Concept of activated sludge

Activated sludge is a sludge-like flocculent formed after continuous aeration of wastewater over a certain period of time, due to the proliferation of aerobic microorganisms. It is inhabited by a microbial community primarily composed of mycelial aggregates, which possess a strong ability to adsorb and oxidize organic substances.

Activated sludge is generally divided into two types: natural cultivation and inoculated cultivation. Most industrial wastewater treatment plants use inoculated cultivation, which results in a shorter cultivation time. Typically, the concentrated sludge from a nearby wastewater treatment plant is used as a strain (seed sludge) for cultivation. For the cultivation of activated sludge in systems for treating urban wastewater and industrially wastewater that is rich in nutrients and low in toxicity, seed sludge can be added directly to the wastewater to be treated and aeration can be carried out; once the sludge turns brownish-yellow, the wastewater can be fed in continuously (the flow rate should be increased gradually). At this point, the sedimentation tank is also put into operation to allow the sludge to circulate within the system. To accelerate the cultivation process, unfermented manure water or other nutrients can be added during the cultivation. The culture development process is complete once the activated sludge concentration reaches the required value for the process. Economically speaking, the amount of sludge used should be as low as possible; generally, it is maintained so that the sludge concentration in the mixed solution is above 0.5 g/L after dilution. When culturing toxic industrial wastewater, river water can be introduced into the aeration tank first, or tap water can be used (which needs to be aerated for a while to remove any residual chlorine). Then, seed sludge and untreated manure wastewater are added and aerated until the sludge turns brownish-yellow; at that point, aeration is stopped, the sludge is allowed to settle and some of the supernatant liquid is removed. An additional amount of manure wastewater is then added to continue the aeration process. Once the amount of sludge increases significantly, the flow rate of the wastewater is gradually increased. In the later stages of culture, the microorganisms in the sludge have been able to adapt well to the quality of industrial wastewater.

Activated sludge is primarily composed of active microorganisms such as bacteria, protozoa, and metazoans; in addition, it contains some inorganic substances, organic matter that has not been decomposed by microorganisms, and residues from the microorganisms’ own metabolism. Activated sludge has a loose structure and a large surface area, possessing strong capabilities for adsorbing, aggregating, oxidizing, and decomposing organic matter. Under appropriate conditions, activated sludge also exhibits good self-coagulation and sedimentation properties, with most flocs ranging in size from 0.02 to 0.2 mm. The activated sludge process uses the organic pollutants present in wastewater as a nutrient source; under conditions of dissolved oxygen, activated sludge is continuously cultivated, and its adsorption, coagulation, and oxidation-decomposition functions are utilized to purify the organic pollutants in the wastewater. Specific information can be found in manuals, books, papers, and other materials related to water treatment

Let’s first talk about the methods for cultivating activated sludge: There are various ways to cultivate activated sludge, but different methods require different amounts of time, as well as human and material resources. The cultivation method should be selected based on factors such as wastewater quality, climate, and actual permit conditions.  1. Preparatory work before cultivation: (1) All structures are constructed, construction debris is removed from the tanks through cleaning, hydrostatic tests confirm the absence of leaks or settlement, and finally, they pass the acceptance checks in accordance with relevant regulations.  (2) All equipment such as electrical appliances, machinery, and pipelines have been installed, and single-unit tests as well as joint tests have been completed successfully. Finally, it passes the acceptance inspection in accordance with the relevant regulations (instructions). (3) To meet the needs of future operation and management, wastewater treatment plants (stations) that have the necessary facilities should carry out basic routine testing, such as pH, water temperature, COD, DO, and biomass, in order to guide the cultivation of activated sludge and daily operations. (4) Investigation of basic data, including the diurnal variations in sewage flow, water quality (pH, water temperature, COD, BOD5/CODCr, nitrogen content, phosphorus content, toxic substances, etc.) and their changes, as well as the technical parameters of various facilities and equipment. Where conditions permit, it is advisable to conduct surveys and record the baseline water quality of the receiving water bodies (such as rivers that receive wastewater), in order to have a basis for assessing the impact on these water bodies after several years. (5) Provide sufficient essential nutrients (carbon source, nitrogen source, phosphorus source) based on the quality of the water to be treated, so that any deficiencies can be addressed. When using the inoculation culture method, it is also necessary to have an adequate supply of dry (or concentrated) sludge from other wastewater treatment plants (stations) with similar wastewater characteristics, to be used as a strain for culturing activated sludge microorganisms. (6) Operators should be familiar with the piping layout of the entire system as well as the aspects related to utility services, and understand the basic process of sludge cultivation and the control requirements. (7) Once the personnel are in place, the system should generally operate continuously after training and acclimatization; no one should be removed from it.  (8) Prepare the necessary original records and reports for testing and operation, as well as establish preliminary rules and regulations. From the beginning of culture cultivation, a more standardized organizational and management framework is gradually established to ensure a smooth start-up and subsequent operation. 2. Natural fermentation: Natural fermentation is also known as the direct fermentation method. It is a cultivation process that makes use of the small amount of microorganisms already present in wastewater to allow them to multiply gradually. For urban sewage and some industrial wastewater with a relatively complete range of nutrients and low toxicity, such as that from food factories and meat processing plants, this cultivation method can be considered; however, the cultivation time is relatively long. Natural fermentation can be further divided into batch fermentation and continuous fermentation. (1) Batch fermentation. Fill the aeration tank with wastewater and carry out anoxic aeration (that is, only aeration without adding wastewater). After a few days, stop aeration, let the mixture settle for 1 hour, then remove about 1/5 of the upper layer of wastewater from the tank and add an equal amount of fresh sewage. These processes of aeration, settling, and water addition are repeated in this manner, with the amount of water added each time being greater than before, and the aeration time being shorter than before. During spring and autumn, sludge can be initially cultivated in about two to three weeks. When the sludge concentration in the aeration tank mixture reaches around 1 gram per liter, continuous water feeding and aeration can proceed. Since the sludge concentration is low in the early stages of cultivation, less sludge accumulates in the sedimentation tank, and thus the amount of recirculated sludge is also lower. As the amount of sludge increases over time, the amount of recirculated sludge increases accordingly. Once the sludge concentration reaches the level required by the process, normal operation can begin, with control carried out in accordance with the process requirements. (2) Continuous culture. First, fill the aeration tank with wastewater, then stop feeding water; after holding it in that condition for half a day to a day, water can be fed continuously. With continuous aeration and the inflow rate being increased gradually from low to high, and after operating continuously for a period of time (similar to the batch method), activated sludge will appear and gradually increase in quantity. When the sludge volume in the aeration tank reaches the concentration required by the process, it is controlled in accordance with the process requirements. Since the natural microbial cultivation method involves directly culturing activated sludge using wastewater, the cultivation process is also one in which microorganisms gradually adapt to the properties of the wastewater and become acclimated. 3. Inoculation cultivation: The cultivation time for this method is short; it is a commonly used technique for culturing activated sludge and is suitable for most industrial wastewater treatment plants. If there is such mud nearby in urban sewage treatment plants, this method can also be used to shorten the incubation time. The two commonly used methods for inoculation culture are as follows: (1) Inoculation culture using concentrated sludge. Concentrated sludge from a nearby sewage treatment plant is used as the strain (seed sludge) for cultivation. For the cultivation of activated sludge in systems for treating urban wastewater and industrially wastewater that is rich in nutrients and low in toxicity, seed sludge can be added directly to the wastewater to be treated and aeration can be carried out; once the sludge turns brownish-yellow, the wastewater can be fed in continuously (the flow rate should be increased gradually). At this point, the sedimentation tank is also put into operation to allow the sludge to circulate within the system. To accelerate the cultivation process, unfermented manure water or other nutrients can be added during the cultivation. The culture development process is complete once the activated sludge concentration reaches the required value for the process. Economically speaking, the amount of sludge used should be as low as possible; generally, it is maintained so that the sludge concentration in the mixed solution is above 0.5 g/L after dilution. When culturing toxic industrial wastewater, river water can be introduced into the aeration tank first, or tap water can be used (which needs to be aerated for a while to remove any residual chlorine). Then, seed sludge and untreated manure wastewater are added and aerated until the sludge turns brownish-yellow; at that point, aeration is stopped, the sludge is allowed to settle and some of the supernatant liquid is removed. An additional amount of manure wastewater is then added to continue the aeration process. Once the amount of sludge increases significantly, the flow rate of the wastewater is gradually increased. In the later stages of culture, the microorganisms in the sludge have been able to adapt well to the quality of industrial wastewater. (2) Inoculation and culture of dry sludge. ““Dry sludge” generally refers to the sludge cake obtained after dehydration using a dehydrator, with a moisture content of around 70–80%. This law applies to remote areas and situations where the distance for transporting the seed sludge is long. The process of inoculating and culturing dry sludge is basically the same as that of culturing concentrated sludge. For sludge inoculation, use fresh sludge cakes that have just been dewatered; before adding them to the aeration tank, a small amount of water should be added and the mixture should be turned into a slurry. The amount of dry sludge added is generally 2 to 5% of the tank volume. Dry sludge may contain a certain concentration of chemical agents (used for sludge conditioning); if the concentration of these agents is too high or their toxicity is significant, it is not suitable to be used as seed sludge for culturing bacteria. To determine whether sludge can be used for inoculation, a small amount of the sludge can be crushed and placed in a small container such as a beaker or plastic bucket; water is added and aeration is provided. If the color of the sludge changes to yellow after some time, it can then be used for inoculation. Precautions for sludge microbial cultivation: (1) During the cultivation of activated sludge, it is necessary to regularly measure parameters such as the pH of the influent water, COD, ammonia nitrogen, dissolved oxygen in the aeration tank, and the sedimentation performance of the sludge. After the activated sludge is initially formed, it is necessary to conduct observations of the biological community; based on these observations, the cultivation status of the sludge is assessed, and the microbial cultivation process is adjusted dynamically. (2) The cultivation of activated sludge should be carried out as much as possible during the season with suitable temperatures. Because the temperature is suitable, microorganisms grow rapidly, resulting in a short incubation time. If culturing can only be carried out in winter, the inoculation method should be used, and more seed sludge is required than in spring and autumn. (3) During the culture process, especially after the sludge begins to form, care should be taken to prevent excessive auto-oxidation of the sludge, particularly in summer. Many factories have experienced such situations. This not only increases the culture time and costs, but may even prevent the wastewater treatment system from being put into operation on schedule. To prevent the self-oxidation of sludge, it is crucial to control the aeration volume and duration; the dissolved oxygen level in the tank should be monitored regularly, and water should be added promptly to meet the nutritional needs of the microorganisms. If the influent concentration is too low, manure and similar substances must be added to supply nutrients; intermittent aeration can be used when such conditions are not available. (4) In the later stage of activated sludge cultivation, appropriately removing some aged sludge is beneficial for further growth and reproduction of microorganisms. (5) Industrial wastewater treatment plants usually have no wastewater inflow before the production facilities come online, but once the facilities start operating, the wastewater generated must be treated promptly. At this time, the culture time should be determined reasonably based on the actual conditions, and seed sludge as well as nutrients should be prepared in advance. (6) If the sludge in the aeration tank has matured but no wastewater is flowing in, aeration should be stopped to put the sludge into a dormant state, or intermittent aeration should be used (by increasing the interval between aeration sessions and reducing the amount of air supplied), in order to minimize the rate of self-oxidation of the sludge. When possible, nutrients such as manure and non-toxic organic waste (such as kitchen scraps) should be added. (7) Most wastewater treatment plants have two or more aeration tanks. In this case, activated sludge can first be cultivated in one aeration tank, and then transferred to adjacent aeration tanks for multi-stage expanded cultivation. This law applies to larger wastewater treatment plants. This post was last edited by cdpulin on 2009-2-25 10:50.]

Let’s talk again about what the activated sludge process is: The activated sludge process is the main method for biological treatment of wastewater, with activated sludge playing a key role in it. The activated sludge process involves the continuous introduction of air into wastewater; after a certain period of time, sludge-like flocs are formed due to the growth of aerobic microorganisms. It is inhabited by a microbial community primarily composed of mycelial aggregates, which possess a strong ability to adsorb and oxidize organic substances. 　　Activated sludge process is a method of biological wastewater treatment. This method involves the continuous mixed cultivation of wastewater and various microbial communities under artificially oxygenated conditions to form activated sludge. Utilize the biological coagulation, adsorption, and oxidation properties of activated sludge to decompose and remove organic pollutants from wastewater. Then the sludge is separated from the water; most of the sludge is returned to the aeration tank, while the excess is removed from the activated sludge system. 　　The main factors affecting the efficiency of the activated sludge process (treatment efficiency and economic benefits) are the choice of treatment method, as well as the design and operation of the aeration tank and sedimentation tank. 　　Process and Principle The typical activated sludge process consists of an aeration tank, a sedimentation tank, a sludge return system, and a excess sludge removal system. 　　The wastewater and the returned activated sludge enter the aeration tank together to form a mixed liquid. The compressed air supplied from the air compressor station enters the wastewater in the form of tiny bubbles through air diffuser devices placed at the bottom of the aeration tank. This is done to increase the dissolved oxygen level in the wastewater and to keep the mixture in a state of vigorous agitation, resulting in a suspended condition. Dissolved oxygen, activated sludge, and wastewater mix together and come into full contact with each other, allowing the reactions of the activated sludge to proceed properly. 　　In the first stage, the organic pollutants in the wastewater are adsorbed onto the surface of the microbial flocs by the activated sludge particles, due to their large specific surface area and polysaccharide-based adhesive substances. At the same time, some large molecular organic compounds are broken down into small molecular organic compounds by bacterial extracellular enzymes. 　　In the second stage, under conditions of sufficient oxygen, microorganisms absorb these organic substances and oxidize them, producing carbon dioxide and water; part of this is used for their own growth and reproduction. As a result of the activated sludge reaction, organic pollutants in the wastewater are degraded and removed, the activated sludge itself proliferates and grows, and the wastewater is purified. 　　The mixture that has been purified by activated sludge enters the secondary sedimentation tank, where the suspended activated sludge and other solid substances settle and are separated from the water. The clarified wastewater is then discharged from the system as treated water. The sludge that has been precipitated and concentrated is discharged from the bottom of the sedimentation tank; most of it is returned to the aeration tank as inoculation sludge in order to maintain the concentration of suspended solids and microorganisms in the aeration tank ； The proliferated microorganisms are removed from the system and are referred to as “excess sludge”. In fact, pollutants are largely transferred from wastewater to these excess sludges. 　　The principle of the activated sludge process can be described vividly as follows: microorganisms “eat” the organic matter in wastewater, thereby turning it into clean water. It is essentially similar to the natural self-purification process of water bodies, except that it is enhanced artificially, resulting in a better effect in purifying wastewater.

It’s quite theoretical; it would be better if it were more targeted. Just personal opinion