Traitement de lixiviats stabilisés de décharge par des membranes de nanofiltration Treatment of stabilized landfill leachates by nanofiltration membranes

Résumé

Le terme "lixiviat" ou "jus de décharge", désigne l'eau qui a percolé à travers les déchets en se chargeant de polluants. Ces effluents pollués doivent être traités.

En raison des exigences croissantes des normes de rejet et de la stabilisation des lixiviats au cours du temps, de nouvelles techniques ont fait leur apparition dans ce domaine. La technologie de l'osmose inverse s'est développée dans de nombreux pays européens. Cependant cette technique sélective et coûteuse, se justifie seulement quand les normes sont drastiques. C'est pourquoi la nanofiltration pourrait constituer une solution intermédiaire.

L'objectif de ce travail est de contribuer à mieux maîtriser cette technique pour l'élimination de la Demande Chimique en Oxygène (DCO) récalcitrante, subsistant après les traitements biologiques classiques.

Nous avons évalué les performances de trois membranes (organiques et minérales) pour l'abaissement de la charge organique, en fonction des conditions hydrodynamiques (vitesse et pression).

Chacune de ces membranes possède un comportement spécifique vis à vis de ces lixiviats stabilisés (adsorption, polarisation de concentration, obstruction des pores).

L'influence d'une coagulation préalable sur les performances d'épuration a également été examinée pour l'une des membranes.

Cette étude constitue une étape préliminaire au dimensionnement d'une installation.

Mots-clés:

• Lixiviat de décharge,
• nanofiltration,
• DCO,
• coagulation

Abstract

Landfill leachate is the name given to water that has passed through solid waste and contains organic and mineral contaminants. Therefore this effluent must be treated before discharge to the environment. Because of new norms and the stabilization of leachates with time, new treatment methods have been designed. Thus, reverse osmosis is used in many European countries. But the use of reverse osmosis is only justified when norms are severe, because the treatment is highly selective (salt rejection >99%) and very expensive. In other cases, nanofiltration may be an interesting alternative.

The purpose of the present work is to propose a process for recalcitrant organic matter in order to optimize the technique. Thus, three membranes (organic and mineral) have been used to evaluate their ability to decrease the Chemical Oxygen Demand (COD) of the leachate. This study helps to determine the size of the device. First, physical parameters were investigated. Each time, the hydraulic regime was turbulent (Re > 2500). Higher permeation fluxes were obtained with organic membranes than with the mineral one (80 L·h^-1·m^-2 compared to 25 L·h^-1·m^-2) under the same experimental conditions (10 bar and 3.4 m·s^-1). Tangential flow rates higher than 2.5 m·s^-1 do not influence COD retention; at lower flow rates polarisation concentration may occur. The removal of COD is achieved in the three cases. At 10 bar an acceptable value of less than 120 mg O₂·L^-1 (norm) is obtained. The inorganic membrane (Tech-Sep) gives the best results (COD rejection: 70 % at 10 bar).

Membranes behave differently toward landfill. The organic membrane MP 20 (Weizmann membrane, cut-off 450 Dalton (Da), polyacrylonitrile) shows low adsorption with landfill leachate. The organic membrane MP 31 (Weizmann membrane, cut off 450 Da, polysulfone) gave a high COD retention ratio; the values for irreversible fouling and static adsorption are of the same order ofmagnitude; a strong membrane-foulant interaction must occur, which improves membrane selectivity. The mineral membrane N01A (Tech-Sep membrane, cut off 1000 Da, zirconium oxide), like MP-31, gives high static adsorption with leachate and irreversible fouling as well. The latter phenomenon can be explained by the obstruction of membrane pores by leachate particles, the size of which is near the membrane cut-off point. Fouling and static adsorption contribute to the increase in the membrane rejection rate.

We studied coagulation as a pretreatment to improve performances of the N01A membrane. Experiments have been carried out with Jar-Test and FeCl₃-like coagulants. The optimal amount of coagulant was 1.4 g Fe·L^-1; 60% COD reduction was achieved. The results obtained with the N01A membrane are improved: reduction of COD rises from 78% to 92 %, concentration polarisation is lower, and therefore the flux increases up to 53 L·h^-1·m^-2. This value still remains lower than the organic membrane fluxes (respectively 80 L·h^-1·m^-2 for organic membranes and 25 L·h^-1·m^-2 for N01A). However, coagulation may not be the appropriate pretreatment because the fouling index of the supernatant after coagulation was similar to that of the raw leachate. Permeability measurements after treatment show that internal fouling is still important (25%). In fact, coagulation does not remove molecules with molecular weights around 500 Daltons, and consequently these particles still obstruct the membrane pores. The phenomenon limits the performance (flux) of this membrane.

Keywords:

• Landfill leachate,
• nanofiltration,
• COD,
• coagulation