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It is difficult to assess the biological consequences of diffuse water contamination by micropollutants which are present in rivers at low, even sublethal levels. River biofilms, which respond quickly to changes of environmental parameters, are good candidates to acquire knowledge on the response of aquatic organisms to diffuse chemical contamination in the field. The study was designed as an attempt to link biofilm metal tolerance and metallic contamination in a field survey covering 13 different sampling sites in the Seine river basin (north of France) with low contamination levels. Cd and Zn tolerance of heterotrophic communities was assessed using a short-term toxicity test based on β-glucosidase activity. Metal tolerance levels varied between sites but there was no obvious correlation between tolerance and corresponding water contamination levels for Cd and Zn. Indeed, metallic contamination at the sampling sites remained subtle when compared to water quality standards (only two sampling sites had either Zn or both Cu and Zn concentrations exceeding the Environmental Quality Standards set by the EU Water Framework Directive). Yet, multivariate analysis of the data using Partial Least Squares Regression revealed that both metallic and environmental parameters were important variables explaining the variability of metal tolerance levels. Automated Ribosomal Intergenic Spacer Analysis (ARISA) was also performed on both bacterial and eukaryotic biofilm communities from the 13 sampling sites. Multivariate analysis of ARISA fingerprints revealed that biofilms with similar tolerance levels have similar ARISA profiles. Those results confirm that river biofilms are potential indicators of low, diffuse contamination levels of aquatic systems., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

This paper presents a methodology for assessing the variability of biodegradable chemical oxygen demand (COD) fractions in urban wastewaters. Thirteen raw wastewater samples from combined and separate sewers feeding the same plant were characterised, and two optimisation procedures were applied in order to evaluate the variability in biodegradable fractions and related kinetic parameters. Through an overall optimisation on all the samples, a unique kinetic parameter set was obtained with a three-substrate model including an adsorption stage. This method required powerful numerical treatment, but improved the identifiability problem compared to the usual sample-to-sample optimisation. The results showed that the fractionation of samples collected in the combined sewer was much more variable (standard deviation of 70% of the mean values) than the fractionation of the separate sewer samples, and the slowly biodegradable COD fraction was the most significant fraction (45% of the total COD on average). Because these samples were collected under various rain conditions, the standard deviations obtained here on the combined sewer biodegradable fractions could be used as a first estimation of the variability of this type of sewer system.

The biodegradability of water samples is usually estimated with bioassays under oxic conditions. In order to overcome some of the drawbacks linked to the incubation of the samples in aerobic batches, a new protocol is proposed and tested, which is based on an organic carbon (OC) balance after a 45 days incubation under anoxic conditions with excess nitrate. The biodegradable fractions of organic matter obtained with the anoxic protocol are slightly lower than those obtained under oxic conditions. Several possible reasons for a systematic underestimation of the biodegradable organic matter under anoxic conditions are evaluated and discussed: a reduced microbial metabolic potential, significantly reduced degradation rates for the slowly biodegradable organic matter, an additional production of refractory organic compounds during the incubation, or the inhibition of the recycling of the organic matter stored in bacterial biomass. Nevertheless, the 7% difference observed on the biodegradable total OC estimations keeps low enough so that the anoxic protocol can be proposed as a convenient alternative to the oxic one.

Two different re-oxygenation techniques (aeration and hydrogen peroxide addition) were compared in respirometric experiments. As similar results were obtained in both cases, it was concluded that the addition of hydrogen peroxide does not modify the oxygen uptake rate of the biomass, under either endogenous or feeding conditions. It was hypothesized that under those experimental conditions (inhibition of nitrification with ATU), hydrogen peroxide alters neither the biomass metabolism nor the biodegradability of the tested substrates. The oxygen uptake rates obtained with the aeration system were often more scattered due to the adhesion of fine bubbles after the switch off of the aeration. Moreover, the transfer rate of oxygen to the solution is faster in the case of hydrogen peroxide addition.