Influence of pH on the release of colloidal and dissolved organic matter from vertical flow constructed wetland surface sludge deposits

Abstract

• Organic matter from VFCW's sludge deposits is less soluble after aging. • Truly dissolved organics (<3kDa) are leached under acidic conditions. • Larger colloids (>3 kDa) are leached under alkaline conditions. • Truly dissolved organics leached around neutrality are very aromatic. • Leached colloids are all poorly aromatic regardless of pH and molecular size. The very rapid development of vertical flow constructed wetlands (VFCW) in Europe and throughout the world has induced a growing concern on the conditions of management of their surface sludge deposits (SD). The leaching of organic components from SD was explored in this study with respect to their potential impact on biochemical oxygen demand (BOD) and their role as complexing agents in the release of trace contaminants. The aromaticity and molecular size of leached organic compounds was investigated over a large range of pHs. Experimental results showed that the pH of the leaching aqueous solution influenced the concentration and characteristics of leached OC. Around the natural pH of SD aqueous suspensions (6 to 8), truly dissolved organics (<3 kDa) exhibited high aromatic content. Under alkaline conditions (pH > 8), large organic colloids (>30 kDa) were found predominant, exhibiting a low aromaticity growing with pH. Under acidic conditions (pH < 6), hydrophilic truly dissolved organics were leached. Comparison of concentration, aromaticity and molecular size of OC leached from 14-year and 7-year SD suggested that ageing of SD reduced their sensitivity to pH conditions and may improve their agronomic properties.