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Six Interfering Factors in the Judgment of Sewage Biodegradability

Time：

2022-10-15

Sewage biodegradability refers to the degree of difficulty of pollutants in sewage to be degraded by microorganisms. Let's understand the six interfering factors in sewage biodegradability judgment.

Sewage biodegradability refers to the degree of difficulty of pollutants in sewage to be degraded by microorganisms. Let's understand the six interfering factors in sewage biodegradability judgment. The biodegradability of sewage refers to the difficulty of pollutants in sewage to be degraded by microorganisms. The biodegradability of sewage depends on the quality of sewage, that is, the nature of pollutants contained in sewage. If the nutrient ratio of sewage is appropriate, the pollutants are easily biodegradable and the content of toxic substances is low, the biodegradability of sewage is strong, and vice versa. Sewage suitable for microbial growth has strong biodegradability, while sewage unsuitable for microbial growth has poor biodegradability.

1. Evaluation method for biodegradability of sewage
The biodegradability of sewage is usually evaluated by the ratio of BOD5 or COD. BOD5 roughly represents the content of biodegradable reducing substances (mainly organic substances), and COD roughly represents the total amount of reducing substances (mainly organic substances).
It is known from BOD5/COD=1/m * CODB/COD (CODB is the content of biodegradable reducing substances) that BOD5/COD is the product of the proportion of biodegradable substances in the reducing substances (CODB/COD) and the biodegradation rate (1/m), which can roughly represent the degree and rate of biodegradation of reducing substances, that is, the biodegradability of sewage.
Generally, the higher the BOD5/COD value, the stronger the biodegradability of sewage. See Table 1 for specific evaluation criteria.

Table 1 Evaluation Standard of Sewage Biodegradability

2. Precautions in sewage biodegradability evaluation

BOD5/COD can only approximately represent the biodegradability of sewage, and the following impacts should be considered when applying BOD5/COD to evaluate the biodegradability of sewage.
⑴ Solid organic matter
Some solid organics can be oxidized by potassium dichromate in the COD determination and expressed in the form of COD, but the contribution to BOD5 in the BOD5 determination is very small and cannot be expressed in the form of BOD5, so the BOD5/COD of sewage is small at this time, but the biological treatment effect is not poor.
⑵ Inorganic reducing substances
Inorganic reducing substances in sewage also consume dissolved oxygen in the determination of BOD5 and COD. The dissolved oxygen consumed by the same inorganic reducing substance in the two determinations is different, indicating that BOD5/COD is reduced, but the biodegradability of sewage is not necessarily poor at this time.
(3) Special organics
Some organic substances are special and can be partially oxidized by microorganisms, but cannot be oxidized by K2Cr2O7. Although BOD5/COD is large, the biodegradability of sewage is actually poor.
⑷ BOD5/TOD
TOD is more accurate than COD in representing the content of organic matter in sewage, and BOD5/TOD is more accurate in evaluating the biodegradability of sewage.
(5) Domestication of inoculated microorganisms
Whether domesticated bacteria are used in determining BOD5 has a great influence on the determination results. The measured results are relatively low when the non domesticated microorganism is used for inoculation. The measured results are more consistent with the actual operation of the treatment facility when the domesticated microorganism is used for inoculation. BOD5/COD measured by inoculating non domesticated microorganisms is low, so it is inconsistent with the actual situation to infer that the biodegradability of sewage is poor. Therefore, domesticated bacteria must be introduced when determining BOD5.
(6) Dilution of water sample
When determining BOD5, it is often necessary to dilute the raw sewage. Because toxic substances have different toxicity at different concentrations, different dilution ratios have a great impact on the determination results. Synthetic organics, inorganic salts, heavy metals, sulfides and SO42 - are toxic to microorganisms at high concentrations and inhibit the growth of microorganisms. At this time, the biodegradability of sewage is poor. If the water sample will be diluted when determining the BOD5 of this kind of sewage, the biodegradability of sewage will be enhanced and the measured BOD5/COD will be increased because the concentration of toxic substances will be reduced and the toxicity will be weakened. It is wrong to infer that the biodegradability of raw sewage is strong.