Wastewater treatment using aerobic biological methods

Wastewater treatment using biological methods

Biological methods are applied to treat dissolved organic substances in wastewater as well as some inorganic substances such as H₂S, Sulfite, Ammonia, Nitrogen,… based on Microbial activity to decompose organic pollutants. Microorganisms use organic matter and some minerals as food to grow and develop. In general, biological treatment methods can be divided into two types:

• The anaerobic method uses a group of anaerobic microorganisms, operating in the absence of oxygen;
• The aerobic method uses a group of aerobic microorganisms, operating under conditions of continuous oxygen supply.

The process of decomposing organic substances by microorganisms is called biochemical oxidation. To carry out this process, dissolved organic substances, both colloidal and small dispersed substances in wastewater need to move inside microbial cells in three main stages as follows:

• Transfer of contaminants from the liquid phase to the microbial cell surface;
• Diffusion from the cell surface through a semi-permeable membrane due to the difference in concentration inside and outside the cell;
• Metabolize substances in microbial cells, produce energy and synthesize new cells.

The rate of biochemical oxidation depends on the concentration of organic matter, the content of impurities and the stability of the wastewater flow into the treatment system. At each given treatment condition, the main factors affecting the biochemical reaction rate are hydrodynamic regime, oxygen content in wastewater, temperature, pH, nutrients and trace elements.

Concept of aerobic biological processes

Aerobic biological process is the oxidation of carbon-containing organic substances that are easily biodegradable (usually expressed by indicators: BOD, COD, TOC – total organic carbon) with the participation of microorganisms. Aerobic organisms in the presence of oxygen.

Principles of aerobic biological processes

The principle of this process is to use aerobic microorganisms to decompose organic substances in wastewater with adequate dissolved oxygen at appropriate temperature, pH, etc.

The process of decomposing organic matter by aerobic microorganisms can be described by diagram:

(FOR)_nNS + O₂ -> CO₂ + H₂O + NH₄ + H₂S + Microbial cells + … ΔH

Under aerobic conditions NH₄⁺ and H₂S are decomposed through nitrification and sulfation by autotrophic microorganisms. :

NH₄⁺ + 2O₂ -> NO₃^– + 2H⁺  + H₂O + ΔH;

H₂S + 2O₂ -> SO₄⁺ + 2H⁺ + ΔH

Aerobic microbial activity includes:

• Nutrition process: microorganisms use organic substances, nutrients and metal trace elements to build new cells to increase biomass and reproduction.

Decomposition process: microorganisms oxidize dissolved organic substances or in the form of small dispersed colloidal particles into water and CO₂ or create other gases

Advantages and disadvantages of aerobic biological wastewater treatment method

 Advantages:

• Compared with the anaerobic method, aerobic technology has the advantages of a more complete understanding of the treatment process, higher treatment efficiency and more thoroughness.
• Aerobic methods do not cause secondary pollution like chemical and physical chemical methods.

Disadvantages:

• But the aerobic method also has disadvantages: the building volume is large and takes up more space.
• Larger construction costs and equipment investment.
• Operating costs, especially costs for aeration energy, are relatively high.
• No ability to recover energy. Not tolerant of sudden changes in organic load.
• After treatment, a residual amount of sludge is generated and this amount of sludge is less stable, thus requiring investment costs to treat sludge.
• Treating wastewater with loading loads not as high as anaerobic methods.

Describe aerobic biological processes

The process of decomposing organic dirt using aerobic biotechnology is a fermentation process with microorganisms in the presence of oxygen to produce products CO₂, H_{2< /sub>O, NO3^‑ and SO4^2-. During aerobic treatment, complex impurities such as proteins, starches, fats… will be decomposed by extracellular enzymes into simple substances such as amino acids, fatty acids, organic acids, applicant… These simple substances will permeate the cell membrane and be further decomposed or converted into new cell building materials by the process of intracellular respiration giving the final product CO2 and H2O. The aerobic process mechanism includes 3 stages:}

Phase 1: Oxidize all organic matter in wastewater to meet the energy needs of cells:

C_xH_yO_zN + (x+ y/4 + z/3 + ¾) O₂ yeast —- > xCO₂ + [ (y-3)/2] H₂O + NH₃

In biological treatment tanks, bacteria play a key role because they are responsible for decomposing organic components in wastewater. In activated sludge tanks, part of the organic matter will be used by aerobic and facultative aerobic bacteria to obtain energy to synthesize the remaining organic matter into new bacterial cells. When the treatment tanks are completed and put into operation, the bacteria present in the wastewater begin to grow according to the growth cycle of bacteria in a batch of microorganisms. In the beginning, to soon put the treatment system into operation, add a new tank as a form of adding bacteria to the treatment tank. The growth cycle of bacteria in the treatment tank includes 4 stages:

• Lag-phase: occurs when the tank begins to operate and sludge from other tanks is added to the tank. This is the stage for bacteria to adapt to the new environment and begin the cell division process.
• Growth phase (log-growth phase): during this phase, bacterial cells divide and increase rapidly in number. The rate of cell division depends on the time required for cell divisions and the amount of food in the environment.
• Equilibrium phase: at this time the bacterial density is kept at a stable number. The reason for this stage is that the nutrients necessary for the growth of microorganisms have been used up, the number of bacteria born is equal to the number of bacteria that have died.
• Log-death phase: during this phase, the number of bacteria that die is greater than the number of bacteria that are born, so the density of bacteria in the tank decreases rapidly. This stage may be caused by species of generally visible size or by characteristics of the environment.

Graph of bacterial growth in a biological treatment tank