6.1 Preliminary Processes: Screening, Grinding, Grit & Equalization

6.1 Preliminary Processes: Screening, Grinding, Grit & Equalization

2025 WPI alignment: This section teaches preliminary treatment processes such as screening, grinding, grit removal, and flow equalization in Treatment Process Evaluation and Adjustment, the 40-question area containing 6 recall, 34 application, and 5 calculation items.

Why this process task matters

Preliminary treatment protects the rest of the plant. Evaluation focuses on whether debris and grit are removed without excessive organic capture and whether equalization actually moderates the flow or load reaching primary and biological treatment.

Process-control model

Evaluation and adjustment sequence

1. Compare headworks influent, downstream flow, channel levels, screen headloss, and available equipment.
2. Inspect captured screenings and grit for quantity, moisture, unusual industrial material, and excess organics.
3. Confirm bypass status and restore parallel units before capacity is exceeded.
4. Use forecast, basin level, usable storage, and downstream capacity to set equalization routing and release.
5. Adjust only the authorized hydraulic or equipment variable and watch both headworks level and downstream response.
6. Document outages, bypasses, disposal quantities, storm timing, storage used, and corrective maintenance.

Diagnostic evidence

Calculation and mass-balance connection

Use flow = area × velocity for a uniform channel when the problem supplies appropriate geometry and units. Use change in equalization storage = inflow volume − outflow volume over the stated interval. Do not add MGD values directly to gallons; convert rate to volume for the time period. A high flow with reduced operating channel area raises velocity and may carry grit forward.

Worked operating scenario

A storm causes high screen differential, one grit unit is offline, and the equalization basin has unused volume. The operator places available headworks capacity in service, uses storage under the wet-weather SOP, clears no jam until LOTO is established, and limits downstream release until the peak passes. Bypassing both screens would relieve upstream level but create pump and basin damage.

Common exam traps

• Grinding is size reduction, not solids or grit removal.
• Equalization does not reduce total pollutant mass; it moderates rate and timing.
• Grit quantity alone is not success if the removed material is mostly organics.
• A bypass may be reportable and is never a routine substitute for maintenance.

Field-to-exam checklist

• Evaluate preliminary treatment by protection achieved and downstream symptoms.
• Separate dense grit while keeping biodegradable organics in the treatment train.
• Use equalization volume and release rate to smooth, not recreate, peaks.
• Treat screen/grinder jams as hazardous-energy work.

Connecting headworks evidence

Screenings and grit records become more useful when normalized to influent flow and storm condition. A larger wet-weather quantity may be expected; a sudden decline while downstream pump clogging increases suggests capture loss rather than a cleaner influent. Compare equipment run time, bypass duration, hauled material, and downstream maintenance. That four-way reconciliation prevents a supervisor from praising low disposal tonnage that actually resulted from an offline rake, plugged grit pump, or material carried into aeration and digestion.

Judging preliminary performance by mass and consequence

Screenings and grit are best trended normalized to flow — commonly cubic feet per million gallons — so that a wet-weather increase is not mistaken for improved capture and a dry-weather decrease is not mistaken for a cleaner sewershed.

Worked normalization. A plant treating 6.0 MGD collects 12 ft³/day of washed grit, which is 2.0 ft³/MG. If the rate falls to 0.8 ft³/MG with no change in influent character, roughly 60 percent of the grit is no longer being captured. That material did not vanish; it is now in the primaries, the aeration basins, or the digesters.

The strongest evidence is downstream consequence, because it is measured whether or not the headworks instruments are working:

• pump clogging frequency and the hours spent clearing rags,
• accumulated grit in digesters, which permanently removes active volume and shortens the effective solids retention time at the same feed rate,
• deposits in aeration basins and channels, and the interval between cleanings,
• abrasion on impellers, augers, and sludge-pump wear parts.

Equalization is evaluated by the same before-and-after logic. Compare the peak-to-average ratio measured at the headworks with the ratio measured on forward flow to primary treatment. Reducing 2.4 to 1.3 demonstrates that the basin genuinely absorbed the peak. If the two ratios are nearly equal, the basin is full, being bypassed, or releasing as fast as it fills — three very different causes with three different responses. Record basin level, gate position, and release rate alongside the ratio so the reason is recoverable later.

Finally, reconcile hauled screenings and grit tonnage against equipment run time and bypass duration. Low disposal tonnage can indicate excellent influent quality or an offline rake; only the four-way comparison tells them apart.