GE Fanuc aids water purification at Formellino

The Formellino Wastewater Treatment plant at Faenza, managed by Hera Imola - Faenza S.r.l., is a medium-sized installation that purifies 1000m3/hour of water and runs constantly.

The plant must ensure all of the water produced by the purification process meets quality regulations.

The Formellino Wastewater Treatment plant diverts the water flowing into the Lamone River, splits the flow onto two parallel lines and directs the two flows to the treatment tanks.

The water is pumped back downstream into the river after purification.

The activated-sludge purification system involves organic substances and ammonia being oxidised in the presence of oxygen by the activated sludge.

The nitrate products, typically eutrophying nutrients, are later removed in absence of oxygen.

Consequently, the oxygen content, the active-sludge concentration, the nitrates and the ammonia, are key data inputs of the plant-process control system.

The first steps are grit removal and de-oiling (not managed by the control system).

The first active step of the plant follows: the equalisation and primary decantation tanks form a vessel for controlling the sewage flow-rate into the various tanks by sluices (a simple level gauge is used for this).

Then sewage reaches the oxidation and pre-denitrification tanks, where the level of oxygen in the slurry is measured at the inlet and at the outlet.

The nitrates and suspended solids are also measured in these tanks (by means of turbidimeters, which are designed specifically for measuring turbidity by implementing optical techniques), along with the phosphorous and ammonia contents, the level of decanted sludge and the inlet and outlet water-flow rate.

Some of the output sludge is re-circulated back to the inlet and re-introduced to improve the biological processes.

After oxidation, the water flows to the secondary decantation tanks, where the sludge deposited on the bottom is collected and conveyed to the thickener.

Here, the sludge is prepared for drying and disposal.

The clarified water is then released into the river.

The water-treatment plant is subject to seasonal variations determined by rainfall.

This means the quality of the water to be treated cannot be determined beforehand.

The plant-collection basin includes many industries that introduce large amounts of waste, meaning wide variety in the water chemistry and flow.

The plant is always switched on to prevent the risk of releasing polluted water into the river.

The old plant ran according to a fixed-time logic, consisting of making the sewage water stand in the various vessels for a predetermined length of time and controlling the operation of the process-related machines (aerators, blowers, pumps) according to dissolved-oxygen measurements and laboratory test-data only.

One of Hera's aims was to use the data collected to control the transit times of the sewage in the tanks and machine operation - according to the values of oxygen, ammonia, suspended solids and nitrates.

This was to improve plant processing and energy efficiency.

Furthermore, the new control system had to allow an operator to work at the plant and relay data to the control room, from where all Hera plants are monitored.

The plants are manned during the day but the control-room alone monitors the operation of all water-treatment plants during the night.

Hera wanted high plant-availability and reliability, data access by operators and improved process management, in terms of better results and more efficient use of energy resources.

To reach these goals, Hera called Pastorelli's environmental engineering firm to establish the project guidelines.

The system was made by Novanet, a company based in Emilia-Romagna, Italy with expertise in the construction of large control and automation systems.

Hera asked Novanet to use GE Fanuc products to implement the control system.

The 'brain' of the system is a GE Fanuc Pacsystems RX3i in redundant hot backup configuration.

It interfaces with all the field instruments on a Profibus network (part optical fibres and part copper wires); there are approximately 600 controlled tags.

The two redundant CPUs ensure the high plant-availability.

The PAC controller establishes the standing times of the slurry in the various stages of the plant.

By means of a direct Modbus/TCP link, the PAC communicates data to the Hera control room, where it is stored in a SQL database and concisely displayed so that the operator (present 24 hours a day) can be warned of faults and act accordingly.

At the Formellino plant, a local computer running GE Fanuc Proficy HMI/Scada Cimplicity software collects, monitors and displays information and data in the form of trend or log.

Ten profiles corresponding to ten different operative and data-access levels have been created according to the privileges established for each user class.

The application allows control parameters to be set up and programmed (the plant has been running only for a few months and the control logic is still being fine-tuned).

Many fault-detecting functions have been implemented in programs running at the water-treatment plant to signal measurements deviating from expected values and to collect and use self-diagnostic data from the field sensors.

Alberto Tabanelli, technical manager of Novanet, said: 'The PLC hot backup function provided default hardware redundancy, which avoided further complications, and the system performance allowed us to introduce a predictive control, which has greatly improved plant performance.

The new system collects plant data for constantly monitoring everything in detail.

Predictive control, sensor-data collection and use and control-system response rapidity have been exploited to optimise machine running-times and consequently decrease energy consumption while keeping the water-quality high.

Before installing the new system, for example, the water was over-oxygenated and this was pointless from a microbiological point of view.

The Formellino plant automation system has been running for only a few months and the implemented logics are still being optimised.

After only 50 days, an energy consumption of 30 per cent has already been observed.

The plant was shut down for approximately half an hour to allow the new system to be installed.

Personnel training was swift, thanks to self-explanatory graphic displays, and was carried out over several shifts to account for staff turnover.

New actuators will be added in the future; these will be controlled continuously instead of in steps.

They will be installed on the Profibus field network and controlled directly by the Pacsystems Rx3i.

These improvements will provide the best results where the processed matter is kept moving: in oxidation and sludge recirculation tanks.

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