Physical and biological changes of suspended particles in a free surface flow constructed wetland

•We monitored suspended particles characteristics in a surface flow CW.•CW residence did not cause removal of the net suspended particles concentrations.•We did observed removal of large dead organic particles and faecal indicators.•Simultaneously addition of inorganic particles and bacteria occurred.•CW residence resulted in the transition from an anthropological to a more natural particles assembly. Suspended particles are considered as contaminants in treated wastewater and can have profound effects on the biological, physical and chemical properties of receiving aquatic ecosystems, depending on the concentration, type and nature of the suspended particles. Constructed wetlands are known to substantially reduce the concentration of suspended particles in treated wastewater, but hardly anything is known about the changes in the type and nature of these particles. Therefore, the aim of the present study was to investigate the changes in the physical and biological characteristics of suspended particles during residence in the full scale surface flow constructed wetland. The constructed wetland consists of unvegetated ponds and reed beds and receives treated municipal wastewater containing low concentrations of suspended particles. It was found that residence in the unvegetated ponds caused no major changes in particle concentration, but the organic content (53–33%) and average size (4.3–3.7μm) of the suspended particles did decrease, caused by sedimentation of large organic particles and addition of smaller inorganic particles most likely resulting from shore erosion. The bacterial species originating from the wastewater treatment plant (quantified by indicator organisms) decreased strongly in abundance (>90% reduction), especially during residence in the reed beds. Simultaneously the total abundance of bacteria gradually increased, indicating the replacement of the bacterial species present in the treated wastewater with other species during residence in the constructed wetland. These observations indicate that constructed wetlands change the nature and type of the suspended particles during residence in a constructed wetland and reduce the input of anthropogenic particles into receiving surface waters.