Elimination of Micropollutants using Engineered Constructed Wetlands – A Laboratory Scale Study

In Switzerland, a new standard for wastewater treatment plants (WWTP) – 80% reduction of micropollutants from raw sewage – is under consideration. Five substances have been proposed as target compounds: Diclofenac, Carbamazepine, Mecoprop, Sulfamethoxazole and Benzotriazole. Although very efficient and reliable, advanced treatment technologies for traditional WWTPs – such as ozonation – are expensive and difficult to implement on relatively small WWTPs, of which there are many in Switzerland. To tackle this problem, we propose to develop a promising technology for supplemental treatment of wastewater: engineered subsurface flow constructed wetlands (ECWs). Classical constructed wetlands (CW) are passive systems, using natural processes to transform and remove contaminants from wastewater. In this work, we have developed and proposed a different design paradigm, i.e., to combine in a single ECW both natural processes (as in CWs) and ad hoc engineered treatments. The resulting system – an ECW – will combine the strengths of both natural and engineered processes, that is, it will be efficient and reliable, with low maintenance and running costs. However, there is still a lack of detailed knowledge of the factors influencing micropollutant degradation and, ipso facto, in the ability to optimize the conditions for contaminant effective elimination. A laboratory scale study was performed to develop a combination of treatments able to achieve the target elimination rate for micropollutants. Based on a literature review, two processes – direct photolysis and adsorption by light expanded clay aggregates (LECA) – were expected to remove more than 80% of all target compounds. Batch experiments were conducted to test the removal efficiency of each process alone. Direct photolysis removed 60 to > 95% of four compounds while adsorption by LECA showed only moderate elimination. A laboratory-scale ECW combining the two removal treatments with biodegradation was subsequently performed to evaluate whether the combined processes could enhance the degradation rate. It was found that removal of just three target compounds was enhanced. A conceptual model was developed and will be used as starting point for modeling micropollutant removal in ECW’s in future studies. Based on the measurements and insights gained from the model, possible reasons for the relative poor performance observed will be discussed, and possible improvements presented.

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