Basic Wastewater Treatment Terms: What Is Anaerobic Biological Treatment?
Anaerobic Process: under anaerobic conditions the nutritional and environmental conditions required by anaerobic microorganisms are created, and these microorganisms are then used to decompose the organic matter in wastewater, yielding products that fall mainly into two kinds.
By the aggregation form of the microorganisms, anaerobic biological treatment of sewage can be divided into the anaerobic activated sludge process and the anaerobic biofilm process. The anaerobic activated sludge process includes the conventional digester, the anaerobic contact digester, the upflow anaerobic sludge blanket (UASB), and the expanded granular sludge bed (EGSB); the anaerobic biofilm process includes the anaerobic biological filter, the anaerobic fluidized bed, and the anaerobic rotating biological contactor.
The anaerobic degradation of macromolecular organics can be divided into four stages: hydrolysis, fermentation (or acidification), acetogenesis, and methanogenesis.
Hydrolysis can be defined as the process in which complex insoluble polymers are converted into simple soluble monomers or dimers.
Because of their very large relative molecular mass, macromolecular organics cannot pass through the cell membrane and therefore cannot be used directly by bacteria. In the first stage they are broken down into small molecules by bacterial extracellular enzymes. For example, cellulose is hydrolyzed by cellulase into cellobiose and glucose, starch is broken down by amylase into maltose and glucose, and protein is hydrolyzed by protease into short peptides and amino acids. These small-molecule hydrolysis products can dissolve in water and pass through the cell membrane to be used by bacteria. Hydrolysis is usually rather slow and is therefore regarded as the rate-limiting stage in the anaerobic degradation of wastewater containing macromolecular organics or suspended solids. Many factors, such as temperature, the composition of the organic matter, and the concentration of the hydrolysis products, may affect the rate and extent of hydrolysis. The hydrolysis rate can be described by the following kinetic equation: p=po/(1+Kh.T)
p - concentration of degradable non-dissolved substrate (g/L);
po - initial concentration of the non-dissolved substrate (g/L);
Kh - hydrolysis constant (d^-1);
T - retention time (d)
Fermentation can be defined as a biodegradation process in which organic compounds act both as electron acceptors and as electron donors. In this process soluble organics are converted into end products dominated by volatile fatty acids, which is why the process is also called acidification.
In this stage, the small-molecule compounds above are converted inside the cells of the fermentative bacteria (that is, the acidifying bacteria) into simpler compounds and secreted outside the cells. The vast majority of fermentative bacteria are strict anaerobes, but about 1% facultative anaerobes are usually present in the anaerobic environment, and these facultative anaerobes protect strict anaerobes such as methanogens from oxygen damage and inhibition. The main products of this stage are volatile fatty acids, alcohols, lactic acid, carbon dioxide, hydrogen, ammonia, and hydrogen sulfide; the product composition depends on the anaerobic degradation conditions, the substrate types, and the microbial populations involved in acidification. At the same time, the acidifying bacteria also use part of the material to build new cell matter, so anaerobic treatment of non-acidified wastewater produces more excess sludge.
During anaerobic degradation, the acid tolerance of the acidifying bacteria must be taken into account. Acidification can still proceed when the pH falls to 4, whereas methanogenesis cannot, so a falling pH reduces methane generation and hydrogen consumption and further changes the composition of the 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 cell 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 the formation of methane, the main intermediate product is methyl-coenzyme M (CH3-S-CH2-SO3-).
The conversion of acetic acid, acetate, carbon dioxide, and hydrogen into methane by methanogenic bacteria is carried out by two physiologically different groups of methanogens: one group converts hydrogen and carbon dioxide into methane, while the other produces methane by decarboxylating acetic acid or acetate. The former accounts for about 1/3 of the total and the latter about 2/3.
The most important methanogenic reactions are:
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
In this stage, acetic acid, hydrogen, carbonic acid, formic acid, and methanol are converted into methane, carbon dioxide, and new cell material.
It should be noted that some books divide anaerobic digestion into three stages, merging the first and second stages into one called the hydrolysis acidification stage. Here we consider that dividing it into four stages reflects the anaerobic digestion process more clearly.
The reaction rates of the four stages above vary with the nature of the wastewater. In wastewater dominated by pollutants such as cellulose, hemicellulose, pectin, and lipids, hydrolysis readily becomes the rate-limiting step; simple sugars, starch, amino acids, and ordinary proteins can all be rapidly decomposed by microorganisms, so for wastewater containing such organics methanogenesis tends to become the rate-limiting stage. Although anaerobic digestion can be divided into the four stages above, in an anaerobic reactor the four stages proceed at the same time and maintain a degree of dynamic balance. Once that balance is upset by external factors such as pH, temperature, or organic loading, the methanogenic stage is inhibited first, which leads to the accumulation of lower fatty acids and abnormal changes in the anaerobic process, and may even bring the whole digestion process to a standstill.
