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Symbols and identification codes used to identify instruments in water and wastewater facilities are typically based on ISA standards, with modifications for the unique nature of water and WRRFs (Meier and Meier, 2011). Standard S5.1 (ISA, 1992) (Figure 4.1), which defines symbols used in P&IDs, was created to help promote uniformity in the instrumentation industry. The symbols in this voluntary, consensus-based standard are adaptable and can be used in many applications. Through regular updates of the standard, ISA has incorporated other symbol standards endorsed by the American National Standards Institute (ANSI) (http://www.ansi.org) and the Institute of Electrical and Electronics Engineers (http://www.ieee.org). Another standard used to develop P&IDs is ISA-5.3-1983, Graphic Symbols for Distributed Control/Shared Display Instrumentation, Logic and Computer Systems (ISA, 1983). The ISA also provides its library of standard symbols in an electronic format that can be easily imported into any computer-aided design (CAD) program.

The ISA’s standards only provide a framework for developing P&IDs; as such, documents prepared by different designers may vary significantly. Therefore, those responsible for developing P&IDs should include a legend defining all lines, symbols, abbreviations, instruments, and equipment-tagging number conventions used in the documents (see Figure 4.3 for a typical legend and symbol drawing).