Types of biological wastewater treatment methods

Biological wastewater treatment is one of the main methods in industrial and sanitary wastewater treatment, which is based on the transfer of organic pollutants from dissolved to suspended solids (biomass or sludge). Biological treatment is also referred to as secondary wastewater treatment, which aims to treat the effluent from the primary treatment by aerobic and anaerobic microorganisms. In the following, various types of biological wastewater treatment methods have been introduced along with the explanation of each step of the process.

What is biological wastewater treatment

Biological wastewater treatment or secondary treatment is defined as a set of biological methods in which the separation of biologically degradable organic substances is possible under certain conditions by using the activity of microorganisms. Among the features of biological wastewater treatment, the following can be mentioned

• very fast
• Has relatively harmless products such as CO2 and H2O
• variable oxygen level during treatment (due to the activity of aerobic and anaerobic microorganisms)
• Need to inject air bubbles

In terms of mechanism, biological wastewater treatment processes are divided into two categories: suspended growth or activated sludge and attached growth processes. For more information, you can refer to the article “What is wastewater treatment? | Classification of wastewater treatment methods”.

Objectives of biological wastewater treatment

• Removal of carbonaceous organic materials (carbonaceous BOD)
• Removal of inorganic materials containing N and P

As mentioned, the main goal of biological wastewater treatment is to remove carbonaceous organic substances and transfer them from the soluble state in wastewater to suspended solids, which is introduced as process sludge.

Types of biological wastewater treatment methods

The most important methods of biological wastewater treatment are:

• Biological treatment with the help of anaerobic microorganisms
• Biological treatment with the help of aerobic microorganisms
• Biological treatment with the help of aerobic nitrifying and anaerobic Denitrifying microorganisms
• Biological treatment with the help of phosphate-removing microorganisms.

Wastewater microbiology in biological treatment

Microorganisms

The activity of microorganisms as the main players in biological wastewater treatment is extremely important. Removal of carbonaceous organic materials and brain materials such as nitrogen and phosphorus is done with the help of microorganisms and especially bacteria. Microorganisms are used to convert soluble, colloidal, and particulate carbonaceous organic materials that have the ability to decompose into various gases and cell production. The resulting cells or the produced microorganisms, because they have a higher specific weight than water, are settled by gravity and can be easily separated. It is obvious that the remaining sludge from the effluents cause BOD, which, if not properly separated, will produce problems as a result of the treatment process. It is very important to use the knowledge of microbiology and biochemistry of microorganisms in designing and choosing the appropriate biological treatment method.

Microbial growth

Microorganisms must have the following factors in order to continue their activity and reproduction:

• energy source
• Carbon, direction of new cell tissue
• Mineral and nutrient elements such as macro elements (N and P) and micro elements (Fe+2, Cu and Zn)

PH range of activity of microorganisms

Microorganisms are able to work in the pH range between 4 and 9.5. But the best and most appropriate range for the activity of microorganisms in aerobic and anaerobic conditions is between 6.5 and 8.5

Types of biological wastewater treatment processes

:In general, based on the type of microorganisms, biological wastewater treatment methods can be classified as follows

• Aerobic wastewater treatment

It includes all wastewater treatment processes that must be carried out in the presence of oxygen. Aerobic process is one of the most influential biological processes. Among the aerobic wastewater treatment processes, we can mention the activated sludge process, which was completed in 1900 by two Englishmen. This wastewater treatment process includes the life of microorganisms along with organic matter in an oxygen-rich (aerobic) environment. This process is similar to the biological process that occurs in the upper layers of the soil and contains a large number of microorganisms, except that the microorganisms in the activated sludge process are kept in a liquid and controlled environment.

• Anaerobic wastewater treatment

.In the anaerobic process, biological treatment is done without the presence of oxygen and anaerobic microorganisms play a major role in this process

• Anoxic wastewater treatment

In this activity, the process environment is similar to anaerobic processes. But nitrification and denitrification are performed by the relevant microorganisms. Also, the required oxygen is supplied through nitrogen oxides. In general, the purpose of this process is to remove the nutrients in the wastewater, including nitrogen and phosphorus. Removal of nutrients in wastewater should be done for the following reasons

• Preventing the phenomenon of eutrification
• Preventing the increase in the oxygen requirement of the receiver flow
• Prevention of contamination and poisoning

• Lagoon processes

Oxygen and the activity of biological agents play an important role in wastewater treatment by lagoon, as in all methods of biological wastewater treatment. The oxygen necessary for the oxidation of pollutants may be provided in one of the following ways

• Dissolved oxygen in wastewater
•  Free air oxygen
•  Oxygen in inorganic and organic compounds
•  Oxygen resulting from photosynthesis of aquatic plants

In the biological treatment of wastewater with the lagoon method, the treatment that is carried out in the primary sedimentation ponds, aeration ponds and final sedimentation are all done in one treatment unit, and the resulting sludge is deposited in the lagoon without the need for special collection and treatment. And they may remove the sludge from the lagoon and use it every few years. Garbage and grain collection are also often done before wastewater treatment with lagoon

Usages of biological wastewater treatment

This treatment method is used in addition to physical and chemical treatment to completely remove organic pollutants and reduce the BOD of wastewater. Also, due to the very high efficiency of the biological method in wastewater treatment and the production of quality sludge for disposal in agricultural lands as fertilizer, this method can be used in the following treatment plants according to the quality characteristics of their effluents:

• Human sanitary wastewater treatment
• Hospital wastewater treatment
• Wastewater treatment of slaughterhouses
• Wastewater treatment of food and textile factories
• Wastewater treatment of detergent factories
• …..

The most important equipment in biological wastewater treatment

In order to prepare the quantitative and qualitative conditions of the wastewater before entering the biological system of the sewage treatment plant, a series of equipment must be used to remove suspended solid particles, etc. Then, a series of wastewater equipment is also needed for aeration in the aerobic part of the biological process

• Manual and mechanical scavengers
• Sedimentation revolving bridge to collect sludge
• Aeration systems, including the use of aeration diffusers

The chosen strategy for wastewater treatment may be different for each production unit. This is determined by the water industry research company in such a way that the employer’s needs and environmental standards are fully covered. PASENCO Company with more than twenty years of successful experience in providing engineering services related to the design, consulting and implementation of industrial and sanitary wastewater treatment systems along with the full guarantee of obtaining environmental standards, will accompany you in this important matter. To get more information and free expert advice, you can communicate with our experts through the communication methods mentioned on this page