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What growth conditions are required for microorganisms commonly found in sewage treatment?

(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 adaptable and grow fast. Bacteria can be divided into two categories, autotrophic and heterotrophic, according to their nutrient requirements. Autotrophic bacteria use various inorganic substances (CO2, HCO3-, NO3-, PO3-4, etc.) as nutrients to convert them into another inorganic substance, release energy, and synthesize cellular substances, its carbon source, nitrogen source and phosphorus source. The sources are all inorganic.
Heterotrophic bacteria use organic carbon as carbon source and organic or inorganic nitrogen as nitrogen source, convert it into inorganic substances such as CO2, H2O, NO3-, CH4, NH3, release energy, and synthesize cellular substances. The microorganisms in sewage treatment facilities are mainly heterotrophic bacteria.

2 fungi

Fungi include mold and yeast. Fungi are aerobic bacteria and use organic matter as carbon source. The growth pH is 2 to 9, and the optimum pH is 5.6. Fungi require less oxygen, half of that of bacteria. Fungi are often found in environments with low pH and low molecular oxygen.

Fungal filaments play a skeleton role in the aggregation of activated sludge, but the presence of excessive filamentous bacteria will affect the sedimentation performance of sludge and cause sludge bulking. 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, and absorbs nitrogen, phosphorus and other nutrients in water to synthesize its own cells.

4 Protozoa

Protozoa are the lowest single-celled animals that can divide and multiply. Protozoa in sewage are both water purifiers and water quality indicators. The vast majority of protozoa are aerobic heterotrophs. In sewage treatment, the role of protozoa is less important than that of bacteria, but since most protozoa can devour 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, clockworms with sufficient dissolved oxygen appeared in large numbers, and when dissolved oxygen was less than 1/L, they appeared less and were not active.

5 Metazoa

Metazoa are multicellular animals. Common metazoans in sewage treatment facilities and stabilization ponds are rotifers, nematodes, and crustaceans.

Metazoa are all aerobic microorganisms and live in a better water quality environment. Metazoa feed on bacteria, protozoa, algae, and organic solids, and their presence indicates better treatment effects and are indicative organisms for wastewater treatment.

(2) Metabolism of microorganisms

The life process of microorganisms is a process in which nutrients are continuously utilized, and cellular substances are continuously synthesized and consumed. 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 realized by the metabolic transformation of pollutants (nutrients) by microorganisms.

1 Nutritional relationship of microorganisms

Bacteria, fungi, algae, protozoa, and metazoans coexist in water bodies. Bacteria and fungi synthesize their own cells by aerobic and anaerobic respiration using organic matter, nitrogen and phosphorus in water as nutrients. Algae use carbon dioxide and nitrogen and phosphorus in water to synthesize their own cells and provide oxygen to the water body. After the cells of the algae die, they become nutrients for the reproduction of fungi.

Protozoa ingest solid organic matter, fungi and algae in water. Metazoa prey on solid organic matter, fungi, algae and protozoa in water.

2 Metabolism of microorganisms

Microorganisms take nutrients from sewage, synthesize their own cells and excrete waste through complex biochemical reactions. This biochemical reaction process for sustaining life activities and growth and reproduction is called metabolism, or metabolism for short.

Metabolism can be divided into catabolism and anabolism according to the type of energy transfer and biochemical reactions. Microbes break down nutrients into simple compounds and release energy, a process called catabolism or energy-generating metabolism; microbes convert nutrients into cellular matter and absorb the energy released by catabolism, a process called anabolism.

When nutrients are deficient, microorganisms oxidatively decompose their own cellular substances to obtain energy. This process is called endogenous metabolism, also known as endogenous respiration. When nutrients are plentiful, endogenous respiration is not evident, but when nutrients are deficient, endogenous respiration is the main source of energy.

There is no life without metabolism. Microorganisms constantly proliferate and die through metabolism. The catabolism of microorganisms provides energy and substances for anabolism, and anabolism provides catalysts and reactors for catabolism. The two metabolisms are interdependent, mutually reinforcing, and inseparable.

Some of the nutrients consumed by microbial metabolism are decomposed into simple substances and discharged into the environment, and the other part is synthesized into cellular substances. Different microorganisms have different metabolic rates, and the ratio of nutrients used for decomposition and synthesis is also different.

Anaerobic Microorganisms decompose nutrients incompletely, release less energy, and metabolize slowly. The proportion of nutrients used for decomposition is large, and the proportion of nutrients used for synthesis is small, and cell proliferation is slow. Aerobic microorganisms decompose nutrients thoroughly, and the final products (CO2, H2O, NO3-, PO43-, etc.) are stable and contain the least amount of energy. Therefore, aerobic microorganisms release more energy and metabolize quickly, and use nutrients for decomposition. The proportion of small, large proportion used for synthesis, the cell proliferation is fast.

(3) The growth environment of microorganisms

The main body of wastewater biological treatment is microorganisms. Only by creating good environmental conditions and allowing microorganisms to multiply in large numbers can a satisfactory treatment effect be obtained. The conditions that affect the growth of microorganisms mainly include nutrition, temperature, pH value, 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 inorganic substances) can be represented by the chemical formula C5H7O2N or C60H87O23N12P. The composition of different microbial cells is not the same, and the requirements for the ratio of carbon, nitrogen and phosphorus are not the same. Aerobic microorganisms require a carbon-nitrogen-phosphorus ratio of BOD5:N:P=100:5:1 [or COD:N:P=(200~300):5:1].

Anaerobic Microorganisms require a carbon, nitrogen, and phosphorus ratio of BOD5:N:P=100:6:1. Among them, N is calculated as NH3-N, and P is calculated as PO43--P. There are many kinds of microorganisms, and the chemical forms of C, N, and P required are also different. For example, heterotrophic bacteria need organic matter as carbon source, while autotrophic bacteria use CO2 and HCO3- as carbon source.

Almost all organic matter is a nutrient source for microorganisms. In order to achieve the desired purification effect, it is very important to control the appropriate C:N:P ratio. In addition to C, H, O, N, P, microorganisms also need elements such as S, Mg, Fe, Ca, K, and trace amounts of Mn, Zn, Co, Ni, Cu, Mo, V, I, Br, B, etc. element.

2 temperature

Different types of microorganisms have different growth temperatures, and the overall temperature range of various microorganisms is 0~80. According to the adaptable temperature range, microorganisms can be divided into three categories: low temperature (good cold), medium temperature and high temperature (hot). The growth temperature of low temperature microorganisms is 20 or less, the growth temperature of mesophilic microorganisms is 20 to 45, and the growth temperature of high temperature microorganisms is 45 or more.

The aerobic biological treatment is mainly at medium temperature, and the optimum growth temperature of microorganisms is 20-37. During anaerobic biological treatment, the optimum growth temperature of mesophilic microorganisms is 25-40°C, and the optimum growth temperature of high-temperature microorganisms is 50-60°C. Therefore, anAerobic microbial treatment often uses two temperature ranges of 33~38 and 52~57, which are called mesophilic digestion (fermentation) and high temperature digestion (fermentation). With the development of science and technology, the anaerobic reaction can be carried out at room temperature of 20~25, which greatly reduces the operating cost.

In a suitable temperature range, the rate of biochemical reaction increases by 1-2 times for every 10 increase. Therefore, the biological treatment effect is better under the condition of higher optimum temperature. 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 normal temperature. The effect of aerobic biological treatment is less affected by climate.

Anaerobic biological treatment is greatly affected by temperature and needs to maintain a higher temperature, but considering the operating cost, it should be operated at room temperature as much as possible (20~25). If the temperature of 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 (high-concentration organic sewage and sludge digestion) is produced during the fermentation process, the heat energy of the waste heat or biogas can be used to achieve medium-temperature and high-temperature fermentation. Under normal circumstances, the fluctuation of temperature within a day should not exceed. Therefore, the appropriate water temperature should be controlled and kept stable during biological treatment.

3pH

Enzyme is an ampholyte, and the change of pH value affects the ionized form of the enzyme, which in turn affects the catalytic performance of the enzyme. Therefore, the 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 range of bacteria, actinomycetes, algae and protozoa is 4~10.

The optimum 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 optimum pH for aerobic biological treatment was 6.5-8.5, and the optimum pH for anaerobic biological treatment was 6.7-7.4 (the optimum pH was 6.7-7.2). It is important to maintain the optimum pH range during biological treatment. Otherwise, the activity of microbial enzymes is reduced or lost, the growth of microorganisms is slow or even death, resulting in treatment failure.

Sudden changes in influent pH can have a large and irreversible effect on biological treatment. So keeping the pH stable is very important.

4 Dissolved oxygen

The metabolic process of aerobic microorganisms uses molecular oxygen as the 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, microorganisms suitable for low dissolved oxygen growth (micro-aerobic thiobacteria) and facultative microorganisms multiply.

They do not decompose organic matter completely, the treatment effect is reduced, and the filamentous bacteria grow dominantly in the low dissolved oxygen state, causing sludge bulking. Excessive dissolved oxygen concentration not only wastes energy, but also causes cell oxidation and death due to relative lack of nutrients. In order to achieve a good treatment effect, the dissolved oxygen should be controlled at 2~3mg/L (the effluent of the secondary sedimentation tank 0.5~1mg/L) during aerobic biological treatment.

Anaerobic microorganisms generate H2O2 under aerobic conditions, but are killed by H2O2 without enzymes to decompose H2O2. Therefore, molecular oxygen must never be present in the anaerobic biological treatment reactor. Other oxidation state 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

Chemical substances that inhibit and poison microorganisms are called toxic substances. It can damage the structure of cells, denaturing enzymes and inactivating them. For example, heavy metals can combine with the -SH group of enzymes, or combine with proteins to denature or precipitate them.

Toxic substances are harmless to microorganisms at low concentrations, and toxic when exceeding 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. The toxicity varies greatly under different conditions, and the tolerance of different microorganisms to the same toxic substance is also different. The specific situation should be determined according to the experiment.

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