The invisible motor of sanitation: how microorganisms clean our waters

Modern basic sanitation depends on an invisible army working uninterruptedly in the tanks and reactors of treatment plants. Far from being merely a mechanical or chemical process, wastewater depollution is, fundamentally, a biological operation carried out by billions of microorganisms. Understanding the dynamics of these microscopic beings reveals how sanitary engineering manages to mimic and accelerate nature's own self-purification cycles.

The microscopic ecosystem of purification tanks

In the biological stages of sewage treatment plants, the environment is designed to maximize the activity of a complex ecological community. This community is not static; it is a dynamic food web where different species compete, cooperate, and interact continuously. The heart of this system lies in the formation of the so-called biological floc, a macroscopic structure composed of bacteria, protozoa, fungi, and extracellular polymers.

Bacteria are absolute protagonists and constitute the largest share of active biomass. They use the organic matter dissolved in the sewage as a source of food and energy. For treatment to be efficient, these bacteria cannot remain dispersed in the water, as it would be impossible to separate them from the treated effluent. Nature solves this problem through the secretion of extracellular polymeric substances, a sort of biological glue, aggregating thousands of bacterial cells into small flocs that have sufficient density to settle.

Around and inside these flocs, a protozoan fauna develops and plays a crucial regulatory role. Ciliated protozoa, which can be fixed or free-living, feed on the bacteria that failed to aggregate into the floc. By consuming these dispersed bacteria, protozoa clarify the water, reducing the turbidity of the final effluent. The presence of different types of protozoa also serves as an excellent biological indicator of system health. A predominance of stalked ciliates indicates a mature, well-oxygenated, and highly efficient sludge, while an abundance of flagellates suggests organic overloading or low oxygen availability.

In addition to protozoa, more complex multicellular organisms, such as rotifers and nematodes, inhabit the more mature phases of treatment. These small animals feed on debris and smaller bacterial flocs, helping to polish the water and consolidate the physical structure of the sludge. Fungi, although less desirable in conventional systems due to their tendency to form filamentous structures that hinder settling, also participate in the degradation of complex organic compounds, especially in industrial effluents with acidic characteristics or deficiencies in essential nutrients. All this biodiversity works synergistically, transforming complex pollutants into simple, inert substances.

The engineering behind aerobic processes

Aerobic sewage treatment processes are based on the activity of microorganisms that require free oxygen to perform their metabolic functions. The most widely used method globally is the activated sludge system, developed in the early 20th century by British engineers who realized that continuous aeration of sewage stimulated the growth of a biomass highly efficient in removing pollutants.

In an activated sludge system, raw sewage, after passing through preliminary bar screening and grit removal treatments, is introduced into an aeration tank. In this location, mechanical blowers or submerged air diffusers constantly introduce oxygen. Under these conditions, aerobic bacteria perform cellular respiration, oxidizing carbonaceous organic matter. Organic carbon is converted into carbon dioxide and water, while a portion is assimilated for the synthesis of new bacterial cells, generating more biomass.

The great efficiency of the activated sludge system lies in biomass recirculation. After the contact period in the aeration tank, the mixture of sewage and microorganisms, known as mixed liquor, flows to the secondary clarifier. In this settling tank, water velocity is drastically reduced, allowing biological flocs to deposit at the bottom by gravity. The clarified water, now free of most of the pollutant load, overflows the surface and proceeds to disinfection or discharge. The concentrated sludge at the bottom of the clarifier is divided: a considerable part is pumped back to the beginning of the aeration tank to maintain a high concentration of microorganisms, while the excess sludge, generated by bacterial growth, is discarded for processing and dewatering.

Another established aerobic alternative is trickling filters. Unlike activated sludge, where biomass grows in suspension, in trickling filters microorganisms attach to a stationary support medium, such as crushed stone or structured plastic modules. Sewage is distributed uniformly over the top of the filter and trickles by gravity through the support media. As water passes, microorganisms adhered to the support consume organic matter. Oxygen enters the system naturally through structural openings, eliminating the need for high-power mechanical blowers, which makes this process energetically more economical, although it requires a larger physical footprint for implementation.

The anaerobic route and the bioenergy of biogas

Unlike aerobic systems, anaerobic processes occur in the absence of free oxygen. This metabolic route is advantageous from an energy and operational standpoint in warm climate countries. Anaerobic digestion is a complex process that occurs in multiple sequential stages, each conducted by specific groups of microorganisms that strictly depend on the product of the previous stage.

The first stage is hydrolysis, where hydrolytic bacteria secrete extracellular enzymes to break down complex, insoluble polymers, such as proteins, carbohydrates, and lipids, into smaller, soluble molecules like amino acids, simple sugars, and fatty acids. Next, in the acidogenesis phase, fermentative microorganisms convert these monomers into intermediate organic compounds, including volatile fatty acids, alcohols, lactic acid, alongside carbon dioxide and hydrogen.

The third stage is acetogenesis, in which acetogenic bacteria transform the products of acidogenesis into acetic acid, carbon dioxide, and molecular hydrogen. Finally, methanogenesis takes place, the most sensitive phase of the process, carried out by methanogenic archaea. These strictly anaerobic organisms use acetic acid or hydrogen and carbon dioxide to produce methane and water. Any thermal or acidity imbalance can inhibit methanogenic archaea, leading to the accumulation of volatile fatty acids and the consequent acidification of the reactor, which ceases biogas production and compromises treatment efficiency.

In Brazil, the use of upflow anaerobic sludge blanket (UASB) reactors has consolidated as one of the main solutions for domestic sewage treatment. In these reactors, sewage enters from the base and flows vertically through a dense sludge blanket composed of highly active microbial granules. As effluent passes through this blanket, microorganisms digest organic matter. At the top of the reactor, a three-phase separator directs the generated biogas for capture, allows biological solids to settle back into the sludge blanket, and releases treated effluent at the upper part.

The great advantage of anaerobic reactors in Brazilian territory is the high average temperature throughout the year. Since anaerobic bacteria and archaea exhibit higher metabolic activity at elevated temperatures, the tropical climate dispenses with the need for artificial reactor heating systems, making the process extremely economical and with very low electrical energy demand when compared to conventional aerobic systems.

Biological nutrient removal and the prevention of eutrophication

Simply removing organic load is not enough to preserve water bodies. Effluents rich in nitrogen and phosphorus cause eutrophication. This phenomenon triggers the uncontrolled growth of algae and cyanobacteria, which block sunlight and deplete dissolved oxygen in the water as they die and decompose, resulting in mass fish kills and severe degradation of the aquatic ecosystem.

To prevent this environmental disaster, modern treatment plants use advanced biological processes for nutrient removal. Nitrogen present in domestic sewage is predominantly in the form of ammonia. Its biological removal occurs in two distinct steps: nitrification and denitrification. Nitrification is a strict aerobic process carried out by chemoautotrophic nitrifying bacteria. In the first phase, specific bacteria oxidize ammonia to nitrite. Subsequently, other bacterial groups oxidize nitrite to nitrate. This process consumes a large amount of oxygen and water alkalinity.

Once converted into nitrate, nitrogen must be transformed into inert gas to be removed from the liquid medium. This occurs in the denitrification stage, carried out under anoxic conditions, meaning environments where dissolved oxygen is absent but nitrate is present. Under these conditions, facultative heterotrophic bacteria use nitrate as the final electron acceptor in their respiratory chain, reducing nitrate to nitrogen gas, which naturally escapes into the atmosphere, where it is already the major component of air.

Biological phosphorus removal, in turn, is based on the action of phosphorus-accumulating organisms. These microorganisms are subjected to alternating cycles of anaerobic and aerobic conditions. In the anaerobic zone, in the absence of oxygen and nitrate, these bacteria consume volatile fatty acids and release phosphorus stored in their cells into the liquid medium. Upon entering the subsequent aerobic zone, these same microorganisms undergo a metabolic stimulus that makes them absorb a significantly greater amount of phosphorus than previously released, storing it as intracellular polyphosphate. Phosphorus is then effectively removed from the system when excess sludge rich in these microorganisms is wasted from the treatment plant.

Operational challenges and microbial population dynamics

Maintaining the balance of a microbial community composed of billions of individuals from hundreds of different species is one of sanitary engineering's greatest challenges. As it is a living biological system, it is highly sensitive to environmental and operational variations. Any sudden change in incoming sewage characteristics can destabilize the process and compromise treated water quality.

One of the most common and feared problems in activated sludge treatment plants is sludge bulking (filamentous bulking). This phenomenon occurs when filamentous bacteria grow disproportionately compared to floc-forming bacteria. These long filaments project outward from the flocs, acting as micro-parachutes that prevent proper sludge settling in the secondary clarifier. As a result, sludge does not concentrate at the bottom and ends up overflowing alongside treated water, compromising system efficiency. Excessive filament growth is usually triggered by factors such as low dissolved oxygen concentration in the aeration tank, a low food-to-microorganism ratio, or nutrient deficiencies like nitrogen and phosphorus.

Another critical challenge is toxic shock caused by clandestine industrial discharges into the domestic sewage network. Heavy metals, organic solvents, agricultural pesticides, or high concentrations of disinfectants and antibiotics can severely inhibit or even decimate the treatment plant's microbiota. When this occurs, the biological process ceases almost entirely, taking weeks for the microorganism population to recover and re-establish operational efficiency.

To mitigate these risks, operators of modern treatment plants use constant biological monitoring tools. Microscopic sludge analysis allows for the early identification of filamentous bacteria or reduced ciliated protozoa activity, enabling rapid adjustments in aeration rates, sludge wasting, or nutrient dosing. In high-tech systems, molecular biology techniques based on genetic sequencing are employed to map microbial community composition with precision, allowing operators to predict operational behaviors and optimize the removal of specific pollutants.

The future of sanitation and the circular economy

The traditional view of sewage treatment plants as simple depollution units is being replaced by the modern concept of resource recovery facilities. In this new paradigm, sewage is no longer viewed as an undesirable waste and is instead considered a valuable source of water, energy, and nutrients, made viable almost entirely by the metabolic activity of microorganisms.

Reclaimed water production is one of the pillars of this transformation. Through the combination of advanced biological processes with membrane filtration systems, it is possible to produce high-quality water suitable for non-potable purposes, such as agricultural irrigation, street washing, industrial boiler cooling, and manufacturing processes. This reduces pressure on drinking water sources, preserving water resources for public supply.

In the energy field, harnessing biogas generated in anaerobic digestion represents an extraordinary opportunity for sanitation sector sustainability. Recovered methane can be burned in cogeneration engines installed at the treatment plant itself to generate electricity and heat. In many large installations, energy produced from biogas is sufficient to supply the entire operational demand of the treatment system, making the plant energetically self-sufficient or even generating surpluses for the electrical distribution grid. Furthermore, biogas purification for biomethane production paves the way for its use as a clean vehicular fuel, substituting petroleum derivatives.

Excess biological sludge, after undergoing sanitization and thermal or chemical stabilization processes, turns into a biosolids product of high agronomic value. Rich in organic matter, nitrogen, and phosphorus, this material can be safely applied in agriculture as a soil conditioner and organic fertilizer, closing the nutrient loop and reducing reliance on non-renewable mineral fertilizers.

The advancement of basic sanitation in Brazil, driven by new universalization guidelines, demands the adoption of efficient, robust, and economically viable technologies. By placing microorganisms at the center of the engineering strategy, the country not only protects public health and preserves its rivers and beaches, but also positions itself at the vanguard of the circular economy, proving that the smallest scale of life is the key to solving our time's greatest environmental challenges.