What Is Anaerobic Biological Treatment? A Detailed Explanation of Wastewater Treatment Terminology
Anaerobic biological treatment (Anaerobic Process) is a process in which, under anaerobic conditions, the nutritional and environmental conditions required by anaerobic microorganisms are formed, and organic matter is biochemically degraded through the metabolic action of anaerobic and facultative bacteria. [1]
Anaerobic biological wastewater treatment processes can be classified by the aggregation form of microorganisms into anaerobic activated sludge process and anaerobic biofilm process. The anaerobic activated sludge process includes the conventional digester, anaerobic contact digester, upflow anaerobic sludge blanket (UASB), anaerobic granular sludge expanded bed (EGSB), etc.; the anaerobic biofilm process includes the anaerobic biofilter, anaerobic fluidized bed, and anaerobic rotating biological contactor.
Aerobic biological treatment has high efficiency and wide application and has become the main method of municipal wastewater treatment. However, aerobic biological treatment consumes more energy and produces more excess sludge, and is especially unsuitable for treating high-concentration organic wastewater and sludge. The significant differences between anaerobic and aerobic biological treatment are:
2. The final product is methane gas with high calorific value, available as a clean energy source;
3. It is especially suitable for treating the sludge of municipal wastewater treatment plants and high-concentration organic industrial wastewater. [1]
1. The treatment process can greatly reduce energy consumption, and can also recover bioenergy (biogas);
2. The sludge yield is very low; the growth rate of anaerobic microorganisms is much lower than that of aerobic microorganisms. The yield Y of acid-producing bacteria is 0.15-0.34 kg VSS/kg COD, the yield Y of methanogens is about 0.03 kg VSS/kg COD, while the yield of aerobic microorganisms is about 0.25-0.6 kg VSS/kg COD;
3. Anaerobic microorganisms may degrade or partially degrade some organic matters that aerobic microorganisms cannot degrade;
4. The reaction process is relatively complex; anaerobic digestion is a continuous microbial process in which many microorganisms of different natures and functions work synergistically;
5. It is relatively sensitive to environmental factors such as temperature and pH;
6. Used alone, anaerobic treatment makes it difficult for the effluent quality to meet standards, and further aerobic treatment is required;
7. It has a relatively strong odor, especially a foul smell;
8. Its removal effect on ammonia nitrogen is poor, etc. [2]
The anaerobic degradation process of macromolecular organic matter can be divided into four stages: the hydrolysis stage, the fermentation (or acidification) stage, the acetogenesis stage, and the methanogenesis stage. [3]
Hydrolysis can be defined as the process by which complex insoluble polymers are converted into simple soluble monomers or dimers.
Because of their huge relative molecular mass, macromolecular organic matters cannot pass through the cell membrane and therefore cannot be directly used by bacteria. In the first stage they are decomposed by bacterial extracellular enzymes into small molecules. For example, cellulose is hydrolyzed by cellulase into cellobiose and glucose, starch is decomposed by amylase into maltose and glucose, and proteins are hydrolyzed by protease into short peptides and amino acids, etc. These small-molecule hydrolysis products can dissolve in water and pass through the cell membrane to be utilized by bacteria. The hydrolysis process is usually slow, so it is considered the rate-limiting stage of anaerobic degradation of waste liquids containing macromolecular organic matter or suspended solids. Various factors such as temperature, composition of organic matter, and concentration of hydrolysis products may affect the speed and extent of hydrolysis. The hydrolysis rate can be described by the following kinetic equation: ρ = ρo / (1 + Kh·T)
The most important methanogenesis reactions are:
ρo — initial concentration of insoluble substrate (g/L);
Kh — hydrolysis constant (d-1);
T — retention time (d)
Fermentation can be defined as a biodegradation process in which an organic compound acts as both electron acceptor and electron donor; in this process soluble organic matter is converted into end products dominated by volatile fatty acids, so this process is also called acidification.
In this stage, the above small-molecule compounds are converted intracellularly by fermentative bacteria (i.e., acidifying bacteria) into simpler compounds and secreted outside the cell. The vast majority of fermentative bacteria are strict anaerobes, but usually about 1% facultative anaerobes exist in the anaerobic environment; these facultative anaerobes can protect strict anaerobes such as methanogens from oxygen damage and inhibition. The main products of this stage include volatile fatty acids, alcohols, lactic acid, carbon dioxide, hydrogen, ammonia, hydrogen sulfide, etc.; the composition of the products depends on the anaerobic degradation conditions, substrate type, and the microbial population involved in acidification. At the same time, the acidifying bacteria also use part of the substances to synthesize new cellular material; therefore, anaerobic treatment of non-acidified wastewater produces more excess sludge.
During anaerobic degradation, the acid tolerance of acidifying bacteria must be considered. The acidification process can proceed when the pH drops to 4. However, for the methanogenesis process, a drop in pH will reduce methane generation and hydrogen consumption, and further cause changes in the composition of acidification end products.
Under the action of hydrogen-producing acetogenic bacteria, the products of the previous stage are further converted into acetic acid, hydrogen, carbonic acid, and new cellular material.
CH3CHOHCOO-+2H2O —> CH3COO-+HCO3-+H++2H2 ΔG’0=-4.2KJ/MOL
CH3CH2OH+H2O－> CH3COO-+H++2H2O ΔG’0=9.6KJ/MOL
CH3CH2CH2COO-+2H2O－> 2CH3COO-+H++2H2 ΔG’0=48.1KJ/MOL
CH3CH2COO-+3H2O－> CH3COO-+HCO3-+H++3H2 ΔG’0=76.1KJ/MOL
4CH3OH+2CO2－> 3CH3COO-+2H2O ΔG’0=-2.9KJ/MOL
2HCO3-+4H2+H+－>CH3COO-+4H2O ΔG’0=-70.3KJ/MOL
In this stage, acetic acid, hydrogen, carbonic acid, formic acid, and methanol are converted into methane, carbon dioxide, and new cellular material.
The process by which methanogenic bacteria convert acetic acid, acetate, carbon dioxide, and hydrogen into methane is accomplished by two physiologically different types of methanogens: one group converts hydrogen and carbon dioxide into methane, and the other produces methane by decarboxylation of acetic acid or acetate; the former accounts for about 1/3 and the latter about 2/3 of the total.
ρ — degradable insoluble substrate concentration (g/L);
CH3COO-+H2O－>CH4+HCO3- ΔG’0=-31.0KJ/MOL
HCO3-+H++4H2－>CH4+3H2O ΔG’0=-135.6KJ/MOL
4CH3OH－>3CH4+CO2+2H2O ΔG’0=-312KJ/MOL
4HCOO-+2H+－>CH4+CO2+2HCO3- ΔG’0=-32.9KJ/MOL
During methane formation, the main intermediate product is methyl-coenzyme M (CH3-S-CH2-SO3-).
It should be pointed out that some books divide the anaerobic digestion process into three stages, combining the first and second stages into one stage called the hydrolysis-acidification stage. Here we consider that dividing it into four stages can more clearly reflect the anaerobic digestion process.
The reaction rates of the above four stages vary depending on the nature of the wastewater. In wastewater dominated by pollutants such as cellulose, hemicellulose, pectin, and lipids, hydrolysis easily becomes the rate-limiting step; simple sugars, starch, amino acids, and general proteins can all be rapidly decomposed by microorganisms, so for wastewater containing such organic matter, methanogenesis easily becomes the rate-limiting stage. Although the anaerobic digestion process can be divided into the above four processes, in an anaerobic reactor the four stages proceed simultaneously and maintain a certain dynamic equilibrium. Once this equilibrium is disrupted by external factors such as pH, temperature, and organic loading, the methanogenesis stage will first be inhibited, resulting in the accumulation of low-level fatty acids and abnormal changes in the anaerobic process, and even leading to a complete stagnation of the entire digestion process.
