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Biological Wastewater Treatment

By May 6, 2023No Comments

Consider the volumes of wastewater produced by nearly 8 billion people on this planet. This wastewater contains toxins and chemicals that are not only harmful to humans but to the natural environment.

If left alone, the natural purification processes are not enough to remove contaminants from wastewater, making wastewater treatment processes necessary to remove or break down things that people have added to the water before it is safely returned to the environment or reused.

While there are many chemical and mechanical treatment processes, biological wastewater treatment is used worldwide and is one of the most effective treatment methods.

What is biological wastewater treatment?

As a natural process, biological wastewater treatment uses organisms, primarily bacteria, protozoa, and other specialty microbes, to break down organic pollutants. It is the secondary treatment in the wastewater treatment process, initiated after removing large solids from raw sewage during the primary treatment stage.

During the biological treatment stage, microorganisms are added to the wastewater tank. These microorganisms use organic waste as nutrients and begin to digest and decompose it. What remains sticks together and settles out of the solution, producing more manageable waste and activated sludge, while the treated water flows into a tertiary treatment process before being discharged into the environment.

Biological treatment eliminates organic pollutants but not all types of pollutants such as detergents, soluble metals, pharmaceuticals, cosmetic wastes, etc. For that, other wastewater treatment processes are needed.

Types of biological wastewater treatment

There are three main types of biological treatment processes—Aerobic, Anaerobic, and Anoxic. The type of biological treatment will depend on a range of factors, such as the type of industry, chemical makeup of wastewater, and compliance and environmental regulations.

Aerobic processes

For aerobic processes, microorganisms require oxygen to break down organic pollutants into carbon dioxide and microbial biomass.

Common aerobic systems include:

Activated Sludge: Wastewater from primary treatment plants enters an aeration tank pumped with oxygen where free-floating microorganisms break down and consume organic matter, while biological solids or sludge settle into larger clumps separating from cleaner water. This system typically requires larger space and generates a large volume of sludge with associated costs for disposing of it. However, capital and maintenance costs are low.

Trickling Filters: While wastewater flows on a bed of rocks, ceramic, gravel, peat moss, and plastic, the microorganisms attach to the bed and form a microbial film. Over time, the microorganisms in this biofilm will break down the organic matter in the wastewater. This system reduces concentrations of organic matter quickly, but it is prone to clogs and needs a trained professional to supervise the process.

Aerated Lagoons: Wastewater sits in a treatment pond into which oxygen is pumped to increase bacterial growth and speed up the decomposition process. The separation of the sludge and clean water takes place within the pond, which is unlike the activated sludge process, where the clean water is transferred into another tank after aeration.

Oxidation Ponds: Although similar to aerated lagoons, the oxidation ponds system is more complicated and takes longer to achieve results. It also requires more space and is not used in densely populated areas.

Anaerobic processes

For anaerobic processes, microorganisms do not require oxygen to break down organic pollutants and convert organic waste into methane, carbon dioxide, and biomass.

Common anaerobic processes are:

Anaerobic Lagoons: Wastewater is introduced to the bottom of large lagoons and it separates into a sludge layer with liquid on the top, which prevents oxygen from reaching the bottom sludge where the anaerobic digestion of organic matter takes place.

Anaerobic sludge blanket reactors: Wastewater passes through a floating blanket-like layer of suspended sludge particles where the microbes break down organic contaminants which settle at the bottom of the tank in chunks and treated effluent flows upward and out of the unit. Throughout the process, biogas is collected into collection hoods.

Common forms of anaerobic sludge blanket reactors include:

  • Upflow anaerobic sludge blanket (UASBs)
  • Expanded granular sludge beds (EGSBs)
  • Anaerobic baffled reactors (ABRs)

Anaerobic filter reactors: A reactor tank is fitted with a filter medium to which microorganisms attach to form a biofilm. This biofilm and its microbes effectively break down organic matter in the wastewater.

Anoxic process

For Anoxic processes, microorganisms use something else other than oxygen for growth, such as sulphate, nitrate, nitrite, selenate, and selenite.

Anoxic means the depletion or deficiency of oxygen. Though anoxic conditions do not contain molecular oxygen, it may contain nitrates or nitrites that have bound oxygen as part of their structure.

The main objective of anoxic treatment is to remove compounds like nitrates, nitrites and phosphorus from wastewater effluent before it is discharged. These nutrients can cause nutrient pollution with serious environmental and human impact. It also causes dense growth of algae, resulting in lower water quality and damage to aquatic life and environmental habitats.

For example, the anoxic process converts nitrates into nitrogen gas in the absence of oxygen. This process is also known as denitrification. While less sludge is produced with the anoxic process, it also releases greenhouse gases (nitrous oxide) into the atmosphere.

Benefits of biological wastewater treatment

Biological wastewater treatment processes have the following benefits compared to other wastewater treatment methods:

  • The treatment methods are based on biological sciences and well understood.
  • The presence of natural soil biology reduces energy costs, making it cost effective. Maintenance and operating costs are also low.
  • It is environmentally friendly and safe, as it relies on natural microbiology to reduce organic waste instead of adding other harmful chemicals and substances. It also has low emissions of nitrogen and phosphorus.
  • The method is efficient because of higher activity in the biomass, resulting in improved sludge reduction, especially through anaerobic digestion which produces fewer biosolids. This increases the efficiency of the system.

In Summary

Biological wastewater treatments are often complemented with additional processes such as filtration, chlorination, or UV processes to ensure proper treatment of wastewater, which can be costly. However, as a biological system, it is economical as it optimises the naturally occurring process of microbial decomposition of organic contaminants in wastewater, so that it can easily be removed.

With over 50 years of service and experience in wastewater management products, we can help you with information on biological wastewater treatments and products. Contact us here.