The Initial Phase of Primary Sewage Treatment: A Full Overview

by Anna

Primary sewage treatment is a critical process in wastewater treatment plants, where raw sewage undergoes preliminary treatment to remove large particles and reduce the organic load before entering the secondary treatment phase. This primary stage plays a vital role in ensuring the efficient and effective treatment of wastewater, ultimately safeguarding public health and environmental well-being. In this article, we will delve into the first step of the primary sewage treatment plant, providing a comprehensive overview of its objectives, processes, and significance.


Screening: The First Step in Primary Sewage Treatment

The first step in primary sewage treatment is known as “screening.” Screening involves the removal of large solid objects and debris from the raw wastewater stream. The primary objective of screening is to protect downstream equipment, prevent clogging in the treatment process, and facilitate smoother operations in subsequent treatment stages.


Equipment and Components Used in Screening

Screening is typically carried out using various equipment and components designed to trap and separate the larger solids from the wastewater flow. The primary components used in screening include:

a. Bar Screens:

Bar screens consist of vertical or inclined bars or rods spaced at regular intervals to create an obstruction in the wastewater flow. As the raw sewage passes through the bars, large solid objects such as sticks, plastics, rags, and other debris are trapped while the wastewater continues its flow.

b. Mechanical Screens:

Mechanical screens employ rotating or moving components to mechanically lift and remove large solids from the wastewater. These screens are especially useful for handling larger volumes of wastewater and efficiently removing bulky debris.

c. Drum Screens:

Drum screens consist of a cylindrical drum with perforated plates or mesh screens. As the drum rotates, wastewater flows through the screens, and solids are collected on the inside of the drum and subsequently removed.

d. Static Screens:

Static screens are simple devices with fixed screens that allow wastewater to flow through while capturing large solids. The captured debris is periodically removed for proper disposal.

Importance of Screening in Primary Sewage Treatment

Screening is a crucial step in the primary treatment process for several reasons:

a. Protecting Equipment:

The removal of large solids at the initial stage prevents damage and clogging of downstream equipment, such as pumps, pipes, and treatment units. This ensures smoother and more efficient operation of the entire wastewater treatment plant.

b. Reducing Maintenance:

By preventing large debris from entering the treatment process, the need for frequent maintenance and cleaning of equipment is significantly reduced, saving both time and costs.

c. Preventing Blockages:

Large solid objects, if not removed in the screening stage, can cause blockages and backups in the treatment process, leading to disruptions and potential overflows.

d. Enhancing Efficiency:

Effective screening optimizes the subsequent treatment processes by removing large solids that may otherwise interfere with settling, aeration, or biological treatment stages.

Additional Processes in Primary Sewage Treatment

While screening is the initial step in primary sewage treatment, the process may involve other essential components and techniques, depending on the specific design and requirements of the treatment plant. Some additional processes often included in primary treatment are:

a. Grit Removal:

Grit, such as sand and small pebbles, may also be removed during the primary treatment stage. Grit removal prevents abrasion and wear in downstream equipment and protects the secondary treatment process from unnecessary stress.

b. Sedimentation:

In certain primary treatment systems, sedimentation tanks or clarifiers may be employed to allow settleable solids to separate from the wastewater. These solids form a sludge layer at the bottom of the tank, while clarified water rises to the surface for further treatment.


The first step in primary sewage treatment, screening, plays a crucial role in efficiently and effectively treating raw wastewater. Through the removal of large solids and debris, screening protects downstream equipment, prevents clogging, and ensures the smooth operation of the entire wastewater treatment plant.

By employing various screening equipment and components such as bar screens, mechanical screens, drum screens, and static screens, wastewater treatment plants can tailor their screening process to meet specific needs and capacities.

Overall, the significance of screening in primary sewage treatment cannot be understated, as it lays the foundation for subsequent treatment processes, promoting the removal of pollutants and contributing to the protection of public health and the environment.


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