What is a Bacteria

Bacteria are microscopic, single-celled organisms that play an essential role in the treatment of wastewater. In the context of sewage systems and treatment plants, bacteria are the primary agents responsible for breaking down organic pollutants in effluent. These microorganisms naturally consume, digest, and convert waste materials into simpler compounds, helping to purify water as it moves through treatment processes.

Without bacteria, modern sewage treatment would be vastly more complex, expensive, and energy-intensive. They are the backbone of biological treatment systems, working silently and continuously to reduce pollution, protect public health, and support environmental sustainability.

What Are Bacteria?

Bacteria are among the most abundant and diverse forms of life on Earth. They exist in virtually every environment, from the depths of oceans to the inside of the human digestive tract. While many people associate bacteria with disease, most bacterial species are harmless or beneficial. In wastewater treatment, bacteria perform vital functions by metabolising organic matter, removing nutrients, and stabilising waste solids.

Bacteria involved in wastewater treatment are broadly classified into:

  • Aerobic bacteria: These require oxygen to survive and function. They are commonly used in treatment processes that include mechanical aeration, such as activated sludge systems. 
  • Anaerobic bacteria: These operate in oxygen-free environments, breaking down waste in septic tanks, anaerobic digesters, and sludge stabilisation systems. 
  • Facultative bacteria: These can adapt to both aerobic and anaerobic conditions, making them useful in variable environments. 

These microbial communities work in synergy, forming complex ecosystems that adapt to the type of waste, temperature, pH levels, and the availability of nutrients and oxygen.

Role of Bacteria in Wastewater Treatment

Bacteria are at the core of most biological treatment stages in a wastewater treatment plant. Their primary function is to consume the biodegradable components of sewage—organic matter such as proteins, fats, carbohydrates, and other dissolved solids.

This microbial digestion process occurs in several stages, depending on the type of treatment system employed. Key processes involving bacteria include:

Primary Treatment Support

Although primary treatment mostly involves mechanical separation of solids from liquids, bacteria can already start acting on organic materials that remain in suspension or on surfaces of settled solids. This is especially relevant in older or combined systems where some biological activity takes place before formal secondary treatment.

Secondary Treatment

This is where bacteria take the lead. In systems such as activated sludge tanks, trickling filters, biofilters, and rotating biological contactors (RBCs), bacteria are supplied with oxygen and a steady flow of effluent. As the bacteria digest the organic waste, they grow and multiply, forming flocs or biofilms. These flocs settle out as sludge, while the cleaned water continues to the next treatment stage.

Key bacterial actions include:

  • Oxidising organic carbon: Converting carbon-based compounds into carbon dioxide and water. 
  • Nitrification: Aerobic bacteria convert ammonia into nitrites and then into nitrates. 
  • Denitrification: In anoxic zones, bacteria convert nitrates into nitrogen gas, which is released harmlessly into the atmosphere. 
  • Phosphorus removal: Certain bacteria, under specific conditions, absorb phosphorus from wastewater and store it intracellularly. 

Tertiary and Sludge Treatment

In advanced stages, bacteria continue to help by digesting residual organics and stabilising biosolids. Anaerobic bacteria are often used in digesters to reduce the volume and odour of sludge, while producing useful biogas (methane) as a by-product.

Types of Treatment Systems Relying on Bacteria

Many wastewater systems are specifically designed to cultivate and support bacterial colonies. Each system creates the right environment for bacteria to thrive and perform their functions effectively. Common bacterial-based treatment technologies include:

  1. Activated Sludge Process This method uses aeration tanks where bacteria remain suspended in the effluent as flocs. Oxygen is constantly supplied through blowers or diffusers, enabling rapid digestion of organic matter. The mixed liquor is then settled in a secondary clarifier. 
  2. Trickling Filters Effluent is sprayed over a bed of stones or synthetic media coated with bacterial biofilm. As the liquid trickles down, bacteria degrade the waste. 
  3. Rotating Biological Contactors (RBCs) These use rotating discs partially submerged in wastewater. Bacteria grow on the discs, alternating between oxygen exposure and submersion to facilitate treatment. 
  4. Sequencing Batch Reactors (SBRs) A time-based variation of the activated sludge process, where treatment steps occur in a single tank through carefully timed cycles of filling, aeration, settling, and decanting. 
  5. Constructed Wetlands and Reed Beds Natural systems where bacteria grow in the root zones of aquatic plants, treating wastewater in a low-energy, ecologically integrated manner. 
  6. Septic Tanks Anaerobic bacteria digest organic waste in small-scale systems, typically serving single properties or remote locations. 

Environmental Factors Affecting Bacterial Activity

The efficiency of bacteria in treating wastewater depends on several environmental and operational factors. For a treatment system to function correctly, conditions must be optimised to support bacterial health and activity:

  • Temperature: Most bacteria involved in treatment are mesophilic, thriving between 20°C and 40°C. Below 10°C, activity slows considerably. 
  • pH: The ideal range for most treatment bacteria is between 6.5 and 8.5. Extremes in pH can inhibit bacterial enzymes. 
  • Oxygen concentration: Aerobic systems must maintain sufficient dissolved oxygen levels (typically 2–3 mg/L) to support bacterial respiration. 
  • Nutrient balance: Bacteria require a balanced supply of carbon, nitrogen, and phosphorus. Imbalances can limit growth or lead to unwanted by-products. 
  • Toxic substances: Chemicals such as disinfectants, solvents, or heavy metals can kill or inhibit bacterial populations, disrupting the treatment process. 

Operators must regularly monitor these parameters and adjust system inputs or conditions to ensure optimal bacterial performance.

Importance of Bacteria in On-site and Small-scale Systems

In domestic and rural settings, bacterial treatment is just as critical, even if on a smaller scale. Septic tanks, package treatment plants, and bio-digesters rely on microbial activity to reduce pollutant loads before effluent is released to soakaways or watercourses.

Some homeowners may use bacterial additives or “starter cultures” to boost the bacterial population, particularly after chemical use, system clean-outs, or periods of low activity. While results vary, maintaining a healthy bacterial ecosystem remains essential to avoid odours, blockages, or system failure.

Maintenance and Support of Bacterial Populations

To keep a biological treatment system working effectively, the bacterial colonies must be protected and maintained. This includes:

  1. Avoiding the discharge of harmful substances Products such as bleach, anti-bacterial cleaners, paint, or pesticides can kill beneficial bacteria. 
  2. Ensuring appropriate loading rates Overloading a system with high volumes or concentrations of waste can overwhelm the bacterial community, leading to incomplete treatment. 
  3. Routine monitoring and desludging Removing excess solids and monitoring parameters such as biological oxygen demand (BOD), ammonia levels, and dissolved oxygen helps maintain a balanced ecosystem. 
  4. Temperature and seasonal adjustment In colder climates, additional insulation or operational changes may be needed to support microbial activity during winter months. 

Conclusion

Bacteria are the unseen workforce of the wastewater treatment world. Through their metabolic processes, they break down pollutants, stabilise waste, and transform contaminated water into an effluent that is safe for discharge or reuse. Whether in municipal treatment plants or private septic systems, these microorganisms are indispensable to achieving environmental compliance, public health goals, and sustainability objectives.

For engineers, operators, and homeowners alike, understanding the role of bacteria—and how to support their function—is fundamental to maintaining an effective and efficient treatment system. From the smallest domestic bio-unit to the largest activated sludge facility, bacteria are at the heart of modern sanitation.