Water Quality Instrumentation Developed to Meet Needs of French Water Supply Industry
The demands of water engineers and scientists in France for better and faster means of measuring the purity of drinking water have resulted in new, accurate instrumentation that gets results in a fraction of the time. Deciding that laboratory analyses are too long and too infrequent, French manufacturers have been developing equipment that continuously monitors the quality of surface water.
For most of France's major urban centers, drinking water has to be taken from rivers that are quite polluted, and yet what comes out of the tap in most French towns has a taste and biological purity fully comparable with mineral water. To achieve this, water sources before and after treatment must be closely monitored.
Up to now, the most common method for measuring water quality has consisted of taking samples from rivers, an expensive method since it requires manual analysis. Above all, it is very time consuming, as it is often difficult to take more than one or two samples per day. With a delay of several hours between sampling and the results of the analysis, it is sometimes too late to act in the event of significant changes in quality indicators. To combat this, a number of French companies have been moving toward automated measurement techniques, either continuous or quasi-continuous.
"To guarantee consistent quality we are developing electronic sensors that can measure physico-chemical and biological data upstream of the water intake and in the water treatment plant," explains Alain Deguin, laboratory manager at Saur, one of the three main water distribution companies in France. It has just installed high performance equipment on an aqueduct that supplies the drinking water for Disneyland Paris.
This equipment is the first of a pre-production run of six units. Developed by Saur's laboratories, it measures Eschetichia Coli levels every two hours. These fecal bacteria act as an indicator for the presence of pathogens. Traditionally, evaluation of their numbers in the laboratory is a very long process. The bacteria from the sample first have to be collected on a filter, then transferred to a Petri dish that is placed in an oven with nutrients. The colonies that form are then counted with the aid of a magnifying glass. The microfiltration and analysis of the sample takes 48 to 72 hours.
With the new equipment installed at Disneyland Paris, the measurement is simpler and quicker, taking just eleven hours. It does this by taking advantage of the property of E Coli to acidify glucose media. The equipment holds 100 of water in a nutrient rich cell and automatically measures the variation in pH, from which it deduces the quantity of bacteria present. It can detect down to one bacterium in 100 ml, which is the European drinking water standard. With a cost per unit of FFr 170,000 to 190,000 ($34,000 to $38,000), the equipment is manufactured and marketed by Ysebaert.
Work is also being carried out by the French company Seres for the measurement of coliform bacteria using an optical method with a colored reagent. Two years ago, this instrumentation company designed a current-based sensor with three electrodes that evaluated the quantity of active chlorine (HClO) in water and from that deduced its disinfecting power in drinking or swimming pool water.
More recently, Seres has marketed equipment for the rapid measurement of the chemical oxygen demand (COD), i.e., the quantity of oxygen dissolved in water that is available to be consumed by oxidizable substances in suspension. By oxidizing the organic material and mineral salts using microwaves, the equipment gives a result in fifteen minutes, compared to two hours using normal oxidation methods. For this reason it is suitable for monitoring industrial effluents liable to rapid change.
Compagnie Générale des Eaux, now part of Vivendi, a worldwide supplier of water distribution services, has been very active in research concerning the quality of drinking water. Its Anjou-Recherche laboratories near Paris have designed automatic equipment capable of measuring the overall toxicity of water, based on a standard test drawn up by the relevant authorities.
The technique is based on the ability of some marine bacteria to emit light. The bacteria are introduced into the effluent whose overall toxicity is required to be measured, and a very sensitive photomultiplier measures the decrease in luminescence caused by the pollution. "The toxicity is determined by measuring the time at which the luminescence has diminished by half," states Pierre Guillot, an instrumentation engineer at Anjou-Recherche. The apparatus produces one measurement every hour. Produced in quantity by Hydro-Environnement and marketed under the name of Automicrotox, it is already in operation at six locations in France, including the Loire, Marne, and Var rivers, monitoring surface water intended for human consumption.
Marc Le Pennec, Hydro-Environnement's marketing manager, also foresees using Automicrotox for the measurement of the toxicity of wastewater. "For that it is only necessary to dilute the samples with clean water," he explains. "In this way we would like to meet the need for more and more frequent analysis of wastewater." The company is adapting its electrode analyzers in the same way to the concentration of ions in wastewater. The equipment is said to be able to react to occasional sudden increases in concentration.
The Anjou-Recherche laboratories also developed equipment designed to measure the clogging power of surface water or wastewater, represented by the concentration of particles in suspension, which are measured in FTU (Formazin Turbidity Units). When this concentration value is high (above 5 FTU) the effluent has an unattractive, cloudy appearance. At lower levels, even if not visible to the naked eye, the particles can clog up water distribution outlets, in particular membrane and sand filters.
Known as the Siltimètre, the new equipment from Anjou-Recherche measures the time that the effluent takes to cross a membrane of a certain porosity at a given pressure. It has the advantage of being much more sensitive than conventional equipment for measuring turbidity, being able for example to measure the concentration of particles from water clearer than that typically obtained from the tap, i.e., less than 0.1 FTU.
For this reason, the first Silfimètre has been installed at the pilot nanofiltration plant at Méry-sur-Oise, to the north of Paris, where it is monitoring the operation of this very delicate installation. "As the equipment is more expensive than a turbidity meter, its range of application is limited to strategic installations of this sort," explains Pierre Guillot. Anjou-Recherche has engaged ENE (Electro-Navale Electronique) to manufacture and market the equipment in the future.
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About the Author: André Larané, is a technical writer living in Paris.