Introduction: Green roofs have great potential to treat domestic greywater on-site while improving the urban microclimate and biodiversity. Preliminary experiments have shown that wetland roof systems are able to efficiently treat greywater which can be used as service water after appropriate hygienization (Rahman, 2023). However, further research is needed to understand underlying removal mechanisms to optimize green roof systems for wastewater treatment.
Objectives: We are investigating which redox conditions are beneficial for the treatment of greywater and the microbial transformation of poorly degradable contaminants.
Methods: Transformation of four wastewater chemicals of concern (namely benzotriazole, diclofenac, sulfamethoxazole and perfluorooctanoic acid) as well as the treatment of synthetic greywater were investigated. For this, two polypropylene boxes (each 37 cm × 26 cm) equipped with helophyte mats planted with Carex acutiformis served as down-scaled model systems under defined conditions. Redox-conditions were changed in the systems via permeation of nitrogen gas or compressed air and monitored using sensitive dissolved-oxygen sensors. Treatment of synthetic greywater was evaluated by measuring standard parameters for wastewater analysis. Transformation of the model chemicals was investigated by using LC-MS/MS technology. De novo proteomics were used to study the microbial community.
Results: We were able to establish different oxic and anoxic subzones within the model systems. Despite the small model size and low amount of plants (three plants per system), the synthetic greywater was treated efficiently with high removal of BOD, COD, TOC, total nitrogen and ammonium – all parameters showing higher removal efficiencies under aeration with compressed air. The transformation of the four recalcitrant model chemicals was highly redox-dependent. Especially benzotriazole and diclofenac showed significantly higher removal when introducing oxic zones, while sulfamethoxazole also showed high removal efficiencies under very anoxic conditions. Identification of transformation products and de novo proteomics gave further insights, highlighting once more the importance of redox-conditions in microbial processes.
Conclusion: This study shows the great potential of redox-dependent microbial transformation for the treatment of greywater in green roof systems.
Rahman, K.Z. et al., Water 2023, 15, 3375. doi: 10.3390/w15193375