Wastewater treatment combines physical, chemical, and biological processes to eliminate pollutants, pathogens, and harmful substances to produce safe water for discharge into the environment or reuse.
Furthermore, methods of treatment in which the application of physical forces predominates are known as unit operations.
In contrast, methods of treatment involving chemical or biological activities are known as unit processes.
There are three types of unit operations and processes:
- Physical unit operations
- Chemical unit processes
- Biological unit processes
However, in water and wastewater treatment, unit operations are those procedures in which changes produced are physical, chemical, or biological.
Also Read: Tips for Subcontractors to Win More Bids: The Quick Guide
Table of Contents
Advantages of Unit Operation in water and wastewater treatment
- It gives a better understanding of processes inherent in the treatment and of the capabilities of these processes in attaining the objectives.
- It helps in the development of mathematical and physical models of treatment mechanisms and the consequent design of the treatment plant.
- It helps coordinate an effective treatment procedure to attain the desired plant performance and effluent quality.
The output of wastewater treatment works is an acceptable effluent through the available unit operations.
Most wastewater treatment processes bring about changes in the concentration of a specific substance by moving the substance either into or out of the wastewater itself. This is called phase transfer.
The principal phase transfers are:
- gas transfer,
- ion transfer,
- solute stabilisation,
- solid transfer,
- nutrient or molecular transfer, etc
The following are the important operations that constitute the physical, chemical, and biological unit operations/processes employed in water and wastewater treatment:
- Gas transfer: aeration
- Ion transfer: chemical coagulation, chemical precipitation, ion exchange, adsorption.
- Soluble stabilization: chlorination, limiting, recarbonation, super-chlorination.
- Nutrient or molecular transfer:
- Interfacial contact:
- Miscellaneous operations: disinfection, copper sulfating, fluoridation, thermal desalination.
- Solid concentration and stabilization: thickening, centrifuging, chemical conditioning, elutriation, biological floating, vacuum filtration, air drying, heat drying, sludge digestion, incineration, wet combustion.
Also Read: Difference Between Architect and Civil Engineer
Physical Unit Operations
Physical unit operations in wastewater treatment use physical and mechanical means to remove pollutants and separate solids from the liquid stream.
The physical unit operations in which application of physical forces predominates consist of the following:
- Screening
- Flow equalization,
- Comminuation,
- Mixing
- Flocculation,
- Sedimentation,
- Floatation,
- Elutriation,
- Vacuum filtration,
- Microscreening,
- Air drying.
The physical unit operations were the first treatment methods because they were derived originally from observations of the physical world.
They form the basis of most process flowsheets. The table below gives the applications of physical unit operations in wastewater treatment:
| Operation | Application |
| 1. Screening | Removal of coarse and settleable solids by surface straining |
| 2. Comminution | Griding of coarse solids |
| 3. Flow equalization | Equalization of flow and mass loading of BOD and suspended solids |
| 4. Mixing | Mixing of chemicals and gases with wastewater and maintaining solids in suspension. |
| 5. Flocculation | Promotion of the aggregation of small particles into large particles. |
| 6. Sedimentation | Removal of settleable solids and thickening of sludges |
| 7. Floatation | Removal of finely divided suspended solids and particles with densities close to that of water. |
| 8. Filtration | Removal of fine residual suspended solids remaining after biological or chemical treatment. |
| 9. Micrscreening | Same as filtration. Also removal of algae from stabilization pond effluents. |
Also Read: 21 Types of Engineering You Need To Know
Chemical Unit Process
Chemical unit processes are those in which the removal of contaminants is brought about by chemical activity.
In the field of wastewater treatment, chemical unit operations are usually used in conjunction with physical unit operations and biological unit processes.
The following chemical unit processes are commonly used for wastewater treatment:
- Chemical precipitation
- Gas transfer
- Adsorption
- Disinfection
- Combustion
- Ion exchange
- Electrodialysis
It should be noted that chemical unit processes are additive processes, whereas physical unit operations and biological unit processes are subtractive processes.
This is an inherent disadvantage of chemical operations because there is usually a net increase in the dissolved constituents in the wastewater due to the chemical unit processes.
In most cases, we add something to the wastewater to achieve the removal of something else.
Another disadvantage of chemical unit processes is that they are all intensive in local operational costs.
The following table gives a summary of the applications of chemical unit processes in wastewater treatment:
| Process | Application |
| 1. Chemical precipitation | Removal of phosphorus and enhancement of suspended solids removal in primary sedimentation |
| 2. Gas transfer | Addition and removal of gases |
| 3. Adsorption | Removal of organics |
| 6. Disinfection | Disinfection of disease-causing organisms |
| 5. Dechlorination | Removal of total combined chlorine residuals |
| 6. Miscellanceous | Achievement of specific objectives in wastewater treatment |
Biological Unit Processes
Biological unit processes are those in which the removal of contaminants is brought about by biological activity.
In the biological treatment of wastewater, the objectives are to coagulate and remove the non-settleable colloidal solids and to stabilize the organic matter.
The wastewater may generally be from three sources:
- domestic wastewater, i.e, sewage
- Agricultural return wastewater
- industrial wastewater
For domestic wastewater, the objective is to remove the various nutrients, specifically nitrogen and phosphorus, which are otherwise capable of stimulating the growth of aquatic plants.
For industry wastewater, the objective is to remove/reduce the concentration of organic and inorganic compounds, which may otherwise be toxic.
Biological processes are classified by their oxygen dependence, of the primary microorganisms responsible for waste treatment.
Biological unit may therefore be classified in the following heads:
1. Aerobic Processes
Aerobic processes are biological treatment processes that occur in the presence of dissolved oxygen.
The bacteria that can survive only in the presence of dissolved oxygen are known as obligate aerobes.
The aerobic process includes the following:
- activated sludge process
- trickling filters
- aerobic stabilization ponds
- aerated lagoons
2. Anaerobic Processes
Anaerobic waste treatment involves the decomposition of organic and/or inorganic matter in the absence of molecular oxygen.
Anaerobic processes consist of the following:
- Anaerobic sludge digestion
- Anaerobic contact processes
- Anaerobic filters
- Anaerobic lagoons or ponds
3. Aerobic-anaerobic processes
In this method, stabilization of the waste is brought about by a combination of aerobic, anaerobic, and facultative bacteria.
Also Read: What is a Bridge?: A Complete Guide To Engineers
Methods of Wastewater Treatment
The wastewater treatment or sewage treatment is a broad term that applies to any process/operation or combination of processes and operations that can reduce the objectionable properties of carried waste and render it less dangerous, with the following:
- Removal of suspended and floatable material
- Treatment of biodegradable organics
- Elimination of pathogenic organisms
Later, a major effort was undertaken to achieve more effective and widespread wastewater treatment.
As a consequence, the required degree of treatment has significantly increased, and additional objectives and goals have been added.
The recently added treatment objectives are the removal of nitrogen, phosphorus, and toxic organic compounds.
Thus, the treatment methods may be divided into the following:
1. Conventional treatment methods
Under Conventional treatment, we have the following:
- Preliminary processes: Pumping, screening, and grit removal. Wastewater contains varying quantities of floating and suspended solids, some of considerable size. The purpose of preliminary processes is to remove floating materials, as well as heavy, settleable inorganic solids, including flow measuring devices.
- Primary treatment includes those physical unit operations and chemical unit processes by which large suspended solids are removed from the wastewater.
- Secondary treatment: Remove the soluble and colloidal organic matter that remains after primary treatment.
2. Advanced Wastewater Treatment
- Tertiary treatment: Includes all operations and processes used to remove the pollutants not removed in preliminary, primary, and secondary treatment. These pollutants may include soluble inorganic compounds such as phosphorus or nitrogen, which may support algal growth in receiving waters, and organic materials contributing biochemical oxygen demand (BOD), chemical oxygen demand (COD), colour, taste and colour, bacterial, viruses, colloidal solids contributing turbidity, or soluble minerals which may interfere with subsequent re-use of the wastewater.
Also Read: 15 Types of Windows Every Homeowner Should Know
Methods of Sludge Processing and Disposal
The bulk of the suspended solids, which enter the waste treatment plant, and the waste solids generated in biological treatment must be handled as sludge at some point in the treatment processes.
Disposal of the accumulated waste sludge is a major economic factor in wastewater treatment, since the construction cost of a sludge processing facility is approximately one-third that of the treatment plant.
Wrapping Up
As discussed above, Wastewater treatment is the process of purifying wastewater to remove contaminants, making it safe to return to the environment or for reuse.
This process typically occurs at wastewater treatment plants and involves several stages, like screening, grit removal, primary and secondary treatment, and disinfection.
I think you enjoyed reading this blog post. If you liked this article, please join us on Telegram. You can also find us on Pinterest, Twitter, and Facebook.
