Biological Wastewater Treatment: Harnessing Microorganisms for Clean Water

Every drop of wastewater that flows from our homes, factories, and farms carries organic pollutants, nutrients, and pathogens. Getting rid of these contaminants before water re-enters the environment is essential – and the most effective workers for this job are not machines, but microorganisms. Biological wastewater treatment leverages bacteria, algae, protozoa, and other tiny organisms to decompose organic matter, remove nutrients, and render water safe for discharge or reuse. This approach forms the backbone of modern sanitation systems worldwide, from large urban treatment plants to small rural lagoons.

Why microorganisms are central to wastewater treatment 🔗

Biological wastewater treatment works because microorganisms naturally consume organic pollutants as their food source. They metabolize dissolved and suspended organic compounds – sugars, fats, proteins – and convert them into simpler, less harmful end products like carbon dioxide, water, and new cellular material. This process reduces the biochemical oxygen demand (BOD) of wastewater, which is a key measure of how much organic pollution is present.

The main groups of microorganisms involved include bacteria, algae, protozoa, and fungi. Each plays a distinct role in what is essentially a miniature food web within the treatment system.

Bacteria: the primary decomposers 🔗

Bacteria do the heavy lifting in biological treatment. They exist in three broad categories based on their oxygen requirements: aerobic (needing oxygen), anaerobic (functioning without oxygen), and facultative (able to work in both conditions). Aerobic bacteria use dissolved oxygen to break down organic matter, producing carbon dioxide and water as byproducts. Anaerobic bacteria, on the other hand, decompose organics in oxygen-free environments, releasing methane and carbon dioxide. Facultative bacteria switch between aerobic and anaerobic metabolism depending on the availability of oxygen, making them versatile participants in treatment systems.

Beyond carbon removal, specialized bacteria handle nitrogen removal through nitrification and denitrification. Ammonia-oxidizing bacteria and nitrite-oxidizing bacteria convert ammonia first to nitrite and then to nitrate under aerobic conditions. Denitrifying bacteria then convert nitrate into harmless nitrogen gas under low-oxygen conditions. This two-step process is critical for preventing excess nitrogen from entering water bodies and triggering eutrophication.

Algae: oxygen producers and nutrient absorbers 🔗

Algae serve a dual function. Through photosynthesis, they produce oxygen that aerobic bacteria need to do their work. At the same time, algae absorb nutrients such as nitrogen and phosphorus from the wastewater, reducing their concentrations in the effluent. This relationship between algae and bacteria is mutualistic: bacteria generate carbon dioxide that algae use for photosynthesis, while algae supply the oxygen bacteria require for respiration. This natural partnership is particularly important in pond-based treatment systems where mechanical aeration may be limited.

Protozoa and higher organisms: the quality controllers 🔗

Protozoa, such as amoebae and ciliates, feed on bacteria within the treatment system. This grazing activity actually improves effluent quality – it helps control the bacterial population, reduces turbidity, and contributes to clearer treated water. In a healthy activated sludge system, a food chain develops where bacteria feed on the organic material, protozoa feed on the bacteria, and even higher organisms like tardigrades and nematodes act as top-level predators. The presence of these higher organisms is actually an indicator that the treatment process is functioning well.

Activated sludge process: the workhorse of biological treatment 🔗

The activated sludge process is the most widely used biological treatment technology in the world. First developed in 1913 and scaled up in the UK in 1920, this method remains the standard for both municipal and industrial wastewater treatment plants.

How the process works 🔗

The system consists of two key components: an aeration tank and a secondary clarifier. In the aeration tank, wastewater is mixed with a concentrated mass of microorganisms – the “activated sludge.” Air or pure oxygen is pumped in using diffusers or mechanical surface aerators. This oxygen supply fuels the aerobic bacteria, encouraging them to grow and consume the organic pollutants in the wastewater.

As bacteria feed and multiply, they form clumps called flocs. These sticky aggregates of microorganisms settle easily, which is essential for separating the treated water from the biological mass. The mixture of wastewater and sludge, known as mixed liquor, flows from the aeration tank to the clarifier. Here, the flocs settle to the bottom under gravity. The clear water above – now significantly lower in organic pollutants – is discharged as treated effluent.

A portion of the settled sludge is returned to the aeration tank to maintain a sufficient population of microorganisms for treating incoming wastewater. The excess sludge is removed and sent for further processing, such as anaerobic digestion, where bacteria can produce methane biogas that may be used to generate electricity.

Monitoring and control 🔗

Operators keep the process running smoothly by tracking metrics like dissolved oxygen levels, mixed liquor suspended solids (MLSS), and the food-to-microorganism (F/M) ratio. If bacterial populations become too high or too low relative to the organic load, treatment efficiency drops. The balance between incoming organic pollution and microbial capacity is the key to consistent performance.

Aerated lagoons: simple yet effective 🔗

Not every community needs or can afford a full activated sludge plant. Aerated lagoons offer a simpler, lower-cost alternative that is especially popular in rural and small-town settings. According to the U.S. EPA, over 4,500 lagoon-based systems serve communities across the United States, the vast majority of which are in small towns of fewer than 3,000 people.

Design and function 🔗

An aerated lagoon is essentially a large, shallow earthen basin where wastewater undergoes biological oxidation with the help of mechanical aeration devices. These devices serve two purposes: transferring oxygen into the water and mixing the contents to ensure contact between microorganisms, oxygen, and organic matter. There are two common types of aeration – surface aeration, where floating mechanical units churn the water surface, and diffused aeration, where blowers push air through devices at the bottom of the lagoon.

Aerated lagoons come in two main configurations. Complete-mix lagoons have enough aeration energy to keep all solids suspended throughout the basin. Partial-mix (facultative) lagoons provide less energy, allowing solids to settle to the bottom where they undergo anaerobic decomposition, while the upper layers remain aerobic. This dual-zone approach within a single lagoon is highly efficient for handling variable waste loads.

Advantages of aerated lagoons 🔗

These systems are attractive because they are cheaper to build and operate than conventional treatment plants. They require less mechanical equipment, less operator expertise, and can handle fluctuating loads – ideal for communities with seasonal tourism, for example. The tradeoff is that they require more land and typically achieve slightly lower treatment efficiency than activated sludge systems. Aerated lagoons also require less land than facultative lagoons and can be engineered to remove ammonia and phosphorus depending on the operational design.

Anaerobic ponds: treatment without oxygen 🔗

Anaerobic ponds are deep, oxygen-free basins where anaerobic bacteria break down high-strength organic waste. They are commonly the first stage in a series of waste stabilization ponds, designed specifically to handle wastewater with heavy organic loads – often from agricultural operations, food processing facilities, or concentrated municipal sewage.

How they work 🔗

These ponds are typically 2 to 5 metres deep, much deeper than other pond types. The depth is deliberate – it limits oxygen penetration and creates the anaerobic conditions needed for specific bacterial communities to thrive. Anaerobic bacteria use oxygen from compounds like nitrates and sulfides rather than dissolved atmospheric oxygen, producing methane and carbon dioxide as end products.

Suspended solids in the incoming wastewater settle to the bottom of the pond as sludge, where they undergo anaerobic digestion over time. This process can remove up to 60% of the BOD from the wastewater. The pond needs to be de-sludged approximately once every 2 to 5 years, when accumulated solids reach about one-third of the pond volume.

Limitations 🔗

Anaerobic ponds work best in warm climates, as anaerobic bacteria require temperatures above 15°C to function efficiently. In colder regions, decomposition slows significantly. Another concern is odour – the production of hydrogen sulfide and other gases during anaerobic breakdown can create unpleasant smells, which is why these ponds need to be sited away from residential areas.

Facultative ponds: the best of both worlds 🔗

Facultative ponds are perhaps the most versatile type of waste stabilization pond. They combine aerobic and anaerobic treatment within a single basin by maintaining distinct oxygen zones at different depths. The upper layer is aerobic, sustained by atmospheric oxygen diffusion, wind mixing, and – most importantly – photosynthetic oxygen production by algae. The bottom layer is anaerobic, where settled solids decompose without oxygen.

The role of algae and bacteria together 🔗

The interaction between algae and bacteria is central to how facultative ponds function. Algae near the surface use sunlight and carbon dioxide to produce oxygen through photosynthesis. Aerobic and facultative bacteria in the upper layers use this oxygen to break down dissolved organic matter. Meanwhile, in the lower depths, anaerobic bacteria decompose the settled sludge. This layered approach is simple, self-regulating, and remarkably effective.

Facultative ponds are generally 1 to 2.5 metres deep and have longer detention times – anywhere from 5 to 30 days – allowing thorough treatment. When combined with an upstream anaerobic pond, overall BOD removal can reach 75 to 85%.

Primary vs. secondary facultative ponds 🔗

A primary facultative pond receives raw wastewater directly. It handles the initial heavy organic load and can be effective for communities with moderate wastewater strength. A secondary facultative pond receives effluent that has already passed through an anaerobic pond. Since much of the organic load has already been reduced, the secondary pond focuses on polishing the water further and reducing remaining BOD and pathogens.

Advantages for developing regions 🔗

Facultative ponds are widely used in tropical and subtropical countries because they are low-cost, low-maintenance, and energy-efficient. They require no mechanical equipment for aeration, relying instead on natural processes. The main requirement is sufficient land area and adequate sunlight. These ponds are well-suited for rural communities, small towns, and developing regions where technical expertise and energy supply may be limited. The U.S. EPA recognizes lagoon-based systems as sustainable and cost-effective options for smaller communities, provided they are properly designed and operated.

Comparing biological treatment systems 🔗

Each biological treatment approach has its strengths and is suited to different contexts. The activated sludge process delivers the highest treatment efficiency and works well in urban areas with limited land but requires significant energy, skilled operators, and ongoing maintenance. Aerated lagoons offer a middle ground – lower cost and simpler operation than activated sludge, but needing more land. Anaerobic ponds are ideal for pre-treating high-strength waste but cannot function as standalone treatment in most cases. Facultative ponds provide robust treatment with minimal energy input but need large surface areas and are climate-dependent.

In practice, many treatment systems combine these approaches in sequence. A typical configuration might involve an anaerobic pond to reduce the initial organic load, followed by a facultative pond for further biological treatment, and finally a maturation pond for pathogen removal. This staged approach maximizes the strengths of each type while compensating for their individual limitations.

The bigger picture: why biological treatment matters 🔗

Biological wastewater treatment is not just an engineering convenience – it is an environmental necessity. Untreated wastewater contributes to waterborne diseases and can cause severe ecological damage through eutrophication and oxygen depletion in rivers and lakes. By harnessing the natural metabolic capabilities of microorganisms, we can treat wastewater effectively while keeping costs manageable – particularly important for the many small and rural communities that lack the resources for advanced treatment technologies.

As global water scarcity intensifies and environmental regulations become stricter, the role of biological unit processes will only grow. Ongoing research into microbial engineering and optimized pond designs continues to improve efficiency, reduce land requirements, and expand the types of contaminants that can be addressed biologically.

What do you think? Given that biological treatment relies heavily on natural processes and microorganisms, how might climate change – with its shifting temperatures and weather patterns – affect the performance of pond-based systems in your region? And as cities grow denser, can innovations in microbial engineering make compact biological treatment systems viable for large urban populations?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10376923/
2. https://asm.org/articles/2020/april/how-microbes-help-us-reclaim-our-wastewater
3. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.801051/full
4. https://courses.lumenlearning.com/suny-mcc-microbiology/chapter/wastewater-treatment-and-water-purification/
5. https://www.epa.gov/small-and-rural-wastewater-systems/lagoon-wastewater-treatment-systems
6. https://www.atlascopco.com/en-us/compressors/air-blowers-landing/aeration-blowers/aeration-blowers-for-lagoon-aeration
7. https://www.wwdmag.com/wastewater-treatment/article/33005608/what-is-a-wastewater-treatment-lagoon
8. https://sswm.info/factsheet/waste-stabilisation-ponds
9. https://www.sciencedirect.com/topics/chemical-engineering/waste-stabilization-ponds
10. https://sswm.info/taxonomy/term/3932/waste-stabilization-ponds-(wsp)