Hpc decoding. COD indicator for wastewater: what it is and standards. Assigned measurement error characteristics

Waste liquids may contain both suspended and soluble substances as hazardous pollutants that have a destructive effect on the environment - the presence of both the former and the latter must be taken into account when performing various types of work.

The main goal of wastewater treatment is to significantly reduce the concentration of pollutants until predetermined pre-standardized indicators are achieved, determined, in the vast majority of cases, by the legislative act currently in force.

The level of contamination of liquids can be reflected by several factors at once, the most important of which can confidently be considered BOD (more understandable to the human mind - biochemical oxygen consumption) and wastewater COD (to put it simply and more clearly - chemical oxygen demand).

Measurements of COD and BOD of wastewater must be carried out to a mandatory extent when carrying out liquid purification processes at various facilities and treatment plants. The layout of treatment facilities can sometimes be completely different, differ from the standards - depending on the current quantitative and qualitative characteristics of the liquid being treated and the degree of contaminants present.

The sequence of reducing BOD and COD in wastewater treatment processes

When carried out by knowledgeable craftsmen, primary wastewater treatment removes oil compounds, large particles, as well as numerous different types of contaminants. At this stage, mechanical and physical methods of cleansing are most often used.

Secondary treatment is the process of separating contaminants and suspended particles that may even be present in dissolved form. Liquid contaminants are organic in nature, and therefore they are purified using classical and innovative methods of biological oxidation.

During this stage, biological methods of wastewater treatment are consistently applied. It is worth noting that determining wastewater COD indicators is important in both the first and second cases.

When carrying out the so-called “tertiary” cleaning, it is necessary to sequentially remove all pollutants, small impurities and metal salts that could remain after the two previous cleanings. It is imperative to pay attention to the chemical oxygen consumption in wastewater. At this stage, physical and chemical methods are actively used: electrodialysis, osmosis, filtration through an adsorbent layer and various others.

During the fourth stage, the sludge is completely (as far as possible) dehydrated to minimize its weight and volume. Carrying out this operation is in no way a so-called “panacea”, which should lead to a decrease in the degree of BOD and COD.

When carrying out any stage of purification, an indicator such as biochemical oxygen consumption in wastewater can be optimized to the required values ​​(this depends, first of all, on the specifics and nature of the contaminated liquids).

Contaminant purification processes are not always carried out using all four stages of processing.

It happens that at the end of the first stage, wastewater treatment facilities discharge wastewater into the city sewer, since the required, permissible pollution standards have already been achieved (the permitted limit has been exceeded).

Europeans, by the way, prefer not to allow extremes, but try to ensure that the method for determining COD in wastewater is more accurate than a similar procedure carried out on the territory of the Russian Federation - it is quite possible that in this (in addition to the material aspect and the instrumental part) , is also the advantage of more advanced countries over our wastewater treatment plants.

Differences between industrial and domestic wastewater

If you try to think about what BOD is in wastewater, and what role this indicator can play in the final version, then you need, first of all, to try to become as thoroughly and in detail familiar with the nature of emissions as possible.

In general, it is always very important to correctly calculate the BOD - without calculations, as they say, “you get nowhere.” In general, pollution can be of domestic and industrial origin (this is the division according to the official principle) - accordingly, the nature of the pollution when different types of water are released will differ.

Domestic wastewater, in the vast majority of cases, is contaminated with organic residues, garbage, and detergents.

In the option of combining household wastewater with industrial wastewater, the organic matter of household wastewater can be confidently considered an additional feeding medium for activated sludge, which will contribute to the improved functioning of biology. At the same time, the COD level of wastewater will also need to be taken into account, since neglecting this indicator entails serious consequences.

FEDERAL SUPERVISION SERVICE
IN THE FIELD OF NATURE MANAGEMENT

QUANTITATIVE CHEMICAL ANALYSIS OF WATER

MEASUREMENT TECHNIQUE
CHEMICAL OXYGEN CONSUMPTION (COD)
IN DRINKING, NATURAL AND WASTE SAMPLES
WATER BY PHOTOMETRIC METHOD

PND F 14.1:2:4.210-2005

The technique is approved for government purposes
environmental control

MOSCOW 2005
(2013 edition)

1 GENERAL PROVISIONS AND SCOPE OF APPLICATION

This regulatory document establishes a photometric method for determining bichromate oxidation (chemical oxygen demand, hereinafter referred to as COD). The methodology applies to the following objects of analysis: drinking water; natural fresh water, including surface and underground water supply sources; industrial, domestic, stormwater and treated wastewater. The technique can be used to analyze samples of melted water, industrial water and snow cover samples.

The range of measured COD values ​​is from 10 to 30,000 mg/dm 3 (by method A - from 10 to 100 mg/dm 3 and by method B - from 100 to 30,000 mg/dm 3).

If the COD value is over 1000 mg/dm 3, preliminary dilution of the sample is necessary.

The technique can be used to analyze water samples with higher COD values, provided they are pre-diluted, but not more than 100 times.

The duration of analysis of one sample is 4 hours, a series of 25 samples is 5 hours. A flow diagram of the analysis is given in.

2 REGULATORY REFERENCES

5.1.2 State standard sample (hereinafter - GSO) of dichromate oxidability of water with an error of the certified value at a confidence probability of P = 0.95 of no more than 2%;

5.1.3 Distiller or installation of any type for producing distilled water in accordance with GOST 6709 or water for laboratory analysis of 2nd degree of purity in accordance with GOST R 52501;

5.1.4 Medical laboratory dispensers, desktop (mounted on a vessel) or manual, single-channel with a fixed or variable dosing volume in accordance with GOST 28311;

5.1.5 Volumetric flasks with a capacity of 100 and 1000 cm 3 according to GOST 1770, accuracy class 2;

5.1.6 Glass cuvettes with screw caps for the spectrophotometer. Cuvette dimensions: height 100 mm, diameter 16 mm;

5.1.7 Graduated pipettes with a capacity of 1; 2; 5; 10 cm 3 according to GOST 29227, accuracy class 2;

5.1.8 Pipettes with one mark, capacity 1; 2; 5; 10; 100 cm 3 according to GOST 29169, accuracy class 2;

5.1.9 Reactor for mineralization with cells for round cuvettes, providing a temperature of (150 ± 5) °C (mineralizer), for example, from NACH (USA);

5.1.10 Thermometer for a mineralizer with a scale range from 100 °C to 200 °C and a division value of 2 °C;

5.1.11 Dark glass bottles with a capacity of 500; 1000 cm 3;

5.1.12 Spectrophotometer, providing measurements at wavelengths of 450 nm and 620 nm, equipped with an adapter for round cells, for example, from NACH (USA);

5.1.13 Weighing cups with a capacity of 50 cm 3 in accordance with GOST 25336;

5.1.14 Drying cabinet of any brand that provides a temperature of (105 ± 5) °C, for example, SNOL-3.5 according to TU 16-681.032;

5.1.15 Household refrigerator of any brand, providing a temperature of (2 - 10) °C;

5.1.16 Spatula;

5.1.17 Rack for storing cuvettes;

5.1.18 Protective screen for the reactor, made of polycarbonate.

It is allowed to use measuring instruments, auxiliary equipment, and laboratory glassware with similar or better metrological and technical characteristics.

5.2 Reagents and materials

5.2.1 Distilled water according to GOST 6709 or for laboratory analysis according to GOST R 52501 (2nd degree of purity), (hereinafter referred to as distilled water);

5.2.2 Potassium dichromate (potassium dichromate), chemical grade. according to GOST 4220 or standard titer, for example, according to TU 6-09-2540;

5.2.4 Mercury (II) sulfate (mercuric sulfate), analytical grade. according to TU 2624-004-48438881;

6.2 When working with equipment, it is necessary to comply with electrical safety rules when working with electrical installations in accordance with GOST R 12.1.019.

6.3 Organization of occupational safety training for workers is carried out in accordance with GOST 12.0.004

6.4 The laboratory premises must comply with fire safety requirements in accordance with GOST 12.1.004 and have fire extinguishing equipment in accordance with GOST 12.4.009.

7 OPERATOR QUALIFICATION REQUIREMENTS

Persons who have a special secondary or higher education in chemistry, who are proficient in photometric analysis techniques and who have studied the rules of operation of the equipment used are allowed to carry out measurements and process their results.

8 CONDITIONS FOR PERFORMING MEASUREMENTS

When performing measurements in the laboratory, the following conditions must be met:

15 CONTROL OF ACCURACY OF MEASUREMENT RESULTS

15.1 In the case of regular analysis according to the method, it is recommended to monitor the stability of measurement results by monitoring the standard deviation of repeatability, the standard deviation of intra-laboratory precision and error in accordance with the recommendations of GOST R ISO 5725 (Part 6). A control sample is prepared using GSO and distilled water. The frequency of monitoring is regulated in the internal documents of the laboratory.

15.2 On-line monitoring of the accuracy of measurement results is recommended to be carried out with each series of samples if the analysis according to the method is performed sporadically, as well as if there is a need to confirm the measurement results of individual samples (if a non-standard measurement result is obtained; a result exceeding the maximum permissible concentration, etc.).

As control samples, samples prepared using GSO and distilled water are used. Control samples with COD values ​​less than 40 mg/dm 3 are used freshly prepared, and samples with COD values ​​of (40 - 1000) mg/dm 3 are stored for 1 month at a temperature of (2 - 10) °C.

Operational control of the measurement procedure is carried out by comparing the result of a single control procedure (K k) with the control standard (K).

The result of the control procedure K k is calculated using the formula:

where Δ l is the error characteristic of the certified COD value in the control sample, established in the laboratory when implementing the method, mg/dm 3 .

Note - At the first stage of control after implementation of the technique, it is allowed to consider Δ l = 0.84·Δ, where Δ is the assigned error characteristic of the technique, which is calculated using the formula

The values ​​of δ are given in the table.

The quality of the control procedure is considered satisfactory when the following conditions are met:

If the condition is not met, the control is repeated. If the conditions are not met again, the reasons leading to unsatisfactory results are clarified.

To assess water pollution with organic compounds, the BOD value is used, however, it takes 5 days to determine the BOD, and sometimes data is required much faster. In this case, instead of microorganisms, potassium dichromate is used to oxidize organic substances in the presence of sulfuric acid (when heated). This mixture oxidizes almost all organic substances contained in contaminated water. The value characterizing the content of organic substances in water that are oxidized by one of the strong chemical oxidizers under certain conditions is called chemical oxygen demand (COD) or water oxidability. COD is expressed in milligrams of oxygen used to oxidize substances contained in 1 dm 3 water. The method of determination is titrimetric.

In accordance with the requirements for the composition and properties of water in reservoirs near drinking water use points, the COD value should not exceed 15 mg O 2 /dm 3; in recreation areas in water bodies, a COD value of up to 30 mg O 2 /dm 3 is allowed.

Dissolved oxygen

Dissolved oxygen is found in natural water in the form of molecules O 2 . Its content in water is affected by two groups of oppositely directed processes: some increase the oxygen concentration, others reduce it. The first group of processes that enrich water with oxygen includes:

the process of absorption of oxygen from the atmosphere;

release of oxygen by aquatic vegetation during photosynthesis;

entry into reservoirs with rain and snow waters, which are usually supersaturated with oxygen.

Absorption of oxygen from the atmosphere occurs on the surface of a water body. The rate of this process increases with decreasing temperature, increasing pressure and decreasing mineralization. Aeration - the enrichment of deep layers of water with oxygen - occurs as a result of mixing of water masses, including wind, vertical temperature circulation, etc.

The release of oxygen as a result of photosynthesis occurs when carbon dioxide is assimilated by aquatic vegetation (attached, floating plants and phytoplankton). The process of photosynthesis proceeds more strongly, the higher the water temperature, the intensity of sunlight and the more nutrients (nutrients) ( P,N etc.) in water. Oxygen production occurs in the surface layer of the reservoir, the depth of which depends on the transparency of the water (it can be different for each reservoir and season, from several centimeters to several tens of meters).

The group of processes that reduce the oxygen content in water includes reactions of its consumption to the oxidation of organic substances: biological (respiration of organisms), biochemical (respiration of bacteria, oxygen consumption during the decomposition of organic substances) and chemical (oxidation Fe 2+ ,Mn 2+ ,NO 2 - ,N.H. 4 + ,CH 4 ,H 2 S). The rate of oxygen consumption increases with increasing temperature, the number of bacteria and other aquatic organisms and substances subject to chemical and biochemical oxidation. In addition, a decrease in the oxygen content in water can occur due to its release into the atmosphere from the surface layers and only if the water at a given temperature and pressure turns out to be supersaturated with oxygen.

In surface waters, the content of dissolved oxygen varies widely - from 0 to 14 mg/dm 3 - and is subject to seasonal and daily fluctuations. Daily fluctuations depend on the intensity of the processes of its production and consumption and can reach 2.5 mg/dm 3 of dissolved oxygen. Oxygen deficiency is more often observed in water bodies with high concentrations of polluting organic substances and in eutrophicated water bodies containing large amounts of nutrients and humic substances.

The oxygen concentration determines the magnitude of the redox potential and, to a large extent, the direction and speed of the processes of chemical and biochemical oxidation of organic and inorganic compounds. The oxygen regime has a profound impact on the life of a reservoir. The minimum content of dissolved oxygen that ensures normal development of fish is about 5 mg/dm3. Reducing it to 2 mg/dm 3 causes mass death (killing) of fish.

The relative oxygen content of water, expressed as a percentage of its normal content, is called the degree of oxygen saturation. This value depends on water temperature, atmospheric pressure and salinity. Calculated by the formula:

M = ,

Where M– degree of water saturation with oxygen, %; A– oxygen concentration, mg/dm3; R– atmospheric pressure in a given area, Pa; N– normal oxygen concentration at a given temperature, salinity (salinity) and total pressure of 101308 Pa.

In accordance with the requirements for the composition and properties of water in reservoirs near drinking and sanitary water use points, the content of dissolved oxygen in a sample taken before 12 noon should not be lower than 4 mg/dm 3 at any time of the year; for fishery reservoirs, the concentration of oxygen dissolved in water should not be lower than 4 mg/dm 3 in winter (during freeze-up) and 6 mg/dm 3 - in the summer.

In the context of the topic of caring for the environment, the issue of keeping rivers and other bodies of water clean is often discussed. Now this is extremely difficult to do, because the wastewater that is discharged into water bodies is highly polluted.

After active participation in one or another industrial process, wastewater accumulates a huge amount of harmful elements, which, when released into an open body of water, lead to the death of aquatic inhabitants and plants, as well as other unpleasant consequences.

To measure the degree of pollution of wastewater, several indicators are taken as a basis, one of which is COD. What is COD and how to reduce this indicator, we will tell you in this material.

Why do we need indicators of the degree of wastewater pollution?

The amount of wastewater pollution can be identified by a number of indicators, the most common among them are:

• COD or chemical oxygen demand;
• BOD is its biochemical consumption.

Measuring an indicator such as COD is necessary to analyze the quality of wastewater or liquid in a reservoir or to study the state of waters in general. COD is a quantitative indicator, it is one of the most informative and detailed.

The following substances act as wastewater pollutants:

• dissolved;
• weighted.

The method for studying the state of a liquid taking into account COD is to determine the amount of oxygen that was spent on the oxidation of organic matter and minerals containing carbon. COD is also called unit of chemical oxidability of water, since organic substances are oxidized by oxygen. After all, it, in turn, is one of the most powerful oxidizing agents.

Oxidability, depending on the origin of the oxidizing agents, can be of the following types:

• iodate;
• bichromate;
• cerium;
• permanganate.

The most accurate indicators are determined by application of the dichromate or iodate method. Oxidability is expressed as the ratio of the volume of oxygen that was spent on the oxidation of mineral and organic substances. It is expressed in milligrams per 1 square meter. dm. liquids.

It is necessary to purify wastewater in order to reduce the concentration of harmful substances to normal levels, which are approved in regulatory documents.

Cleaning is carried out at special treatment facilities or stations. Their layout depends on the quantity and quality of wastewater, as well as the level of its contamination. However, the wastewater treatment scheme will be the same and the main goal of the work is to reduce COD and BOD.

COD and BOD as criteria for water pollution

The COD value includes the total content of organic substances in the liquid in the volume of consumed bound oxygen for their oxidation. COD is a general indicator of industrial and natural water pollution.

But such an indicator as BOD determines the amount of dissolved oxygen that is spent on the oxidation of organic substances by bacteria in the required volume of liquid.

For identical samples, the COD value will be higher than the BOD value, since more substances are subject to chemical oxidation.

What factors influence COD

There are a lot of factors that can affect the composition of harmful substances and the acidity of a liquid. One of the key factors is a set of biochemical processes occurring in the reservoir itself. As a result of these processes, substances react with each other and form new ones, which may differ in structure from the previous ones and have a different chemical composition.

These substances can enter the reservoir as follows:

• along with precipitation;
• together with domestic or industrial wastewater;
• with underground and surface wastewater.

Their structure and composition can be very different, in particular, which of them can be resistant to oxidizing agents. Depending on this factor, you need to choose the most effective oxidizing agent for certain substances.

In surface waters, organic matter can be suspended, dissolved, or colloidal. Oxidability is different for filtered and unfiltered samples. Natural waters are less susceptible to pollution by organic matter of natural origin.

Surface waters have a higher degree of oxidation compared to such types of water as:

• underground;
• ground and others.

For example, mountain rivers and lakes have oxidation in the region of 2–3 mg per cubic decimeter, rivers fed by swamps - 20 mg/cubic meter. dm and flat reservoirs - from 5 to 12, respectively.

A significant factor that affects oxidation is seasonal changes occurring in hydrobiological and hydrological regimes.

Also, the oxidation of a reservoir can change under the influence of human activity; depending on the sphere of human activity, pollution of one type or another enters the reservoir.

Requirements for the COD indicator according to the standard

According to the standard, COD indicators should fluctuate ranging from 15 to 30 mg/cu.m. dm. The degree of wastewater pollution according to COD indicators looks like this:

• very pure – up to 2 mg/cu. dm;
• relatively pure – 3 mg/cu. dm;
• average pollution – 4 mg/cu. dm;
• contaminated – 15 mg/cu.d.m. and higher.

Stages of wastewater treatment and reduction of pollution levels

Wastewater treatment includes the following stages:

• primary cleaning – This is the removal of oil films, large pieces of dirt and numerical contaminants that are easily removed. This stage involves cleaning using a physical-mechanical method;
• secondary treatment. At this stage, suspended parts and pollutants, which are contained even in dissolved form, are separated. Some pollutants are organic in origin and must be removed through biological oxidation. This stage involves a biological method of wastewater treatment;
• tertiary treatment – This is the removal of all remaining small particles and contaminants, including metal salts. Purification is carried out by osmosis, electrodialysis, filtration through an adsorbent, etc.;
• fourth stage – At this stage, the sludge is dewatered, which reduces its volume and weight to a minimum.

The level of COD and BOD is gradually reduced to certain values ​​at each stage, the amount of their reduction depends on the characteristics of the wastewater.

Wastewater is not always treated in all four stages. Very often, treatment plants discharge wastewater into the collector after the first stage of treatment, and this brings COD levels back to normal. In some countries, purification is carried out in only two stages, the third stage being used only as a last resort.

The difference between domestic wastewater and industrial wastewater

Wastewater can be of industrial or domestic origin; the nature of the contaminants in them is also different. So, as a rule, household wastewater is contaminated with such things as:

• garbage;
• organic residues;
• detergents.

But industrial drains are filled with industrial waste, if it is the food industry, then there Most of all there will be suspended substances and fats. COD and BOD values ​​in industrial wastewater will be higher than in domestic wastewater.

Sometimes wastewater is combined, as a result of which organic matter from domestic wastewater becomes a breeding ground for activated sludge from bioremediation.

Ranges of criteria ratios for different waters

An analysis of such an indicator as COD is carried out to determine how much oxygen equivalent to dichromate is contained, which was used to oxidize all organic and inorganic substances in the sample.

As mentioned earlier, a value such as COD, which evaluates the reducing activity of chemicals, will be greater than BOD, the value of which depends solely on the amount of organic matter subject to biochemical decomposition. The relationship between these two indicators reflects the completeness of biochemical oxidation of substances, which are contained in wastewater. The greater the difference between these indicators, the greater the increase in biologically active masses. In particular, this ratio can be used to determine how suitable the wastewater is for biological treatment.

If there are few substances susceptible to biochemical oxidation, then it is best to use physicochemical methods for research that can bring the ratio of indicators to the required figure.

Optimal range the ratio of BOD and COD is from 0.4 to 0.75 units. The optimal value for the ratio between the chemical and biological demand for oxygen is 0.7, with which the biological treatment process can proceed fully and fully.

Once the wastewater is separated by gravity, it removes mainly those substances that are difficult to oxidize. After this stage, the ratio of indicators increases.

Then follows biological treatment stage, as a result of which the ratio of indicators decreases by 0.2, since organic substances that undergo biochemical oxidation disappear in wastewater.

Also, in order to assess the presence of biologically degradable particles in waters, the inverse ratio of indicators can be used. For example, according to sanitary requirements, which imply that the COD for wastewater suitable for biotreatment, this indicator should not exceed the BOD value by more than one and a half times.

If we talk about biological treatment facilities that purify mixtures of domestic and industrial wastewater, then, as a rule, they have a ratio of both parameters in the incoming liquid for treatment is somewhere in the region of 1.5 to 2.5. When wastewater is mixed with industrial waste, this figure increases to 3.5, and when water flows from some production facilities it can reach up to 8.

As you can see, the COD value will allow you to analyze the state of the liquid in reservoirs and make it possible to find out how suitable it is for purification and to what extent. Detailed research into this and other values ​​will make the environment around us much cleaner.

One of the most common methods for assessing the degree of contamination of wastewater is the COD indicator (chemical oxygen absorption - Lurie Yu. Yu. Analytical chemistry of industrial wastewater. - M.: Chemistry, 1984.)

In the USSR, the bichromate method for determining COD was adopted as an arbitration method. However, this method is time-consuming (about 6 hours) and requires a large consumption of sulfuric acid (165 ml for each analysis), so it is not very suitable for mass analyzes in factory laboratories and wastewater treatment plants.

There are simpler, accelerated versions of this method, which, however, give somewhat lower results compared to the arbitration method. In addition, the known accelerated methods are not unified and need to be adjusted in relation to the studied wastewater from different industries.

We studied the average daily flows of various breweries: Kharkov No. 1 and No. 2, Izyumsky, Kupyansky, Poltava, Melitopol and Belgorod.

The optimal conditions for the oxidation of wastewater with solutions of potassium dichromate were studied and an accelerated method for determining COD was proposed, according to which the analysis takes about 20 minutes, the consumption of sulfuric acid is 45 ml per water sample.

Considering that the results of COD determination by the accelerated method are somewhat lower than those obtained by the arbitration method, it was of interest to establish the relationship between the COD values ​​found by the two methods, and thus make adjustments to the calculation when analyzing COD by the accelerated method.

COD was determined in wastewater samples using two methods. The relationship between COD indicators found by accelerated (X) and arbitration (y) methods, expressed in graphical form. To do this, in the general linear regression equation Y=a+bx determined the coefficients A And b solving a system of two equations: