AP Syllabus focus:
‘Secondary treatment uses bacteria to break down organic matter into carbon dioxide and inorganic sludge. Aeration increases the rate of bacterial decomposition.’
Secondary treatment is the core biological stage of sewage treatment.
Flow diagram of a conventional activated sludge plant showing how wastewater moves from pretreatment and primary settling into aeration tanks and then to final settling before discharge as effluent. The diagram emphasizes the recycle loop of return activated sludge, a key operational feature that keeps the microbial community dense and effective. Source
It relies on oxygen and carefully managed microbial communities to remove dissolved and fine suspended organic wastes, producing clearer effluent and concentrated sludge for further handling.
Purpose of secondary treatment
Secondary treatment targets the organic pollution that remains after primary settling, especially dissolved and very small particulate organics that would otherwise deplete oxygen in receiving waters.
Main goals:
Lower biochemical oxygen demand (BOD) and remaining suspended solids
Convert biodegradable organics into carbon dioxide and new microbial biomass (sludge)
Prepare wastewater for later polishing and disinfection steps (not part of this subsubtopic)
Core concepts: microbes + oxygen
Secondary treatment works because microorganisms use organic matter as “food” (energy and carbon source). In most modern systems, the key pathway is aerobic decomposition, which requires a steady supply of dissolved oxygen.
Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen microorganisms need to biologically decompose organic matter in water over a set time; higher BOD indicates more biodegradable pollution.
Aeration is added to prevent oxygen from becoming the limiting factor for microbial activity, allowing faster and more complete breakdown of organics.
Aeration: The mechanical addition of air (or pure oxygen) to wastewater to raise dissolved oxygen and keep microorganisms mixed with the waste they are decomposing.
Common secondary treatment system: activated sludge
The most widely used approach is the activated sludge process, designed to cultivate dense microbial populations that rapidly metabolise organic wastes.
Activated sludge: A mixture of wastewater and concentrated microbial biomass (often forming clumps called floc) used to biologically break down organic matter under aerated conditions.
Major components and flow (typical)
Schematic of the activated sludge secondary-treatment loop, highlighting the aeration tank (bioreactor) followed by a clarifier where biomass settles. It explicitly shows the return activated sludge (RAS) stream that maintains a high microbial concentration and the waste sludge stream removed for further handling. Source
Aeration basin (bioreactor)
Wastewater is mixed with activated sludge (microbes)
Diffusers or surface aerators add oxygen and maintain mixing
Secondary clarifier (settling tank)
Microbial floc settles out as secondary sludge
Clearer treated water exits from the top
Sludge recycling
A portion of settled biomass is returned to the aeration basin (return activated sludge, RAS) to maintain a high microbe concentration
Excess biomass is removed as waste activated sludge (WAS)
What “breakdown” produces
Secondary treatment converts organic matter into:
Carbon dioxide (CO₂) from microbial respiration
Inorganic sludge (as emphasised in the syllabus wording) meaning the stabilized, less-putrescible solids and microbial biomass produced after much of the easily degradable organic fraction has been consumed
Why aeration increases decomposition rate
Aeration increases decomposition by:
Preventing low-oxygen conditions that slow aerobic microbes
Increasing contact between microbes and dissolved organics via mixing
Supporting dense microbial populations that can respond to variable inflows
Operational factors that affect performance
Even without memorising engineering details, AP Environmental Science students should connect treatment outcomes to biological conditions.
Dissolved oxygen and mixing
Too little oxygen:
Slower decomposition, odours, poorer settling, higher BOD in effluent
Adequate oxygen + mixing:
Faster conversion of organics to CO₂ and biomass
More stable treatment under changing flow
Temperature and toxicity
Colder temperatures generally slow microbial metabolism, reducing treatment efficiency.
Toxic inputs (e.g., industrial solvents, heavy disinfectants) can inhibit or kill microbes, causing a sudden drop in performance.
Sludge production as a tradeoff
Secondary treatment moves pollution from water to biomass:
Less organic load in the effluent
More sludge that must be treated, stabilized, and disposed of or reused elsewhere in the facility’s overall process
How secondary treatment success is evaluated (conceptually)
Performance is inferred from measurable water-quality changes across the process:
Decreased BOD from influent to effluent
Reduced cloudiness and fine solids after clarification
Stable settling behaviour of biomass in the secondary clarifier
FAQ
Operators adjust blower output and diffuser airflow based on dissolved oxygen probes.
Common controls include variable-speed blowers and on/off cycling during low-flow periods.
Sludge age (solids retention time) is the average time biomass stays in the system.
It affects microbial community structure, settling behaviour, and how completely organics are stabilised.
Poor settling can occur when filamentous bacteria dominate, creating “bulking” floc.
This can increase effluent turbidity and carry microbes out with treated water.
Activated sludge keeps microbes suspended and aerated in a mixed tank.
Trickling filters grow microbes as a biofilm on media while wastewater trickles past, with aeration occurring through airflow in the filter.
Oxygen transfer into water is energy-intensive, so blowers run continuously or near-continuously.
Energy demand increases with higher BOD loads and stricter effluent targets.
Practice Questions
State what secondary treatment does and explain how aeration affects it. (2 marks)
Secondary treatment uses bacteria/microorganisms to break down organic matter (1)
Aeration adds oxygen, increasing the rate of bacterial decomposition/respiration (1)
Describe the activated sludge secondary treatment process, including the role of the aeration basin, secondary clarifier, and sludge recycling, and link each to how organic matter is removed. (6 marks)
Aeration basin mixes wastewater with microbial biomass and supplies oxygen (1)
Microbes biologically break down dissolved/fine organic matter, producing and biomass/sludge (1)
Secondary clarifier allows microbial floc/solids to settle, separating treated water (1)
Return activated sludge is recycled to maintain a high concentration of microbes in the aeration basin (1)
Excess sludge is removed as waste activated sludge to prevent over-accumulation (1)
Overall outcome: lowered BOD/organic load in effluent due to biological decomposition and solids separation (1)
