Filamentous bacteria are a diverse group of microorganisms defined by their unique physical structure, which is elongated and thread-like. Unlike most bacteria that are spherical or rod-shaped, these organisms grow as long strings of cells, known as filaments or trichomes. This morphology is shared across numerous bacterial phyla, including some species of Cyanobacteria and Actinobacteria. The filamentous architecture allows these microbes to colonize surfaces and form complex, interconnected communities. While this morphology is a successful survival strategy in nature, it can pose significant challenges when these bacteria proliferate in human-engineered systems like water treatment facilities.
Defining the Shape and Growth
The unique morphology of filamentous bacteria is based on a long chain of individual cells joined end-to-end, which is technically called a trichome. This chain forms when cells divide repeatedly in the same direction but fail to separate completely after division. In many species, the entire chain of cells is encased in a protective, non-living sheath, and the term “filament” often refers specifically to a sheathed trichome.
The appearance of the filament varies significantly between species, with individual cells inside the chain ranging in shape from rectangular to oval or discoid. Some species exhibit a complex, tree-like structure known as branching, categorized as either true or false. True branching, seen in genera like Nocardia, occurs when a cell within the filament divides along a plane that is not perpendicular to the main axis, creating a branch where the cellular fluids remain connected to the main body. False branching, exemplified by Sphaerotilus natans, is a simpler process where the trichome breaks within the sheath, and the resulting strand pushes out through the sheath wall. The ability to form these long, often branched, chains provides a greater surface area for nutrient absorption and allows the bacteria to anchor themselves firmly within their environment.
Ecological and Environmental Roles
Filamentous bacteria occupy a wide range of natural habitats, including soils, freshwater lakes, and marine environments, where their morphology supports important ecosystem functions. Their thread-like shape gives them a high surface-area-to-volume ratio, which enhances their efficiency in absorbing low concentrations of nutrients from their surroundings. This characteristic makes them effective decomposers, breaking down complex organic matter in soil and water and returning simpler compounds to the ecosystem.
Certain filamentous species, such as some cyanobacteria, play a significant role in global nutrient cycling by fixing atmospheric nitrogen into biologically usable forms like ammonia. This process enriches the soil and water, making nitrogen available to other organisms. The genus Streptomyces, a filamentous Actinobacteria, is particularly notable for its ecological role in soil, utilizing its thread-like growth to penetrate the soil matrix and search for nutrients. These genera also produce a wealth of secondary metabolites, including many antibiotics used in medicine, such as streptomycin and tetracycline. Their ability to form complex, interconnected networks also helps stabilize soil structure and contributes to biofilm formation in aquatic systems.
The Problem of Sludge Bulking
The problem arises when filamentous bacteria proliferate in the controlled environment of an activated sludge wastewater treatment plant. The activated sludge process relies on the formation of dense, compact microbial clusters called flocs, which settle easily in the final clarifier to separate clean water from the microbial biomass. When filamentous bacteria grow excessively, they extend from the flocs like a tangled net, physically preventing the formation of these dense clusters.
This condition is known as sludge bulking, dramatically increasing the sludge volume index (SVI)—a measure of how well the sludge settles. The tangled, low-density flocs trap water and remain buoyant, causing the sludge blanket to rise and overflow into the treated effluent. This results in poor effluent quality and a loss of valuable microbial biomass, potentially compromising the entire treatment process.
Excessive growth is often triggered by changes in the wastewater environment, such as low concentrations of dissolved oxygen (DO) in the aeration basin or an imbalance in the food-to-microorganism (F/M) ratio. Low DO conditions favor many filamentous species because their high surface area allows them to scavenge oxygen more effectively than floc-forming bacteria. Conversely, high F/M ratios (abundance of food) can favor certain filamentous types, while very low F/M ratios (nutrient starvation) can favor others.
Specific culprits frequently identified in bulking incidents include Microthrix parvicella, associated with the uptake of long-chain fatty acids, and Sphaerotilus natans, which thrives under low DO conditions. These organisms, along with others like Nocardia and Type 0092, compete with beneficial floc-formers, and their thread-like structure is the mechanical cause of the poor settling characteristics. The resulting poor sludge settling is the primary operational challenge filamentous bacteria pose to wastewater facilities worldwide.
Mitigation and Control Strategies
Controlling excessive filamentous growth requires a combination of chemical and operational adjustments designed to favor floc-forming bacteria. A common and rapid chemical approach is chlorination, where a chlorine solution is dosed into the return activated sludge (RAS) line. The chlorine preferentially contacts and damages the exposed filaments extending from the floc, while the protected floc-forming bacteria remain largely viable.
Operational strategies focus on changing environmental conditions that allow the filaments to outcompete floc-formers. Increasing the dissolved oxygen concentration in the aeration basin suppresses species that thrive in low-oxygen environments. Another strategy involves installing selector tanks, which are small, highly mixed zones placed at the head of the aeration basin. These tanks expose incoming wastewater to a high F/M ratio for a short period, allowing fast-growing floc-formers to rapidly absorb food before the slower-growing filamentous organisms can access it. Adjusting the overall F/M ratio and the sludge retention time (SRT) provides a longer-term method for managing the microbial community. For specific filaments like Microthrix parvicella, specialized chemical coagulants, such as polyaluminum chlorides, can be used to prevent the uptake of required fatty acid nutrients.