Controlling the purification process

Dec 29, 2011 | Flow & Level

A GE control system is the ‘brain’ behind a solution designed to ensure that all of the water produced by a purification treatment process meets or exceeds required quality regulations. The Formellino Wastewater Treatment plant which is managed by Hera Imola – Faenza purifies 1000 m3/hour of water and runs 24/7 and is 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 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.

Fixed time logic

The old 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 (aerators, blowers, pumps, etc) according to dissolved oxygen measurements and laboratory test data only. The goal set by water treatment manager of Hera Giovanni Tedioli 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 we decided to refurbish the plant, we asked ourselves how to make sure that the new automation system would guarantee our peace of mind,” explains electrical maintenance, automation and remote control manager of Hera, Massimo Zanoni.

“The water treatment plant releases water into our own rivers and this implies additional responsibilities toward society: we need to guarantee fault-less operation for ourselves and for our environment.”

The ‘peace of mind’ was then to be translated into 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.

Control system

Novanet was asked to develop a system using GE products for implementing the control system as these products are standard at Hera plants because they are reliable, competitively priced, the construction technology is good and assistance in case of need is prompt and conclusive.

The ‘brain’ of the system is a GE 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 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 they are 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.

Improved purification

As a result of this installation, all plant data is collected and used for predictive calculations and for optimising process efficiency; the company is benefiting from an enhanced water purification process and a 30% energy saving; water quality is much improved as is control of crucial river habitat parameters.

“We used GE products in this plant for the first time and despite the complexity of the logics and the installations, we encountered no problems at all,” concludes Novanet technical manager Alberto Tabanelli.