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, DOC, BDOC, organohalides, aminoacids, bacterial regrowth, liquid and gas chromatography.
B. Legube, Laboratoire de Chimie de l'Eau et des Nuisances (URA CNRS 1468), École Supérieure d'Ingénieurs de Poitiers, Université de Poitiers, 40 avenu du Recteur Pineau, 86022 Poitiers Cedex, FRANCE