Preliminary Treatment (Screening, Grit Removal, Flow Measurement)
Screening
Bar screens (coarse, 1–4 in openings) and fine screens remove rags, plastics, and debris that would damage pumps and clog downstream equipment. Mechanical rakes move screenings to a hopper or washer. Channels are designed for a velocity near 2 ft/s through the bars so grit stays suspended without being carried over. Rising differential head loss means the screen needs cleaning. Screenings typically amount to roughly 0.5–2 ft³ per million gallons.
Grit removal
Grit is inorganic sand and eggshell that abrades equipment. Aerated and vortex grit chambers slow flow to near 1 ft/s so grit settles while organic solids stay suspended. Grit quantities are typically 3–8 ft³/MG. Over-removal drags organics into the grit and creates odors; under-removal sends grit to digesters and clarifiers.
Flow measurement
Parshall flumes measure flow from depth at a fixed point upstream of the throat; they are self-cleaning and tolerate solids. Weirs (V-notch, rectangular) require a stilling well and a free-falling nappe. Magnetic and ultrasonic meters give instantaneous flow but need calibration and clean sensors. Totalized daily flow drives every loading calculation.
Traps
Measuring head in the wrong place on a Parshall flume; using average flow when peak was asked; forgetting conversions (1 MGD = 1.547 cfs = 694 gpm).
Worked example
A channel 3.0 ft wide carries flow 2.0 ft deep at 4.0 MGD. Convert: 4.0 × 1.547 = 6.19 cfs. Area = 3.0 × 2.0 = 6.0 ft². Velocity = 6.19/6.0 ≈ 1.03 ft/s - right at the target for a grit chamber.