Programmable logic controllers find a home in wastewater treatment

Abstract

Efforts to computerize wastewater treatment plants have had mixed results. A wastewater treatment plant presents significant obstacles to computer automation. One of the most challenging problems is coping with the aggressive environment of the plant. Industrially hardened programmable logic controllers (PLCs) have found a home in the wastewater treatment field. These small, reliable, and rugged computers are gaining acceptance in this conservative industry. Standard process control and data acquisition software packages are available from most PLC manufacturers for use on a supervisory personal computer workstation networked to their PLCs. In addition, similar third-party generic software packages, which support the PLCs of multiple manufacturers, can be procured. These packages provide functions such as interactive graphics, database, trending, historic storage, and report generation. Networks containing personal computers and PLCs offer an attractive alternative to the conventional large distributed computer system (DCS). A compomplex PLC network can equal a DCS in capability, while a simple PLC network can offer fast, reliable control of a single process. In each case, PLCs offer the advantage of a rugged input/output (I/O) system designed for the factory floor. Process control applications with fail-safe provisions are suitable to PLCs and can be demonstrated throughout the plant. Although they are industrially hardened, caution must be exercised in the application of PLCs. The corrosive nature of the atmosphere at a wastewater treatment plant still presents challenges. However, properly installed, PLCs maintain their reputation for reliability.