What is wastewater treatment? | Classification of wastewater treatment methods

Wastewater collected from rural and urban communities as well as industrial units must be returned to water or soil sources. In each case, This complex question must be answered: in order to preserve the environment, which of the wastewater pollutants and to what extent should be removed? The answer to this question requires an examination of local conditions and needs, along with the application of practical knowledge, engineering judgments based on experience, and compliance with environmental regulations and standards. In fact, wastewater treatment is a process during which domestic and industrial wastewaters are purified and disinfected and can be reused and discharged into the environment.

Before the emergence of the microbial theory by Koch and Pasteur in the second half of the 19th century, the relationship between pollution and disease was only vaguely known, and the nascent science of bacteriology was not used for sewage treatment. The issue of wastewater treatment in its modern form was raised from the beginning of the 20th century. At a time when sanitary conditions had caused the demand to apply more efficient methods in wastewater control, there was an increasing growth. The impossibility of providing large and sufficient surfaces for ground disposal of untreated wastewater, especially for big cities, led to the use of more effective methods in sewage treatment. In this article, various methods of wastewater treatment are also reviewed.

Objectives of wastewater treatment:

The reason for wastewater treatment was to solve public concerns in the field of health and community health and to improve unfavorable conditions caused by unprincipled disposal of wastewater in the environment.

• Separation and removal of suspended and floating materials
• Purification of biodegradable organic substances
• Removal of pathogenic microorganisms (pathogens)

Today, other goals based on aesthetic and environmental issues have been added to these goals, which include the removal of nutrients such as nitrogen and phosphorus, and are particularly important in areas where wastewater is discharged into surface water sources. Likewise, today, due to the increase in human knowledge, the goals of wastewater treatment in some advanced countries are focused on the issues caused by toxic and potentially toxic chemicals, and the removal of toxic compounds and organic substances resistant to biological decomposition and heavy metals from wastewater has a special priority.

Types of wastewater and their origin:

In general, there are three types of wastewater: domestic wastewater, industrial wastewater, and wastewater from floods. Domestic wastewater, which is also called sanitary wastewater, is discharged from residential and commercial units. Industrial wastewater is caused by the use of water in the processes of industrial units, which is the most variable type of wastewater, and depending on the type of industries and factories, the nature of wastewater also changes. Therefore, sufficient expertise and experience is needed for industrial wastewater treatment. wastewater caused by floods is the runoff resulting from atmospheric precipitation, which is collected using a piping system and open channels.

Quantitative and qualitative characteristics of wastewater:

99.9% of wastewater by mass is made up of water and only 0.1% of it is made up of solids, these solids are classified into two general groups of organic and inorganic solids. Organic materials of wastewater include proteins, carbohydrates and fats, and inorganic solids are sediments, salts and metals. Proteins are mainly amino acids and carbohydrates can be sugars, starch and cellulose. There are different types of fats, such as industrial oils (grease). These organic materials all contain carbon that can be converted into carbon dioxide biologically. In addition, proteins contain nitrogen, which are considered nutrients and must be separated from wastewater, otherwise the amount of oxygen consumed in the place where the wastewater is discharged will increase, and this will cause the pollution of the water environment and the phenomenon of eutrification.

Wastewater treatment quantities:

BOD: The most widely used quantity to measure the level of wastewater pollution is the amount of oxygen that aerobic microorganisms need to break down biodegradable organic materials into final products.

cellular tissue (sludge)  CO₂+   organic matter + Dissolved oxygen

Uses in:

• Estimating the amount of oxygen required for reaction with organic substances
• Estimation of the dimensions of the wastewater treatment plant
• Measuring and calculating the efficiency of some purification processes
• Most wastewater disposal standards refer to it.

• COD: The amount of dissolved oxygen required for the chemical oxidation of an organic substance by strong oxidizers. This quantity is larger than BOD and has a faster calculation than it. This quantity is usually highly used in industrial wastewater, for example, cellulose from the waste water of paper factories is biologically oxidized slowly, but it is chemically oxidized at a high rate.

Classification of wastewater treatment methods:

Wastewater pollutants are removed through physical, chemical, and biological methods, and each of these methods is classified under the title of physical unit operations, chemical unit processes, and biological unit processes. In wastewater treatment systems, these methods are used together with special arrangements and combinations.

Physical unit operation

Treatment methods in which the use of physical forces is dominant are called physical unit operations. For example, the sedimentation of solid materials due to the force of gravity or the floating of fat or oil in wastewater due to its lower specific weight than water are examples of physical treatment. Other methods of physical purification include garbage collection, filtration, and gas transfer.

Screening Unit

The first operation we deal with in wastewater treatment plants is screening unit. The purpose of this unit is to remove large and heavy floating objects to protect the next units of the treatment plant from possible damages. The Screening collector is made of stainless steel iron bars and mesh, which can be different depending on its openings, which may be circular, rectangular or square.

Grit Removal:

The purpose of grit removal unit in the wastewater treatment plant is to separate heavy inorganic particles such as sand and gravel and heavy granular materials such as corn kernels. It is necessary to use this unit to prevent clogging of pipes, reduce accumulation in settling tanks and also protect mechanical equipment against wear.

Flow Equalization

Flow Equalization is used to overcome operational problems caused by fluctuations in the flow rate and concentration of wastewater pollutants, improve the performance of the subsequent units of the treatment plant, and also reduce the dimensions of the units and the cost of wastewater treatment. The main application of the flow equalization unit is to minimize or eliminate organic and hydraulic load shocks in secondary wastewater treatment. In this unit, the PH of the wastewater is also stabilized.

Chemical unit processes

The treatment methods in which the separation or transformation of wastewater pollutants are done due to the addition of chemicals or as a result of chemical reactions are called chemical unit processes. The most common examples of chemical treatment include chemical precipitation, surface absorption process, chlorine disinfection and PH adjustment. In the table below, you can see the use of each of the chemical processes in wastewater treatment:

Biological unit processes

The treatment methods in which the separation of wastewater pollutants is done through biological activity are called biological unit processes. These methods are primarily used for the separation of biodegradable organic substances that exist in the form of solution and colloid in wastewater. These organic materials either turn into a mass of cellular tissue (we call them “sludge” in wastewater treatment), which can be separated from the wastewater by sedimentation, or they turn into CO2 and other gases and enter the air. Biological treatment is also used to remove nutrients from wastewater.

Application of wastewater treatment methods

In the following, you will learn about the main and common methods of wastewater treatment. In the table below, you can see the important wastewater pollutants and the operations and processes that are carried out to remove these pollutants:

“Unit operations and processes and wastewater treatment systems used in separating the main pollutants in wastewater”

Wastewater treatment steps

Preliminary wastewater treatment

Preliminary wastewater treatment is the separation of components from wastewater that can cause erosion or clogging of devices and occupy the useful volume of treatment units. Garbage collection and granulation and crushing processes are included in this stage of wastewater treatment.

Primary wastewater treatment

In primary treatment, the goal is to remove suspended solid particles from wastewater. This separation is usually done by physical operations such as sedimentation. Since the sedimentation in this step occurs due to the force of weight, a long residence time is required, so the primary treatment is a relatively slow process. The primary treatment effluent usually contains a significant amount of organic matter and has a relatively high BOD content. Primary treatment is often a precursor to secondary treatment.

Secondary wastewater treatment or biological treatment

Secondary wastewater treatment includes biological treatment such as aeration and the use of microorganisms to transfer organic pollutants from dissolved to suspended solids (BioMass), which has relatively harmless products such as CO₂ and H₂O.

organic matter  + oxygen + bacteria            Biomes + CO₂ , H₂O , NH₃

Advanced wastewater treatment

In this stage of treatment, the goal is to separate components from wastewater such as nutrients, toxic compounds, and large amounts of organic matter and suspended solids. Nitrogenization, phosphorus removal, use of active carbon and sand filters are among this stage.

Types of wastewater treatment methods

The main application of biological wastewater treatment processes is the separation of carbonaceous organic matter in wastewater, which is usually measured in terms of BOD or COD. These processes are carried out both aerobically and anaerobically. Biological treatment in the system of wastewater treatment plants can be implemented in three ways: suspended growth, attached growth or a combination of the two.

Aerobic processes of biological wastewater treatment

Aerobic processes of biological wastewater treatment are implemented in two ways: suspended growth and attached growth:

• Activated sludge process
• Extended Aeration Activated Sludge Process (EAAS)
• Fixed Growth Activated Sludge Process (IFAS)
• Activated sludge process with attached growth on moving bed (MBBR)
• Wastewater treatment process with continuous batch reactor (SBR)
• Rotary biological strainer (RBC)

Anaerobic processes of biological wastewater treatment

In anaerobic biological treatment processes, wastewater is treated without the presence of oxygen, and anaerobic microorganisms play a major role in this process. In general, in the process of anaerobic purification, complex organic substances are affected by acid-producing microorganisms, and they are the result of the process of organic acids. Then, organic acids are reacted with methane producing microorganisms and the result of this process is water, methane gas and CO2. If the above process is not done completely, it is possible to produce gases such as H2S, which will cause a drop in pH in this type of treatment and will eventually disrupt the process.

• UASB (Up Flow Anaerobic Blanket )
• UABR ( Up Flow Anaerobic Baffle Reactor)
• EGSB (Expended Granule Sludge Bed)
• UAFB ( Up Flow fixed Bed)

Factors affecting the choice of wastewater treatment method:

• General cost (operation and initial)
• Applicability
• Operating specifications
• Required equipment
• Manpower
• Ease of management and use of the wastewater treatment plant system

Choosing the right equipment and designing a suitable treatment plant can help reduce energy needs and significantly reduce operational and operating costs.

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