Bacteria | Activated Sludge Process | Wastewater Treatment Plant

BACTERIA

Bacteria are microscopic prokaryotic organisms that feed on nutrients dissolved in water and divide into two equal cells via binary fission. Bacteria come in many different sizes, shapes, types, and forms. Although not all types and forms are relevant in most wastewater treatment plant operations, the following are.

HETEROTROPHIC BACTERIA

Organic matter is used by heterotrophic bacteria as an energy source and a carbon source for synthesis. These components are derived from the wastewater constituents. Because organic contaminants make up the majority of contaminants in most domestic wastewater, heterotrophic bacteria predominate in most wastewater treatment plants. Except in anaerobic digesters or anaerobic lagoons specifically designed to operate under anaerobic conditions, all organic matter is degraded by heterotrophs, usually aerobic heterotrophs.

AUTOTROPHIC BACTERIA

Autotrophic bacteria generate energy from inorganic substances and use carbon dioxide as a carbon source during synthesis. These bacteria are more common in wastewater dominated by certain industrial flows than in domestic wastewater. They are, however, an important component of wastewater and should not be overlooked. Certain inorganic matter is best degraded by autotrophs, such as nitrifying bacteria (important in nitrogen removal), sulphur reducing bacteria, and iron reducing bacteria, which are all anaerobic bacteria.

AEROBIC BACTERIA

Aerobic bacteria, or aerobes, require oxygen for synthesis in the same way that humans do. When possible, the bacteria use oxygen bound up in organic matter, but they also use excess dissolved oxygen (DO) directly from the wastewater during synthesis. Wastewater with a sufficient concentration of DO will operate as an aerobic system, indicating that aerobes dominate the bacteria mass, whereas wastewater with insufficient DO will operate as an anaerobic system. Because of the unacceptable odor problems caused by anaerobic operation, it is only used in completely closed portions of the treatment process. Maintaining adequate DO in wastewater for effective aerobic treatment is a critical component of treatment plant operation.

ANAEROBIC BACTERIA

Anaerobic bacteria, also known as anaerobes, synthesize using nitrogen, sulphur, or iron (generally in that order of preference) rather than oxygen. They also produce hydrogen sulphide, as well as other undesirable gaseous byproducts of organic degradation. Although anaerobic organic matter degradation is faster than aerobic degradation, the objectionable by-products are significant enough to discourage their use. Except for phosphorus removal, sludge digestion, or both, the extra time required for aerobic degradation is insufficient to justify the odor problems associated with anaerobic degradation.

FACULTATIVE BACTERIA

Facultative organisms can live in both aerobic and anaerobic environments, but they prefer one over the other. When referring to facultative organisms that prefer aerobic conditions but can function in anaerobic environments, the term facultative aerobes will be used. Facultative anaerobes prefer anaerobic environments but can also thrive in aerobic ones.