Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs)

Abstract

The design of French VFCWs leads to the formation of a sludge layer at the surface of the first filters due to the retention of suspended solids from the percolation of unsettled wastewater. This layer plays a major role in the system but still little is known on its characteristics and evolutions. In this study, suspended solids and sludge deposits sampled from two French VFCW plants were analyzed by different methods in the objective to assess the evolution of particulate organic matter (POM) along the treatment chain and within the sludge layer, and identify relevant analytical indicators of these phenomena. The treatment chain included an aerobic trickling filter followed by FeCl injection and two successive stages of filters. Thermal analyses showed that OM contents of suspended solids decreased along the treatment chain. POM in inflow suspended solids was predominantly composed of reactive, biodegradable compounds which were partly hydrolyzed and mineralized notably at the trickling filter stage. 3D fluorescence spectra collected from aqueous POM extracts confirmed the evolution of organic matter from low-molecular reactive compounds to more complex and stable structures such as humic-like substances. FTIR confirmed the mineralization of POM's reactive constituents along the treatment chain by the decrease in the intensities of the characteristics bands of aliphatic compounds or proteins, and its humification in the sludge deposits through the relative increase of the bands at 1634cm (v) and 1238cm (δ and/or δ). Isotopic ratios δH/H and δN/N were found to be good indicators of POM evolutions. The higher values of δH/H and δN/N ratios measured in sludge deposits as compared to inflow suspended solids were related to POM humification and to microbial processes of POM hydrolysis and mineralization, respectively.

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

“…So-called "Classical French" vertical flow constructed wetlands (VFCWs) are based on the percolation of unsettled wastewaters through 2 successive stages of filters filled with gravels or sand and planted with reeds. This process induces the formation of a sludge deposit (SD) layer at the surface of the first-stage filter which has been shown to play a major role in the retention and degradation of a large variety of contaminants during regular conditions of operation [25,26].…”

“…terrestrial organic residues from the catchment area). The surface sludge layer is the siege of numerous reactions which induce the evolution of organic matter [26,28] that could in turn influence the quantity and the chemical nature of the DOM. Some studies have already monitored the concentration of DOM in VFCW systems, from inflow to outflow [29][30][31].…”

“…The general objective of this study was thus to generate scientific knowledge on the release of organic colloids from the SOM of VFCW surface sludge deposits, and more specifically on the effects of pH on the quantities and physical-chemical characteristics of the leached organic compounds. Previous work from our group compared the characteristics of 14 SDs samples from different origins and identified 2 typologies of samples according to their age, namely the older SDs group beyond 3 years of age, and the younger SDs group of 1 year or less [26]. The present study focused on sludge deposits of the older SDs group (beyond 3 years of age) which was considered as representative of a steady state mode of operation of the VFCW unit.…”

Use of a clay mineral and its nonionic and cationic organoclay derivatives for the removal of pharmaceuticals from rural wastewater effluents

“…Constructed wetlands and lagoon-based systems represent elegant and low-cost water remediation technologies used by numerous small communities in Europe (Zhang and Li, 2011;Verlicchi and Zambello, 2014;Thiebault et al, 2016a;Thiebault et al, 2016b;Kania et al, 2018). While constructed wetlands assisted by phyto-planted filters appear quite sophisticated with the design of several stages of vertical-flow filters including the action of sludge, the lagoon type is quite simple leaning against the presence of micro-organisms combined with solar UV radiation for the removal of diverse organic contaminants.…”

Comparative characterization of surface sludge deposits from fourteen French Vertical Flow Constructed Wetlands sewage treatment plants using biological, chemical and thermal indices

“…Kim et al (2013), reported that although mainly organic, sludge deposits contained significant amounts of inorganic constituents originating from the local geological background or generated by possible operations of pretreatment of the influent such as phosphate precipitation by FeCl3 addition. Kim et al (2013) and Kania et al (2018Kania et al ( , 2018a investigated the extent of organic matter evolution within the sludge deposits, such as mineralization. Finally, the authors of this study reported structural properties data of surface sludge deposits in a recent article (Kania et al, 2018b).…”