Sewage Treatment


What are the three stages of sewage treatment

• Primary Treatment:

This sewage treatment involves removing floating solids by gravitational settling. The large solids are screened first then grease and scum are removed. After this, sand and other coarse substances are removed by grit chambers. The grit is then removed and screened. Wastewater is then run directly into sedimentation tanks.

The primary waste is usually bulky and contains a lot of water. Sometimes, water is filtered out after drying the waste. Sludge at the bottom of the tank is usually removed using equipment.

• Secondary Treatment

This is where the biological degradation of organic material takes place. indigenous aquatic micro-organisms carry out this process under controlled conditions. A sewage treatment plant is used in secondary sewage treatment is order to achieve a certain degree of effluent quality.

This is usually applied to the liquid part of the sewage recovered after primary treatment. Bacteria and protozoa break down the organic material while they reproduce to form cells of biological solids.

Tertiary Treatment

This aims at purifying the water further and also recycling it. As a result, the end water can be discharged into the environment. The first step is the filtration. This helps to remove any suspended matter in the wastewater. Sand and Carbon may be used to carry out filtration.

The wastewater is then transferred to lagoons for further removal of remaining particulate matter. Nitrogen and Phosphorus are removed as they may be harmful to the environment. Odour is also removed using various chemicals.

Primary sewage treatment

This is the very first step in sewage treatment.
It usually removes substances that will float or just settle out. Wastewater is held temporarily in a tank and heavier solids settle at the bottom. Eventually, the floating materials are removed while the settled solids need equipment to be removed. This is because they are bulky. The remaining liquid is released or put through a secondary treatment process.

Every wastewater tank in this process has its own features and equipment that can be selected according to the requirements of each individual site.
Primary treatment involves screening, comminution, grit removal and sedimentation. The screens usually block floating materials that could clog pipes and pumps hence they are cleaned mechanically. A comminutor grinds and shreds residue that manages to pass through the screens. However, this residue is later removed in the sedimentation process.

Grits slow down the flow so that solids can settle out of the water. It is very important to remove grit. This is due to the fact that it causes excess wear and tear on plant equipment and pumps.
Sedimentation tanks are also called primary clarifiers. Solids gradually sink to the bottom while sewage passes through them. The resulting sludge is removed mechanically.

Large particles can be bound into clumps by flocculation. Flakes are formed by colloids. They are then dissolved into a liquid. This is done by mixing them with coagulated water through the hydrodynamic process. Once the flakes are big enough, they can be removed by sedimentation.

Secondary sewage treatment

This is the second step in treating sewage. It involves removing dissolved compounds. The liquid portion of the sewage resulting from primary treatment is usually taken through secondary sewage treatment. This treatment uses biological processes to remove water contaminants.

Bacterial decomposition usually removes nutrients and remaining solids. The levels of Oxygen are interchanged at different stages. This is because the process requires both aerobic and anaerobic conditions. Hence, different communities of bacteria thrive and remove different pollution components.

Anaerobic process – Here organisms function without oxygen. They convert organic contaminants into fuel hence; they are used to treat high strength wastewaters. Anaerobic processes use less energy than the mechanical aeration process because they are loosely attached.
Aerobic process – These are most common in municipal wastewater treatment. Organic contaminants are converted into carbon dioxide because there is Oxygen. In addition, water and other end products may also be formed.

Aerobic lagoons – These are large, shallow earthen basins. They provide enough time for bacteria and algae to treat wastewater. This is because it is done naturally.

Rotating biological contactor – This is a process consisting of closely spaced parallel discs. The discs are partially submerged in the wastewater. As a result, microorganisms grow on the disc. Anaerobic degradation then takes place.

Trickling filter – Wastewater is distributed over a media surface. Microorganisms then form a layer of biomass over this surface. As a result, they consume contaminants in the water. The media could be rocks, gravel, etc.

Finally, Chlorine is used to kill pathogenic bacteria and get rid of the odour.

Tertiary sewage treatment

This is the final stage in the sewage treatment process. It ensures that the resulting water is of good quality before it is reused, recycled or discharged into the environment.

Tertiary sewage treatment removes any remaining inorganic components and substances like Nitrogen and Phosphorus. It also removes any health threats like bacteria and viruses.

Chlorination – This is a method that removes dissolved substances from the water. It could be iron, manganese or hydrogen sulphide which causes the odour. In addition, it also destroys harmful bacteria, parasites, and other organisms.

Ozonation – This involves the infusion of ozone. High electrical voltages on Oxygen molecules usually form Ozone gas. This infusion results in the destruction of harmful bacteria and other toxic microorganisms.

Ultraviolet radiation – This involves passing Ultraviolet light through wastewater. This is a very powerful disinfection method. The UV rays completely destroy microorganisms. Moreover, they also reduce the number of dissolved materials in the wastewater.

Activated Carbon Absorption – This is a process that removes low concentrations of contaminants from water. This is because they are difficult to remove by other means. Activated carbon is very porous and thus it has a large surface area for adsorption of contaminants.

Ion exchange – This is an irreversible chemical reaction that involves removing ions in wastewater. One ion solution is exchanged for another of the same charge using an immobile solid ion. These particles are either naturally occurring or synthetically produced.

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