Equipment, Maintenance, and Instrumentation
Pumps and blowers
Centrifugal pumps handle most clear and mixed liquors; positive displacement pumps (progressive cavity, piston, diaphragm) handle sludge, grit slurries, and polymer. Cavitation sounds like gravel, cuts flow and head, and erodes impellers - the usual cause is insufficient NPSH available from excessive suction lift, a clogged strainer, or a hot liquid near its vapor pressure. Blowers may be positive displacement or centrifugal; diffusers, VFDs, and dissolved oxygen control tie the air supply to demand.
Maintenance programs
Preventive maintenance is scheduled; predictive maintenance uses vibration analysis, thermography, oil analysis, and ultrasound to catch faults early; corrective and reactive work follow. A work-order system tracks history and spare parts. Lockout/tagout precedes any repair, and alignment, lubrication, seals, and packing are routine checks.
Instrumentation
Level: ultrasonic, radar, bubbler or pressure, and float switches. Flow: magnetic meters require a conductive liquid (poor for non-conductive fluids and low flows), ultrasonic and Doppler meters clamp on, and weirs or flumes suit open channels. Analyzers include pH, ORP, DO, turbidity, and chlorine. Calibrate against known standards, verify with a second buffer, and check for fouling, drift, and wiring faults. A reading that is too steady or too clean is a warning sign.
Traps
Confusing precision with accuracy; calibrating without checking the standard first; assuming a magnetic flow meter works on a dry or empty pipe.
Worked example
Affinity laws: a pump slowed from 1,750 to 1,400 rpm keeps 80% of speed. Flow falls to 0.80, head to 0.80² = 0.64, and power to 0.80³ = 0.512 - a small speed change saves a lot of energy.