/Understanding the 3 stages of Wastewater Treatment
Wastewater Treatment

Understanding the 3 stages of Wastewater Treatment

The growth of the urban population in major cities and towns around the world has created several social challenges. Many cities have not been able to build social amenities to keep up with the growth rate of their populations. As a result, there are several people living in and around major cities who do not have access to clean water and safe sanitation facilities.

Additionally, the changes in global weather patterns and climate have led to prolonged droughts in some regions and life-threatening floods in others. The combination of natural disasters and rapid population growth in cities has led to the development of various ways to conserve, recycle and share the limited water resources available.

Global Wastewater Treatment

One of the main ways the municipalities try to make clean water available is by treating wastewater from industries and residential areas. According to the 2017 UN world water development report on wastewater, it is estimated that about 70% of the wastewater in high-income goes through some form of wastewater treatment processes. The same report highlighted that middle income, and lower-middle-income countries treated 38% and 28% of their wastewater, respectively. Unfortunately, only 8% of the wastewater in low-income countries receives any kind of treatment.

The wastewater is taken through three fundamental treatment stages so as to make it safe for reuse or to be released into the environment to replenish the natural water cycle of the region. There are several wastewater treatment solutions and technologies in use today, but most of them fall into the following categories.

•    Primary Wastewater Treatment

•    Secondary Wastewater Treatment

•    Tertiary Wastewater Treatment

1 Primary Wastewater Treatment

The primary stage of wastewater treatment can also be referred to as the sedimentation stage. This is because the wastewater is allowed to flow through a series of large sedimentation tanks where solids are separated from the water.

The wastewater is kept in the primary sedimentation tanks or clarifiers so that the suspended solids can sink to the bottom as grease and oils rise to the water surface. The clarifiers have skimmers which take away the floating oils and mechanical scrapers at the bottom to move the accumulated sludge to be processed in digesters. The clear water is then allowed to move into the secondary treatment stage.

2 Secondary Wastewater Treatment

The Secondary treatment of wastewater is mainly aimed at dealing with biological contaminants. This is done mainly by adding oxygen to the wastewater. There are three main ways that air is added to the wastewater in the secondary treatment stage.

i.    Biofiltration

Biofiltration employs the use of tricking filters, sand filters and contact filters to eliminate sediments from the wastewater.

ii.    Aeration

In aeration treatment, microorganisms are introduced into the wastewater. The mixture is then aerated for several hours while the microorganisms feed on the residual organic matter.

iii.    Oxidation Ponds

The wastewater is retained in oxidation ponds for up to two or three weeks while the biodegradable matter breaks down.

3 Tertiary Wastewater Treatment

The third stage of the wastewater treatment process makes use of special equipment and chemicals to sanitize the water. In this stage, 99% of the impurities in the wastewater are eliminated. After this stage, water can safely be reintroduced to natural water bodies such as seas and rivers. The list below highlights a few of the processes used in this stage of wastewater treatment.

•    Carbon Adsorption

•    Biological Nutrient Removal (BNR)

•    Phosphorus removal

•    Nitrogen Removal

•    Disinfection

There are various other more advanced treatment practices carried out on wastewater aimed at purifying it further. For example, the pharmaceutical industry takes the water through a fourth stage that eliminates micro-pollutants such as environmental persistent pharmaceutical pollutants (EPPP) and other chemicals. Nevertheless, the three stages mentioned discussed here cover the conventional, accepted wastewater treatment practices.

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