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Chemical indicators of water pollution by organic substances. Water pollution with organic substances. Drinking water quality

To judge the epidemiological danger of water, bacteriological and chemical indicators of pollution are used.

Bacteriological indicators of water pollution. From an epidemiological point of view, it is predominantly pathogenic microorganisms that matter when assessing water. However, even with modern advances in microbiological technology, testing water for the presence of pathogenic microorganisms, and even more so viruses, is a rather labor-intensive process. Therefore, it is not carried out during mass water tests and is carried out only if there are epidemiological indications, for example, during outbreaks of infectious diseases in which water transmission is suspected.

In assessing water quality in sanitary practice, indirect bacteriological indicators of water pollution are widely used. It is believed that the less water is contaminated with saprophytes, the less dangerous it is from an epidemiological point of view.

One of the indicators of water pollution by saprophytic microflora is the so-called microbial number.

The microbial number is the number of colonies that grow when 1 ml of water is inoculated onto meat-peptone agar after 24 hours of cultivation at a temperature of 37°.

The microbial number characterizes the total bacterial contamination of water. When assessing the quality of water according to this indicator, they use observational data that in the water of unpolluted and well-equipped artesian wells the microbial number does not exceed 10-30 per 1 ml, in the water of unpolluted mine wells - 300-400 per 1 ml, in the water of relatively clean open reservoirs - 1000-1500 in 1 ml. With effective purification and disinfection of tap water, the number does not exceed 100 in 1 ml.

Of even greater importance is the determination of the presence of E. coli in water, which is excreted in the excrement of humans and animals. Therefore, the presence of E. coli in water signals fecal contamination and, therefore, possible contamination of water with pathogenic microorganisms of the intestinal group (typhoid fever, paratyphoid fever, dysentery, etc.).

Testing water for E. coli content allows us to foresee the possibility of contamination of water with pathogenic microflora in the future and, therefore, creates the opportunity to prevent it through timely implementation of the necessary measures.

The degree of contamination of water with E. coli is expressed by the value of the coli titer or coli index.

The coli titer is the smallest amount of test water in which, using the appropriate technique, E. coli is detected (grown). The lower the coli titre, the more significant the fecal contamination of water.

Coli index - the number of E. coli in 1 liter of water.

In clean water from artesian wells, the coli-titer is usually above 500 (coli-index less than 2), in uncontaminated and well-equipped wells the coli-titer is not lower than 100 (coli-index no more than 10).

A number of experimental studies have shown that E. coli is more resistant to disinfectants than the causative agents of intestinal infections, tularemia, leptospirosis and brucellosis, and therefore can serve not only as an indicator of water contamination, but also as an indicator of the reliability of its disinfection, for example, in a water supply system.

If, after water disinfection, the titer of E. coli rises to 300 (coli index no more than 3), then such water can be considered safe against the main pathogens of diseases spread by water.

Chemical indicators of water pollution. Chemical indicators of water pollution include organic substances and their breakdown products: ammonium salts, nitrites and nitrates. Apart from nitrates, these compounds themselves, in the quantities in which they are usually found in natural waters, do not affect human health. Their presence can only indicate contamination of the soil through which the water flows, feeding the water source, and that along with these substances pathogenic microorganisms could have entered the water.

In some cases, each of the chemical indicators may have a different nature, for example, organic substances are of plant origin. Therefore, a water source can be recognized as polluted only if the following conditions are met: 1) the water contains not one, but several chemical indicators of pollution; 2) bacterial indicators of contamination, such as E. coli, were simultaneously detected in the water; 3) the possibility of contamination is confirmed by a sanitary inspection of the water source.

An indicator of the presence of organic substances in water is oxidability, expressed in milligrams of oxygen spent on the oxidation of organic substances contained in 1 liter of water. Artesian waters have the lowest oxidability - up to 2 mg 02 per 1 liter; in the waters of mine wells, oxidability reaches 3-4 mg 02 per 1 liter, and it increases with increasing color of the water. In water from open reservoirs, oxidation can be even higher.

An increase in water oxidation above the above values ​​indicates possible contamination of the water source.

The main source of ammonia nitrogen and nitrites in natural waters is the decomposition of protein residues, animal corpses, urine, and feces.

With fresh pollution by waste, the content of ammonium salts in the water increases (exceeds 0.1 mg/l). Being a product of further chemical oxidation of ammonium salts, nitrites in quantities exceeding 0.002 mg/l also serve as an important indicator of contamination of a water source. It must be taken into account that in deep underground waters the formation of nitrites and ammonium salts from nitrates is possible during reduction processes. Nitrates are the end product of the oxidation of ammonium salts. Their presence in water in the absence of ammonia and nitrites indicates that nitrogen-containing substances, which have already been mineralized, entered the water relatively recently.

Chlorides are some indicator of the contamination of a water source, since they are contained in urine and various waste, but it must be taken into account that the presence of large quantities of chlorides in water (more than 30-50 mg/l) can also be caused by the leaching of chloride salts from saline soils.

To correctly assess the origin of chlorides, it is necessary to take into account the nature of the water source, the presence of chlorides in the water of neighboring water sources of the same type, as well as the presence of other indicators of water pollution.

DIRECT CYCLE OF DECOMPOSITION OF NITROGEN-CONTAINING ORGANIC COMPOUNDS

It is represented by undecomposed protein substances, often of animal origin, as well as nitrogen, which is part of microorganisms, low plants and undecomposed remains of higher plants.

At the beginning of decomposition, ammonia is formed, then under the action of nitrifying bacteria in the presence of a sufficient amount of oxygen, ammonia is oxidized to nitrous acid (NO 2 -) ( nitrites) and then enzymes of another microbial family oxidize nitrous acid into nitric acid (NO 3 -) (nitrates).

With fresh pollution by waste, the content of water increases. AMMONIUM SALTS, that is, the ammonium ion is 1. An indicator recent pollution water with organic substances of protein nature. 2. Ammonium ion can be found in clean waters containing humic substances and in waters of deep ground origin.

Detection of NITRITES in water indicates recent contamination of the water source with organic matter (the content of nitrites in the water should be no more than 0.002 mg/l).

NITRATES- this is the final product of the oxidation of ammonium compounds; the presence in water in the absence of ammonium and nitrite ions indicates long-standing pollution water source. The nitrate content in mine well water should be 10 mg/l; in drinking water from centralized water supply up to 45 mg/l).

The detection of the simultaneous presence of ammonium salts, nitrites and nitrates in water indicates constant and long-term organic pollution of water.

CHLORIDES- are extremely widespread in nature and are found in all natural waters. A large amount of them in water makes it undrinkable due to its salty taste. In addition, chlorides can serve as an indicator of possible contamination of a water source with wastewater, therefore chlorides as sanitary indicator substances can be important if tests for their content are carried out repeatedly, over a more or less long period of time. (GOST "Drinking water not >> 350 mg/l).

SULPHATES- are also important indicators of organic water pollution, since they are always contained in household wastewater. (GOST "Drinking water" not >> 500 mg/l).

OXIDIZABILITY- this is the amount of oxygen in mg consumed for the oxidation of organic substances contained in 1 liter of water.

DISSOLVED OXYGEN

Due to the lack of contact with air, groundwater very often does not contain oxygen. The degree of saturation of surface waters varies greatly. Water is considered clean if it contains 90% of the maximum possible oxygen content at a given temperature, Medium purity - at 75-80%; Doubtful - at 50-75%; Contaminated - less than 50%.

According to the “Rules for the Protection of Surface Waters from Pollution,” the oxygen content in water at any time of the year must be at least 4 mg/l in a sample taken before 12 noon.

Due to significant fluctuations in the absolute oxygen content in natural waters, a more valuable indicator is the amount of oxygen consumption during a certain storage period of water at a certain temperature (BIOCHEMICAL DEMAND FOR OXYGEN for 5 or 20 days - BOD 5 - BOD 20).

To determine it, the test water is saturated with atmospheric oxygen by vigorous shaking, the initial oxygen content in it is determined and left for 5 or 20 days at a temperature of 20 0 C. After this, the oxygen content is determined again. Most often the indicator BOD 5 used to characterize the processes of self-purification of water bodies from pollution by industrial and domestic wastewater.

MAIN SOURCES OF RESERVOIR POLLUTION, CONSEQUENCES OF RESERVOIR POLLUTION

The main sources of water pollution are:

1. industrial and domestic wastewater (domestic water has high bacterial and organic contamination)

2. drainage water from irrigated lands

3. wastewater from livestock complexes (may contain pathogenic bacteria and helminth eggs)

4. organized (storm drainage) and unorganized surface runoff from the territory of settlements, agricultural fields (use of various chemicals - mineral fertilizers, pesticides, etc.)

5. mole wood rafting;

6. water transport (3 types of wastewater: fecal, domestic and water obtained in engine rooms).

In addition, additional sources of water contamination by pathogens of intestinal infections can be: hospital wastewater; mass bathing; washing clothes in a small pond.

Pollution entering water bodies:

1. violate the normal living conditions of the biocenosis of the reservoir;

2. contribute to changes in the organoleptic parameters of water (color, taste, smell, transparency);

3. increase bacterial contamination of water bodies. Human consumption of water that has not undergone purification and disinfection methods leads to the development of: infectious diseases, namely bacterial, dysentery, cholera, viral (viral hepatitis), zoonoses (leptospirosis, tularemia), helminthiasis, as well as human infection with protozoa (amoeba, ciliates slipper);

4. increase the amount of chemicals, the excess of which in drinking water contributes to the development of chronic diseases (for example, the accumulation of lead, beryllium in the body)

Therefore, the following hygienic requirements are imposed on the quality of drinking water:

1. Water must be epidemiologically safe against acute infectious diseases;

2. must be harmless in chemical composition;

3. water must have favorable organoleptic characteristics, must be pleasant to the taste, and must not cause aesthetic objection.

To reduce human morbidity associated with water-borne transmission, it is necessary:

implementation of an environmental complex of measures (enterprises are sources of pollution) and control over its implementation (controlling bodies of the Ministry of Natural Economy, FS Rospotrebnadzor);

application of methods to improve the quality of drinking water (vodokanal);

drinking water quality control.

Natural water has a slightly alkaline reaction (6.0-9.0). An increase in alkalinity indicates pollution or blooming of the reservoir. An acidic reaction of water is observed in the presence of humic substances or the penetration of industrial wastewater.

Rigidity. The hardness of water depends on the chemical composition of the soil through which the water passes, the content of carbon monoxide in it, and the degree of contamination with organic matter. It is measured either in mEq/L or in degrees. According to the degree of hardness, water can be: soft (up to 3 mg-eq/l); medium hardness (7 mg = eq/L); hard (14 mg=eq/l); very hard (over 14 mg-eq/L). Very hard water has an unpleasant taste and can worsen the course of kidney stones.

The oxidability of water is the amount of oxygen in milligrams that is consumed for the chemical oxidation of organic and inorganic substances contained in 1 liter of water. Increased oxidation may indicate water contamination.

Sulfates in quantities exceeding 500 mg/l give water a bitter-salty taste; at a concentration of 1000-1500 mg/l they adversely affect gastric secretion and can cause dyspepsia. Sulfates can be an indicator of contamination of surface waters by animal waste.

An increased iron content causes coloring, cloudiness, gives the water a smell of hydrogen sulfide, an unpleasant inky taste, and in combination with humic compounds - a marshy taste.

Ammonia in water is regarded as an indicator of epidemiologically dangerous fresh water pollution with organic substances of animal origin. An indicator of more recent contamination are salts of nitrous acid - nitrates, which are products of ammonia oxidation under the influence of microorganisms during the process of nitrification. The presence of nitrates in water without ammonia and salts of nitrous acid indicates the completion of the mineralization process and, with a high content of them in water, indicate long-standing contamination of it . However, the content of all three components in water - ammonia, nitrites and nitrates - indicates the incompleteness of the mineralization process and epidemiologically dangerous water pollution.

52. Methods for improving water quality .

I.Basic methods

1. Clarification and decolorization (purification): settling, filtration, coagulation.

2. Disinfection: boiling, chlorination, ozonation, irradiation with UV rays, the use of the oligodynamic action of silver, the use of ultrasound, the use of gamma rays.


II.Special processing methods: deodorization, degassing, deferrization, softening, desalination, defluoridation, fluoridation, decontamination.

At the first stage of water purification from an open water source, it is clarified and discolored. Clarification and decolorization refers to the removal of suspended substances and colored colloids (mainly humic substances) from water and is achieved by settling and filtration. These processes are slow and the bleaching efficiency is low. The desire to accelerate the sedimentation of suspended particles and speed up the filtration process led to preliminary coagulation of water with chemicals (coagulants) that form hydroxides with quickly settling flakes and accelerating the sedimentation of suspended particles.

Aluminum sulfate – Al2(SO4)3 – is used as coagulants; ferric chloride – FeCl3; iron sulfate - FeSO4, etc. Coagulants, when properly treated, are harmless to the body, since the residual amounts of aluminum and iron are very small (aluminum - 1.5 mg/l, iron - 0.5 - 1.0 mg/l).

After coagulation and settling, the water is filtered using fast or slow filters.

With any scheme, the final stage of water treatment at a water treatment plant should be disinfection. Its task is to destroy pathogenic microorganisms, i.e. ensuring epidemic water safety. Disinfection can be carried out by chemical and physical (reagent-free) methods.

Boiling is a simple and reliable method. Vegetative microorganisms die when heated to 800C within 20–40 seconds, so at the moment of boiling the water is actually disinfected.

Ultrasound is used to disinfect domestic wastewater. It is effective against all microorganisms, including spore forms, and its use does not lead to foaming when disinfecting household wastewater.

Gamma radiation is a very reliable and effective method that instantly destroys all types of microorganisms.

Reagents that do not change the chemical composition of water during disinfection include ozone.

Currently, the main method used for water disinfection at water supply stations due to technical and economic reasons is the chlorination method.

The effectiveness of water disinfection depends on the selected dose of chlorine, the time of contact of active chlorine with water, the temperature of the water and many other factors.

Modifications of chlorination include: double chlorination, chlorination with ammoniation, rechlorination.

Conditioning the mineral composition of water can be divided into removing salts or gases from water that are in excess quantities (softening, desalting and desalination, deferrization, defluoridation, degassing, decontamination, etc.) and adding minerals in order to improve the organoleptic and physiological properties of water (fluoridation, partial mineralization after desalination, etc.).

To disinfect individual water supplies, tablet forms containing chlorine are used. Aquasept, tablets containing 4 mg of active chlorine monosodium salt of dichloroisocyanuric acid. Pantocide is a drug from the group of organic chloramines, solubility is 15-30 minutes. Releases 3 mg of active chlorine.

22.12.2016

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Today we tell you everything you wanted to know about organic water pollutants.

Organic water pollutants

In addition to inorganic substances (iron, manganese, fluorides), water also contains organic substances. In our blog you will learn about the types of organic pollutants and how to detect their excess.

Sources of water pollution:

There are 3 main types of sources of water pollution:

  • Settlements. In this case, sewage drains are the main place of accumulation of household waste. Every day, people use huge amounts of water for consumption, cooking, hygiene and cleaning, after which this water, along with detergents and food waste, ends up in the sewer system. Then the water is cleaned by municipal facilities and the water is returned for reuse.
  • Industry. It is a major pollutant in developed countries with a huge number of enterprises. The amount of wastewater they emit is three times higher than municipal wastewater.
  • Agriculture. In this area, crop production intensively pollutes water bodies due to the use of fertilizers and pesticides. About a quarter of nitrogen fertilizers, a third of potassium and 4% of phosphorus fertilizers end up in water bodies.

Impact of organic pollutants on human health

There are many diseases caused by water pollution. For example, washing your face with contaminated water can cause conjunctivitis. Shellfish and algae living in water can cause schistosomiasis (fever, liver pain).

How to determine the amount of organic matter in water

The value characterizing the content of organic and mineral substances in water is called oxidability. To estimate chemical oxygen demand, i.e. water oxidability, the dichromate and permanganate methods are used. Determining bichromate oxidability requires quite a long time, so it is not convenient for mass monitoring of the operation of treatment facilities. It is permanganate oxidation that regulates the quality of drinking water according to SanPiN.

What is permanganate oxidation?

Permanganate oxidability is an indicator obtained to evaluate COD using the permanganate method, in other words, it is an indicator of the total amount of organic substances in water. Permanganate oxidability is expressed in milligrams of oxygen used to oxidize these substances contained in 1 dm3 of water. This indicator does not indicate the organic substances contained in water, but only indicates the excess of their quantity.

Signs of excess permagane oxidation

→ Wastewater treatment

Sanitary and chemical indicators of wastewater pollution


The composition of wastewater and its properties are assessed based on the results of sanitary-chemical analysis, which, along with standard chemical tests, includes a number of physical, physico-chemical and sanitary-bacteriological determinations.

The complexity of the composition of wastewater and the impossibility of determining each of the pollutants leads to the need to select indicators that would characterize certain properties of water without identifying individual substances. Such indicators are called group or total. For example, the determination of organoleptic indicators (smell, color) allows one to avoid the quantitative determination in water of each of the substances that have an odor or give color to the water.

A complete sanitary-chemical analysis involves the determination of the following indicators: temperature, color, odor, transparency, pH value, dry residue, dense residue and losses on ignition, suspended substances, settling substances by volume and mass, permanganate oxidation, chemical demand for oxygen demand (COD), biochemical oxygen demand (BOD), nitrogen (total, ammonium, nitrite, nitrate), phosphates, chlorides, sulfates, heavy metals and other toxic elements, surfactants, petroleum products, dissolved oxygen, microbial count, coli bacteria (coliforms), helminth eggs. In addition to the listed indicators, the mandatory tests of a complete sanitary-chemical analysis at urban wastewater treatment plants may include the determination of specific impurities entering the drainage network of settlements from industrial enterprises.

Temperature is one of the important technological indicators; a function of temperature is the viscosity of the liquid and, consequently, the force of resistance to settling particles. Therefore, temperature is one of the determining factors in the sedimentation process. Temperature is of utmost importance for biological purification processes, since the rates of biochemical reactions and the solubility of oxygen in water depend on it.

Color is one of the organoleptic indicators of wastewater quality. Domestic fecal wastewater is usually slightly colored and has a yellowish-brownish or gray tint. The presence of intense coloring of various shades is evidence of the presence of industrial wastewater. For colored wastewater, determine the color intensity by diluting it to colorless, for example 1:400; 1:250, etc.

Odor is an organoleptic indicator characterizing the presence of odorous volatile substances in water. Usually the smell is determined qualitatively at a sample temperature of 20°C and is described as fecal, putrefactive, kerosene, phenolic, etc. If the odor is unclear, the determination is repeated by heating the sample to 65°C. Sometimes it is necessary to know the threshold number - the smallest dilution at which the smell disappears.

The concentration of hydrogen ions is expressed by the pH value. This indicator is extremely important for biochemical processes, the speed of which can significantly decrease with a sharp change in the reaction of the environment. It has been established that wastewater supplied to biological treatment facilities must have a pH value in the range of 6.5 - 8.5. Industrial wastewater (acidic or alkaline) must be neutralized before being discharged into the drainage network to prevent its destruction. Municipal wastewater usually has a slightly alkaline reaction (pH = 7.2-7.8).

Transparency characterizes the general contamination of wastewater with undissolved and colloidal impurities, without identifying the type of contamination. The transparency of municipal wastewater is usually 1-3 cm, and after treatment it increases to 15 cm.

Dry residue characterizes the total contamination of wastewater with organic and mineral impurities in various states of aggregation (in mg/l). This indicator is determined after evaporation and further drying of the wastewater sample at t = 105 °C. After calcination (at t = 600°C), the ash content of the dry residue is determined. Based on these two indicators, one can judge the ratio of the organic and mineral parts of contaminants in the dry residue.

Solid residue is the total amount of organic and mineral substances in a filtered wastewater sample (in mg/l). Determined under the same conditions as dry residue. After calcination of the dense residue at t = 600°C, it is possible to roughly estimate the ratio of the organic and mineral parts of soluble wastewater contaminants. When comparing calcined dry and dense municipal wastewater residues, it was determined that most of the organic pollutants are in an undissolved state. In this case, mineral impurities are mostly in dissolved form.

Suspended solids is an indicator characterizing the amount of impurities that is retained on a paper filter when filtering a sample. This is one of the most important technological indicators of water quality, allowing one to estimate the amount of sediment generated during wastewater treatment. In addition, this indicator is used as a design parameter when designing primary settling tanks. The amount of suspended solids is one of the main standards when calculating the required degree of wastewater treatment. Losses on ignition of suspended solids are determined in the same way as for dry and dense residues, but are usually expressed not in mg/l, but as a percentage of the mineral part of suspended solids to their total amount in dry matter. This indicator is called ash content. The concentration of suspended solids in municipal wastewater is usually 100 – 500 mg/l.

Settling substances are part of the suspended substances that settle to the bottom of the settling cylinder after 2 hours of settling at rest. This indicator characterizes the ability of suspended particles to settle, allows one to evaluate the maximum settling effect and the maximum possible volume of sediment that can be obtained under resting conditions. In urban wastewater, settling substances on average account for 50-75% of the total concentration of suspended solids.

Oxidability refers to the total content of organic and inorganic reducing agents in water. In urban wastewater, the overwhelming majority of reducing agents are organic substances, so it is believed that the oxidability value is entirely related to organic impurities. Oxidability is a group indicator. Depending on the nature of the oxidizing agent used, a distinction is made between chemical oxidation, if a chemical oxidizer is used in the determination, and biochemical, when aerobic bacteria play the role of an oxidizing agent - this indicator is the biochemical oxygen demand - BOD. In turn, chemical oxidation can be permanganate (oxidizer KMn04), dichromate (oxidizer K2Cr207) and iodate (oxidizer KJ03). The results of determining oxidability, regardless of the type of oxidizer, are expressed in mg/l 02. Dichromate and iodate oxidability are called chemical oxygen demand or COD.

Permanganate oxidability is the oxygen equivalent of easily oxidized impurities. The main value of this indicator is the speed and ease of determination. Permanganate oxidation is used to obtain comparative data. However, there are substances that are not oxidized by KMp04. By determining COD, it is possible to fairly fully assess the degree of water contamination with organic substances.

BOD is the oxygen equivalent of the degree of contamination of wastewater with biochemically oxidizable organic substances. BOD determines the amount of oxygen required for the life of microorganisms involved in the oxidation of organic compounds. BOD characterizes the biochemically oxidizable part of organic pollutants in wastewater, which are primarily in dissolved and colloidal states, as well as in the form of suspension.
To mathematically describe the process of biochemical oxygen consumption, the first-order kinetic equation is most often used. To derive the equation, we introduce a number of notations: La – the amount of oxygen required for the oxidation of all organic matter, i.e. BODtotal mg/l; Lt – the same consumed at time t, i.e. BOD mg/l; La – Lt – the same remaining in solution at time t, mg/l.

Nitrogen is found in wastewater in the form of organic and inorganic compounds. In urban wastewater, the main part of organic nitrogenous compounds consists of protein substances - feces, food waste. Inorganic nitrogen compounds are represented by reduced – NH4+ and NH3 oxidized forms N02” and N03”. Ammonium nitrogen is formed in large quantities during the hydrolysis of urea, a human waste product. In addition, the process of ammonification of protein compounds also leads to the formation of ammonium compounds.

In urban wastewater before its treatment, nitrogen in oxidized forms (in the form of nitrites and nitrates) is usually absent. Nitrites and nitrates are reduced to molecular nitrogen by a group of denitrifying bacteria. Oxidized forms of nitrogen can appear in wastewater only after biological treatment.

The source of phosphorus compounds in wastewater is physiological excretions of people, waste from human economic activities and some types of industrial wastewater. The concentrations of nitrogen and phosphorus in wastewater are the most important indicators | tools for sanitary-chemical analysis that are important for biological treatment. Nitrogen and phosphorus are necessary components of the composition of bacterial cells. They are called biogenic elements. In the absence of nitrogen and phosphorus, the biological treatment process is impossible.

Chlorides and sulfates are indicators whose concentration affects the total salt content.

The group of heavy metals and other toxic elements includes a large number of elements, which is increasingly increasing as knowledge about purification processes accumulates. Toxic heavy metals include iron, nickel, copper, lead, zinc, cobalt, cadmium, chromium, mercury; toxic elements that are not heavy metals - arsenic, antimony, boron, aluminum, etc.

The source of heavy metals is industrial wastewater from engineering plants, electronics, instrument-making and other industries. In wastewater, heavy metals are contained in the form of ions and complexes with inorganic and organic substances.

Synthetic surfactants (surfactants) are organic compounds consisting of hydrophobic and hydrophilic parts that cause the dissolution of these substances in oils and water. Approximately 75% of the total amount of surfactants produced is accounted for by anionic active substances; the second place in production and use is occupied by nonionic compounds. Surfactants of these two types are determined in urban wastewater.

Petroleum products are non-polar and low-polar compounds extracted with hexane. The concentration of petroleum products in reservoirs is strictly standardized, and since the degree of their retention at urban wastewater treatment plants does not exceed 85%, the content of petroleum products in the wastewater entering the station is also limited.

There is no dissolved oxygen in the wastewater entering the treatment plant. In aerobic processes, the oxygen concentration must be at least 2 mg/l.

Sanitary and bacteriological indicators include: determination of the total number of aerobic saprophytes (microbial number), coliform bacteria and analysis of helminth eggs.

The microbial number assesses the total contamination of wastewater with microorganisms and indirectly characterizes the degree of water contamination with organic substances - food sources for aerobic saprophytes. This indicator for municipal wastewater ranges from 106 to 108.