 The disinfection step is a very important component of the entire treatment process of public drinking water. The choice of the ideal process is among others dependent from the raw water quality, the water flow rate, the length of the piping network up to the sampling points and its features as well as the legal requirements. These vary according to the respective country. Furthermore, cost reflections are also an important criterion. Usual processes, basing on chlorine compounds are chlorination by means of chlorine gas, sodium hypochlorite or chlorine dioxide. Further processes are ozonation as well as UV radiation. Ideal results are often achieved by combining these processes.
Challenge: Switching to alternative processes
Mainly in larger drinking waterworks providing several thousand people with water, disinfection by means of chlorine gas is the process with the highest popularity, mainly for cost reasons. Main disadvantage of the process: the storage and dosing of chlorine gas is a danger source for staff and environment that is not to be neglected. For this reasons, currently, many drinking water suppliers replace disinfection with chlorine gas with the alternative process listed above. Since the end of 2003 this happens in all drinking waterworks around St. Petersburg in Russia, too. Here, the switch is made to disinfection by sodium hypochlorite.
With a capacity of 36 000 m³ drinking water per day, the waterworks Kronstadt near St. Petersburg is one of the smallest of these waterworks. Here, too, drinking water had been disinfected by chlorine gas dosing up to that date. This process, effected manually and not automatically, was to be switched to a fully automatic disinfection by means of sodium hypochlorite dosing – a typical process of chemical fluid handling. The total conception comprises the supply with the concentrated chemical, its storage and its transfer to the dosing points up to a controlled dosing, including the necessary measuring and control technology as well as the automatic process control, monitoring and visualizing.
Peak charges under control
A correct design and dimensioning of the plant required first the water quality and the water flow rate as well as the chronological fluctuations of both measurands to be analysed and evaluated over an extended period. This resulted in the necessity to disinfect the drinking water in normal operation with a 12%-sodium hypochlorite solution. Additionally, the possibility of disinfection with a 19%-solution was to be enabled in order to achieve the requested disinfection effect even at irregular peak load of the raw water, for example at increased pollution charge during strong raining. Basing on these principal data, a modularly designed chemical fluid handling system was desgined. In the present case only one single module was used, this being sufficient for treating 36 000 m³. In other waterworks in the regionof St. Petersburg, the system was adjusted to higher water flows with the installation of several modules.
Design of the module
The 19%-sodium hypochlorite solution is transferred via a magnetically-coupled chemical transfer pump type ProMinent® vonTaine® from the supply point to a storage tank of polyethylene with a filling capacity of 8m³. This concentrate is diluted to 12% by an automated dilution station. Additionally, the water flow is registered by an inductive flow meter. The measuring signal is processed in a local control unit and controls two metering pumps type ProMinent® Makro TZ. The latter are equipped with a speed controlled motor and an integrated frequency converter thus reaching a dosing accuracy of +/- 2 per cent. Beside the dosing task, the pumps also effect the metering of the diluted solution into two storage tanks with a volume of 20 m³ each. During normal operation the controlled dosing of the 12% sodium hypochlorite solution in each of the two drinking water lines is effected by three motor metering pumps type ProMinent® Sigma.
During peak load of the raw water, the undiluted solution is directly dosed with two further motor metering pumps type Sigma from the concentration reservoir into the drinking water line. For a controlled dosing and to monitor the chlorine concentration in the drinking water, the chlorine concentration is measured after the dosing points in each drinking water line by means of a chlorine sensor type ProMinent® CTE and a measuring transducer type ProMinent® DMTa.
 All metering processes are controlled with a control device type ProMinent® D1Ca. The control and monitoring of the total system including filling level indicator and leakage monitoring is effected by a PLC type Siemens S7. The PLC as well as the measuring transducer and the control device are installed into a central control cabinet. A big colour display in the control cabinet shows a piping and instrumentation diagram that allows to view the status of all plant parameters at any time: for example, which valves are open or closed, which pumps are in operation or not operating, how is the current filling level of the storage tanks or which chlorine concentration is currently present in the drinking water. The plant operator may chose between Russian and German language for display and control functions.
For a highly fail-safe process running, a stand-by-pump is integrated into the process for all pumps. The process is controlled fully automatically. At critical plant statuses, the operation staff is warned. All requirements of water protection are fulfilled, for example by using reservoirs with drip pans according to regulations of DVS and overfill protection and leakage sensors with WHG approval (please see site products/storage tanks for detailed information on our storage tanks). Moreover, all reservoirs are equipped with an ultrasound-filling level measuring. The total conception is completed with an intensive support of the waterworks during the installation and commissioning of the plant on the spot by ProMinent®.
Result
In this project, from the absolute beginning, special attention was paid to ideally integrate and link all system components as well as to consider the entire process. Starting with the chemical supply and storage up to the point where the sodium hypochlorite in the drinking water leaves the waterworks. By optimizing the entire process, a range of advantages could be realised with regard to costs, process quality and process safety:
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No danger potential as chemicals are stored in uncritical concentrations
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The requirements for water protection are obeyed
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No operation staff necessary as plant works fully automatically and almost free of maintenance requirements
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No specialized staff necessary for the monitoring of the plant, as problems that might occur can be realized very easily
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Plant is highly available as all plant components are designed redundantly
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Problem-free commissioning is granted as all main components of the plant are provided by one manufacturer and thus problem-free linking is granted
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Moreover, commissioning is supported by the manufacturer
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The waterworks could realize the switch of chemicals with one partner, decreasing the adjustment and integration efforts substantially
The benefits could be realised especially thanks to the fact that the entire plant was designed, supplied and commissioned from one source. |