Phosphorus retention in constructed wetlands enhanced by zeolite‐ and clinopyroxene‐dominated lava sand

Abstract

Constructed wetlands (CWs) are engineered systems for treating wastewater by sequestering nutrients and contaminants. Our aim was to assess the main phosphorus (P) binding states in operating CWs to assess P saturation and indications on P recycling potential of filter materials, which might be necessary under future peak P scenarios. The investigated vertical flow CWs (operation time up to 16 years) are based on either fluviatile (Fluv) sand or zeolite-(Ze-LS) and clinopyroxene (Cl-LS)dominated lava sand. Organic and inorganic P accumulated in all CWs independent of filter materials and showed a considerable increase with operation time. Concentrations of P decreased sharply with depth in the Fluv-CWs compared to only a slight decrease in the lava sand CWs, with P concentrations of deeper horizons approximating the relatively P enriched original lava sand substrates. Orthophosphate was the dominant pool in all CWs, while the sum of organic fractions ranged between 11% and 33%. Sequential extraction indicated that P was mainly associated with Fe and Al (hydr)oxides for Fluv-CWs and Ze-LS-CWs, while Ca and Mg bound mineral phosphates dominated in Cl-LS-CWs. Oxalate extractions pointed to a clear dominance of P fractions associated with poorly crystalline Fe-and Al-(oxy)hydroxides. Solution 31 P NMR analyses revealed that inositol hexakisphosphates were a major pool of organic P in surface layers of CWs, which increased with operation time. With a maximum of 0.5% P content, filter sands do not appear to be a suitable fertilizer for direct application to agricultural fields. The dominance of inorganic, poorly crystalline P species point to potentially high desorption capacity which might be investigated further, to assess recycling potential of P or usage of filter materials as soil amendments with relatively high plant available P. The latter might become feasible and economically attractive under future P scarcity. Simultaneously, P saturation indexes (DPS) did not indicate an imminent P saturation of filters, since P accumulation was not restricted by binding to Al and Fe minerals. Christine Alewell and Jen-How Huang shared first authorship.

Elevated Vertical-Flow Constructed Wetlands for Light Greywater Treatment

“…Elimination rates for P tot load varied from 83.4% in Phase 2, 56.8% in Phase 4b to 18.4% in Phase 5c (see Table 1). In contrast, the lava sand compartment exhibited P tot effluent values <0.05 mg/L (LOD) at all times (P elimination rates >97%), which corroborates literature findings of enhanced phosphorus removal in zeolite-containing lava sand CW that treat municipal wastewater from combined sewer systems in Germany, even after operating times of years [30,33]. P removal is improved by the high natural zeolite content of lava sands [33] and the increased contact time between VFCW and porous media in the filter [30].…”

Soil phosphomonoesters in large molecular weight material comprise multiple components

“…Interestingly, Alewell et al. (2021) reported the accumulation of complex phosphomonoesters in constructed (non‐soil) wetlands after 16 yr of operation, which suggests that microbial turnover of organic material may be an important factor in their synthesis.…”

“…Several studies on soil Po have already found an association with complex phosphomonoesters and that of soil organic C, which appear to accumulate on a decadal timeframe (McLaren et al, 2017;McLaren, Smernik, McLaughlin, McBeath, et al, 2020;Schefe et al, 2015). Interestingly, Alewell et al (2021) reported the accumulation of complex phosphomonoesters in constructed (non-soil) wetlands after 16 yr of operation, which suggests that microbial turnover of organic material may be an important factor in their synthesis.…”

Women advancing research on hydrological processes: Preface

“…One focus of her research is on the development of indicators for wetland degradation and regeneration. In their paper,Alewell et al (2021), they assessed the main phosphorus binding states in operating constructed wetlands to assess phosphorus saturation and indications on phosphorus recycling potential of filter materials. They showed that phosphorus accumulation increased with age of the investigated vertical flow constructed wetlands in all three investigated filter materials (fluviatile sands, zeolite-and clinopyroxene-dominated lava sands) and most of the phosphorus was accumulated in the upper horizons.Dr.…”