Wastewater Treatment Basics: What Is the B/C Ratio?
What Is the B/C Ratio?
The B/C ratio is the ratio of BOD to COD.
BOD (biochemical oxygen demand) is the mass concentration of dissolved oxygen required by the biochemical process in which microorganisms decompose organic matter in water under aerobic conditions. To make BOD measurements comparable, a standard time period is prescribed and the consumption of dissolved oxygen is measured - usually five days, giving the five-day biochemical oxygen demand, written BOD5, which is the value in common use. The larger the BOD value, the more organic matter the water contains and the more serious the pollution. BOD is an environmental monitoring indicator used to track organic pollution in water. All organic matter can be decomposed by microorganisms, a process that consumes oxygen; if the dissolved oxygen in the water is insufficient to supply the microorganisms, the water body is in a polluted state.
COD (chemical oxygen demand) is the amount of oxidant consumed when a water sample is treated with a strong oxidizing agent under defined conditions. It reflects the degree to which the water is polluted by substances: the larger the COD, the more serious the organic pollution. COD is expressed in mg/L. Based on the COD measured by water quality monitoring instruments, water can be classified into five classes: Class I and Class II with COD ≤15 mg/L basically meet drinking water standards; water above Class II cannot be used as drinking water; Class III (COD ≤20 mg/L), Class IV (≤30 mg/L) and Class V (≤40 mg/L) are considered polluted water. The higher the COD value, the more serious the pollution.
Why Are BOD and COD Used as Pollution Indicators?
Wastewater contains many kinds of organic matter - sometimes a dozen, dozens or even hundreds of different organic substances. Analyzing each one qualitatively and quantitatively is both time-consuming and reagent-consuming. Could a single pollution indicator represent all the organic matter in wastewater and its quantity? Environmental researchers have found that all organic substances share two common features: first, they all consist of at least carbon and hydrogen; second, the vast majority can be chemically oxidized or oxidized by microorganisms, their carbon and hydrogen combining with oxygen to form harmless carbon dioxide and water.
Whether in chemical oxidation or biological oxidation, the organic matter in wastewater consumes oxygen: the more organic matter, the more oxygen is consumed, and the two are directly proportional. This is why chemical oxygen demand (COD) and biochemical oxygen demand (BOD) are used to represent the content of reducing substances in sewage.
The Relationship Between BOD and COD
The oxygen consumed by oxidizing sewage with chemical reagents is called COD (chemical oxygen demand), and the oxygen consumed by microbial oxidation in sewage is called BOD (biochemical oxygen demand). Since COD and BOD comprehensively reflect the quantity of all organic matter in water, and suitable testing instruments are available with simple methods that deliver results quickly, both are widely used in water quality testing and analysis as key indicators of water quality monitoring and important references for environmental monitoring of water bodies.
In fact, COD reflects not only organic matter in water but also reducing inorganic substances, such as sulfides, ferrous ions and sodium sulfite. For example, if ferrous ions in the sewage are not completely removed in the neutralization tank, their presence in the biochemical treatment effluent may cause the effluent COD to exceed the limit.
Some organic matter in sewage can be fully biologically oxidized (such as glucose and ethanol), some can only be partially biodegraded (such as methanol), and some cannot be biodegraded at all and is even toxic (certain surfactants). Organic matter in sewage can therefore be divided into biodegradable and non-biodegradable fractions. By convention, COD roughly represents all the organic matter in sewage while BOD represents the biodegradable organic matter; the difference between COD and BOD therefore indicates the non-biodegradable organic matter.
What Does the B/C Ratio Indicate?
The B/C ratio (BOD/COD) is commonly used to indicate the biodegradability of sewage. When BOD/COD is greater than 0.3, the wastewater is generally considered biologically treatable. The larger the BOD5/COD value, the higher the assessed biodegradability of the wastewater; anaerobic and anoxic conditions rely on anaerobic bacteria to digest the organic matter and achieve purification. Generally, only sewage with a ratio above 0.3 is suitable for biological treatment.
The BOD5/COD indicator - the ratio of five-day biochemical oxygen demand to chemical oxygen demand - is an index of the biodegradability of sewage, expressed as:
BOD5/COD = (1 - α) × (K/V)
α is the ratio of the biochemically hard-to-degrade fraction (CODNB) to COD;
K is the ratio of BOD5 to the ultimate biochemical oxygen demand BODU, a constant.
As the formula shows, the BOD5/COD value decreases as α increases, so the ratio reflects the biodegradability of the sewage. Reference bands: B/C > 0.58 - fully biodegradable; B/C = 0.45-0.58 - good biodegradability; B/C = 0.30-0.45 - biodegradable; B/C = 0.1-0.3 - poorly biodegradable. In practice, BOD5/COD = 0.3 is commonly taken as the lower limit for biodegradable sewage.
