Influence of cation exchange capacity (CEC) in a tidal flow, flood and drain wastewater treatment wetland

The cation exchange capacity (CEC) of aggregate (or media) affects treatment performance in flood and drain wastewater wetland treatment systems. In column studies, an electrostatically neutral, high density polyethylene (HDPE) was compared to a lightweight expanded shale aggregate (LESA) that had a CEC of approximately 4.0 meq/100 g. Two columns of LESA and two columns of HDPE treated equal mass loadings of manufactured wastewater for 8 months. One column of each aggregate or medium type was planted. The LESA and LESA-planted columns decisively outperformed the HDPE and HDPE-planted columns for COD removal (LESA 91 and 94% versus HDPE 54 and 60% removal), nitrification (LESA 96 and 96% versus HDPE 42 and 43%), and total nitrogen removal when the COD:TKN was 3.3:1 (LESA 95 and 96% versus HDPE 46 and 68%). Significantly, the treatment role of plants depended on the aggregate or medium CEC. In the LESA columns, plants positively affected COD and TSS treatment, but had no effect on nitrification. In the HDPE columns, plants generally harmed treatment. Results of this study demonstrate that the CEC of aggregate or media in flood and drain wetlands should be a critical design criterion. Moreover, study results also demonstrate that role of plants in vegetated submerged bed (VSB) wetland treatment systems can fundamentally depend on a system state condition such as aggregate CEC.