Wastewater treatment is a sophisticated process that cleans water used in homes and industry before it is returned to the environment. This purification process is a necessity for public health and environmental protection, preventing pollution of natural water bodies. At the core of this engineering effort are various microorganisms, primarily bacteria, which act as natural recyclers to break down contaminants. These microscopic agents perform the heavy lifting, transforming harmful substances into simpler, less damaging compounds. Biological methods are widely used because they offer a sustainable and effective way to clean water. The role of bacteria is so central that the biological stage is responsible for the majority of pollutant removal in modern treatment plants.
The Biological Necessity
The biological treatment phase is used because of Biological Oxygen Demand (BOD). BOD represents the amount of oxygen that microorganisms consume while breaking down the organic matter dissolved in the wastewater. If untreated water containing a high load of organic matter were released directly into a river, the bacteria present would rapidly consume the organic material. This rapid consumption would use up the dissolved oxygen needed by fish and other aquatic life. The treatment plant uses bacteria to consume this organic matter in a controlled environment instead. By breaking down the pollutants into simpler compounds like carbon dioxide and water, the bacteria significantly reduce the water’s BOD before discharge. This process mitigates the risk of oxygen depletion and subsequent environmental damage in the receiving waterways.
Types of Bacteria and Their Environments
The biological treatment stage relies on a diverse microbial community classified by how they handle oxygen.
Aerobic Bacteria
Aerobic bacteria require free, dissolved oxygen to metabolize organic pollutants. These organisms are the backbone of systems like the activated sludge process, where air is intentionally introduced to create an oxygen-rich environment. Examples of these bacteria include species from the Pseudomonas and Bacillus genera.
Anaerobic Bacteria
Anaerobic bacteria thrive in environments completely devoid of oxygen. They utilize alternative metabolic pathways to break down organic compounds, often in specialized vessels called anaerobic digesters. These processes are slower but can produce biogas, primarily methane, which can be harnessed for energy.
Facultative Bacteria
Facultative bacteria represent a versatile group that can switch their metabolism, functioning with or without dissolved oxygen. They adapt to fluctuating conditions, using oxygen when available but switching to other electron acceptors, such as nitrate, when oxygen levels drop.
Key Microbial Processes
Carbonaceous BOD Removal
The primary function of the bacterial community is the degradation of carbon-based organic matter. Heterotrophic bacteria consume the organic compounds in the wastewater as a food source, converting them into new cellular biomass, carbon dioxide, and water. This fundamental step is responsible for the bulk reduction of organic pollution and the resulting decrease in the water’s oxygen demand. A significant portion of this consumed organic material is incorporated into the microbial floc, which is then separated from the clean water.
Nitrogen Removal
Bacteria also perform the complex task of Nitrogen Removal, which is essential because excess nitrogen can cause excessive algae growth, or eutrophication, in natural waters. This is a two-step process starting with nitrification, which occurs in oxygen-rich zones. Specific autotrophic bacteria, such as Nitrosomonas, first convert ammonia into nitrite, and then bacteria like Nitrobacter oxidize the nitrite into nitrate.
The second step, denitrification, requires an environment where oxygen is absent, known as anoxic conditions. Denitrifying bacteria use the nitrate produced during nitrification as a substitute for oxygen. They reduce the nitrate and nitrite compounds back into harmless nitrogen gas, which simply vents into the atmosphere. This cycle effectively removes nitrogen from the water stream.
Biological Phosphorus Removal
Biological Phosphorus Removal is achieved by Phosphate-Accumulating Organisms (PAOs). PAOs are enriched by cycling the water between anaerobic and aerobic zones. In the anaerobic phase, they release phosphorus while consuming and storing organic acids. When exposed to the aerobic phase, these PAOs rapidly uptake excess phosphorus from the water and store it within their cells as polyphosphate granules, far beyond their basic nutritional needs. The phosphorus is removed from the water when the sludge biomass containing the PAOs is separated and sent for disposal.
Managing the Microbial Ecosystem
Maintaining the health and efficiency of the bacterial population requires careful operational control of the reactor conditions. Dissolved Oxygen (DO) is a heavily controlled parameter, particularly in aerobic treatment basins. The DO level must be sustained through mechanical aeration for optimal function, ensuring rapid degradation of organic matter. Conversely, the DO must be driven down to create the necessary anoxic environment for denitrification.
Wastewater temperature also influences microbial activity, with the mesophilic range (typically 20 to 40 degrees Celsius) supporting optimal performance. The pH level must be kept within a stable range, as bacteria are sensitive to environments that are too acidic or too alkaline.
Sludge Management is a crucial part of controlling the microbial ecosystem. A portion of the settled bacterial flocs, known as activated sludge, is continuously returned to the aeration basin to seed the incoming wastewater. This ensures a consistently high concentration of performing microorganisms for effective water purification.