Sodimate discusses water-treatment processes

Water-quality refining is the most essential step in water-treatment processes and, when done properly, it guarantees palatable taste characteristics suitable for drinking water.

The reliability and accuracy of the processes for storing, metering, and dissolving the required reagents are indispensible for successfully completing this final treatment.

Outlined below is a case story about Viry-Chatillon, a water-treatment plant, where Sodimate installed a powder-activated-carbon (PAC) dosing unit for Eau et Force, a subsidiary of Suez - Lyonnaise des Eaux.

Due to a 4,500km interconnected piping network, three towns in the South of Paris region, Morsang-sur-Seine, Vigneux-sur-Seine, and Viry-Chatillon, provide and supply up to 440,000m3 of drinking water per day to approximately one million inhabitants.

Built in 1931 and gradually expanded over several years, the plant in Viry-Chatillon can produce 120,000m3 of drinking water each day from raw water taken from the River Seine and, to a lesser extent, underground water drawn from aquifers of the Albien and Sparnacien rivers.

To produce a perfect water quality, the plant uses the most modern process.

After fine screening, the water goes through ozone pre-oxidation before entering the settlement tanks.

The water is then filtered through granular-activated carbon to absorb the organic residue.

An ozone treatment precedes a second filtration with PAC to process and neutralise micro pollutants, improving the water quality.

Before proceeding to the final chlorination, the water's calcium-carbon ratio is adjusted to an adequate ratio.

Eau du Sud Parisien is in charge of servicing and maintaining all equipment and treatment processes.

Additionally, they ensure that the workers implement processes and organise the water-treatment plant to ensure the proper sanitation of the water produced.

However important sanitation monitoring may be, ensuring taste consistency receives the most attention - as Jean-Louis Le Hir points out: 'It is essential that the consumer should have water with the proper amount of nutrients and taste quality.

For this reason, the water-treatment division of the plant in Viry-Chatillon includes a refining step on granular-activated carbon located between the ozone pre-oxidation and the final disinfection.

Injecting powdered PAC in the settling tanks improves water quality by eliminating the odour-causing compounds, pesticides and their byproducts, algal toxins, chlorinated hydrocarbons, trihalomethanes and other byproducts from the disinfection process.

At Viry-Chatillon, the powder-activated-carbon process is used periodically: 'Between seasons, the spring or autumn rains cause deterioration of the raw water quality.

The addition of activated carbon and its porous structure allows the absorption of a wide range of compounds,' said Jean-Louis Le Hir.

Since the middle of the 1960s, the plant in Viry-Chatillon used an activated carbon feeding system to deal with occasional deterioration of the raw water quality; however, the unit was outdated and no longer met current regulations.

To upgrade this equipment, Eau du Sud Parisien contacted Sodimate to design and install a new activated carbon preparation and distribution unit.

The objective was clear: design and install a new system taking into account the technical, safety and environmental dimensions of the project.

To do this, Sodimate chose to use the proper equipment to design a customised installation to meet the needs of their client.

Sodimate specialises in the design and installation of equipment for storing, dosing, dissolving, and mixing powdered reagents.

Their services range from providing reliable and efficient equipment, integrating designs, automation and technical assistance.

Although the goal is often the same in designing plants, the characteristics vary from one project to another and Sodimate offers custom designs and turnkey solutions to meet the customer's needs.

For Eau du Sud Parisien, Sodimate was commissioned to replace the old 1960s PAC dosing and preparation plant in Viry-Chatillon.

Three factors were considered while designing the system, including: compatibility with the existing equipment, completion with the Atex regulations and accordance with the specifications of Eau du Sud Parisien.

'The first difficulty consisted of removing all the old equipment,' said Stephane Zuddas, Project Manager for Eau du Sud Parisien.

After much consideration, the choice was made to keep the old plant layout despite their particular configuration: two 70m3 flat-base concrete silos each with three levels; the highest, for storing carbon.

The second and lower levels, about 20m each, house the silo dischargers and feeding system and slurry mixing into the three settling tanks respectively.

'Submitting the entire plant to Atex specifications created considerable installation constraints, generating significant increases in costs,' Zuddas added.

Dividing the facility into three working areas (PAC storage, Atex discharge and feeding system, and slurry mixing and injection) was the best compromise to meet Atex norms and reduce costs.

The two concrete silos overlooking the station were upgraded by installing anti-explosion valves.

The lower room had to meet ATEX regulations.

'This lone room,' said Zuddas, 'had to match different requirements: the existing equipment, Atex regulations, food-sanitation norms and size constraints.

To allow the plant to function 24 hours a day, 365 days a year, the two silos alternate in discharging carbon.

To remedy the problem created by their flat bottoms, two discharge systems are installed in each silo and split into two different feeders.

'If a breakdown occurs on a line, the process is capable of automatically switching to the second,' Zuddas added.

At this level, each piece of equipment, each motor and each seal is strictly compliant with Atex requirements, as well as the food quality requirements.

'We follow a standard that is very close to the agro-food industries,' Zuddas said, 'each piece of equipment installed must have a certificate and be validated by the owner.

'For example, each valve casing is both ACS certified and compliant with ATEX requirements.

To remove the constraints commonly associated with the use of different quality PAC, a preparation device was installed with weighing cells to control exactly the concentration of the solution.

'The advantage is two fold: it avoids the problems linked to use of different quality carbons and lets an aqueous solution leave the Atex area at the lower room,' Zuddas added.

The normal preparation cycle, which lasts 15min (if necessary, an emergency procedure reduces this to less than 3min), has several distinct phases to refine the dosing.

Depending on the concentration needed by the operator, the quantity of carbon required is automatically set to the gram.

The amount of carbon calculated is then introduced in a small amount of water before being dissolved in a previously weighed volume of water, in general 300l, to produce the slurry.

This preparation is then automatically distributed in two tanks that are used alternately.

'The main difficulty consisted in finding a weight indicator capable of weighing the carbon to the gram, then defining the times and speeds adapted so the powder would soak properly,' said Francois Jacob, sales director with Sodimate.

Regardless of the carbon used, this technique of preparing the activated carbon ensured accuracy, safety and autonomy without having to comply with Atex requirements on the lower retention tanks.

When the slurry preparation is finished, the desired quantity must be distributed in each of the three settling tanks at a specific dosing rate.

To do this, four distribution lines (progressive cavity pumps) were installed - three remain on duty and one on standby for maintenance purposes.

Sodimate chose four booster pumps with offset rotors from PCM to adapt their solution to the process.

In the first layout of the customer specifications, the skid pumps were positioned horizontally.

As a result, Sodimate considered installing the skid pumps vertically to free up floor space and ensure easy access for maintenance.

After several attempts, the solution was validated and implemented.

Now, the pumps operate by suction rather than under pressure.

'The only constraint was that the pump had to be completely watertight to be operational,' said Jacob.

Because the access doors were too narrow, the preparation tanks and cisterns were assembled and welded inside the facility.

The installation was then configured to the technical, safety and environmental requirements in accordance with the specifications of the operator.

Among these were: a station capable of processing from 0 to 50 g/m3 of PAC to treat from 800 to 5,000m3/hr of water, compliant with Atex norms, easy to maintain, fully automated with weekly cleaning cycles and with zero waste.

In case of PAC slurry overflow, the processed water can be reintroduced at the head of the plant.

The completed unit can be monitored locally or within the control room on control screens that display the levels, the preparation and the concentration statuses.

All production/consumption process reports are archived in Lyonnaise des Eaux database for review and statistics.

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