The Basic Principle And Purification Process Of Activated Sludge
The Basic Principle And Purification Process Of Activated Sludge
The activated sludge method is essentially an artificial enhancement of the self-purification of natural water bodies. It can remove dissolved and colloidal biodegradable organic matter, suspended solids and other substances that can be adsorbed by activated sludge from sewage, and is adaptable to water quality and quantity The characteristics of wide range, flexible and diverse operation modes, and good controllability have become the main body of biological treatment methods.
1. Basic principles
Activated sludge is formed by the mixture of bacteria, fungi, protozoa, metazoa and other microbial groups with suspended substances and colloidal substances in sewage. It has a strong ability to adsorb and decompose organic matter and good settling performance. Sludge particles are called activated sludge because of their biochemical activity.
Properties of activated sludge
From the appearance point of view, activated sludge is flock particles like alum flowers, also known as biological flocs. The diameter of the flocs is generally 0.02-0.2 mm, which can immediately condense into larger flock particles when standing still. Shen. The color of activated sludge varies with the quality of the sewage. It is generally yellow or dark brown. It is black when the oxygen supply is insufficient or in an anaerobic state. When the oxygen supply is too much and the nutrients are insufficient, it is star-grey, slightly acidic, and has a slight soil odor. Entrained-some musty smell. The water content of activated sludge is very high, generally above 99%, and its specific gravity varies depending on the water content. The relative density of the mixed liquid in the aeration tank is 1.002-1.003, while the relative density of the return sludge is 1.004-1.006. Activated The surface area of sludge is generally 20-100 cm2/mL.
Composition of activated sludge
The solid matter in activated sludge is less than 1%, which is composed of organic matter and inorganic matter, and its composition ratio varies with the nature of the raw sewage. The organic component is mainly the microbial population living in the activated sludge, and also includes some inert sewage in the inflowing sewage that is difficult to be ingested and utilized by the bacteria, the so-called "refractory organic matter", and the microbial self-oxidation residues.
The activated sludge microbial population is a mixed group dominated by aerobic bacteria. Other microorganisms include yeast, actinomycetes, molds, protozoa, metazoans, etc. The bacterial content of normal activated sludge is generally 107-108 /mL, protozoa is about 100/mL.
Among activated sludge microorganisms, protozoa feed on bacteria, and metazoans feed on protozoa and bacteria. A food chain is formed between them, forming an ecologically balanced biological group. Activated sludge bacteria often exist in the form of bacterial micelles, which are less in a free state, which makes the bacteria have the performance to resist external unfavorable factors.
Free bacteria are not easy to precipitate, but can be preyed by protozoa, which makes the effluent of the sedimentation tank clearer. The inorganic components of activated sludge are all carried in by the raw sewage. As for the inorganic salts in the microorganisms, the amount is negligible due to the very small amount.
In short, activated sludge is composed of the following four parts:
① Microbial populations with metabolic functions (M);
② Microbial (mainly bacteria) self-oxidation residues (M);
③The refractory organic matter carried by the raw sewage (M;);
④ Inorganic substances carried in by raw sewage (M;). Among them, the active microbial population is the main component of activated sludge.
2. Activated sludge purification reaction process
The purification reaction process of activated sludge is relatively complicated. There are physical, chemical or physicochemical processes such as the adsorption and flocculation of organic pollutants by the activated sludge itself, as well as the biological transformation and absorption of organic pollutants by microorganisms in the activated sludge. The biochemical process can be roughly divided into the following two stages.
Initial adsorption and removal stage
Within a short time (5-10 min) after the sewage and activated sludge are contacted and mixed, the organic pollutants in the sewage, especially the suspended and colloidal organic matter, show a high removal rate. The initial high-speed removal phenomenon is the result of the combined effect of physical adsorption and biological adsorption. During this process, the organic substrate in the mixed solution rapidly decreases, and the BOD decreases rapidly, as shown in the curve of the adsorption zone in Figure 2. This is because the activated sludge has a large surface area and is enriched with a large number of microorganisms on the surface. The outside is covered with a clay layer of polysaccharides. When the sewage is suspended, colloidal organic substrates and activated sludge flocs When in contact, it is quickly condensed and removed by adsorption. This phenomenon is the "initial adsorption removal" effect.
The initial adsorption process is very fast and can generally be completed within 30 minutes. The adsorption removal rate of sewage BOD can reach 70%. For sewage containing more suspended and colloidal organics, BOD can be reduced by 80%-90% . The initial adsorption rate mainly depends on the activity of microorganisms and the degree of hydraulic diffusion in the reactor and the law of hydraulic dynamics. The former determines the adsorption and coagulation efficiency of activated sludge microorganisms, while the latter determines the degree of contact between activated sludge flocs and organic substrates. . The high adsorption activity of activated sludge microorganisms depends on a larger specific surface area and a suitable microbial proliferation period. Generally speaking, the endogenous respiration stage microorganisms in a "starvation" state have the strongest adsorption activity.
Metabolic stable phase
The organic pollutants adsorbed on the surface of activated sludge microbial cells, under the action of permease, the dissolved and small molecular organic matter directly enters the cell body through the cell wall, while the colloidal and suspended macromolecular organic matter such as starch and protein Then it is firstly hydrolyzed into dissolved small molecules under the action of extracellular enzyme-hydrolase, and then enters the cell body. At this time, part of the simple soluble organic matter produced by hydrolysis will diffuse into the mixed solution, causing the BOD value of the mixed solution to increase. High, as shown by the extracellular hydrolysis zone curve in Figure 2.
The organic pollutants that enter the cell body are oxidized and decomposed into intermediate products under the catalysis of various intracellular enzymes (such as dehydrogenase, oxidase, etc.). Some intermediate products are synthesized into new cell materials, while others are oxidized. In order to stabilize inorganic products, such as CO2 and H2O, and release energy for the synthesis of cells, this process is the oxidation and decomposition process of substances, also known as the stabilization process. In this process, unstable high molecular weight organic substances are transformed into simple and stable low molecular weight inorganic substances through biochemical reactions, and the BOD of the mixed solution gradually decreases, as shown in the curve of intracellular biological oxidation zone in Figure 2. The time required for the stabilization process depends on the degree of organic conversion, which is much longer than the adsorption process.