The degree of contamination of both wastewater and water in reservoirs with organic substances contained in dissolved form and in the form of non-sedimented suspended and colloidal particles can be determined from the oxygen content consumed by the biochemical oxidation of these substances in the course of the life activity of aerobic bacteria. This quantity is called the biochemical oxygen demand, is designated as BOD and is expressed numerically by the oxygen concentration in mg / l or g / m3. In some cases it is necessary to calculate the total biochemical oxygen demand for the entire mass of organic contaminants discharged into the reservoir with sewage.
BOD is determined depending on the purpose of the analysis both in the pre-bedded and in the non-waste water at a temperature of 20 ° C. Thus, BOD shows the concentration of oxygen required to oxidize colloidal and dissolved contaminants, as well as that part of the insoluble substances that are not retained in the sedimentation tanks. This method is based on the fact that in the treatment plants, as will be seen below, only these substances are subjected to the oxidation process; Insoluble precipitating substances are released in the sedimentation tanks. In domestic waters, these substances constitute about one-tenth of all organic contaminants.
Biochemical oxygen demand is determined in 20 days and is designated as BOD20; For many types of sewage, the BOD-20 is equal to the BOD-complete and is adopted for the calculation of treatment facilities. Biochemical oxygen consumption is often determined in 5 days (the so-called five-day BOD sample), which is indicated in the analyzes, respectively. The determination of BODs is recommended as standard in the operation of treatment plants. Periodically, BOD is determined for longer periods prior to the onset of nitrification, which usually occurs on the 10th-12th day. These terms are conditional, since the rate of oxidation depends on the type of oxidizable substances and the rate constant of oxygen consumption.
About 21% of oxygen is consumed in household and industrial wastewater for the first day, about 87.5% for about 5 days, and about 100% of oxygen required for oxidation in 20 days (BOD). This is consistent with the equation describing the process of oxygen consumption during biochemical oxidation of organic substances and the average value of the rate constant of oxygen consumption & i = 0.18.
For different values of k, the ratio of BOD5 to BOD will be different. Thus, for a constant value of j = 0, l BOD5 is only 68.4% of the total biochemical oxygen demand.
In the design of treatment plants, the determination of BODs of wastewater should be considered mandatory. For wastewater in the absence of experimental data with sufficient accuracy, it is possible to take the conversion factor from BOD to BOD1 equal to 1.5.
The total BOD, or BOD 20, for the domestic water is 40-50 g oxygen per person per day.
The concentration of all pollutants, including organic ones, as noted above, decreases with an increase in the rate of wastewater disposal. Consequently, the BOD (La) of domestic sewage can be determined depending on the rate of water disposal <7, l / day, per person by formula
However, studies have shown that even BOD20 does not fully determine the total mass of organic substances contained in wastewater, since organic matter, which goes to the growth of bacteria, and persistent organic substances not affected by the biochemical process are not taken into account.
For a more complete assessment of the content of organic substances in wastewater, especially if it is a mixture of domestic and industrial waters, chemical oxygen demand (COD) has recently been determined (except BOD). The COD value is determined by heating organic compounds with chemically pure concentrated sulfuric acid, to which potassium iodate or chromic acid salts are added, giving their oxygen for oxidation. For domestic sewage BOD 20 is 86% COD; However, many production waters have COD exceeding BOD 20 by 50% or more.