AP Syllabus focus:
‘Primary treatment removes large objects with screens and grates, then allows solid waste to settle to the bottom of a tank.’
Primary treatment is the first major step in wastewater (sewage) treatment. It relies on simple physical processes—screening and gravity settling—to remove debris and suspended solids before later treatment steps.
Purpose and placement in wastewater treatment
Primary treatment aims to quickly reduce the solid load in incoming wastewater so downstream processes are less likely to clog, overload, or fail. It targets:
Floatable materials (trash, rags, plastics, oils/grease)
Settleable solids (heavier particles that sink)
Some suspended solids that aggregate and settle during quiescent conditions
It does not primarily remove dissolved nutrients, dissolved chemicals, or pathogens; its strength is physical separation.
Key terminology
Primary treatment: The initial, physical stage of sewage treatment that removes large debris by screening and removes settleable solids by gravity settling in a tank.
Primary treatment is often described using “influent” (incoming wastewater) and “effluent” (outflow from a process).
Influent: Wastewater entering a treatment unit (such as a screen or settling tank).
In practice, the quality of primary effluent depends strongly on flow rate and how long wastewater remains in the settling tank.
Step 1: Physical removal with screens and grates
The first operation removes large, visible materials that could damage equipment or obstruct flow.
Mechanically cleaned bar screen (headworks) showing the bar rack that intercepts large debris while allowing water to pass through. The labeled parts (e.g., rake assembly, discharge chute, drive unit) illustrate how captured “screenings” are lifted out for disposal, reducing clogging risk to downstream equipment. Source
Facilities commonly use screens and grates designed to intercept debris while allowing water to pass.
What screening captures (and why it matters)
Large objects (e.g., sticks, wipes, cloth, plastics)
Materials that can entangle pumps or block pipes
Debris that would otherwise accumulate in settling tanks and reduce capacity
Operational considerations
Screens must be cleaned (manually or mechanically) to prevent backups.
Captured material (“screenings”) must be handled and disposed of appropriately to avoid odours and vector attraction.
During storm events, higher flows can push more debris into the system, increasing maintenance demands.
Step 2: Settling of solids in a tank (primary clarification)
After screening, wastewater enters a settling tank (often called a primary clarifier) where water movement slows.
Under low turbulence, gravity separates materials by density.
What happens in the settling tank
Heavier solids sink and accumulate as sludge at the bottom.
Lighter materials (especially oils and greases) rise, forming a surface layer.
The clearer middle layer exits as primary-treated wastewater.
Why settling works
Settling is driven by:
Particle size and density (larger/denser particles settle faster)
Hydraulic retention time (how long water stays in the tank)
Flow conditions (excess turbulence keeps solids suspended)
Outputs of primary settling
Primary sludge: concentrated organic and inorganic solids collected from the bottom
Primary effluent: partially clarified water sent onward for additional treatment
A surface layer of floatables that must be removed to keep the outflow clear
Performance: what primary treatment typically removes
Primary treatment can substantially reduce:
Total suspended solids (TSS) (often a major fraction of settleable material)
Biochemical oxygen demand (BOD) to a limited extent, because some oxygen-demanding organic matter is removed with the solids
Effectiveness varies with influent composition, tank design, and loading conditions; high flows can reduce settling time and lower removal efficiency.
Limitations and environmental significance
Primary treatment alone is insufficient for safe discharge because:
Many dissolved pollutants remain in the water.
Pathogens are only partially reduced.
Fine and colloidal particles may not settle without additional processes.
Even with these limits, primary treatment is environmentally important because removing solids early reduces downstream oxygen demand and lowers the risk of visible pollution and sediment accumulation in receiving waters.
FAQ
Some systems include a dedicated grit-removal step ahead of primary settling to prevent abrasion and deposition.
Common approaches:
Velocity-controlled channels
Aerated grit basins
Separated grit is washed and disposed of to reduce odour and organic content.
It is set by design targets balancing settling efficiency and tank size.
Key influences include:
Average and peak flow rates
Wastewater temperature (affects viscosity)
Expected solids characteristics (size/density)
Design aims to avoid short-circuiting while keeping conditions calm enough for settling.
Floatable oils/grease rise to the surface and can be removed by skimming mechanisms.
Operational factors that help:
Low turbulence
Regular skimmer maintenance
Preventing excess grease inputs from industrial sources
Recovered scum is handled separately from settled sludge.
Primary sludge is often rich in organic matter and water, making it:
Heavy and costly to transport
Odorous if stored improperly
Attractive to pests if exposed
Plants usually thicken it to reduce volume before further handling or disposal.
Short-circuiting occurs when influent flows directly to the outlet without adequate settling time.
It matters because:
Solids are carried out with the effluent
Clarification efficiency drops sharply
Baffles, inlet design, and proper flow control are used to reduce this risk.
Practice Questions
Describe two physical processes used in primary sewage treatment. (2 marks)
Screening with screens/grates removes large objects/debris (1)
Settling/sedimentation in a tank allows solid waste to sink to the bottom (1)
A wastewater plant reports that after heavy rainfall, more suspended solids are observed leaving the primary settling tank. Explain, using primary treatment processes, why this can happen and suggest two operational responses. (6 marks)
Heavy rainfall can increase influent flow, reducing retention time in the settling tank (1)
Higher flow increases turbulence, keeping particles suspended and preventing settling (1)
Screens/grates may be overloaded, allowing more debris/solids to pass onward (1)
Response: increase frequency/automation of screen cleaning or add temporary screening capacity (1)
Response: adjust flow distribution (e.g., use additional tanks if available) to increase effective settling time (1)
Response: remove accumulated sludge/floatables more frequently to maintain tank volume and performance (1)
