The quality of wastewater effluents causes degradation of receiving water bodies such as lakes, rivers and streams.
Chemical and biological treatment are the two basic techniques for removing contaminants from wastewater influents, although biological treatment is now employed due to various limitations of chemical treatment.
Microbes have a vital role in the environment, economy and society.
These have been used for centuries to produce a wide variety of products including enzymes, probiotics and biofuels such as bioethanol, hydrogen gas and so on.
The breakdown of organic compounds in wastewater is mainly controlled by microbes and their enzymes.
They play an important role as the primary engineers in controlling all ecological processes.
They act as a universal catalyst for ecological alterations.
While many different microorganisms are utilized in sewage treatment, three well-known bacteria play a significant role in keeping sewage clean.
All of these microorganisms contribute to the treatment process in different ways, ensuring that there is minimal influence on the surrounding environment.
The following microorganisms are employed in wastewater treatment:
Aerobic Bacteria
Aerobic bacteria are frequently used during the construction of new treatment plants in an aerated atmosphere.
This bacterium destroys contaminants in wastewater by utilizing free oxygen in the water, which it then converts into energy for survival and reproduction.
To properly use this sort of bacterium, oxygen must be introduced mechanically.
This assures that the bacteria may continue to do their work and grow and multiply on their food source.
Anaerobic Bacteria
In wastewater treatment, anaerobic bacteria are often utilized.
The primary purpose of these bacteria in sewage treatment is to reduce the volume of sludge and generate methane gas from it.
The beautiful thing about this form of bacteria, and why it is utilized more frequently than aerobic bacteria, is that if cleaned and treated properly, methane gas can be used as an alternative energy source.
Given the already rising energy costs of wastewater treatment, this is a significant gain.
In contrast to aerobic bacteria, the bacteria described above can receive more than enough oxygen from their food supply and do not require additional oxygen to function.
Another advantage of using anaerobic bacteria in wastewater treatment is that they eliminate phosphate.
Facultative microorganisms in wastewater treatment are bacteria that can flip between aerobic and anaerobic metabolism depending on their surroundings.
It is worth noting that these bacteria prefer an aerobic environment.
what are anaerobic processes?
Anaerobic processes are those in which neither oxygen nor nitrate is present.
These processes are carried out by a large and varied group of microorganisms that normally live in a symbiotic relationship.
Anaerobic digestion has contributed a lot to the conservation of natural resources which cannot be replenished back.
It involves a series of biological processes which takes place naturally when a bacterium breaks down the organic content in the absence of oxygen.
Anaerobic processes have been used in wastewater treatment systems for more than a century, initially to stabilize the solids produced.
These bioreactors, called anaerobic digesters, were simple concrete tanks in which the solids were placed as a slurry and allowed to decompose anaerobically.
Gradually, it was discovered that the decomposition could be accelerated by heating the digester to a consistent temperature of about 35°C and mixing it to provide uniform reaction conditions.
These discoveries led to the current high-rate anaerobic digestion process, which uses HRTs of 15 to 20 days.
Anaerobic digestion remains an extremely popular and widely used solids stabilization process, particularly in municipal wastewater treatment.
Biological activities have been identified as the main root for more than 70% of methane production in the atmosphere.
Anaerobic digestion is considered to take place in a series of three to four steps namely.
-Hydrolysis
-Acidogenesis (acid forming)
-Acetogenesis
-Methanogenesis
What is the product?
The rate-limiting reaction in anaerobic digestion is usually the conversion of volatile acids to methane.
The breakdown of volatile acids provides very little energy to methane-forming microorganisms.
Most of the energy released from the volatile acids is transferred to methane.
Because of the low energy yield obtained from volatile acids by methane-forming bacteria, their growth rate is restricted, that is, the amount of substrate utilization per unit of organisms is high.
Therefore, bacterial growth or sludge production is low, and optimum operational conditions must be maintained for satisfactory rates of solids destruction and methane production.
These factors are responsible for the rate-limiting reaction of the conversion of volatile acids to methane.
However, if the substrates fed to the anaerobic digester were mostly slowly degrading particulate materials, then the rate-limiting reaction would be the hydrolysis of the particulate material.
Factor influence the processes
Methane-forming bacteria are strict anaerobes and are extremely sensitive to changes in alkalinity, pH and temperature.
Therefore, operational conditions in the digester must be periodically monitored and maintained within optimum ranges.
In addition to alkalinity, pH, and temperature, several other operational conditions should be monitored and maintained within optimum ranges for the acceptable activity of methane-forming bacteria.
These conditions are gas composition, hydraulic retention time (HRT), oxidation-reduction potential (ORP) and volatile acid concentration (Table 1).
Process control of anaerobic digesters is often difficult because numerous operational conditions are interrelated and changes in one condition may directly or indirectly affect others.
Also, the relatively low concentrations of solids and short solids retention times (SRTs) maintained in completely mixed digesters render the process susceptible to toxic upsets and shock loadings.
Another difficulty in achieving proper digester operation is the presence of different bacterial groups that have different optimum values or ranges of values for operational conditions.
For example, there are two optimal temperatures for the anaerobic digestion of solids.
The acid-forming bacteria have an optimum temperature of 30°C
The mesophilic, Methane-forming bacteria have an optimum temperature of 35°C.
TABLE 1 Operational Conditions for Acceptable Activity of Methane-forming Bacteria and Methane Production
| Condition | Optimum | Marginal |
| Alkalinity, mg/l as CaCO3 | 1500–3000 | 1000–1500
3000–5000 |
| Gas composition | ||
| Methane, % volume | 65–70 | 60–65 & 70–75 |
| Carbon dioxide, % volume | 30–35 | 25–30 & 35–40 |
| Hydraulic retention time, days | 10–15 | 7–10 & 15–30 |
| pH | 6.8–7.2 | 6.6–6.8 & 7.2–7.6 |
| Temperature, mesophilic | 30–35°C | 20–30° & 35–40°C |
| Temperature, thermophilic | 50–56°C | 45–50° & 57–60°C |
| Volatile acids, mg/l as acetic acid | 50–500 | 500–2000 |
Benefits of Anaerobic process
Anaerobic wastewater digestion technologies address not only the requirement for businesses to properly clean wastewater discharged into the environment but also the necessity for the industry to break free from the cost and pollution associated with fossil fuels.
Anaerobic digestion facilities have been acknowledged by the United Nations Development Program as one of the most useful decentralized forms of energy supply due to their efficiency, as they require less capital than major power plants.
Given the increased emphasis on climate change mitigation, waste reuse as a resource and new technology strategies that have reduced capital costs, anaerobic digestion has stoked the interest of governments in several countries, particularly those in arising regions with high infrastructure investment.
References
[1] Mogens Henze. Poul Harremoes J es la Cour J ansen . Erik Arvin, Wastewater Treatment, Biological and Chemical Processes, Second Edition, Springer-Verlag Berlin Heidelberg GmbH,1996.p100.
[2] Malina, J. F., Jr., Anaerobic sludge In Design of Anaerobic Processes for the Treatment of Industrial and Municipal Wastes, J. F. Malina, Jr. and E G. Pohland. eds. Technomic Publishing, Lancaster, Pennsylvania, pp. 167-21 2, 1992.
[3] S. Environmental Protection Agency, Process Design Manual for Sludge. Treatment and Disposal, EPA 625/1-7! I-O 1 1, U. S. Environmental Protection Agency. (‘Cincinnati, Ohio, 1979.
[4] Source: The Microbiology of Anaerobic Digesters, by Michael H. Gerardi, John Wiley & Sons, Inc, 2003.
[5] Maximizing opportunities for anaerobic digestion from wastewater, https://www.watertechonline.com/wastewater/article/15550717/maximizing-opportunities-of-anaerobic-digestion-from-wastewater
[6] What is the role of microorganisms in wastewater? https://www.netsolwater.com/what-is-the-role-of-micro-organisms-in-waste-water.php?blog=1434
