Inorganic particle accumulation promotes nutrient removal of vertical flow constructed wetlands: Mechanisms and implications

Phosphorus attenuation and mobilization in sand filters treating onsite wastewater

The effectiveness of phosphorus (P) removal by sand filters is limited during septic tank effluent (STE) treatment. The elevated effluent P concentrations pose threats to drinking water quality and contribute to eutrophication. The concern of P leaching from sand filters is further exacerbated by the increased frequency of flooding and natural precipitation due to climate change. This study aimed to understand P attenuation and leaching dynamics, as well as the removal mechanisms in sand filters treating STE, offering insights into the design and implementation of P removal/recovery modules to onsite wastewater treatment systems. P attenuation and leaching during STE treatment and rainfall were studied in bench-scale columns (new vs. aged sand). At standard STE loading (1.2 gallon d-1 ft-2), 24-32% removal of total phosphorus (TP) was achieved, while increased P removal efficiency (35-53%) was observed at low loading (0.6 gallon d-1 ft-2) with influent containing 10.3-20.0 mg P L-1. Complete breakthroughs were observed in both aged (12-70 days) and new columns (27-73 days) at test hydraulic loadings. The maximum TP attenuation level was 20.6-45.3 mg P kg-1 and 25.3-33.0 mg P kg-1, in aged and new sand columns, respectively. When simulated rain was applied (15-60 mm h-1), 80-97% of the attenuated P leached out and the leaching dynamics were impacted by rainfall duration rather than the intensity. The highest concentrations of TP (15.6-15.9 mg L-1) were leached out from both columns within the first 2-6 h. Orthophosphate was the dominant P species in treated effluent (83-84%) and leachate (69-88%), demonstrating its significance as the major P form in the discharge. In addition, aged sand (>5 years) accumulated higher levels of Mg, Al, Ca, and Fe, thus enhancing the P attenuation level during STE treatment. Collectively, this study underscored the importance of frequent field monitoring for reliable long-term P removal estimates.

Enhanced adaptability of pyrite-based constructed wetlands for low carbon to nitrogen ratio wastewater treatments: Modulation of nitrogen removal mechanisms and reduction of carbon emissions

Pyrite-based constructed wetlands (CWs) stimulated nitrate removal performance at low carbon to nitrogen (C/N) ratio has been gaining widely attention. However, the combined effects of pyrite and C/N on the nitrate removal mechanisms and greenhouse gases (GHGs) reduction were ignored. This study found that pyrite-based CWs significantly enhanced nitrate removal in C/N of 0, 1.5 and 3 by effectively driving autotrophic denitrification with high abundance of autotrophs denitrifiers (Rhodanobacter) and nitrate reductase (EC 1.7.7.2), while the enhancement was weakened in C/N of 6 by combined effect of mixotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) with high abundance of organic carbon-degrading bacteria (Stenotrophobacter) and DNRA-related nitrite reductase genes (nrf). Moreover, pyrite addition significantly reduced GHGs emissions from CWs in all stages with the occurrence of iron-coupled autotrophic denitrification. The study shed light on the potential mechanism for pyrite-based CWs for treating low C/N ratio wastewater.

Impact of clogging on accumulation and stability of phosphorus in the subsurface flow constructed wetland

Substrate clogging is one of the major operation challenges of subsurface flow constructed wetlands (SSF-CWs). And the phosphorus (P) removal performance and stability of P accumulation of SSF-CWs would be varied with the development of substrate clogging. In this study, three horizontal SSF-CWs microcosms with different clogging degrees were conducted to explore the mechanism of P accumulation behavior influenced by substrate clogging. Increase in clogging degree resulted in hydraulic retention time (HRT) diminution and adsorption sites increase, which jointly led to reduced P removal efficiency at low clogging degree (L-CW), however, higher P removal efficiency was obtained as adsorption sites increase offset HRT diminution at high clogging degree (H-CW). Substrate adsorption was the primary removal pathway in all SSF-CW systems. It accounted for 77.86 ± 2.63% of the P input in the H-CW, significantly higher than the control (60.08 ± 4.79%). This was attributed to a higher proportion of Fe/Al-P accumulated on the substrate of H-CW, since clogging aggravated the anaerobic condition and promoted the generation of Fe ions. The increase in clogging degree also elevated the release risk of the accrued P in SSF-CWs, since Fe/Al-P was considered bioavailable and readily released under environmental disturbance. The obtained results provide new insights into the P transport and transformation in SSF-CWs and would be helpful to optimize substrate clogging management.

Immobile Iron-Rich Particles Promote Arsenic Retention and Regulate Arsenic Biotransformation in Treatment Wetlands

Remediation of arsenic (As)-contaminated wastewater by treatment wetlands (TWs) remains a technological challenge due to the low As adsorption capacity of wetland substrates and the release of adsorbed As to pore water. This study investigated the feasibility of using immobile iron-rich particles (IIRP) to promote As retention and to regulate As biotransformation in TWs. Iron-rich particles prepared were immobilized in the interspace of a gravel substrate. TWs with IIRP amendment (IIRP-TWs) achieved a stable As removal efficiency of 63 ± 4% over 300 days, while no As removal or release was observed in TWs without IIRP after 180 days of continuous operation. IIRP amendment provided additional adsorption sites and increased the stability of adsorbed As due to the strong binding affinity between As and Fe oxides. Microbially mediated As(III) oxidation was intensified by iron-rich particles in the anaerobic bottom layer of IIRP-TWs. Myxococcus and Fimbriimonadaceae were identified as As(III) oxidizers. Further, metagenomic binning suggested that these two bacterial taxa may have the capability for anaerobic As(III) oxidation. Overall, this study demonstrated that abiotic and biotic effects of IIRP contribute to As retention in TWs and provided insights into the role of IIRP for the remediation of As contamination.

Fate of phosphorus from treated wastewater in soil-based constructed wetlands

In France, soil-based constructed wetlands for the discharge of treated wastewater have become a popular technique to both reduce flow to surface receiving water bodies and perform complementary treatments. This study focuses on the fate of phosphorus in three different soils, as well as its assimilation by Phragmites australis. The experimental set-up consisted of three lysimeters containing three soils selected to be representative of those typically found near wastewater treatment plants (i.e. a silt loam Fluvisol, a sandy loam Fluvisol and a sandy-clay loam Technosol). Lysimeters are undisturbed soil monoliths (1.5 m³ in volume), whose masses are continuously monitored in order to obtain an accurate water mass balance. The lysimeters here were intermittently fed for 3.5 days and then left to rest for 3.5 days. The experiment lasted 26 months, including 18 months of feeding with phosphorus (PO₄-P, TP) fluxes in and out being monitored along with water content, oxygen content and redox potential at various depths. The quantities of phosphorus stored in the soils and assimilated in the Phragmites australis were measured. Phosphorus fractionation in soils was performed to better understand its distribution and potential remobilization. Low phosphate concentrations were measured at the outlets of all three lysimeters, thereby highlighting satisfactory phosphorus retention in the three soils (removal efficiencies >90%). A significant amount of phosphorus can be exported by harvesting Phragmites australis aerial parts (26%, 17% and 13% of the yearly incoming phosphorus mass for the silt loam Fluvisol, sandy loam Fluvisol and sandy-clay loam Technosol, respectively). The fractionation step served to determine that the phosphorus retained in the soil was primarily bound to iron oxides/hydroxides, calcium and clay. Moreover, it was found to be preferable to hold oxidizing (aerobic) conditions and pH close to neutral in order to maintain conditions under which the complexes formed with phosphorus remain stable.