Phosphorus removal in surface flow treatment wetlands for domestic wastewater treatment: Global experiences, opportunities, and challenges.

Treatment Wetlands (TWs) are widely used for the treatment of domestic wastewater, with an increasing emphasis on provision of multiple co-benefits. However, concerns remain regarding achieving stringent phosphorus (P) discharge limits, system robustness and resilience, and associated guidance on system design and operation. Typically, where P removal is intended with a passive TW, surface flow (SF) systems are the chosen design type. This study analysed long-term monitoring datasets (2-30 years) from 85 full-scale SF TWs (25 m2 to 487 ha) treating domestic sewage with the influent load ranging from 2.17 to 54,779 m3/d, including secondary treatment, tertiary treatment, and combined sewer overflows treatment. The results showed median percentage removals of total P (TP) and orthophosphate (Ortho P) of 28% and 31%, respectively. Additionally, median areal mass removal rates were 5.13 and 2.87 gP/m2/yr, respectively. For tertiary SF TWs without targeted upstream P removal, 80% of the 44 systems achieved ≤3 mg/L annual average effluent total P. Tertiary SF TWs with targeted upstream P removal demonstrated high robustness, delivering stable effluent TP < 0.35 mg/L. Seasonality in removal achieved was absent from 85% of sites, with 95% of all systems demonstrating stable annual average effluent TP concentrations for up to a 30-year period. Only two out of 32 systems showed a significant increase in effluent TP concentration after the initial year and remained stable thereafter. The impact of different liner types on water infiltration, cost, and carbon footprint were analysed to quantify the impact of these commonly cited barriers to implementation of SF TW for P removal. The use of PVC enclosed between geotextile gave the lowest additional cost and carbon footprint associated with lining SF TWs. Whilst the P-k-C* model is considered the best practice for sizing SF TWs to achieve design pollutant reductions, it should be used with caution with further studies needed to more comprehensively understand the key design parameters and relationships that determine P removal performance in order to reliably predict effluent quality.

Phosphorus removal in emergent free surface wetlands.

Constructed and natural wetlands are capable of absorbing new phosphorus loadings, and, in appropriate circumstances, can provide a low-cost alternative to chemical and biological treatment. Phosphorus interacts strongly with wetland soils and biota, which provide both short-term and sustainable long-term storage of this nutrient. Soil sorption may provide initial removal, but this partly reversible storage eventually becomes saturated. Uptake by biota, including bacteria, algae, and duckweed, as well as macrophytes, forms an initial removal mechanism. Cycling through growth, death, and decomposition returns most of the biotic uptake, but an important residual contributes to long-term accretion in newly formed sediments and soils. Despite the apparent complexity of these several removal mechanisms, data analysis shows that relatively simple equations can describe the sustainable processes. Previous global first order removal rates characterize the sustainable removal, but do not incorporate any biotic features. This article reviews the relevant processes and summarizes quantitative data on wetland phosphorus removal.

Nitrogen farming for pollution control.

The use of free water surface treatment wetlands for nitrate reduction has an extensive basis in data from dozens of operating systems. Marshes are effective for denitrification, with first order areal annual rate constants centered on thirty-four meters/year. Performance improves at higher water temperatures, with a modified Arrhenius temperature factor of 1.090. Performance also increases with increasing hydraulic efficiency, created by prevention of short-circuiting, and reflected in values of the tanks-in-series parameter N > 5. Higher efficiencies are associated with submergent and emergent soft tissue vegetation, and lower efficiencies with unvegetated open water and forested wetlands. Hydraulic loadings of 2-7 cm/day can produce 30% nitrate load reductions, over the temperature range 6-20 degrees C. Carbon availability limits denitrification at high nitrate loadings, however, wetlands produce carbon in sufficient quantities to support the loads anticipated in the upper midwest. The conversion of agricultural lands to treatment wetlands focused on nitrate reduction is termed nitrogen (N) farming. (D.H. Hey, Nitrogen farming: harvesting a different crop. Restoration Ecology, 2002, 10 (1), 1-11). A demonstration project is indicated to address local issues and scale-up considerations. Such a project would require thorough monitoring for the purpose of optimizing and refining design models. Significant ancillary benefits of ecological diversity and wildlife habitat are certain to accompany the project, but are of secondary importance until the water quality functions are demonstrated. Regulatory issues include permitting and wetland classification. Economic issues include proper pricing of services and methods of revenue generation. Resolution of these potential difficulties may require modification of existing policies and institutions.