Water purification control
This article talks about the purification plant control and supervision in Trescore Balneario, province of Bergamo (Italy) that serves water to sixteen Val Cavallina towns.
By Massimo Giussani
The growing sensibility towards environmental problematics allows people to concentrate in the quality of control and purification processes of urban and industrial drainings. An accurate and coordinated control of the refluent water deposits may give significant mid and long-time advantages for the communities that are deciding to invest in saving the environment.
The control of the water integrated cycle in the mountain community of Val Cavallina represents an example of that kind. The coordination of the internal distribution networks in each town, the creation of important infrastructures such as the Acquedotto dei Laghi, the circumlacustral collector and the community purification plant have primed a significant improvement of Cherio river and Endine lake conditions.
The present article illustrates how the control and supervision problems of the refluent water purification plant in Trescore Balneario, which is the complex that serves water to sixteen towns in the mountain community of Val Cavallina, have been faced.
The plant was built by "La Biodepuratrice", a company located in Clusone, province of Bergamo (Italy) and specialized in the planning and production of purification plants. The mountain community of Val Cavallina is the owner of the mentioned purification plant even if the management is in charge of the Val Cavallina Servizi from Casazza, province of Bergamo (Italy), in connection with "La Biodepuratrice". The control and supervision system of the plant has been developed recently by P & K New Electric, a company located in Casnigo, province of Bergamo (Italy) using Automa's Super-Flash as development system.
A single system for Val Cavallina towns
The Trescore Balneario purification plant was built in the early nineties in order to serve three adjacent towns with a consumption basin stimated in less than fifteen thousands consumers. In 2001 the water gathering and control system was enlarged allowing the mountain community of Val Cavallina to serve more than fifty thousands consumers. In fact all Casazza and Entratico towns have been included and the plant is now processing urban and industrial drainings for almost twenty thousands daily cubic metres.
The presence of a single aqueduct for the sixteen community towns has helped to intercept the spring water. In a year almost 300.000 cubic metres of water were being free to flow naturally towards Endine lake. The new gathering system has also permitted to increase the total amount of drinkable water introduced into the lake arriving up to half a million cubic metres in a year. The joined action of water exchange and refluent water removal helped to improve Cherio river and Endine lake conditions with positive results for the local ecosystem. The telecontrol and supervision aspects are considered important by the Val Cavallina Servizi because drainages and purification plants are managed remotely through the Val Cavallina control center thanks to the Acquatel and Decotel system based on local PLCs. The control center is also equipped with a programmed maintenance system and an alarm messenger system via SMS. The Trescore Balneario purification plant is an exception considering that the telecontrol has not been already activated. Anyway, it is always possible to activate this option anytime as it is an optional function included in the supervision software.
Finally, the Val Cavallina Servizi has the construction, management and maintenance of purification plants approved according to ISO 9002 certifications since 2000.
The Trescore Balneario purification plant is located away from the built-up areas near Cherio river in the lower point of the mountain community, and even if it has a diameter of almost a hundred metres it is hidden allowing a reduction of enviromental impact.
The water treatment consists in passing through a group of phases in cascade, including the recycling of organic material in order to guarantee an effective demolition of pollution agents.
The refluent water course is usually measured through the use of magnetic meters. At the start the refluent water is filtered through the use of two grids and transferred to a first basin where sand collectors and oil extractors start to remove the deposits from the bottom of the basin letting them surface. The liquid sewage is then transferred to a second basin where after being mixed with biological mud coming from a successive section, is forced to an oxidation process because of the exposure to pumped air from appropriate compressors. In this basin is expected the 50% of the material recycling along with the denitrification and dephosphation processes. The water passes then to a successive distribution basin where the liquid sewage is divided in four sedimenters allowing the mud to be deposited at the bottom of the basin. The pumps at the bottom of the sedimenters start to pick up the biological mud putting it back in the oxidation basin. The sedimenter water passes then to a chlorination basin and in the end to a filtering one where once it has been disinfected, it comes out to be introduced into Cherio river. The excessed mud accumulated in the sedimenters is extracted and introduced into a pre-thickener where is being digested and stabilized before passing to the pro-thickener and to a dehydratation final phase. In this phase the mud loses water for almost the 80% of its weight and in the end is stored in appropriate cases for the discharging process. In all the phases some magnetic and ultrasound meters measure the liquid sewage course, the treated water and mud in order to provide along with the fluid physical and chemical results, the elements needed to control the purification process.
In the complex the plant gazes at more than five hundred digital inputs and outputs (350 input and 200 outputs), and ten analogical I/O. Sensors and actuators are connected through 4-20 mA lines, but are mainly used in impulsive conditions. For example, the course meters are calibrated to give a boost for each measured mc/h and the conversion of the result values from a counter occur at a PLC level.
The process control is carried out through a Siemens S7 PLC 300 serial with a 315-2 DP CPU to which are connected four ET 200 decentralized units through DP Profibus that are involved in the engine control and signal collection. The supervision system resides in a personal computer and communicates with the PLC through a Applicom board using a MPI bus. A further PLC works on data exchange with the system installed in Val Cavallina cooperative headquarters.
Purification plant supervision
The supervision system is supported by Super-Flash, Automa's SCADA, and is divided into multiple pages that bring back the various sections of the synoptic available in the PLC control system main board. The navigability among pages and device selection is simple and intuitive and permits to supervise the entire plant with just few mouse clicks.
Although this option is available, it is always better not to carry out the control operations directly from a PC to a remote workstation, leaving this task to the PLC control. Nevertheless it is possible to set up some actuator timers such as drives for grids, fans and biofilters, through the use of recipes in order to control their estimated breaking and working times. Anyway the security sensors permit to exclude the timers in order to force the working or restarting operations allowing to operate directly from the plant in case of maintenance or critical situations. A key selector permits to indicate the maintenance disabled status to the control and supervision system for the recent added pumps through an appropriate icon display.
All the pumps and engines indicate their status - working or thermal restarting - . In the control synoptic the anomaly conditions are represented through various flashing speeds coming from an indicating light near the engine.
The system offers appropriate total and partial counters that help in the tracing of used devices reducing the risk of unexpected failures. Another counter registers the cubic meters of the treated products. In this case there are two counter types: one that counts the daily cubic meters and another one that has the option of manual resetting in order to analyze the water course in different time intervals. The results can be easily visualized with graphics in the mentioned pages allowing to show the state of the oxigen concentration, the pH and the course in different plant points. At midnight the courses passed through the plant along with the engine counters are stored in a file history. A simple program wrote in MicroC permits to extract the data regarding a specified period by entering just the date intervals. Finally, the system allows a complete alarm management permitting to warn remotely the operator on duty via modem or cell phones in a near future.
Super-Flash 's choice
Thanks to software packages like Super-Flash, many developers equipped themselves with PLC-based control systems in order to expand their offering possibilities with advanced supervision systems capable to be adapted to multiple requirements coming from diversified italian industries.
From a technical point of view Super-Flash is a development system with a basic language and a syntax guide accessible to those with PLC programming knowledge that are interested in developing reliable applications.
The basical functionalities allow the development of typical SCADA tasks: reading and writing of variables in fields, displaying synoptics, trends, alarm management, recipe management and file history saving.
On the other hand Automa anticipated the possibility of being supported by a programming language that allows the management of complex situations for advanced developers. MicroC is not a traditional compiler, even if it has a syntax similar to Ansi C. MicroC code is processed by a compiler and converted in metacode to later be performed in the execution phase by a Super-Flash internal engine capable of running 250 parallel tasks. This operating way guarantees the operating system independence and the total run code management, increasing the reliability benefits and the application stability.
Super-Flash is able to execute or stop many MicroC programs simultaneously. Automa's C language compiler is supplied with a package of libraries oriented to supervision problem solving, that contains lots of functions for alarm management, recipes, trends and external user interfaces including those needed for communication protocols development.
Super-Flash does not need to have other applications running under the operating system thanks to the existence of integrated printing and data sheet management, allowing in this way the reduction of instability risks and the costs regarding software licenses. In this way if the software package is running under Windows and if there are stability problems when running applications such as Word or Excel simultaneously, it is always possible to use Super-Flash internal applications in order to increase security and reliability levels.
Automa's software, which latest version is 3.6, is available in two commercial solutions: a ROYALTY-FREE version with just a paid development system, and a RUNTIME version with a free development system but with costs regarding the final applications.
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