1. What are bacterial flocs?

In the field of microbiology, it is customary to refer to the bacterial masses formed by the genus Actinomycetes as bacterial flocs. In the field of water treatment engineering, all flocs formed by the mutual flocculation and integration of flocculating bacteria with capsules or mucus are called flocs.

2. The role of bacterial flocs

1. Adsorb and oxidize and decompose organic substances; Bacterial flocs are the form in which bacteria exist. Bacteria account for 99% of the total microorganisms in activated sludge, reaching 10^7 to 10^8 per ml. They are the main force in biological treatment. Once bacterial flocs are affected and damaged by various factors, the removal rate of organic matter by the activated sludge process will significantly decrease, or even have no removal capacity at all.

2. The adsorption and decomposition of organic matter by bacterial flocs provide a favorable living environment for protozoa and micrometazoa. For instance, the bacterial flocs themselves provide attachment sites for protozoa and micrometazoa. The bacteria in the bacterial flocs can remove toxins and serve as food for animals themselves.

3. It has an indicating function; The performance of aerobic activated sludge can be measured by the color, transparency, quantity, particle size and tightness of the structure of the bacterial flocs. If the newly formed bacterial flocs are light in color, colorless and transparent, and have a tight structure, it indicates that the bacterial flocs have vigorous vitality, strong adsorption and oxidation capabilities, and strong regeneration ability. The aged bacterial flocs are dark in color, loose in structure, weak in activity, and have poor adsorption and oxidation capabilities.

3. Formation of bacterial flocs

1. The adhesive effect of viscous polysaccharides

Many bacteria can secrete sticky polysaccharide substances that adhere to the cell wall, forming what is called a capsule. When these capsules adhere to each other, they form a large capsule shared by many bacteria. When bacteria enter old age, the amount of sticky polysaccharide polymers secreted by them increases, further accelerating the enlargement of the large bacterial capsule, thus forming the rudimentary form of bacterial flocs.

2. The linking effect of cellulose-based polysaccharides

When the bacterial flocs were observed under an electron microscope, it was found that while the large capsule enlarged, many substances similar to cellulose networks appeared on its outer side. It was analyzed that the components of these substances were also polysaccharides. They may either be the secretions of bacteria or the evolutionary products of viscous polysaccharides. They not only have adhesive properties but also have the function of entanglement and connection. Under their influence, various bacteria are densely distributed in these grid-like fibers. As the quantity of these cellulose-like polysaccharides keeps increasing, true bacterial flocs are formed.

4. The biological significance of bacterial flocs

A stable bacterial symbiosis has been formed

The macromolecular organic substances adsorbed by the bacterial flocs are first hydrolyzed and absorbed by the surface bacteria. The products after metabolism are not completely degraded and serve as food for the bacteria below. On the other hand, as granulation occurs from the surface to the inside, the local oxygen concentration gradually decreases, and bacteria with different requirements for oxygen concentration can each find the most suitable position.

2. Enhance the ability of bacteria to resist environmental changes and the predation of micro-animals

Under the shielding effect of multiple layers of bacteria, the microenvironment in the bacterial flocs is much more stable than that in the solution. The particles of the bacterial flocs are larger, and protozoa can only consume a very small part of the surface bacteria, thereby enhancing the ability of most bacteria to resist the predation of micro-animals.

3. The bacterial flocs also store carbon sources for bacteria

The structural polysaccharides in bacterial flocs are not only their skeletons but also the carbon storage sources for the bacteria. When nutrients in the environment are scarce, these polysaccharides are utilized by bacteria as carbon sources. Although the bacterial flocs gradually disintegrate at this time, to a certain extent, it delays the decline of bacteria. Therefore, the formation of bacterial flocs is a symbiotic product of bacteria to maintain common interests and has significant biological significance for the survival of bacteria.

5. Observation of bacterial flocs

Bacterial flocs are the main form of bacteria in wastewater treatment. In some sludges that are not suitable for the growth of protozoa, the treatment effect can be judged by observing the size and quantity of bacterial flocs. Through microscopic observation, we can determine the properties of bacterial flocs based on the following factors:

1. The quantity of bacterial flocs

Counting based on the current situation, it is determined by the density of the bacterial flocs within a certain volume. Affected by factors such as sampling and sludge concentration, it is generally difficult to conduct a correct analysis. Under normal circumstances, no description is given, and only records are made when significant changes occur.

2. The shape of the bacterial flocs

According to the shape of the bacterial flocs, they are described as four forms: spherical, irregular, open and closed. True spherical flocs do not exist in activated sludge. Usually, nearly spherical bacterial flocs are referred to as "spherical". If the shape is very different and spherical, it is called "irregular shape", at which time there is a large protrusion on the side of the bacterial floc. In a closed bacterial floc, there is almost no open space. On the contrary, in an open bacterial floc, a part of the bacterial floc is clearly separated from another part through one space.

3. The compactness of the bacterial flocs

Tightness is expressed as weak or strong. In weakly compact bacterial flocs, the binding of bacterial cells is very low and lacks a tight center. Such bacterial flocs are easily damaged when the side of the cover slip is lightly pressed. There is no clear boundary between the bacterial floc and the liquid because there are many cells at its edge that are both in the liquid and in the bacterial floc. It is usually obvious that there are many dispersed substances.

In compact bacterial flocs, the combination of bacterial cells is strong, and there is a clear boundary between the bacterial flocs and the liquid. Sometimes, compact bacterial flocs form large, solid granular flocs. When observed under a microscope, due to the large particle size and low light transmittance of the sludge, dark images are often presented.

4. The size of the bacterial flocs

It is classified by diameter into three types: large (d>500μm), medium (150μm<d<500μm), and small (d<150μm). The diameter is generally determined by the edge where the bacterial flocs are farthest apart.

5. Composition of bacterial flocs

The composition of bacterial flocs mainly refers to: the amount of aged sludge, the shape and size distribution of bacterial flocs, whether there are inorganic particles and non-biological organic particles, color, etc.

The degree of sludge aging refers to the proportion of dead bacterial cells in activated sludge. The new bacterial flocs are light in color, colorless and transparent, with a compact structure, vigorous vitality, and strong adsorption and oxidation capabilities. The aged bacterial flocs are dark in color, loose in structure, weak in activity, and have poor adsorption and oxidation capabilities. Adjust the amount of excess sludge discharged based on the observed degree of sludge aging to keep the bacterial flocs in the optimal active state.

The shapes and sizes of bacterial flocs vary, which to a certain extent reflects the richness of the types of bacteria in the flocs. Flocs with a rich variety have a higher resistance to shock when the water quality changes. The color of the bacterial flocs turning black might be due to insufficient dissolved oxygen in the aeration tank. If the color of the bacterial flocs turns pale and white, it may be due to excessive dissolved oxygen in the aeration tank or low influent concentration and load. The microorganisms in the sludge oxidize themselves due to lack of nutrients.