Biological processes in wastewater treatment plants are one of the first and oldest technologies in the history of wastewater treatment plants. The Biological process also known as the activated sludge process was invented by Ardern and Lockett after several testing in 1913. The first large-scale wastewater plant with an activated sludge process was introduced in Stockport, UK in 1920.
The basic process of the activated sludge process is introducing the numerous microorganisms that consume the organic waste and oxides the ammonia into nitrate and nitrite.
The overall purpose of the activated sludge process is as follows:
- Dissolve and particulate the biodegradable component of wastewater into the acceptable end product.
- Capture and settle the suspended and non-settle particulate into biofilms or flocs.
- Transform nutrients like phosphorus and nitrogen from the wastewater stream, which otherwise cause Eutrophication.
- In some cases, the activated sludge process is deployed to remove the trace organic product compounds.
Whenever the activated process technology is introduced in the industry, it is always recommended that pretreatment should be applied to the wastewater stream before biological treatment as the toxic component in the wastewater stream affects the sensitive microorganisms in the biological process.
Role of Microorganisms in Wastewater Treatment Plant
The removal of dissolved and particulate carbonaceous BOD matter and stabilization of organic matter found in wastewater is accomplished by various types of microorganisms. The microbes are used to convert the biodegradable biomass into i.e. carbonaceous BOD into a simple end product and additional biomass as represented by the following equation for aerobic treatment of biological matters.
Organic matter + v2O2 + v3NH3 + v4PO43 ————> New Cells +v6CO2 + v7H20
Vn stoichiometry value, where the ammonia and phosphate represent the organic food required by the microbes more specifically bacteria to survive and maintain the population for treatment of future coming wastewater while producing the acceptable simple product i.e., water and carbon dioxide.
whereas bacteria are also capable of removing nitrogen and phosphorus in the wastewater treatment process. Specific bacteria oxidize the ammonia into nitrate and nitrite, while other bacteria nitrogen to gaseous nitrogen. The biological process allows microbes to take up and store large amounts of inorganic phosphorus
Metabolic process type in the Biological domestic wastewater
The metabolic process variation in biological wastewater is very important to make the biological process effective and get the desired pollutant removal. There are many combinations of metabolic processes being used to get the effective treatment of the coming wastewater stream.
| Sr.No | Metabolic Process | Description |
|---|---|---|
| 01 | Aerobic/Oxic Process |
Biological process that occurs in the presence of oxygen. |
| 02 | Anaerobic Process |
Biological process that occurs in the absence of oxygen. |
| 03 | Anoxic Process |
The process by which the nitrate nitrogen is converted into nitrogen gas in the absence of oxygen, this process also known as denitrification. |
| 04 | Facultative Process |
Biological process in which organisms can function in the presence and absence of oxygen. |
| 05 | Combined Process |
Various combinations of aerobic, anaerobic and anoxic processes group together to achieve a specific treatment objective. |
Treatment type in the Biological domestic wastewater
The biological process involves different methods to grow the microorganism and each method has its own benefit and removal efficiency. The adaptation of any particular treatment technology depends upon numerous factors like flow rate, incoming wastewater, space availability.
| Sr. No | Treatment Process | Description |
|---|---|---|
| 01 | Suspended growth process |
Biological treatment processes in which the microorganisms responsible for the conversion of the organic matter or other constituents in the wastewater to gasses and cell tissue are maintained in suspension within the liquid |
| 02 | Attached growth process |
Biological treatment processes in which the microorganisms responsible for the conversion of the organic matter or other constituents in the wastewater to gasses and cell tissue are attached to some inert medium, such as rocks, slag, or specially designed ceramic or plastic materials. |
| 03 | Combined process |
Term used to describe combined processes (e.g., combined suspended and attached growth processes) |
| 04 | lagoons process |
A generic term applied to treatment processes that take place in ponds or lagoons with various aspect ratios and depths |
Type of bacteria Biological domestic wastewater
The microorganisms are responsible for the degradation and conversion. The term microorganisms describes a very heterogeneous group of living organisms, the characteristic of which is that they are formed of one or a few cells.
Microorganisms can be classified into two groups:
- Prokaryotes with a relatively simple cell construction
- Eukaryotes with a more complex cell construction.
Bacteria, including actinomycetes and cyanobacteria (“cyanophyte”) belong to the group of prokaryotes, whereas fungi, including yeast, mold fungi, protozoans and algae can be classified as eukaryotes.
An important classification is the categorization of the bacteria according to the tolerated temperature stages which are described below stages which are described below
| Type | Minimum (0C) | Optimum (0C) |
Maximum (0C) |
|---|---|---|---|
| Psychrophilic | 0‐10 | 15‐20 | 25‐30 |
| Mesophilic | 10‐15 | 25‐35 | 35‐45 |
| Thermophilic | 25‐45 | 50‐55 | 75‐80 |
Principal growth curve of a bacteria culture
The curve in figure shows the principal the growth curve of a bacteria culture, during activated sludge process it is dependent on different factors:
- In the starting phase the microorganisms adjust themselves to the new external conditions
- The greatest additional growth of microorganisms is realized in the exponential phase. The essential precondition is that all environmental conditions like pH value, temperature, aeration and the quality of the substrate is optimal.
- A further growth in the stationary phase is not possible. Reasons are mostly a shortage of substrates of the oxygen or mostly toxic metabolic products
- The dying phase characterizes a higher dying rate than the regeneration phase.
Conclusion
The growth of microorganisms is a critical factor in determining the overall effluent quality and whether the desired results will be achieved. The microbes involved in biological processes are highly sensitive to external conditions such as pH, temperature, and the characteristics of the incoming wastewater stream in a biological wastewater treatment plant. Therefore, all necessary measures should be taken to avoid shock loading the microbes during treatment.
