The control of nitrogen in wastewater begins with nitrification for oxidisation of ammonia-nitrogen and ends with denitrification for reduction of nitrates and nitrites to nitrogen gas. Both nitrification and denitrification are accomplished through a biological treatment, accelerated by the injection of compressed air.
During the process, wastewater temperature and pH are the two most important parameters to be measured and controlled because they affect the distribution of ammonia and ammonia ions. The water temperature must be kept to a specified range, regardless of the season.
The innovative technology at the Dusseldorf water plant was to bring the nitrification and denitrification together in a simultaneous process. The wastewater swirls, through five 23,200 [m.sup.3] flat, open-air tanks, each tank divided into four narrow channels where compressed air is blown into the bottom to supply oxygen. The compressed air is supplied by four large turbines. Controlling the turbines, the amount of air injected into the process, the flow of water, water temperature, pH, bacteria, and many other parameters associated with the process is a science that is not completely understood.
Technology and co-operation
Engineers use the SCADA software to track and plot the variables with the intention of converting what is now largely an empirical process into engineering science. Some of the process variation can be reduced through technology and co-operation of local industry, and this reduction is important to the cost-effectiveness of the plant.
An example of this co-operation is Henkel, a major detergent manufacturer located 10 kilometres away. Henkel’s liquid effluent is a substantial portion of the incoming wastewater, and so it has agreed to measure critical wastewater parameters such as pH, conductivity, temperature, and water volume, and transmit these to the Dusseldorf plant. There they are combined with other measurements of the input stream to form a complete picture of the incoming water to be processed.
By keeping good track of input variables and the results, engineers hope to transform what is now a continuous process into a batch control operation. They want to formulate a set of recipes for the different “batches” of water they receive, which will vary according to the season, the amount of rainwater, and the proportions coming from private homes, commercial establishments, and manufacturing facilities.
It is estimated that nearly 20% of total project costs were saved as a result of industry co-operation and the use of state-of-the-art PLC and PC SCADA technology.
Careful monitoring of the input stream provides another layer of protection for the plant. It helps to prevent the accidental introduction of highly toxic substances into the water system, which would damage the biological process and perhaps shut it down for weeks. In an emergency highly toxic substances are contained and shunted to a holding tank.
The control system
The main contractor for the control system was Schneider Electric Germany, who in turn chose Alcon Projects (84) Ltd. as a subcontractor together with systems integrator Inotronics GmbH to carry out the process control system with its P-CIM software.
Twenty workstations running P-CIM are used for supervision, management, collection and displaying of process data. Half of them are in the central control room and the others are scattered about the site. The control system currently handles 16,000 I/O points and will be expanded soon to 35,000. The entire control system upgrade with communication links was installed in the first five months of 1999.
Eighteen redundant pairs of Telemecanique TSX-7 PLCs working in a redundancy configuration carry out the job of process control. Each controller is backed up by an additional controller programmed to take over in case the primary one fails. All controllers are linked to the central control room with fibreoptic cable.
At night, when input variation is lowest, the entire plant is controlled by three operators. The facility covers a large area; a bicycle trip from the control room to the most distant part takes 20 minutes.
Afcon specialises in turnkey control systems and says it has completed wastewater treatment projects in Budapest, Tel-Aviv, Athens, and Bombay as well as a number of other cities around the world.
The PC-based visualisation system consists of 20 operator workstations connected to two sets of data servers which are also working in a redundancy configuration. A redundant 100 MB Ethernet fibreoptic communications network connects all stations to the data servers. Response time, it is claimed, is less than half a second.
Two of the workstations project large images onto the wall of the control room. A single operator can work with as many as ten different screens.
Reliability and performance of the communication system is understandably a major requirement. To ensure high reliability, dual network cables, hubs, PLCs and above all, software redirecting support, were implemented. Real time data are read into the communication server, stored in the archive server, and distributed to the operation workstations.
Many enhanced features of P-CIM point out the advantages of a PC-based SCADA system. Historical data collecting into an SQL data server, for example, allows engineers to store and analyse vast quantities of data, which will allow them to study and refine the process. And as the process moves from continuous control to batch control, recipe management becomes more important. Recipe management allows users to dynamically modify a recipe’s
Producing reports for various government agencies is also a major and obvious requirement. It is relatively easy to configure reports that satisfy the new ATV M-260 standard with the software’s Windows-based system, according to the site’s process control manager.
The software has built-in Internet access, which allows authorised users to view plant data from across the globe. Although this feature is not currently used, it may become important in the future as site engineers travel away from the plant, or if they want to observe its operation while in another location-such as their homes.
City Hall in Dusseldorf is one remote location that is keenly interested in the operation of the plant. To follow day-by-day operations it maintains a workstation with a direct communications link with the control room. The city must pay for the operation of the plant and more importantly, it must pay for nitrogen and phosphorous released into the Rhine that exceed ATV’s limits.
So far, advanced wastewater treatment technology, coupled with good co-operation with industrial neighbours and modern control systems have produced a very efficient system, and Dusseldorf residents enjoy a low water usage rate of 2 per cubic [European Dollar] meter.