Primary treatment is the first line of defense — physical processes that remove settleable solids and floatables before biology takes over. Understanding it sets the foundation for everything that comes after.
Every wastewater treatment plant follows the same basic sequence: preliminary treatment, primary treatment, secondary treatment, and — at many facilities — some form of disinfection or tertiary polishing before discharge. Primary treatment is the second stage, and understanding what it does — and what it doesn't do — is foundational knowledge for the operator exam and for understanding everything that follows.
This article covers the complete picture of primary treatment: what it is, what equipment is involved, what gets removed, what doesn't, how performance is measured, and what the operator exam specifically tests.
Primary treatment uses physical processes — primarily gravity settling — to remove settleable solids and floatable material from wastewater. No biology. No chemicals (in basic primary treatment). Just physics: heavy stuff sinks, light stuff floats, and the clarified water in the middle moves forward to secondary treatment.
The primary goal is to reduce the load on the biological secondary treatment process. By removing a significant fraction of the suspended solids and associated BOD before the aeration basin, primary treatment makes secondary treatment more efficient and reduces aeration costs.
Primary treatment does not remove dissolved pollutants — dissolved BOD, ammonia, nitrate, phosphorus, or pathogens. It only removes what can be settled or floated out physically. This is why secondary treatment exists: to handle what primary can't.
Most primary treatment systems consist of the same sequence of physical unit processes, each targeting a different size or type of material:
The first barrier. Bar screens or mechanical screens remove large solids — rags, wipes, sticks, plastics, and other debris — that would damage downstream pumps and equipment. Coarse screens (bar spacing 1–3 inches) catch the biggest material. Fine screens (1–6mm openings) catch smaller solids. Screenings are typically compacted and landfilled. Clogged or bypassed screens are a major source of rag accumulation in digesters and pump clogs throughout the plant.
Grit chambers remove dense inorganic material — sand, gravel, eggshells, coffee grounds, and similar heavy particles. Grit is abrasive and will wear out pumps, impellers, and diffusers if allowed to pass through the plant. Grit chambers are designed to settle grit while keeping organic solids (which are lighter) in suspension so they can be treated biologically downstream. Common designs include aerated grit chambers, vortex grit separators, and horizontal-flow grit channels. Grit is washed, classified, and landfilled.
The heart of primary treatment. Wastewater flows slowly through a large, circular or rectangular settling tank — called the primary clarifier — where reduced velocity allows settleable solids to sink to the bottom as primary sludge, while floatable material (grease, oils, scum) rises to the surface to be skimmed off. The clarified effluent in the middle zone flows over a weir to secondary treatment. Detention time in the primary clarifier is typically 1.5–2.5 hours. Primary sludge is pumped from the bottom to digestion or thickening.
The BOD removal numbers deserve a closer look. Primary treatment removes 25–40% of BOD — but only the particulate BOD that's attached to settleable solids. The soluble (dissolved) BOD passes right through the clarifier unchanged. This is why secondary treatment removes so much more BOD — it's targeting the dissolved fraction that primary can't touch.
Primary treatment removes 50–70% of TSS but only 25–40% of BOD. The reason: not all suspended solids have proportional BOD. Some TSS is inorganic (grit, mineral particles) with essentially no BOD. And a large fraction of the total BOD is dissolved — invisible to a clarifier. This discrepancy between TSS and BOD removal percentages is a commonly tested exam concept.
The primary clarifier is the most important unit in primary treatment. Understanding its design and how it's operated is essential for the exam.
Settling in a clarifier depends on surface overflow rate (SOR) — the flow rate divided by the clarifier surface area, expressed in gallons per day per square foot (gpd/ft²). A lower SOR gives particles more time to settle before the water reaches the outlet weir. Typical primary clarifier SOR is 600–1,200 gpd/ft² under average flow conditions.
The other key design parameter is detention time — the average time wastewater spends in the clarifier. At typical SORs, primary clarifier detention time is 1.5–2.5 hours. Too short and solids don't have time to settle. Too long and the sludge sitting on the bottom can go septic, releasing dissolved BOD back into the effluent.
The weir overflow rate (WOR) measures the flow leaving the clarifier per linear foot of outlet weir — expressed in gallons per day per linear foot (gpd/lf). A high WOR creates turbulence near the weir that can pull settled solids back up into the effluent. Typical maximum WOR for primary clarifiers is 10,000–15,000 gpd/lf.
Grease, oils, and other floatable material rise to the surface of the primary clarifier and accumulate as scum. A rotating scum blade or scum baffle directs this material to a scum trough, where it's pumped to digestion or disposal. Poor scum removal leads to scum carryover into the secondary process, grease buildup in aeration basins, and foaming in digesters. Checking that scum removal equipment is operating correctly is part of every shift inspection.
Settled solids accumulate at the bottom of the primary clarifier as primary sludge. It's typically 1–6% solids and has a strong odor due to the readily biodegradable organic material it contains. Primary sludge is pumped continuously or on a schedule to thickening or directly to anaerobic digestion. Key operational point: don't let primary sludge sit too long in the clarifier. Septic primary sludge is harder to digest, releases dissolved BOD back into the effluent, and causes serious odor problems.
Standard primary treatment relies on gravity alone. Chemically enhanced primary treatment (CEPT) adds coagulants and flocculants — typically ferric chloride, alum, or polymer — to the primary clarifier influent. These chemicals destabilize colloidal particles and cause fine suspended solids to clump together (flocculate) into larger particles that settle faster and more completely.
CEPT can boost TSS removal to 80–90% and BOD removal to 50–60% — significantly higher than conventional primary treatment. It's used at facilities that need better primary performance, have limited secondary treatment capacity, or are targeting phosphorus removal (ferric chloride removes phosphorus as well as solids).
| Primary Treatment | Secondary Treatment | |
|---|---|---|
| Process type | Physical (gravity settling) | Biological (activated sludge, trickling filters) |
| What it removes | Settleable solids, floatables, particulate BOD | Dissolved BOD, colloidal BOD, ammonia (if nitrifying) |
| TSS removal | 50–70% | 85%+ from total influent TSS |
| BOD removal | 25–40% | 85%+ from total influent BOD |
| Energy use | Low — gravity driven | High — aeration is the largest energy cost |
| Solids produced | Primary sludge (raw, odorous) | Waste activated sludge (WAS) |
| Permit compliance | Cannot meet secondary treatment standards alone | Required to meet NPDES effluent limits |
A poorly operating primary clarifier doesn't just produce bad primary effluent — it stresses everything downstream. Here's what operators see when primary treatment struggles:
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| High effluent TSS | Hydraulic overload, short-circuiting, worn weirs, sludge buildup | Check SOR, inspect for short-circuiting, increase sludge pumping frequency |
| Scum carryover | Scum blade not rotating, scum baffle damaged, excessive scum accumulation | Inspect and repair scum removal equipment, increase removal frequency |
| Septic sludge / odors | Sludge sitting too long in clarifier, low temperatures slowing decomposition | Increase sludge pumping frequency, check pump operation |
| Sludge blanket rising | Pump failure, high hydraulic loading pushing sludge up, denitrification in clarifier | Check sludge pumps, reduce hydraulic loading if possible, inspect for gas production |
| Screen blinding / bypass | Heavy debris loading, mechanical failure, inadequate cleaning frequency | Increase cleaning frequency, inspect mechanical components, check bypass gates |
| Topic | What to Know |
|---|---|
| Purpose | Physical removal of settleable solids and floatables — reduces load on secondary treatment |
| TSS removal | 50–70% in primary clarification |
| BOD removal | 25–40% — only particulate BOD; dissolved BOD passes through |
| Why TSS% > BOD% | Some TSS is inorganic; much BOD is dissolved and invisible to settling |
| Surface overflow rate | Flow ÷ clarifier surface area; typical 600–1,200 gpd/ft² |
| Detention time | 1.5–2.5 hours typical |
| Primary sludge | 1–6% solids; raw, odorous; pumped to digestion or thickening |
| Septic sludge | Caused by sludge sitting too long; fix by increasing pumping frequency |
| Screening purpose | Protect downstream equipment from rags, debris, large solids |
| Grit removal purpose | Remove abrasive inorganic material before it damages pumps and equipment |
| CEPT | Adds coagulants/flocculants to improve TSS and BOD removal beyond gravity alone |
The WastewaterAce Complete Exam Guide covers primary treatment, secondary treatment, solids handling, and all 12 core exam topics — 200 questions with full explanations for every answer.
Get the Study Guide — $17