EXEC/OP is a Fortran computer program that performs preliminary design synthesis of municipal wastewater treatment and sludge disposal systems. It selects the components in the wastewater treatment and sludge disposal trains from a list of candidate process units with fixed design characteristics so that criteria on effluent quality, cost, energy, land utilization, and subjective undesirability are best satisfied. The computational procedure uses implicit enumeration coupled with a heuristic penalty method that accounts for the impact of return sidestream from sludge processing. The program can consider eight different types of wastewater treatment processes and thirteen different types of sludge treatment processes. This computer magnetic tape file contains the Fortran source code for Version 1.2 of the EXEC/OP program.