Performance de la nanofiltration pour l'élimination de la matière organique naturelle: essais sur l'usine de Méry/Oise

L'intérêt croissant que les traiteurs d'eaux portent à l'élimination de la matière organique naturelle (MON) a abouti au développement de nouvelles technologies de traitement. Dans ce but, un prototype de nanofiltration à l'échelle industrielle (2 x 1400 m 3 j-¹) est installé à l'usine de Méry sur Oise depuis juillet 1992. Utilisé en traitement de finition après clarification et filtration sur sable, il alimente depuis février 1993 un réseau test de la commune d'Auvers sur Oise (6 000 hbts) en région parisienne.L'objectif de cette publication est de présenter quelques uns des résultats de caractérisation de la MON obtenus pendant 9 mois d'expérimentation (octobre 1992 à juillet 1993), et en particulier ceux concernant les rendements d'élimination de la matière organique naturelle et par voie de conséquence de la demande en chlore.Ces rendements sont généralement supérieurs à 90 % en termes de COD,CODB et d'absorbance UV à 254 et 270 nm éliminés. L'analyse des potentiels de réactivité avec le chlore (taux de chloration: 2,5 mg Cl2/mg C, temps de réaction: 72 heures, pH = 7,5, 20 °C) montre que le perméat est peu consommateur de chlore (demande en chlore < 0,2 mg l-¹ Cl2) et peu précurseur de chloroforme et de trihalométhanes (PFCHC13 < 3 µg l-¹, PFTHM < 11 µg l-¹). Les rendements d'élimination des PFTHM et PFTOX sont généralement supérieurs à 90 %.L'analyse spécifique des constituants majoritaires du perméat montre que les acides aminés totaux (hydrolyse acide puis dérivation à l'OPA/HPLC) constituent une proportion importante du COD (25 à 60 % selon les saisons). Ces composés représentent la quasi totalité de la demande en chlore du perméat si l'on se réfère aux données bibliographiques.Compte tenu de ces résultats, la nanofiltration apparaît comme un excellent procédé de traitement de finition des eaux à potabiliser. En effet, bien qu'elle constitue une barrière de sécurité contre les germes pathogènes, la très faible charge organique du perméat obtenu par nanofiltration (COD~0,15 à 0,3 mg l-¹ C, CODB<0,1 mg l-¹ C) rend plus aisée la maîtrise du résiduel de chlore (lorsqu'une chloration est nécessaire pour maintenir la qualité de l'eau dans les réseaux) et constitue une limitation importante de la formation des sous-produits de chloration.Increasing interest in removing natural organic matter (NOM) has lead to the development of new drinking water treatment technologies. Since July 1992, a nanofiltration demonstration plant (2 x 1400 m3 d-1) has been used to treat sandfiltered water from the Oise river. The permeate has been distributed since February 1993 to the 6000 inhabitants of Auvers/Oise in the Paris suburb. The purpose of this paper is to present and discuss some ofthe results obtained over nine months of operation of this full scale plant, particularly yields of NOM removal and consequently the decreasing of chlorine reactivity (chlorine demand, TTIM and TOX formation potentials).Dissolved organic carbon (DOC) and UV-absorbance were determined using DOC analyser and a spectrophotometer. Biodegradable dissolved organic carbon (BDOC), which represents the biologically assimilable portion of DOC, was determined using the method of JORET et LEVI (1986). Chlorine demand, trihalomethane and total organohalide formation potentials (THMFP and TOXFP) were carried out under the following experimental conditions: applied chlorine dose of 2.5 mg Cl2/mg DOC, pH = 7,5 72 h-contact time and 20°C. Ultrafiltration experiments involved the use ofa laboratory ultrafiltration cell, Total amino-acids were analysed by HPLC after hydrolysis and orthophtaldialdehyde (OPA) derivatization. Aldehyde and ketone determination was based on the method developedby GLAZE et al. (1989) involving pentafluorobenzyl hydroxyl amine (PFBHA) derivatization.Characterization of sand-filtered water (SFW): The sand-frltered water (SFW) upstream of the nanofiItraton membranes has a DOC between 2.4 and 4.l mg l-1, depending on the season (table 1). Its BDOC ranges from 0.7 to l.l mg l-1 C. In fact, a BDOC value higher than 0.3 mg l-1 C has been mentioned by several authors as the limit above which possible bacterial regrowth can take place in the distribution network.The chlorine consumption curves, shown in figure 2 for five sampling campaigns, indicate that the chlorine demand of the SFW can reach 3.4 to 5.2 mg l-1 depending on the season (table 2). The THMFP and the TOXFP are 108-149 ug l-1 and 344-446 ug l-1 Cl- respectively. Note that the ratio of chlorine demand over DOC varies from 1.0 to 1.7 mg Cl2/mg DOC while the THMFP/DOC and TOXFP/DOC ratios present average values of 47.5 ug/mg DOC and 160 ugCl-/mg DOC respectively.The distribution of the SFW (table 3) indicates that the fraction with apparent mo lecular weight Iess than 3 kilodatons contains the major compounds at this stage of the water treatment. This fraction presents the highest chlorine consumption. Specific total amino acids (TAA) analyses demonstrate that TAA represent 3 to 8% of the DOC of the sand-filtered water. The most abundant arnino acids are glycine, aspartic acid, glutamic acid, serine and alanine. The chlorine consumption attributed to these amino acids is evaluated as 1 mg l-1 Cl2, that is to say 1/5 to 1/3 of the SFW chlorine demand. Formaldehyde and acetaldehyde seem to be the major aldehydes present in the SFW with a level of 7 ug l-1 and 20 ug l-1 of formaldehyde and acetaldehyde respectively. They represent only about 0.5 to 0.6 % of the SFW DOC.Characterization of the permeate: The nanofiltration permeate presents a very low NOM level in terms of DOC, BDOC and UV absorbance at 270 nm, that is to say 0.14 to 0.34 mg l-1 C, < 0.1 mg l-1 C and < 0.006 cm-1-l respectively (table 6).The chlorine consumption curves, showt in figure 4 for five sampling campaigns, demonstrate the low permeate reactivity with chlorine. The chlorine demands (table 7) after 72 hours are between 0.12 and 0.32 mg l-1. Moreover chlorine demand/DOC ratios have a value from 0.46 to 0.93 mg Cl2/mg DOC, i.e. half the values measured for SFW. The THMFP and TOXFP (72hours) range from 7 to 11 ug l-1 and 26 to 31 ug l-1 Cl- respectively.Total amino acid (TAA) analyses showed that TAA represent 35 to 60% of the permeate DOC and can account almost entirely for the chlorine consumption. Formaldehyde and acetaldehyde (the major aldehydes analysed) represent 7 to 8% of the permeate DOC.According to the results presented in this paper, nanofiltration appears to be an excellent technolory as a polishing step in surface water treatment. Whereas the level of sand-fïltered water (SFW) DOC varies from 2.4 to 4.1 mg l-1 C (depending on the season), the permeate DOC is consistently lower than 0.3 mg l-1 C. The efficiency of nanofiltration is about 90% for DOC, BDOC and consequently for chlorine demand, THMFP and TOXFP. The high retention of NOM is probably in relation with the percentage (75%) of compounds with apparent molecular weight above 500 daltons in the SFW. In fact the low values of BDOC and chlorine demand justify the use of nanofiltration for the production of a water which represents a very low risk of bacterial regrowth and a low risk of formation of disinfection by product in the network when distributed with a low concentration of residual chlorine

Analyse par HPLC et CG/SM des constituants du carbone organique dissous (COD), du COD biodégradable (CODB) et des composés organohalogénés (TOX) d'un perméat de nanofiltration

Pour limiter la formation de composés organohalogénés des eaux traitées et la reviviscence bactérienne des réseaux, il est important d'éliminer la majeure partie du carbone organique dissous (COD) et du carbone organique dissous biodégradable (CODB) contenus dans les eaux naturelles. Des travaux récents nous ont permis de montrer que la nanofiltration est une technologie de choix pour répondre à ces contraintes.L'objectif de cet article est de présenter à partir de travaux de laboratoire un inventaire détaillé du carbone organique résiduel d'un perméat prélevé le 21/04/93 sur le prototype industriel de nanofiltration de Méry/Oise en banlieue parisienne. Pour atteindre cet objectif il a été nécessaire de mettre en œuvre des 'techniques analytiques impliquant l'utilisation de la chromatographie liquide haute performance (CLHP) et de la chromatographie en phase gazeuse (CG) soit équipée d'un détecteur à ionisation de flamme (FID) ou d'un détecteur à capture d'électrons (ECD), soit couplée à la spectrométrie de masse (SM).Les résultats obtenus ont montré que le COD du perméat étudié est constitué d'environ 60% d'acides aminés libres et combinés, de 7% d'aldéhydes et de 10 à 20% de composés divers identifiables en CG/SM. Ces derniers composés comprennent majoritairement des acides gras aliphatiques et des acides aromatiques de faibles masses. La concentration de chacun de ces composés a été estimée à 0,3 µg l-¹ C. On peut raisonnablement penser, d'après la bibliographie que les hydrates de carbone (non analysés dans cette étude) représenteraient une part importante de COD du perméat. En outre, cette étude a montré que la part prise par les acides aminés totaux dans le CODB du perméat est importante.Seul le tiers des potentiels de formation d'organohalogénés totaux (PFTOX) a été identifié comme étant des trihaloméhanes (THM) et des acides haloascétiques. Toutefois, étant donné que les acides aminés totaux représentent à eux seuls la quasi totalité de la demande en chlore du perméat, les autres sous-produits de chloration non identifiés seraient probablement des nitriles chlorés, des chloramines et des chloroaldéhydes qui sont parmi les principaux intermédiaires réactionnels de la coloration des acides aminés.Removal of dissolved organic carbon (DOC) and biodegradable dissolved organic carbon (BDOC) is one of the most important means to prevent disinfection by-products (DBPs) formation during water treatment and bacterial regrowth in distribution systems. In previous investigations, the authors have shown that nanofiltration, over nine months of operation at industrial scale in Paris suburbs, was an effective technology to meet the new guidelines concerning chlorine DBPs.This paper reports laboratory investigations aimed to identify and quantify the main organic components included in the low DOC, BDOC and TOXFP (Total - Organohalides Formation Potential) residuals of a nanofiltration permeate sampled on April 21, 1993.Details on DOC, BDOC, organohalides, amino acids and aldehydes analysis procedures were described elsewhere (AGBEKODO et al., 1994). Chlorination was undertaken in potential conditions k: 2.5 mg Cl2/mg DOC, pH=7.5 (phosphate buffer), 72 hours contact time, in dark at 20°C. Haloacetic acids determination consisted (after chlorination in potential conditions) in liquid-liquid extraction, methylation with diazomethane and gas chromatography analysis. Extraction procedure based on XAD8/XAD4 adsorption prior to gas chromatography/Mass Spectrometry (GC/MS) analysis (Fig. 3), allowed a 19000 fold concentration of the permeate. To prevent possible contamination of the permeate, the extraction system consisted of four glass columns and teflon materials. The flow through the columns was performed under high purity nitrogen gas pressure.Analysis involving high performance liquid chromatography (HPLC) and gas chromatography (GC) in combination with Mass Spectrometry (MS), showed that the studied permeate DOC (- 0.15 mg l-¹ c) consisted of amino acids at an average of 60% of DOC, aldehydes (7%) and 10 to 20% of several other compounds (analyzed in GC/MS) including primarily fatty and aromatic acids of low molecular weight (Table 4a and Table 4b). The maximum concentration of each compound (identified hy GC/MS) has been roughly assessed to 0.3 µg l-¹ C. According to literature, sugars represent probably an important portion of the remaining DOC of the permeate. Moreover, the authors have shown that amino acids represented a large portion of permeate BDOC.Only 34% of the total organohalide potentials were identified as trihalomethanes and haloacetic acids. However, since amino acids represent almost the entire chlorine demand of the permeate, the non- identified chlorination DBPs are likely chlorinated nitriles, chloramines and chloraldehydes which are known as the main reactionnal intermediates of aminoacid chlorination

Nanofiltration of hormone mimicking trace organic contaminants

The removal mechanisms of three hormone mimicking organic compounds by nanofiltration (NF) membranes have been examined. Two NF membranes having different pore size were used in laboratory-scale nanofiltration experiments with feed solutions spiked with a hormone mimicking compound ¾ nonylphenol, tert-butyl phenol, or bisphenol A. Retention of the compounds was determined at various solution chemistries, namely aqueous solution pH, ionic strength, and presence of natural organic matter. The nanofiltration behavior of the selected hormone mimicking compounds appears similar to that of natural hormones as reported in our previous work. While the solution pH can dramatically influence the retention of hormone mimicking compounds by a loose NF membrane, ionic strength does not affect the nanofiltration of such contaminants. However, in the presence of natural organic matter in the feed solution, ionic strength appears to play a significant role in solute-solute and solute-membrane interactions, resulting in increased retention due to partitioning of the hormone mimicking compounds onto organic matter at a higher ionic strength