Modelling of arsenic retention in constructed wetlands

Mathematical modeling of subsurface flow constructed wetlands performance for arsenic removal: Review and perspectives

Constructed wetlands are nature-based solutions able to remove different pollutants from water, including arsenic. Arsenic is a pollutant of concern given its toxicity and its presence in water sources worldwide. Despite the increased interest in investigating the performance of constructed wetlands in the treatment of arsenic-contaminated water at the laboratory scale, the application of these solutions at the pilot and full scale is still limited. To understand and predict the removal of arsenic in constructed wetlands, some numerical models have been developed. Among black box models, only first-order models have been proposed, with unsuccessful results. The model that best describes arsenic retention processes in constructed wetlands is RCB-ARSENIC, a mechanistic model adapted from Retraso-CodeBright that simulates arsenic reactive transport. This model includes arsenic precipitation, arsenic sorption on supporting media, arsenic sorption on plants roots and arsenic uptake by plants; represented in the reactive term of the reactive transport equation. Thus, it includes two of the three main processes that remove arsenic in constructed wetlands: precipitation, sorption, and coprecipitation. Despite this, and what is known about arsenic geochemistry, the formulation of these reactive rates requires improvement. In addition, this model was calibrated and validated using data from a single horizontal subsurface flow constructed wetland system, which treated one type of synthetic water. Therefore, it cannot be applied to other types of arsenic-contaminated water or other constructed wetland systems. Moreover, the reactive transport of relevant species -especially iron- and their role in arsenic removal, along with relevant redox reactions associated to the presence of organic matter, oxides or bacteria-, must be included. A comprehensive mechanistic model able to simulate different design, environmental and operation conditions may be used to guide the design of constructed wetlands targeting the removal of arsenic.

Advances in the process-based models of constructed wetlands and a way forward for integrating emerging organic contaminants

This research examines advancements in the development of process-based models of constructed wetlands (CWs) tailored for simulating conventional water quality parameters (CWQPs). Despite the promising potential of CWs for emerging organic contaminant (EOC) removal, the available CW models do not yet integrate EOC removal processes. This study explores the need and possibility of integrating EOCs into existing CW models. Nevertheless, a few researchers have developed process-based models of other wastewater treatment systems (e.g., activated sludge systems) to simulate certain EOCs. The EOC removal processes observed in other wastewater treatment systems are analogous to those in CWs. Therefore, the corresponding equations governing these processes can be tailored and integrated into existing CW models, similarly to what was done successfully in the past for CWQPs. This study proposed the next generation of CW models, which outlines 12 areas for future work: integrating EOC removal processes; ensuring data availability for model calibration and validation; considering quantitative and sensitive parameters; quantifying microorganisms in CWs; modifying biofilm dynamics models; including pH, aeration, and redox potential; integrating clogging and plant sub-models; modifying hydraulic sub-model; advancing computer technology and programming; and maintaining a balance between simplicity and complexity. These suggestions provide valuable insights for enhancing the design and operational features of current process-based models of CWs, facilitating improved simulation of CWQPs, and integration of EOCs into the modelling framework.

Domestic wastewater treatment efficiency of the pilot-scale trickling biofilter system with variable flow rates and hydraulic retention times

In this study, a pilot-scale trickling biofilter (TBF) using pebbles and gravels media was evaluated for the treatment of domestic wastewater. The TBF system was installed in an open environment at residential area of Quaid-i-Azam University, Islamabad, Pakistan, and was operated at three different recirculation flow rates (Q), i.e. 0.04, 0.072 and 0.1 m³/day and under three different HRTs, i.e. 48, 72 and 96 h. It was observed that the efficiency of pilot-scale TBF system in terms of pathogens removal was significant, i.e. at flow rates of 0.04, 0.072 and 0.1 m³/day, an average reduction of 39.8-62.5% (p = 0.007), 35.9-48.6% (p = 0.01) and 25.8-57.3% (p = 0.009) respectively were attained in CFU/mL under different HRTs. Moreover, it has been observed that due to high void spaces up to 30%, pebbles and gravels filter media in co-ordination allowing good microbial growth and increased the diversity of bacterial species. Furthermore, it also facilitate the removal of different pollutant indicators, i.e. chemical oxygen demand (COD) (74.2-80.5%), total dissolved solids (TDS) (60.3-69.5%), electric conductivity (EC) (62.8-68.6%) and phosphates (PO₄) (45.3-60.3%). A significant reduction in total nitrogen (TN) (59-63.3%) was observed at flow rates of 0.04 and 0.072 m³/day (p = 0.005). The experimental data of this research study will be helpful for further modification in the TBF system using different filter media in association and selecting optimal HRTs and flow rates in future study to get maximum efficiency of TBF system while treating domestic wastewater.

Winter flooding of California rice fields reduces immature populations of Lissorhoptrus oryzophilus (Coleoptera: Curculionidae) in the spring

BACKGROUND

In California, rice fields are flooded over the winter months (November to March) to facilitate degradation of post-harvest rice straw and to provide temporary habitat for migratory waterfowl. Prior research showed that winter flood rice fields had fewer rice water weevil (Lissorhoptrus oryzophilus), larvae and pupae during the rice production season than fields that were left unflooded in the winter. A series of experiments were conducted to provide further support for these trends under controlled conditions and to find a mechanism for this phenomenon.

RESULTS

Under winter flooded conditions there was a 50% reduction in populations of weevil immatures compared with the untreated control (no straw or winter flood). These same conditions corresponded to a 20% increase in the amount of silicon found in plant tissues in 2014 and a 39 to 90% decrease in methane production in the soil from 2013 to 2014, respectively.

CONCLUSION

Evidence from previous field research and these controlled studies supports winter flooding as an appropriate tactic for controlling L. oryzophilus populations in the spring. However, the mechanism that would explain why winter flooding adversely affects L. oryzophilus immatures remains unclear. © 2016 Society of Chemical Industry.

Simulation of arsenic retention in constructed wetlands

The software RCB-arsenic was developed previously to simulate the metalloid behavior in a constructed wetland (CW). The model simulates water flow and reactive transport by contemplating the major processes of arsenic (As) retention inside of CW. The objective of this study was to validate the RCB-arsenic model by simulating the behavior of horizontal flow CW for As removal from water. The model validation was made using data from a 122-day experiment. Two CWs prototypes were used: one planted with Eleocharis macrostachya (CW_planted) and another one unplanted (CW_unplanted) as a control. The prototypes were fed with synthetic water prepared using well water and sodium arsenite (NaAsO₂). In the RCB-arsenic model, a CW prototype was represented using a 2D mesh sized in accordance with the experiment. For simulation of As retention in CW, data addition was established in two stages that considered the mechanisms in the system: (1) aqueous complexation, precipitation/dissolution, and adsorption on granular media and (2) retention by plants: uptake (absorption) and rhizofiltration (adsorption). Simulation of As outlet (μg/L) in stage_1 was compared with CW_unplanted; the experimental mean was 40.79 ± 7.76 and the simulated 39.96 ± 6.32. As concentration (μg/L) in stage_2 was compared with CW_planted, the experimental mean was 9.34 ± 4.80 and the simulated 5.14 ± 0.72. The mass-balance simulation and experiment at 122 days of operation had a similar As retention rate (94 and 91%). The calibrated model RCB-arsenic adequately simulated the As retention in a CW; therefore, it constitutes a powerful tool of design.