Secondary Wastewater Treatment: Biological Methods for Pollution Removal

Before wastewater can be safely released into the environment, it must undergo biological treatment to reduce harmful pollutants. Secondary treatment-the stage following primary sedimentation-uses living microorganisms to break down organic matter that would otherwise deplete oxygen levels in rivers and lakes. This biological approach forms the backbone of modern wastewater treatment, converting dissolved and suspended organic pollutants into stable compounds through natural metabolic processes.

Understanding secondary biological treatment 🔗

Secondary biological treatment specifically targets the reduction of biochemical oxygen demand (BOD) and suspended solids in wastewater. BOD measures the amount of oxygen that microorganisms need to decompose organic matter-high BOD levels in discharged water can suffocate aquatic life by depleting dissolved oxygen. According to research published on ScienceDirect, combining primary and secondary treatment can remove over 90% of BOD from wastewater.

Biological treatment systems fall into two main categories: suspended growth systems, where microorganisms float freely in the wastewater, and attached growth systems, where microorganisms form biofilms on fixed media surfaces. Both approaches rely on aerobic bacteria that consume organic pollutants as food, converting them into carbon dioxide, water, and additional biomass. After biological treatment, effluent typically requires disinfection or further tertiary treatment before discharge.

Activated sludge process for BOD removal 🔗

The activated sludge process is the most widely used suspended growth method for municipal and industrial wastewater treatment. Developed over a century ago, this technology remains popular because of its effectiveness and adaptability to various conditions.

How activated sludge works 🔗

The process begins when wastewater enters an aeration tank where air diffusers inject oxygen from the bottom. This aeration serves two critical functions: supplying dissolved oxygen for aerobic bacteria and providing mixing energy to keep microorganisms in contact with organic matter. The microorganisms-primarily bacteria, protozoa, and fungi-consume organic pollutants as their food source, producing more biomass in the process.

As noted in Water Technology, the activated sludge process generates approximately 0.65 pounds of biomass for every pound of BOD removed. This biological mass forms flocs-clumps of bacteria bound together by sticky substances they secrete-which settle readily in subsequent clarification tanks.

The role of return and waste activated sludge 🔗

After aeration, the mixed liquor flows to a secondary clarifier where biological flocs settle by gravity. A portion of this settled sludge, called return activated sludge (RAS), is recycled back to the aeration tank to maintain adequate biomass concentration. Without this recycling, the system would lack sufficient microorganisms to treat incoming wastewater effectively.

Because microorganisms continuously reproduce, excess biomass must be removed as waste activated sludge (WAS). Proper sludge wasting is essential-failure to remove excess solids results in poor treatment performance and solids escaping over the clarifier weir. This process typically achieves 85% to 95% BOD removal from aeration influent.

Trickling filters and biofilters 🔗

Trickling filters represent one of the oldest and most reliable attached growth treatment technologies. Despite their name, these systems do not actually filter wastewater mechanically-instead, they promote biological treatment through microbial biofilms growing on support media.

Design and operation principles 🔗

A trickling filter consists of a tank filled with high surface-area media such as rocks, gravel, plastic, or ceramic materials. Wastewater is distributed over the media surface using rotating sprinkler arms that ensure even coverage. As the water trickles downward through the bed, it contacts the biofilm-a thin layer of microorganisms (typically 0.1 to 0.2 mm thick) attached to the media surfaces.

According to the U.S. EPA, trickling filters are aerobic treatment systems where microorganisms attached to the medium remove organic matter from wastewater. Oxygen reaches the biofilm through natural air circulation in the porous bed or through forced ventilation. As Britannica notes, these systems can remove up to 85% of organic pollutants from sewage.

Sloughing and secondary clarification 🔗

As the biofilm grows thicker, the innermost layers become starved of oxygen and nutrients. This causes periodic sloughing-the detachment of biomass from the media surface. The sloughed material flows with the effluent to a secondary clarifier, where it settles and is removed. Some systems recirculate a portion of the clarified effluent back over the filter to improve distribution and treatment efficiency.

Trickling filters offer advantages over activated sludge systems, including lower energy consumption, simpler operation, and better resistance to shock loads. However, they generally achieve lower treatment efficiencies and are more sensitive to temperature variations.

Rotating biological contactors 🔗

Rotating biological contactors (RBCs) provide another attached growth option for secondary treatment. This technology uses a series of closely spaced plastic discs mounted on a horizontal rotating shaft, partially submerged in wastewater.

Operation mechanism 🔗

The discs-typically 2 to 4 meters in diameter-rotate slowly at 1 to 2 revolutions per minute. Approximately 40% of each disc’s surface area remains submerged at any time. As the discs rotate, microorganisms attached to the surfaces alternately contact wastewater (for food uptake) and atmosphere (for oxygen absorption). This continuous cycling creates ideal conditions for aerobic biological treatment.

A biological slime layer develops on the disc surfaces, similar to trickling filter biofilms. According to sustainable sanitation resources, RBCs offer advantages including simplicity of maintenance, low power consumption, and ability to withstand shock or toxic loads. Multiple disc assemblies arranged in series can achieve both BOD removal and nitrification, with visual color changes indicating the transition from carbon-metabolizing to nitrogen-oxidizing conditions.

Sequencing batch reactors and oxidation ditches 🔗

Unlike continuous-flow systems, sequencing batch reactors (SBRs) perform all treatment steps in a single tank through timed cycles. This fill-and-draw approach offers excellent process control and flexibility.

SBR operating cycles 🔗

As explained by Wastewater Digest, SBRs operate through five basic phases: fill, react, settle, draw, and idle. During the fill phase, wastewater enters the tank containing active biomass from the previous cycle. The react phase involves aeration to promote biological oxidation of organic matter. After sufficient reaction time, aeration stops and solids settle to the bottom. Clarified effluent is then decanted from above the sludge blanket, and the cycle repeats.

SBRs eliminate the need for separate clarifiers and return sludge pumping systems, reducing both footprint and equipment costs. They also accommodate biological nutrient removal through simple adjustments to aerobic, anoxic, and anaerobic phase durations.

Oxidation ditches 🔗

Oxidation ditches are oval or racetrack-shaped channels where wastewater circulates continuously past brush aerators or surface rotors. These extended aeration systems achieve both BOD and nitrogen removal while maintaining simple operation. The looped reactor design promotes efficient mixing and aeration while accommodating varying flow conditions. Oxidation ditches are particularly suitable for locations with available land and where operators have limited specialized training.

Membrane biological reactors 🔗

Membrane bioreactors (MBRs) combine biological treatment with membrane filtration to produce high-quality effluent suitable for water reuse applications. This advanced technology has grown significantly since its introduction in the 1960s.

MBR configuration and advantages 🔗

MBRs integrate microfiltration or ultrafiltration membranes with activated sludge bioreactors. The membranes replace secondary clarifiers, physically separating treated water from biomass. According to PCI Membranes, this approach offers several advantages: up to 50% smaller footprint, effluent free of suspended solids, reduced bacteria and viral content, and minimal disinfection requirements.

MBRs can operate at much higher biomass concentrations (8,000-12,000 mg/L) than conventional activated sludge systems, enhancing treatment efficiency and reducing sludge production. However, membrane fouling remains a significant operational challenge requiring regular cleaning and maintenance.

Stabilization ponds and lagoons 🔗

For communities with available land and limited resources, stabilization ponds offer a low-technology treatment alternative. These large, shallow basins rely on natural processes-sunlight, wind, algae, and bacteria-to treat wastewater over extended detention periods.

Types of stabilization ponds 🔗

Pond systems typically include three types arranged in series. Anaerobic ponds are the deepest (3-4 meters), receiving raw wastewater and allowing solids to settle while anaerobic bacteria digest organic matter. Facultative ponds (1.5-2.5 meters deep) support aerobic conditions near the surface and anaerobic conditions at the bottom, with intermediate zones housing facultative organisms that function in either environment. Maturation ponds provide final polishing and pathogen removal through extended exposure to sunlight.

As noted in WaterWorld, pond systems stabilize organic material through natural processes involving algae-bacteria symbiosis. Algae produce oxygen through photosynthesis, which aerobic bacteria use to decompose organic matter, releasing carbon dioxide that algae then utilize-creating a continuous treatment cycle. These systems are commonly used for small communities and agricultural operations processing livestock wastewater.

What do you think? As cities face increasing water scarcity, should wastewater treatment plants prioritize technologies like MBRs that enable water reuse, even if they cost more to operate? How might emerging smart city initiatives integrate real-time monitoring to optimize these biological treatment processes?

References

1. https://www.sciencedirect.com/topics/chemical-engineering/activated-sludge-process
2. https://en.wikipedia.org/wiki/Activated_sludge
3. https://www.watertechonline.com/wastewater/article/15550311/aerated-activated-sludge-basics
4. https://water.mecc.edu/courses/Env149/lesson7b.html
5. https://www.epa.gov/system/files/documents/2022-10/trickling-filters-factsheet.pdf
6. https://www.britannica.com/technology/trickling-filter
7. https://en.wikipedia.org/wiki/Rotating_biological_contactor
8. https://sswm.info/water-nutrient-cycle/wastewater-treatment/hardwares/semi-centralised-wastewater-treatments/rotating-biological-contactors
9. https://www.wwdmag.com/what-is-articles/article/55237334/what-is-a-sequencing-batch-reactor
10. https://www.newterra.com/technology/tri-oval-oxidation-ditch-system/
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC4931528/
12. https://www.pcimembranes.com/articles/membrane-bioreactors-mbr-for-wastewater-treatment/
13. https://en.wikipedia.org/wiki/Waste_stabilization_pond
14. https://www.waterworld.com/home/article/16192273/introduction-to-wastewater-treatment-ponds