F. Brissaud

Positive gadolinium anomalies in wastewater treatment plant effluents and aquatic environment in the Hérault watershed (South France).

Anthropogenic gadolinium (Gd), used as a contrast agent in magnetic resonance imaging, may enter rivers and groundwaters with the effluents of wastewater treatment plant (WWTP). Such contaminations, which are mainly found in densely populated areas with highly developed medical systems, induce positive gadolinium anomalies in waters. This study reports on the occurrence of positive Gd anomaly in wastewaters, surface and groundwaters in a slightly populated Mediterranean watershed. Water samples have been collected along the Hérault River, in its tributaries, in wells and springs supplying drinking water and in WWTP effluents during two sampling campaigns in February and July 2003. Systematically pronounced positive gadolinium anomalies (Gd/Gd( *)) were observed in WWTP effluents with values reaching 306. These observations have shown that Gd/Gd( *) can also be found in wastewater drained from rural communities, not equipped with MRI facilities. Positive gadolinium anomalies were detected in two tributaries of the Hérault River and in some wells supplying drinking water, corresponding to an excess of anthropogenic Gd in water up to 15.4pM. A monthly monitoring on one well has confirmed the persistence of gadolinium anomalies all along the year, suggesting a continual wastewater contamination on this site. A spatial monitoring on one tributary showed that wastewater contribution modifies completely the normalized REE pattern of river water, resulting in a decrease of REE amount correlated to the Gd anomaly appearance.

Non-steady-state modelling of faecal coliform removal in deep tertiary lagoons.

In Noirmoutier, a French island off the Atlantic coast, secondary effluents flow into a series of four lagoons, 1.4-2.8 m deep, and are reused for agricultural irrigation. The excess water is disposed of to the sea. The aim of this study was to provide a model capable of predicting the microbiological quality of the water pumped for irrigation or discharged to the sea. Meteorological variables, flow rates, physical-chemical characteristics and faecal coliform (FC) contents were monitored for a year and a half. The hydraulic pattern of each lagoon was assumed to be that of completely mixed reactor because of the calculated dispersion numbers and the wind mixing effect. Coliform decay was assumed to follow first order kinetics in each lagoon. Die-off coefficients were calculated in each lagoon using a non-steady-state model. The main bacterial removal mechanism was shown to be solar irradiation. Empirical equations were established to calculate die-off coefficients as a function of received solar energy and temperature. FC die-off rates were higher in the first lagoon and then decreased successively in those following. FC numbers in the different lagoons were predicted with reasonable accuracy in spite of high variation in inlet water quality. The model will facilitate the prediction of water quality under various climatic conditions and different water reuse scenarios and will help to optimise reclamation and storage facilities.

Residence Time Distribution and Disinfection of Secondary Effluents by Infiltration Percolation

Infiltration percolation is used as a tertiary treatment in order to meet the WHO’s microbiological standards applying to unrestricted agricultural wastewater reuse. Faecal coliform removal, ∆fc, was investigated in laboratory columns and on a 565 m pilot plant. ∆fc observed in laboratory columns was shown to be closely related to water detention time distribution, DTD. The relationship between ∆fc and DTD, which has been determined from column tests, allowed a good prediction of the disinfection performances of the pilot plant for hydraulic loads of 0.54 and 0.66 m/d. For 0.82 m/d, the maximum load that could be tested on the plant, the mean faecal coliform removal was more than 1 log. unit higher than predicted. These unexpected good performances, though calling for more comprehensive explanation, speak for the widespreading of a reliable and cost-effective extensive technique.

Taking wind into account in the design of waste stabilisation ponds.

Up to now, most investigations on the dependency of the fluid flow patterns and performance of waste stabilisation ponds (WSPs) on wind speed and direction and pond layout have been performed using 2D and 3D CFD steady state isothermal models. 3D non steady state models integrating thermal processes and boundary conditions taking into account the full influence of meteorological factors are likely to provide more realistic predictions of WSP performance. Such modelling was undertaken for 4 pond layouts, 2 without baffles and 2 with baffles. Wind speed and direction were kept constant throughout each simulation while other meteorological forcings were derived from field measurements. Twelve wind directions and 2, 4 and 6 m s(-1) wind speeds were considered for each WSP layout. Simulations allowed verifying that the pond performance is dependent on the wind direction and velocity, that baffles may improve WSP performance and that the addition of well-designed baffles has the advantage of reducing its sensitivity to the wind.

Fluid flow pattern and water residence time in waste stabilisation ponds

As treatment processes are kinetic-dependent, a consistent description of water residence times is essential to the prediction of waste stabilization ponds performance. A physically-based 3D transient CFD model simulating the water velocity, temperature and concentration fields as a function of all influent meteorological factors--wind speed and direction, solar radiation, air temperature and relative humidity--was used to identify the relationships between the meteorological conditions and the hydrodynamic patterns and water residence times distributions in a polishing pond. The required meteorological data were recorded on site and water temperatures recorded at 10 sampling sites for 141 days. Stratification events appear on very calm days for wind speeds lower than 3 m s(-1) and on sunny days for wind speeds lower than 5 m s(-1). De-stratification is related to two mixing processes: nightly convection cells and global mixing patterns. Numerical tracer experiments show that the results of the flow patterns can be evaluated using the dispersed flow regime approximation and, for wind speeds exceeding 6 m s(-1), the completely stirred tank reactor assumption.