The basic principle and purification process of activated sludge

The activated sludge method is essentially an artificial enhancement of the self purification effect of natural water bodies, which can remove dissolved and colloidal biodegradable organic matter, suspended solids and other substances that can be adsorbed by activated sludge from wastewater. It has the characteristics of wide adaptability to water quality and quantity, flexible and diverse operation modes, and good controllability, and has become the main body of biological treatment methods.

Basic principle: Activated sludge is a flocculent sludge particle formed by mixing microbial communities such as bacteria, fungi, protozoa, and metazoa with suspended and colloidal substances in wastewater, which has strong adsorption and decomposition ability for organic matter and good settling performance. Due to its biochemical activity, it is called activated sludge.

From the appearance, activated sludge is a flocculent particle like alum flower, also known as a biological flocculent. The diameter of the flocculent is generally 0.02-0.2 mm, and it can immediately condense into larger flocculent particles and sink when left standing.

The color of activated sludge varies depending on the water quality of the sewage, usually yellow or tea brown. When there is insufficient oxygen supply or anaerobic state, it appears black. When there is excessive oxygen supply or insufficient nutrition, it appears gray white and slightly acidic, with a slight soil odor and a slightly moldy odor. Activated sludge has a high moisture content, usually above 99%, and its specific gravity varies depending on the moisture content. The relative density of the mixed liquid in the aeration tank is 1.002-1.003, while the relative density of the reflux sludge is 1.004-1.006. The surface area of activated sludge is generally 20-100 cm2/mL. The solid matter in the activated sludge is less than 1%, which is composed of organic matter and inorganic compound, and the proportion varies with the nature of the raw sewage.

The organic components mainly consist of microbial communities residing in activated sludge, as well as some inert and difficult to be absorbed and utilized by bacteria in the incoming wastewater, known as "difficult to degrade organic matter", and residues from microbial oxidation.

The microbial community of activated sludge is a mixed population mainly composed of aerobic bacteria, with other microorganisms including yeast, actinomycetes, molds, protozoa, metazoa, etc. The bacterial content of normal activated sludge is generally 107-108 pieces/mL, while that of protozoa is around 100 pieces/mL. In activated sludge microorganisms, protozoa feed on bacteria, while metazoa feed on protozoa and bacteria, forming a food chain between them, forming an ecologically balanced biological community. Activated sludge bacteria often exist in the form of bacterial micelles, with fewer in a free state, which gives them the ability to resist adverse external factors. Free bacteria are not easy to settle, but can be preyed upon by protozoa, making the effluent of the sedimentation tank clearer. The inorganic components of activated sludge are all carried in by raw sewage, and the inorganic salts present in microorganisms are negligible due to their small quantity.

In summary, activated sludge is composed of the following four components:

① Microbial population with metabolic activity (M);

② Microbial (mainly bacterial) self oxidation residues (M);

③ Difficult to biodegradable organic matter carried in by raw sewage (M;);

④ Inorganic substances entrained by raw sewage (M;).