The Modular Bioreactor platform consists of different types of bioreactors that can be used in sequential and/or parallel configuration. The platform is specifically suitable for investigating sustainable solutions for environmental challenges, such as sustainable energy generation, recovery of resources from complex waste streams, and degradation of (micro)pollutants. In this case study, we illustrate the latter application with the use of constructed wetlands.
By The Modular Bioreactor Platform / September 7, 2021
In the last decades, micropollutants have been observed at trace concentration (ng/L to µg/L) in the aquatic environment. These compounds are a potential hazard to environmental and human health and are found in important water resources, such as water used for drinking water production. Concerns about the presence of these micropollutants in the environment indicates the removal of them from the water phase. Therefore, technologies need to be designed for their removal throughout the water cycle and to understand the fate and transformation of these compounds. There is a need for combinations of microbial, chemical and physical processes in innovative reactors, and these systems need to be designed, developed and validated.
Constructed wetlands (CWs) are designed to mimic natural processes that use plants, microorganisms and soil to treat wastewater in a controlled environment. Dominant contaminant removal processes in CWs are aerobic and anaerobic biodegradation, photodegradation, plant uptake and transformation by plants, and sorption to the sediment. Especially aerobic and anaerobic biodegradation by undefined microbial communities, attached to the sediment and plants and present in biofilms, is a key removal mechanism for compounds in CWs. The operational and maintenance costs of CWs are low compared to conventional wastewater treatment plants. Therefore, the application of CWs as a tertiary treatment step for the removal of low concentrations of micropollutants from wastewater effluent is studied more and more.
Case study: Pilot-scale hybrid CWs
In this case study, hybrid-constructed wetlands with three CW water flow types in different combinations were installed: (1) vertical subsurface flow CW, (2) horizontal subsurface flow CW, and (3) surface flow CW. In total, six different hybrid CWs were installed, allowing to test all the possible sequences with the 3 CW types in a modular set-up. The CWs consisted of sand and gravel, and were planted with Phragmites australis. They were fed with a synthetic medium at a flow rate of 140 L/d, resulting in an hydraulic retention time of 7.5 days. Removal of phosphate, benzoic acid, and benzotriazole was studied in these systems.
The CWs were operated for a full year, which allowed to monitor the removal efficiency of the CWs over time and through different seasons. Benzoic acid and nitrate were removed by biodegradation, while phosphate was removed by adsorption and plant uptake. Benzotriazole was removed by adsorption and biodegradation, and its removal efficiency steadily increased from 40% to 100%. It was hypothesized that this was due to microbial adaptation towards biodegrading benzotriazole. In summer, the removal efficiency of compounds that are removed by microbial processes was higher than in winter, likely as a result of a higher microbial activity.
In autumn, the plants were removed and as a lower temperature was observed, this resulted in less input of oxygen and nutrients for microbial processes. As a consequence, a lower removal of compounds that rely on microbial degradation (benzoic acid, nitrate and benzotriazole) was observed in the horizontal flow CW. The vertical flow CW performed similarly as in summer. The surface flow CW was frozen during 2 weeks in winter, and no operation of the hybrid-CWs was possible. When not frozen, the low temperatures and absence of plants resulted in a low microbial activity, especially in the horizontal flow CWs. Nevertheless, the vertical flow CWs performed better. However, the overall performance of the wetlands depended on the figuration (the used sequences) tested.
Research of the Modular Bioreactor Platform
CWs are often approached as ‘black box’ systems, and the removal efficiency of a CW is determined by comparing influent and effluent concentrations. When biodegradation is an important removal mechanism, determining the dominant microbial removal mechanisms for specific compounds in CWs allows the prediction of removal efficiency and the formation of possible toxic transformation products. This is crucial to design a CW for the removal of specific compounds , and identify if pre- or post-treatment in other treatment units is needed, e.g. photooxidation as pre-treatment or biological polishing as post-treatment. Furthermore, the different designs of a CW can steer specific removal processes, e.g the presence of various redox conditions. This flexibility in design and sequential combinations are part of the Modular Bioreactor platform.
Interested in the use of constructed wetlands? Read our research paper and review here:
Thomas V. Wagner, Vinnie de Wilde, Bert Willemsen, Muhamad Mutaqin, Gita Putri, Julia Opdam, John R. Parsons, Huub H.M. Rijnaarts, Pim de Voogt, Alette A.M. Langenhoff, 2020, Pilot-scale hybrid constructed wetlands for the treatment of cooling tower water prior to its desalination and reuse, Journal of Environmental Management, 271: 110972
Thomas V. Wagner, John R. Parsons, Huub H. M. Rijnaarts, Pim de Voogt & Alette A. M. Langenhoff (2018) A review on the removal of conditioning chemicals from cooling tower water in constructed wetlands, Critical Reviews in Environmental Science and Technology, 48:19-21, 1094-1125.
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