (i.e., FTW treatment train composed of Stage 1 and Stage 2 FTW cells ($\mathrm{S}1\text{-}\mathrm{FTW}\to \mathrm{S}2\text{-}\mathrm{FTW}$), with an overall volume of 84 ${\mathrm{m}}^3$ and $41.85{\mathrm{m}}^3$ per cell). An identical system without FTW served as a control (i.e., control treatment train of $\mathrm{S}1\text{-}\mathrm{C}\to \mathrm{S}2\text{-}\mathrm{C}$). Overall, the FTW-augmented WSP system achieved greater treatment of 5-day biochemical oxygen demand (${\mathrm{BOD}}_5$), total suspended solids (TSS), total nitrogen (TN), ammonium nitrogen (${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$), dissolved reactive phosphorus (DRP), and total phosphorus (TP) than the control. The corresponding removal efficiencies were 91.5% versus 85.0%, 91.3% versus 87.7%, 85.0% versus 75.6%, 93.9% versus 86.1%, 77.6% versus 57.7%, and 75.8% versus 58.7%, with a statistically significant difference ($p<0.05$) for all except TSS. This difference in performance resulted from the better performance of the Stage 1 FTW cell for ${\mathrm{BOD}}_5$, TN, and ${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$ removal, and the Stage 2 FTW cell for DRP and TP removal. Simultaneous nitrification-denitrification (SND) occurred in the FTW treatment train, whereas only nitrification occurred in the control treatment train, which resulted in statistically higher TN removal efficiency in the former. Contribution of plant uptake to phosphorus removal was marginal (6.76%) and adsorption-desorption on solid particles was assumed to be the major removal pathway. The FTWs generally decreased the physicochemical parameters, with stronger effects on diurnal variations than seasonal fluctuations. The effect of FTW on daily physicochemical conditions played a key role in superior performance of FTW treatment.

Calcium silicate hydrate was synthesized from rice husk and used to recover phosphorus from model wastewaters. The phosphorus removal capacity of calcium silicate hydrate was substantially lower in a high alkalinity model animal wastewater ($\mathrm{5,000}\mathrm{mg}/\mathrm{L}$ as ${\mathrm{CaCO}}_3$ at pH 9), compared to deionized water, due to the formation of calcite, which reduced the concentration of free calcium available to react with phosphorus. By gradually decreasing the alkalinity, a proportionate increase in phosphorus uptake was observed, and model wastewaters with alkalinity levels of up to $307\mathrm{mg}/\mathrm{L}$ as ${\mathrm{CaCO}}_3$ showed a phosphorus uptake ($\ge 99\%$) similar to deionized water with no alkalinity. Phosphorus was extracted from spent calcium silicate hydrate by modified soil phosphorus tests. For calcium silicate hydrate exposed to phosphorus in deionized water, the best recovery was under acidic and alkaline conditions, and for calcium silicate hydrate exposed to phosphorus in high alkalinity model wastewaters, the best phosphorus release was under acid and neutral conditions. The results indicate that calcium silicate hydrate synthesized from rice husk shows promise for phosphorus recovery from wastewaters and subsequent reuse as fertilizer.

Waste stabilization ponds (WSPs) rely on natural processes to improve water quality. WSPs require less capital or operational resources than traditional treatment and are commonly used in rural communities or for secondary disinfection. Typically, WSPs designs use empirical equations without three-dimensional (3D) circulation, thermal stratification, or varying water levels. In the present study, a 3D numerical model (Delft3D version 4.04) was applied combined with observations of an operational WSP for nine months (May–November 2017). After calibrating light attenuation and eddy viscosity, model results were in agreement with temperature profiles at five locations, with an average root mean square errors of 1.3°C. The results indicate minimal effects from inflows temperatures and vertical temperature differences of up to 8.0°C between the surface and bed, which inhibited vertical mixing. The buoyancy frequency, a measure of density stratification, was inversely proportional to wind speed and was used to determine that the threshold wind speed required for mixing varied seasonally between 1 and $3\mathrm{m}/\mathrm{s}$. The results from a simulated tracer indicate that empirical methods for estimating hydraulic retention times—the average length of time water remains in a WSP—are inaccurate and varied by an average of 22% from the simulated tracer.