In industrial fermentation, the choice of carbon source requires a combination of factors,
the following are some common carbon sources and their characteristics.
1. Sugar:
Glucose: commonly used monosaccharide in industrial fermentation, widely used in the fermentation production of antibiotics, amino acids, organic acids and so on. However, it is a multi-molecular compound, easy to cause a large number of bacterial reproduction, resulting in sludge expansion, increasing the value of COD in the effluent water, affecting the quality of effluent water, and industrial glucose contains many impurities, food glucose price is expensive.
Starch: includes polysaccharides, etc., such as starch paste. Starch cannot be used directly by most micro-organisms and needs to be saccharified to make starch hydrolysate for use.
2. Organic acid: such as lactic acid, citric acid, acetic acid, etc., not only can be used as a carbon source, but also can regulate the pH value of fermentation liquid. When acetic acid is used as a carbon source, it is similar to sodium acetate, but it is more wasteful to be used as an industrialised product as a carbon source.
3. Alcohols: e.g. methanol, ethanol, glycerol, etc.
Methanol: It has the advantages of low operating cost and small sludge production, and fast denitrification rate. However, it is flammable and is a Class A hazardous chemical, with strict requirements for storage and use; the response time of microorganisms to it is slow, and it cannot be utilised by all microorganisms; and it has a certain poisonous effect, and its long-term use has an impact on the discharge of tail water.
Glycerol: For example, we can use glycerol-containing carbon sources, such as the cheap new carbon source of ‘Changjiang Jiangyu’, which is initially purified, with a COD of 1.5 million after complete de-watering, controllable moisture, adjustable concentration, few impurities, high glycerol content, neutral, no special odour and low freezing point.
4. Fats and oils: Soybean oil, corn oil, cottonseed oil and lard are common, and moulds and actinomycetes can use fats and oils as a carbon source.
5. Biomass carbon source: non-toxic and harmless bioproducts produced by fermentation of sugar, agricultural wastes, etc. through the principle of bioengineering, and the main components are small-molecule organic acids, alcohols and sugars.
It is easier to be utilised by microorganisms than a single chemical, and the cost of using it is lower, which makes it more cost-effective, but the stability of the product needs to be improved, and the COD equivalent of each batch of the product needs to be tested before use.
6. Sludge hydrolysis supernatant: The bioconversion volatile acid VFA comes from the supernatant of sludge hydrolysis, which has a high denitrification rate, and can be provided directly from within the sewage plant, which reduces the problem of carbon source transport. However, different sludge, different hydrolysis conditions produced by the VFA component has a big difference, denitrification rate is also different, and direct hydrolysis of sludge as an external carbon source, need to consider the release of nitrogen and phosphorus in the process of sludge hydrolysis.
Factors to consider when choosing a carbon source include：
The cost of carbon source dosing is determined by combining the equivalent COD price of the carbon source and the dosing volume.
Carbon source sludge production rate, the addition of carbon source will increase sludge production and sludge treatment cost is higher.
Ensure the stability of wastewater operation, taking into account the avoidance of sludge expansion, elevated effluent COD, nitrite nitrogen accumulation and other problems.
In addition, different microorganisms have different abilities and needs to utilise carbon sources, and it is also necessary to choose the appropriate carbon source based on the characteristics of specific microorganisms, fermentation methods, different requirements of production practices and scientific experiments, and economic benefits.
At the same time, it is important to pay attention to the principles of media preparation such as the proper ratio of nutrients, osmotic pressure and pH value.
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