Primary sedimentation is usually the second stage of the Wastewater Treatment (WWT) process. It occurs after the water passes through the Inlet Works/Screening process where all non-organic solids such as non-dissolved materials, wet-pipes and debris are removed from the water. Once wastewater leaves the Inlet Works/Screening process, organic solids still remain in it. The rate of wastewater flow is reduced dramatically before entry into the primary sedimentation tanks; this has two effects on the wastewater:
1. Fats, oil and greases (commonly known as FOG) float to the surface of the sedimentation tank are collected by a scraper bar which continuously rotates across the surface of the tank. The FOG is removed from the wastewater and disposed of.
2. Organic solids are heavier than water and due to the effects of gravity fall to the bottom of the tank. Once they reach the bottom of the tank, the sediment (often referred to as sludge) is collected by a rake arm and moved to a nearby storage tank (similar process as poin (1) above).
At this stage the sediment is a concentrated mix of water solids with a high content of HS2, methane and bacteria - this is where the sludge reprocessing process begins. The wastewater (now virtually solid free and considerably cleaner) enters the aeration stage of the process where small organic particles (too small to be affected by gravitation in the sediment tanks) and ammonia are removed.
The majority of the sedimentation process is of mechanical nature, however throughout the various stages of wastewater treatment, the aim is to keep the wastewater moving (at various speeds) from the beginning to the end of the process.
Still, some portions of the water remain in a static state, and can cause a build-up of solids that create blockages and sedimentation problems for the mechanical equipment used in the treatment process. To alleviate this, small amounts of air are introduced into the process to move the water along with mechanical surfaces or submerged mixers. In some cases, chemical dosing is also used to help break down the sludge.
Lobe and side channel blowers in the primary sedimentation stage of wastewater treatment include equalization tanks and grit/sand removal devices. The process requires small concentrated amounts of air generally in the form of coarse bubble aeration for short periods of time. This application is suited for side channel or small lobe blowers.
The dosing process is also a part of the primary process. Here, chemicals are added to the wastewater in a controlled manner. The process is intended to increase the efficiency of the treatment process and help with the removal of undesirable elements such as phosphorus. The introduction of chemical dosing can also help with the coagulation and flocculation processes.