14.2 Preliminary, Primary, and Secondary Treatment
Key Takeaways
- Preliminary treatment protects downstream equipment by removing large debris, rags, grit, and sometimes grease before biological or settling processes are stressed.
- Primary treatment removes settleable solids and floatables, so performance is usually described with hydraulic loading, detention time, surface overflow rate, and percent removal.
- Secondary treatment removes biodegradable organics by biological conversion, physical settling, and sometimes chemical support rather than by screening alone.
- Activated-sludge questions often reduce to BOD loading, mixed liquor solids, food-to-microorganism ratio, sludge age, return sludge, waste sludge, and clarifier behavior.
- PE WRE treatment questions commonly ask for process role, load, removal efficiency, hydraulic residence time, or a failure diagnosis from effluent BOD, TSS, sludge blanket, or dissolved oxygen clues.
Treatment Train Logic
Wastewater treatment is not a list of isolated tanks. It is a sequence that removes materials in the order that protects equipment, stabilizes flow, lowers solids and oxygen demand, and produces 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 what each step does before calculating.
Unit Roles
| Unit or process | Primary purpose | Typical exam clue |
|---|---|---|
| Bar screen or fine screen | Remove rags, wipes, plastics, and large solids | Pump clogging, screenings handling |
| Grit chamber | Remove sand, gravel, eggshells, and grit | Abrasion, deposition, velocity control |
| Primary clarifier | Settle heavy solids and skim floatables | Surface overflow rate, detention, sludge blanket |
| Aeration basin | Biologically oxidize soluble and fine particulate BOD | Dissolved oxygen, MLSS, F/M, sludge age |
| Secondary clarifier | Separate biological floc from treated water | Return activated sludge, wasting, bulking, TSS |
Preliminary treatment is mostly protection. Screens keep large material out of pumps and tanks. Grit removal is different from primary clarification because grit is dense inorganic material that should settle while most organics stay in suspension. If grit velocity is too high, grit passes through. If velocity is too low, organics settle with the grit and create odor.
Primary Treatment
Primary clarifiers remove settleable solids and scum. They also reduce some biochemical oxygen demand (BOD) because particulate organics leave with the sludge. Exam questions may give a percent removal, surface overflow rate, weir loading, or detention time. Do not apply a primary removal percentage to a different basis than the prompt uses. If flow is unchanged, concentration removal and load removal are numerically the same. If flow changes, use loads.
A simple primary calculation: influent BOD is 220 mg/L and primary removal is 30 percent. Primary effluent BOD is 220(1 - 0.30) = 154 mg/L. If flow is 4 MGD, the influent BOD load is 4 x 220 x 8.34 = 7,339 lb/day and the remaining load is 5,137 lb/day.
Secondary Treatment
Secondary treatment targets biodegradable organics and suspended biological solids. Activated sludge uses aeration, microorganisms, return activated sludge (RAS), and waste activated sludge (WAS). Trickling filters, rotating biological contactors, oxidation ditches, lagoons, and sequencing batch reactors are alternative biological systems with different operating patterns.
Core activated-sludge quantities:
- Food load = Q x influent substrate concentration x 8.34.
- Biomass inventory = basin volume x mixed liquor volatile suspended solids x 8.34.
- F/M ratio = food load / biomass inventory.
- Hydraulic residence time = basin volume / flow.
- Solids retention time is controlled by wasting and solids inventory, not by water detention time alone.
Dissolved oxygen matters because aerobic organisms need oxygen to oxidize carbonaceous BOD and, if nitrification is intended, ammonia. Low DO can cause poor treatment, odors, filamentous growth, or loss of nitrification. Too much aeration wastes energy and can contribute to floc shear or settling problems.
Clarifier and Effluent Clues
Secondary clarifiers are biological solids separators. High effluent TSS can mean hydraulic overload, poor sludge settleability, rising sludge from denitrification, excessive sludge blanket, or mechanical problems. High effluent BOD with low TSS points more toward soluble organics escaping biological treatment. High BOD and high TSS may point to both biological and solids-separation problems.
PE exam workflow:
- Sketch the train from headworks to effluent.
- Identify whether the question is hydraulic, solids, organic load, or process diagnosis.
- Convert concentration and flow to load when comparing units.
- Use HRT, overflow rate, F/M, and removal efficiency only with compatible units.
- Reject answers that remove grit in a secondary clarifier or solve soluble BOD with a bar screen.
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?