Preliminary Treatment of Waste Water

The first step in wastewater treatment is known as preliminary treatment.

Wastewater pre-treatment is the initial stage of wastewater treatment and is among the most important stages. It entails the process of excluding coarse particles, sand, gravel, and other materials that are likely to harm or block other treatment processes. The main purpose is to make the wastewater ready for other stages of treatment while maintaining the functionality of the system. Here’s an overview of preliminary treatment and its components:

1. Preliminary treatment objectives

• Protect Equipment: Eliminate items that may cause harm or hinder the operations of pumps, pipes, and other equipment in the treatment facility.
• Improve Efficiency: Minimize the burden on other treatment processes thus enhancing their efficiency.
• Prevent Clogging: Reduce the accumulation of wastes that may hinder the treatment processes in the systems.

2. The major components of preliminary treatment are as follows:

1. Screening
   • Purpose: To filter out large particles like rags, plastics, sticks and other solid materials from the wastewater.
   • Types of Screens:
     • Bar Screens: It is made of parallel bars that capture large objects.
     • Fine Screens: Have small apertures to collect more refined particles.
   • Operation: Wastewater passes through screens where solids are caught and are later scooped or washed out.
2. Grit Removal
   • Purpose: To exclude materials that are likely to exert mechanical wear and tear on the equipment such as sand, gravel, and small stones.
   • Methods:
     • Grit Chambers: Slowing down the rate of the flow of wastewater so that the heavier particles can settle at the bottom.
     • Aerated Grit Chambers: Air can be used to generate a rolling action in water which aids in the sedimentation of grit.
   • Equipment: Grit chambers and aerated grit tanks where settled grit is collected and removed.
3. Pre-Aeration
   • Purpose: To add air into the wastewater, which aids in the flotation of grease and oils and also the elimination of odors.
   • Process: In the pre-aeration tanks, air is passed through the wastewater to facilitate the floating of lighter materials that are skimmed off the surface.
4. Flow Equalization
   • Purpose: To control the rate of flow of the wastewater so that it can be easily managed in the next stages of treatment.
   • Method: Flow equalization basins are used to store the incoming wastewater for some time and then discharge it at a slow rate.
   • Benefits: Minimizes the effects of flow fluctuations and contributes to maintaining stable conditions in subsequent processes.

3. Process Overview

• Influent Screening: Sewage flows into the treatment works and is initially subjected to screening to remove any large objects.
• Grit Removal: The screened water is then discharged to grit chambers where sediments and other heavy materials settle.
• Pre-Aeration: Air is added to facilitate the process of separation of grease and oils as well as to eliminate bad smells.
• Flow Equalization: Sewage is collected and held for a certain period to allow for the proper flow of the water.

4. Preliminary treatment outcomes

• Protection of Equipment: Reduces wear and tear on pumps, pipes and other mechanical parts of the system.
• Improved Efficiency: Improves the efficiency of the subsequent treatment stages by minimizing the concentration of solids and other materials.
• Reduced Maintenance: Reduces the chances of blockage and the frequency of cleaning.

Conclusion

The first stage of wastewater treatment is known as preliminary treatment. It helps in preventing large solids, grit, and debris from entering the subsequent stages of treatment, thus, safeguarding equipment and enhancing efficiency for the next stages of secondary and tertiary treatment. Preliminary treatment should be strong enough to sustain the performance and durability of the wastewater treatment plant.