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1. What is activated sludge?
Activated sludge is composed of active microorganisms, residues of microbial self-oxidation, organic matter adsorbed on activated sludge and inorganic matter that cannot be biodegraded, of which microorganisms are the main components of activated sludge.
Activated sludge microorganisms are an ecosystem composed of bacteria, fungi, protozoa, metazoans and other microbial groups.
Bacteria are the center of activated sludge in composition and purification function, is the most important component of microorganisms, the nature of organic matter in sewage determines that those species of bacteria are dominant, protein-containing sewage is beneficial to Alcaligenes and Bacillus spp., and carbohydrate sewage or hydrocarbon sewage is beneficial to Pseudomonas spp. At a certain energy level (i.e., the activity capacity of bacteria), most of the bacteria form the floc of activated sludge and form colloids, which have good self-coagulation and sedimentation properties.
In the process of activated sludge treatment, the first and main bearer of purified sewage is bacteria, followed by protozoa, which are the first predators of bacteria, and then metazoa, which are the second predators of bacteria.
2. Purification process and mechanism
Activated sludge microorganisms are capable of continuously removing organic matter from effluent, which is generally divided into three stages:
1. Initial removal and adsorption
In many activated sludge systems, when the sewage and activated sludge contact within a short time (3-5 minutes) there is a high organic matter (BOD) removal rate, this initial high-speed removal phenomenon is caused by adsorption, because the sludge surface area is large (between 2000-10000m2/m3 mixture), and the surface has a polysaccharide viscous layer, therefore, the suspended and colloidal substances in the sewage are removed by flocculation and adsorption, The BOD that is initially removed is stored on the surface of the microbial cells as a standby food source, and is then aerated for several hours before being ingested and metabolized.
In the initial stage, the amount of organic matter removed by the unit sludge is a certain limit, it depends on the type of sewage and the sludge performance when it comes into contact with sewage, for example, there are more suspended and colloidal organic matter in the sewage, then the initial removal rate is large, on the contrary, if there are many dissolved organic matter, the initial removal rate is small, and for example, the refluxed sludge is not sufficiently aerated, the organic matter stored in the sludge in advance will not be metabolized sufficiently, the sludge has not been regenerated, and the activity can not be well recovered, so the initial removal rate will be reduced, but, If the reflux sludge is aerated for a long time, the sludge will be in the endogenous respiration stage for a long time, and it will lose its activity due to excessive self-oxidation, which will also reduce the initial removal rate.
2. The metabolic role of microorganisms
Activated sludge microorganisms use various organic matter in sewage as nutrients, and under aerobic conditions, part of the organic matter is synthesized into new cellular matter (protoplasm); The other part of the organic matter is catabolized, that is, oxidized and decomposed to obtain the energy needed to synthesize new cells, and finally to form stable substances such as CO2 and H2O. In the process of new cell synthesis and microbial growth, in addition to oxidizing a part of the organic matter to obtain energy, there is also a part of the microbial cell material that is also oxidizing and decomposing and supplying energy.
The metabolic process of activated sludge microbial removal of organic matter from sewage is mainly composed of the synthesis of microbial cell matter (activated sludge growth), the oxidative decomposition of organic matter (including a part of cellular matter) and the consumption of oxygen. The period of sludge growth is the period of rapid removal of organic matter.
3. Formation and coagulation and precipitation of flocs
The organic matter in the sewage is biodegraded, a part of the oxidation and decomposition to form carbon dioxide and water, and a part of the synthesis of cellular matter becomes the bacteria, if the organic matter that forms the bacteria is not separated from the sewage, such purification can not be regarded as the end, in order to separate the bacteria from the water, gravity precipitation method is now used, if each bacteria is in a loose state, because its size is about the same as the colloidal particles, then it will remain in a stable suspension state, and precipitation separation is impossible, for this reason, The bacteria must be allowed to condense into flocs that are easy to precipitate.
Bacteria that are easy to form flocs include Kinetobacterium, Alkaloidogenic Bacillus, Colorless Bacillus, Flavobacterium, Pseudomonasporus, etc., but no matter which kind of bacteria can only coagulate under certain conditions.
3. Evaluation index of activated sludge
In addition to observing the biological phase of the microbial composition of activated sludge, the following indicators are used in the evaluation of activated sludge.
1、混合液悬浮固体（MLSS）
Mixed liquid suspended solids refers to the number of mixed suspended solids after the mixture of sewage and activated sludge in the aeration tank, the unit is mg/L, also known as the concentration of mixed liquid sludge, it is an index to measure the number of activated sludge in the aeration tank, and in the activated sludge method, MLSS is generally 2-4g/L.
2、混合液挥发性悬浮固体（MLVSS）
Mixed liquid volatile suspended solids refers to the weight of organic matter in the mixed liquid suspended solids, unit mg/L, in general, the ratio of MLVSS/MLSS is relatively fixed, for domestic sewage, often around 0.75, for industrial wastewater, the ratio varies depending on the water quality.
3. Sludge sedimentation ratio (SV%)
Sludge sedimentation ratio refers to the volume ratio (%) of the sedimented sludge to the mixed solution after 30min of static sedimentation in the aeration tank mixture in a 100ml graduated cylinder, because the normal activated sludge can generally be close to its maximum density after 30min of static sedimentation, so the sludge sedimentation ratio can reflect the amount of sludge during the normal operation of the aeration tank, which can be used to control the discharge of the remaining sludge, and it can also reflect the abnormal situation such as sludge expansion in time, so as to facilitate early identification of the cause and take measures, and the determination of sludge sedimentation ratio is relatively simple. And it can explain certain problems, so it has become one of the important indicators for evaluating activated sludge.
4. Sludge index (SVI)
The full name of sludge index is sludge volume index, which refers to the volume occupied by 1g of dry sludge after 30min of static sedimentation of the mixture at the outlet of the aeration tank is measured in ml, that is
SVI value can better reflect the looseness (activity) and cohesion and sedimentation performance of activated sludge, SVI value is too low, indicating that the sludge is fine and compact, inorganic matter, lack of activity and adsorption capacity, SVI value is too high, indicating that the sludge is difficult to precipitate and separate, and the concentration of reflux sludge is reduced, and even "sludge expansion", resulting in sludge loss and other consequences, it is generally believed that when the SVI of domestic sewage is < 100, the sedimentation performance is good; When the SVI is 100-200, the precipitation performance is average; When the SVI > 200, the precipitation performance is not good.
For example, if the sedimentation ratio of the mixed liquid sludge in the exposure pool is 20% and the sludge concentration is 2.5g/L, then SVI=20×10/2.5=80
5. Sludge age (ts)
Sludge age is the ratio of the total amount of activated sludge working in the aeration tank to the amount of remaining sludge discharged per day, the unit is days, when the operation is stable, the remaining sludge volume is the amount of newly increased sludge, so the sludge age is the average residence time of the newly increased sludge in the aeration tank, or the average time required for the sludge to double the size.
Fourth, the influence of environmental factors
1. Dissolved oxygen
The activated sludge method is an aerobic process, for the traditional activated sludge method, the maximum demand for oxygen appears at the head of the aeration tank where the sewage and sludge begin to mix, and the oxygen supply is often insufficient, and the oxygen supply is insufficient, which will cause an anaerobic state, hinder the normal metabolic process, and breed filamentous bacteria. The amount of oxygen supplied is generally controlled by the concentration of dissolved oxygen in the mixture. Due to the different sizes of activated sludge flocs, the minimum dissolved oxygen concentration required is not the same, the smaller the flocs, the larger the contact area with sewage, and the more suitable for oxygen uptake, the required dissolved oxygen concentration is small; On the contrary, if the floc is large, the dissolved oxygen concentration required is large, and in order to make the precipitation separation performance good, a larger floc is expected, so the dissolved oxygen concentration is about 2mg/L.
2. Nutrients
In the activated sludge system, the metabolism of microorganisms requires a certain proportion of nutrients, in addition to the carbon source represented by BOD, nitrogen, phosphorus and other trace elements are also needed, domestic sewage contains various elements required by microorganisms, but some industrial wastewater lacks some key elements - nitrogen, phosphorus, etc. The demand for nitrogen and phosphorus should meet the following ratio, that is, BOD:N:P=100:5:1.
3. PH value
For aerobic biological treatment, the pH value is generally 6.5-9.0, the PH value is lower than 6.5, the fungus begins to compete with the bacteria, when it is reduced to 4.5, the fungus will be completely dominant, seriously affecting the precipitation separation, when the PH value exceeds 9.0, the metabolic rate is hindered.
For the activated sludge method, the PH value refers to the mixture, and for alkaline wastewater, the biochemical reaction can play a buffering role; For acidic wastewater dominated by organic acids, biochemical reactions can also play a buffering role, and if the PH value factor is taken into account in the acclimation process, the activated sludge can also be gradually adapted, for the occurrence of impact load, when the PH value changes sharply, it will give the activated sludge a serious blow, and the purification effect will deteriorate sharply. In order to make the sewage treatment plant run stably, the impact of the sudden change in PH value should be avoided, and the acid and alkali wastewater should be pretreated before biochemical treatment to adjust the PH to a suitable range.
4, Water temperature
Water temperature is an important factor affecting the growth of microorganisms, and the main reason is that urban sewage is easy to be treated biologically in summer, while the purification effect is reduced in winter. In the temperature range of the microbial enzyme system that is not affected by denaturation, the increase in water temperature will increase the microbial activity, which can increase the reaction speed. In addition, the increase in water temperature is also conducive to physical processes such as mixing, stirring, and precipitation, but is not conducive to oxygen transfer.
For biochemical processes, it is generally believed that the water temperature is best at 20-30 °C, and the purification effect is reduced above 35 °C and below 10 °C. Therefore, cooling measures should be taken for high-temperature industrial wastewater, and necessary insulation measures should be taken for sewage in cold areas. At present, for small biological treatment devices, measures are generally taken to insulate indoors, and for large-scale sewage treatment plants, such as water temperature can be maintained at 6-7 °C, measures such as increasing sludge concentration and reducing sludge load rate are taken, and activated sludge can still effectively play its purification function.
5. Toxic substances
There are many toxic substances for biological treatment, and poisons can be roughly divided into heavy metals, inorganic substances such as H2S and organic substances such as cyanide and phenol. These substances have a toxic effect on bacteria, either destroying some necessary physiological structures of bacterial cells, or inhibiting the metabolic process of bacteria. The toxic effect of poisons is also closely related to PH value, water temperature, dissolved oxygen, the presence or absence of other poisons and the number of microorganisms, and whether they are domesticated.