Earthworm's enhancement potential of constructed wetland wastewater treatment process in a REUSE perspective

International audience

anglais. Constructed wetlands (CW) for wastewater treatment, nature-based solution1, is a promisingtechnology for REUSE purpose, as it is cheap, robust, energy efficient and showing optimistic resultson the elimination of micropollutants2. Our work focuses on the contribution of faunal biodiversity invertical unsaturated/saturated FPs (ns/s FPVs). This design enables aerobic and anaerobic processes,and synergy between substrate, plant roots, interstitial biofilm microorganisms for water purification1.In an ns/s FPV, water is delivered from above, in batches, to increase water infiltration speed andpromote oxygenation of the surface layers between two batches. This type of FPV delivers stable, highlevels of COD and nitrogen removal in particular3.The main drawback of FPs is the space requirement due to both clogging issues and kinetic limits ofbiodegradation, within the bacterial biofilm developed by the bioremediation. To remedy this, we arestudying the potential of introducing earthworms into CW. Earthworms are known for their ability toremediate many pollutants, lower clogging4,facilitate soil oxygen transfer, increase soil contaminantsbioavailability and enhance soil microbial activity5. Our work involves studying the effect of worms onpurification performance in relation to changes in hydrodynamic and microbiological properties.Our experiments take place on Toulouse III University campus, treating raw WW from a teachingadministrativebuilding generating effluent from 20 to 50 PE. Effluents’ physico-chemical parametersare on the average of a domestic WW with the exception of a high dissolved nitrogen load. Experimentsare carried out at mesocosm scale (0.2 m2). Nine mesocosms are operated simultaneously, testingthree experimental conditions replicated three times (control hydraulic load 1.2 m2/PE; wormshydraulic load 1,2 m2/PE; worms hydraulic load 2.4 m2/PE). In parallel, the four pilots (5 m²) areoperated with four different experimental conditions (control; worms; worms+plants; plants). Classicalphysico-chemical parameters are monitored (COD, BOD5, SS, EC, DO, pH, NH4+-N, NO3--N, PO43--P…).Tracer tests (fluoresceine) and falling head permeametry are performed for hydrodynamiccharacterization. Finally, metagenomic (16sRNA) analysis characterize the microbial communitystructure and diversity assessment.The results from the mesocosm experiment revealed a significant effect of earthworm activity on thehydraulic conductivity of the substrate limiting clogging extend. Despite the increase in hydraulicconductivity, the residence time of water in the system remained constant with or without worms.This result suggests that worms have a positive effect on clogging, without reducing biodegradationpotential by maintaining an equivalent residence time. In all 3 conditions, the residence time is (≃ 2h),lower than it would be in a full-scale VSSF-CW due to the feeding mode chosen. While satisfactoryremoval rates were achieved for particulate load (>80%), those for dissolved load were significantlylower than the standards expected for this filter design (<< 50%). A slight improvement in COD removalwas observed in the presence of worms. The hypothesis put forward to explain the low levels ofdissolved load removals is the feeding method used, with infrequent feeding equivalent to 0.15 mwater depth batches (while technical guides recommend < 0.04 m)6 . These excessively large batchesforce flows and shorten residence times. These results suggest that a more regular mode of feedingshould enable us to demonstrate the influence of worms on purification performance.

français. Les filtres plantés (FP) pour le traitement des eaux usées (EU), technologie fondée sur la nature 1 prometteuse pour la REUSE, car peu chère, robuste, peu consommatrice d'énergie et montrant des résultats encourageants sur l'élimination de micropolluants 2 . Notre travail se focalise sur l'apport d'une biodiversité faunistique dans les FPs verticaux de type non saturés/saturés (FPV ns/s). Ce design permet des processus aérobies et anaérobies, et une synergie entre le substrat, les racines des plantes, les microorganismes du biofilm interstitiel pour la purification de l'eau 1 . Dans un FPV ns/s, l'eau est apportée par le haut, par bâchée pour augmenter la vitesse d'infiltration des eaux et favoriser l'oxygénation des couches de surface entre deux bâchées. Ce type de FPV permet des niveaux stables et élevés d'abattement de la DCO et de l'azote notamment 3 . La principale limite des FP est son emprise au sol, liée à des problèmes de colmatage et de limites des cinétiques de biodégradation. Pour y remédier, nous étudions l'effet de l'introduction de vers de terre dans les FP. Ces derniers sont connus pour leur capacité à réduire le colmatage 3 , faciliter la pénétration de l'oxygène dans le sol, augmenter la biodisponibilité des contaminants et stimuler l'activité microbienne 4 . Notre travail consiste à étudier l'effet des vers sur les performances épuratoires en relation avec l'évolution des propriétés hydrodynamiques et microbiologiques.