14.2 Preliminary, Primary, and Secondary Treatment
Key Takeaways
- Preliminary treatment (screening and grit removal) protects downstream pumps and tanks by removing rags, grit, and grease before settling or biological processes are stressed.
- Primary clarifiers remove settleable solids and floatables; performance is described with surface overflow rate (SOR), detention time, weir loading, and percent removal (typically 25-40 percent BOD, 50-70 percent TSS).
- Secondary treatment removes biodegradable organics biologically; activated sludge is governed by BOD load, mixed liquor suspended solids (MLSS), food-to-microorganism (F/M) ratio, and solids retention time (SRT).
- Apply concentration removal directly only when flow is unchanged; otherwise convert to load (lb/day = MGD x mg/L x 8.34) before comparing units.
- Effluent clues diagnose the failing unit: high TSS points to clarifier or settleability problems; high soluble BOD points to under-loaded or oxygen-starved biology.
Treatment Train Logic
Wastewater treatment is a sequence, not a list of isolated tanks. Materials are removed in the order that protects equipment, stabilizes flow, lowers solids and oxygen demand, and yields an effluent that can be disinfected or polished. The April 2024 WRE specification separates preliminary, primary, secondary, nutrient removal, solids handling, disinfection, and advanced treatment, so you should be able to state each step's purpose before you calculate.
Unit Roles
| Unit or process | Primary purpose | Typical exam clue |
|---|---|---|
| Bar screen / fine screen | Remove rags, wipes, plastics, large solids | Pump clogging, screenings handling |
| Grit chamber | Remove sand, gravel, grit | Abrasion, deposition, ~1 ft/s control velocity |
| Primary clarifier | Settle solids, skim floatables | SOR, detention, sludge blanket |
| Aeration basin | Oxidize soluble and fine particulate BOD | Dissolved oxygen (DO), MLSS, F/M, SRT |
| Secondary clarifier | Separate biological floc from water | Return activated sludge, wasting, bulking, TSS |
Preliminary treatment is mostly protection. Aerated and vortex grit chambers are sized to hold a control velocity near 1 ft/s so dense inorganic grit settles while lighter organics stay suspended. Too fast, grit passes; too slow, organics settle and turn septic.
Primary Treatment
Primary clarifiers remove settleable solids and scum and incidentally reduce BOD because particulate organics leave with the sludge. Typical removals are 25-40 percent BOD and 50-70 percent TSS. Design metrics:
- Surface overflow rate (SOR) = Q / surface area, often 800-1,200 gpd/ft^2 at average flow.
- Detention time = volume / Q, commonly 1.5-2.5 hours.
- Weir loading rate = Q / total weir length, often under 20,000 gpd/ft.
Do not apply a primary removal percentage on the wrong basis. If flow is unchanged, concentration removal and load removal are numerically identical; if flow changes, use loads. Worked example: influent BOD 220 mg/L, removal 30 percent, so effluent = 220(1 - 0.30) = 154 mg/L. At 4 MGD, influent load = 4 x 220 x 8.34 = 7,339 lb/day; remaining load = 5,137 lb/day; removed = 2,202 lb/day, which is the dry solids feeding the digester from this stream.
Secondary Treatment
Secondary treatment targets biodegradable organics and biological solids. Activated sludge uses aeration, microorganisms, return activated sludge (RAS), and waste activated sludge (WAS). Trickling filters, rotating biological contactors (RBCs), oxidation ditches, lagoons, and sequencing batch reactors (SBRs) are alternative biological systems.
Core activated-sludge quantities:
- Food load = Q x influent substrate (mg/L) x 8.34 (lb BOD/day).
- Biomass inventory = basin volume x mixed liquor volatile suspended solids (MLVSS) x 8.34 (lb).
- F/M ratio = food load / biomass inventory (typically 0.2-0.5/day conventional).
- Hydraulic residence time (HRT) = basin volume / flow (4-8 hr conventional).
- Solids retention time (SRT) = solids in system / solids leaving per day; set by wasting, not by HRT.
Dissolved oxygen drives the aerobes; aeration basins are typically held at about 2 mg/L DO. Low DO causes poor treatment, odors, filamentous bulking, and loss of nitrification; excess DO wastes energy and can shear floc.
Clarifier and Effluent Clues
Secondary clarifiers separate biological solids. High effluent TSS can mean hydraulic overload, poor settleability (high sludge volume index, or SVI), rising sludge from denitrification, a deep sludge blanket, or mechanical failure. High effluent BOD with low TSS points to soluble organics escaping biology. High BOD and high TSS together point to both biological and separation problems.
PE workflow: sketch the train from headworks to effluent; decide whether the question is hydraulic, solids, organic-load, or process diagnosis; convert concentration and flow to load when comparing units; use HRT, SOR, F/M, and removal efficiency only with compatible units; and reject answers that remove grit in a secondary clarifier or solve soluble BOD with a bar screen.
Comparing Biological Process Options
The exam frequently asks which secondary process fits a constraint. Conventional plug-flow activated sludge gives strong BOD removal but needs skilled operation. Extended aeration (oxidation ditches) runs at long SRT (20-30 days) and low F/M (0.05-0.15/day), producing well-stabilized sludge and tolerating load swings, which suits small communities. Trickling filters and RBCs are fixed-film systems with low energy and simple operation but limited cold-weather nitrification. SBRs combine reaction and settling in one tank on a timed cycle, saving footprint where flow is variable. Lagoons are land-intensive but cheap.
When a prompt stresses operator skill, footprint, energy, or flow variability, the answer is usually the process whose strength matches that single constraint.
Oxygen Transfer and Aeration Sizing
Aeration is the largest energy cost at most plants. The oxygen demand a basin must satisfy is the carbonaceous BOD oxidized plus, if nitrifying, the nitrogenous demand. A useful exam relationship is that field oxygen-transfer rate equals standard transfer rate times correction factors for alpha (wastewater vs. clean water), beta (salinity), temperature, and the driving-force ratio of saturation minus operating DO. Blower or diffuser sizing therefore follows from total pounds of oxygen per day divided by the field transfer efficiency.
If a question gives BOD load, an ammonia load, and a transfer efficiency, total the oxygen demand first (carbonaceous plus 4.57 lb O2 per lb ammonia-N) before converting to air or power, and do not forget the nitrogenous share when nitrification is required.
A primary clarifier receives wastewater with BOD5 = 220 mg/L. If primary treatment removes 30 percent of BOD and the flow does not change, what is the primary effluent BOD5 concentration?
An aeration basin receives 3.0 MGD at 180 mg/L BOD. The basin volume is 1.2 MG and the mixed liquor volatile suspended solids concentration is 2,500 mg/L. What is the approximate F/M ratio?
A 1.5 MGD primary clarifier has a surface area of 1,200 ft^2. What is its surface overflow rate, and is it within a typical primary-clarifier range?