The Formellino Wastewater Treatment plant at Faenza is a medium-sized installation, which purifies 1000 m3/hour of water and runs non stop. This waste water plant must ensure that all of the water produced by the purification treatment process meets or exceeds the required 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 the purification treatment process. The 'brain' of this wastewater treatment system is a GE Fanuc PACSystems RX3i control system from GE Intelligent Platforms .
The activated sludge purification system is a biological type, where organic substances and ammonia are oxidized 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.
Once the first steps of grit removal and deoiling are completed, the 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, 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 recirculated 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 and the clarified water is released into the river.
The water treatment plant is, due to its intrinsic nature, subject to seasonal variations determined by rainfall. Consequently, one of the process criticalities is that the quality of the water to be treated cannot be determined beforehand.
Furthermore, the plant collection basin includes a number of industries which introduce large amounts of waste, thus the water chemistry and flow varies greatly. Another criticality of a plant like this, with such an extensive coverage, is that it is always on. This is essential to prevent the risk of releasing polluted water into the river and to prevent being fined by the water quality monitoring authority.
The old wastewater treatment plant was run according to a fixed time logic. This consisted 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 according to dissolved oxygen measurements and laboratory test data only.
The goal was to use the data collected by various sensors 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 to improve plant processing and energy efficiency. Furthermore, the new control system had to allow an operator to work at the plant as well as relaying 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.
When the decision to refurbish the plant was made the question of how to make sure the new automation system would guarantee pease of mind was asked. This peace of mind refers to 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.
In order to reach these goals, the new wastewater treatment system was made by Novanet, an Italian based company with major expertise in the construction of large control and automation systems. Novanet used GE Fanuc products for implementing the control system because they are reliable, competitively priced, the construction technology is good and assistance is prompt and conclusive.
The “brain” of the new wastewater treatment system is a GE Fanuc PACSystems RX3i in redundant hot backup configuration, which interfaces with all the field instruments on a Profibus network (part optical fibres and part copper wires); there are several new and old sensors in total, amounting to approximately 600 controlled tags.
The two redundant CPUs ensure the high plant availability required by the application criticality. The PAC Contoller 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 control room, where they are stored in a SQL database and concisely displayed so that the operator 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, in addition to alarms, which may be silenced or not by the users according to their access levels. 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 to set up and program control parameters (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 program running at the water treatment plant today to signal measurements deviating from expected values and to collect and use self-diagnostic data from the field sensors.
The new wastewater treatment 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.
The plant was shut down for approximately half and hour to allow the new system to be installed, and after only 50 days, an energy consumption of 30% had already been observed. Personnel training in the new GE Fanuc PACSystems RX3i control system was swift, thanks to intuitive, self-explanatory graphic displays, and was carried out over several shifts to account for staff turnover.