Representatives of microorganisms that are directly useful for removing BOD from wastewater are bacteria. Bacteria are the tiniest microorganisms and morphologically the simplest. Although its presence can be confirmed with a light microscope, it cannot be identified without physiological tests.
There are a large number of protozoa and some micro-metazoa in the activated sludge of sewage treatment, and their weight can account for 5%-10% of the total biomass.
1. Classification of microorganisms in sewage treatment
There are many types of microorganisms in sewage treatment, mainly fungi, algae and animals.
1. Bacteria
Bacteria are highly adaptable and grow quickly. According to the different nutrient needs, bacteria can be divided into two categories: autotrophs and heterotrophs. Autotrophic bacteria use various inorganic substances (CO2, HCO3-, NO3-, PO3-4, etc.) as nutrients to convert them into another inorganic substance, release energy, synthesize cellular matter, and its carbon, nitrogen and phosphorus sources are all inorganic.
Heterotrophic bacteria use organic carbon as the carbon source and organic or inorganic nitrogen as the nitrogen source, convert it into CO2, H2O, NO3-, CH4, NH3 and other inorganic substances, release energy, and synthesize cellular matter. Microorganisms in sewage treatment facilities are mainly heterotrophic bacteria.
2. Fungi
Fungi include mold and yeast. Fungi are aerobic bacteria that use organic matter as a carbon source, with a growth pH of 2?9 and an optimal pH of 5.6. Fungi require less oxygen, only half of bacteria. Fungi often appear in environments with low pH and less molecular oxygen.
Fungal filaments play a skeleton role in the agglomeration of activated sludge, but the appearance of too many filamentous bacteria will affect the sedimentation performance of the sludge and cause sludge expansion. The role of fungi in sewage treatment cannot be ignored.
3. Algae
Algae are unicellular and multicellular plant microorganisms. It contains chlorophyll, which uses photosynthesis to assimilate carbon dioxide and water to release oxygen, absorb nitrogen, phosphorus and other nutrients in the water to synthesize its own cells.
4. Protozoa
Protozoa are the lowest single-celled animals that can divide and proliferate. Protozoa in sewage are both water purifiers and water quality indicators. The vast majority of protozoa belong to the aerobic heterotrophic type. In sewage treatment, the role of protozoa is not as important as bacteria, but because most protozoa can swallow solid organic matter and free bacteria, they have the effect of purifying water quality.
Protozoa are more sensitive to changes in the environment, and different protozoa appear in different water quality environments, so they are water quality indicators. For example, when dissolved oxygen is sufficient, clockworms appear in large numbers, but when dissolved oxygen is less than 1/L, they appear less and are not active.
5. Metazoa
Metazoans are multicellular animals. Common metazoans in sewage treatment facilities and stabilization ponds are rotifers, nematodes and crustaceans.
Metazoans are aerobic microorganisms and live in a good water quality environment. Metazoans feed on bacteria, protozoa, algae and organic solids, and their appearance indicates good treatment effect and is an indicator organism for sewage treatment.
2. Metabolism of microorganisms
The life process of microorganisms is the process of continuous use of nutrients, continuous synthesis and consumption of cellular materials. This process is accompanied by the birth of new life, the death of old life and the transformation of nutrients (substrate). The biological treatment of sewage is achieved by using the metabolic transformation of pollutants (nutrients) by microorganisms.
1. Nutritional relationship of microorganisms
Bacteria, fungi, algae, protozoa, metazoa live in symbiosis in water bodies. Bacteria and fungi use organic matter, nitrogen and phosphorus in water as nutrients for aerobic and anaerobic respiration to synthesize their own cells. Algae use carbon dioxide and nitrogen and phosphorus in water to synthesize their own cells and provide oxygen to the water body. After the cell death of algae, it becomes a nutrient for fungi to reproduce.
Protozoa swallow solid organic matter, fungi and algae in water. Metazoans prey on solid organic matter, fungi, algae and protozoa in water.
2. Metabolism of microorganisms
Microorganisms take nutrients from sewage and synthesize their own cells and excrete waste products through complex biochemical reactions. This biochemical reaction process to maintain life activities and growth and reproduction is called metabolism, referred to as metabolism.
Metabolism can be divided into catabolic and anabolic according to the transfer of energy and the type of biochemical reaction. Microorganisms break down nutrients into simple compounds and release energy, a process called catabolism or capacity metabolism; Microorganisms convert nutrients into cellular matter and absorb energy released by catabolism, a process called anabolism.
When nutrients are deficient, microorganisms oxidize and decompose their own cellular material to obtain energy, which is called endogenous metabolism and endogenous respiration. When nutrients are sufficient, endogenous respiration is not obvious, but when nutrients are deficient, endogenous respiration is the main source of energy.
There is no life without metabolism. Microorganisms continuously multiply and die through metabolism. The catabolism of microorganisms provides energy and substances for anabolism, which provides catalysts and reactors for catabolism. The two metabolisms are interdependent, mutually reinforcing and inseparable.
Some of the nutrients consumed by microbial metabolism are broken down into simple substances and excreted into the environment, and the other part is synthesized into cellular matter. Different microorganisms have different metabolic rates, and the proportion of nutrients used for decomposition and synthesis is also different.
Anaerobic microorganisms do not decompose nutrients thoroughly, release less energy, and metabolize slowly. Aerobic microorganisms decompose nutrients thoroughly, the final products (CO2, H2O, NO3-, PO43-, etc.) are stable and contain the least energy, so aerobic microorganisms release more energy in metabolism, the metabolism speed is fast, the proportion of nutrients used for decomposition is small, the proportion of synthesis is large, and the cell proliferation is fast.
3. The growth environment of microorganisms
The main body of biological treatment of wastewater is microorganisms, and only by creating good environmental conditions for microorganisms to multiply can satisfactory treatment results be obtained. The conditions that affect the growth of microorganisms mainly include nutrition, temperature, pH, dissolved oxygen and toxic substances.
1. Nutrition
Nutrition is the material basis for the growth of microorganisms, and the energy and substances required for life activities come from nutrition. The composition of microbial cells (excluding H2O and inorganics) can be expressed by the chemical formula C5H7O2N or C60H87O23N12P. The composition of different microbial cells is different, and the requirements for carbon, nitrogen and phosphorus ratios are not exactly the same. Aerobic microorganisms require a carbon-nitrogen-phosphorus-phosphate ratio of BOD5:N:P=100:5:1 [or COD:N:P=(200~300):5:1].
Anaerobic microorganisms require a carbon-to-nitrogen to phosphorus ratio of BOD5:N:P=100:6:1. where N is counted as NH3-N and P is measured as PO43--P. There are many kinds of microorganisms, and the chemical forms of C, N, and P required are also different. For example, heterotrophs need organic matter as carbon sources, while autotrophs use CO2 and HCO3- as carbon sources.
Almost all organic matter is a source of nutrients for microorganisms, and it is very important to control the appropriate C:N:P ratio to achieve the desired purification effect. In addition to C, H, O, N, and P, microorganisms also need elements such as S, Mg, Fe, Ca, and K, as well as trace elements such as Mn, Zn, Co, Ni, Cu, Mo, V, I, Br, and B.
2. Temperature
The overall temperature range of various microorganisms is 0~80. According to the temperature range of adaptation, microorganisms can be divided into three categories: low temperature (cold-loving), medium-temperature and high temperature (thermophilic). The growth temperature of low-temperature microorganisms is below 20, the growth temperature of mesophilic microorganisms is 20~45, and the growth temperature of high-temperature microorganisms is above 45.
Aerobic biological treatment was mainly at medium temperature, and the optimal growth temperature of microorganisms was 20~37. The optimal growth temperature of mesotemperature microorganisms and 50~60 was 50~60 for anaerobic biological treatment. Therefore, anaerobic microbial treatment often uses two temperature ranges, 33~38 and 52~57, which are called medium-temperature digestion (fermentation) and high-temperature digestion (fermentation) respectively. With the development of science and technology, anaerobic reactions can be carried out at room temperature of 20~25, which greatly reduces operating costs.
In the suitable temperature range, for every 10 increases, the biochemical reaction rate increases by 1~2 times. Therefore, the biological treatment effect is better under higher optimal temperature conditions. Artificially changing the temperature of sewage will increase the treatment cost, so aerobic biological treatment is generally carried out at natural temperature, that is, at room temperature. The effect of aerobic biological treatment is less affected by climate.
Anaerobic biological treatment is greatly affected by temperature, and it is necessary to maintain a high temperature, but considering the operating cost, it should be operated at room temperature as much as possible (20~25). If the temperature of the raw sewage is high, medium temperature fermentation (33~38) or high temperature fermentation (52~57) should be used. If there is enough waste heat or enough biogas is produced during fermentation (high concentration of organic sewage and sludge digestion), medium and high temperature fermentation can be achieved by using waste heat or the heat energy of biogas. Under normal circumstances, the temperature fluctuation in a day should not be exceeded. Therefore, it is necessary to control the appropriate water temperature and maintain stability during biological treatment.
3. pH value
Enzyme is an amphoteric electrolyte, and the change of pH value affects the ionization form of the enzyme, which in turn affects the catalytic performance of the enzyme, so pH value is one of the important factors affecting the activity of the enzyme. Different microorganisms have different enzyme systems and have different pH adaptation ranges. The pH adaptation range of bacteria, actinomycetes, algae and protozoa is 4~10.
The optimal pH for yeast and mold is 3.0~6.0. Most bacteria are suitable for neutral and alkaline environments with pH=6.5~8.5. The optimal pH of aerobic biological treatment was 6.5~8.5, and the appropriate pH of anaerobic biological treatment was 6.7~7.4 (optimal pH was 6.7~7.2). It is important to maintain the optimal pH range during biological processing. Otherwise, the activity of microbial enzymes is reduced or lost, and microorganisms grow slowly or even die, leading to treatment failure.
Sudden changes in the pH of the influent can have a large impact on biological treatment, and this effect is irreversible. Therefore, it is very important to keep the pH level stable.
4. Dissolved oxygen
The metabolic process of aerobic microorganisms uses molecular oxygen as a receptor and participates in the synthesis of some substances. Without molecular oxygen, aerobic microorganisms cannot grow and reproduce, so a certain concentration of dissolved oxygen (DO) should be maintained during aerobic biological treatment. Insufficient oxygen supply is suitable for the proliferation of microorganisms with low dissolved oxygen (trace aerobic sulfogens) and facultative microorganisms.
They do not decompose organic matter thoroughly, the treatment effect is reduced, and the dominant growth of filamentous bacteria in the state of low dissolved oxygen causes sludge expansion. Too high a concentration of dissolved oxygen not only wastes energy, but also oxidizes and dies cells due to relative nutrient deficiency. In order to achieve good treatment results, it is appropriate to control the dissolved oxygen at 2~3mg/L (0.5~1mg/L in the effluent of the secondary sedimentation tank) during aerobic biological treatment.
Anaerobic microorganisms produce H2O2 under aerobic conditions, but are killed by H2O2 without the enzyme that breaks down H2O2. Therefore, molecular oxygen must not be present in anaerobic biotreatment reactors. Other oxidized substances such as SO42-, NO3-, PO43- and Fe3+ will also have adverse effects on anaerobic biological treatment, and their concentrations should also be controlled.
5. Toxic substances
Chemicals that inhibit and poison microorganisms are called toxic substances. It can destroy the structure of cells, denature enzymes and make them inactive. For example, heavy metals can bind with the -SH group of enzymes, or bind to proteins to denature or precipitate.
Toxic substances are harmless to microorganisms at low concentrations, and toxic occurs when they exceed a certain value. Certain toxic substances can become nutrients for microorganisms at low concentrations. The toxicity of toxic substances is affected by factors such as pH value, temperature and the presence or absence of other toxic substances, and the toxicity varies greatly under different conditions, and the tolerance of different microorganisms to the same poison is also different, and the specific situation should be determined according to the experiment.
4. The indicative role of microorganisms in sewage treatment
(1) When the number of activated sludge reaches more than 1000/mL, accounting for more than 80% of the total number of organisms, it can be concluded that this activated sludge has a high purification effect.
(2) When the purification performance of activated sludge deteriorates, the organisms that appear are multi-wave worms, lateral trichomoniasis, house trichomoniasis, legionoplasmosis and other fast-swimming organisms. At this time, the floc is very broken, about 100um in size. In severe deterioration, only polygonia and trichomoniasis appear In extreme deterioration, neither protozoa nor metazoans appear.
(3) The organisms that appear when the activated sludge is restored from the deteriorated state are slow-swimming or creeping organisms such as infectious swimmers, oblique leafworms, oblique tube worms, and pointed caterpillars. It has been observed that these microorganisms become dominant organisms for about a month.
(4) The organisms that appear when the activated sludge fraction disintegrates are carbopods such as active legumes and radiant amoeba. When more than tens of thousands of these organisms appear, the flocs become smaller, making the treated water cloudy. When these organisms are found to increase dramatically, this phenomenon can be suppressed to some extent by reducing the amount of backflow sludge and the amount of air delivery.
(5) The microorganisms that appear when the activated sludge expands are jersey fungi, various molds, etc., these filamentous microorganisms cause sludge expansion, and when the SVI is above 200, these filamentous microorganisms are filamentous. There are fewer microscopic animals in expanded sludge than in normal sludge.
(6) Microorganisms that appear when dissolved oxygen is insufficient are sulfur bacteria bae's body. These microorganisms are adapted to survive at low dissolved oxygen concentrations. When these microorganisms appear, the activated sludge is black, rotten and smelly.
(7) Microorganisms that appear during excessive aeration, if the over-aeration time lasts for a long time, various amoebas and rotifers are the dominant organisms.
(8) When the concentration of wastewater is too low, a large number of microorganisms appear as wandering insects.
(9) Microorganisms that appear when the BOD load is low. Epidermal insects, scale insects, rotifers, oligochaetae, etc. are dominant organisms, and these organisms are also indicators of nitrification when they are numerous.
(10) Organisms that appear when shock loads and poisons are inflowing. Because protozoa respond faster to changes in environmental conditions than bacteria, the effects of shock loads and poisons on activated sludge can be observed by observing the changes of protozoa. Shield fiberworms, which are the most sensitive to shock loads and poisons among protozoa, indicate that shock loads and small amounts of poisons flow into the protozoa when the number of shield fibers decreases sharply.
