A wastewater treatment plant treats sewage water from various sources and makes it reusable. It comprises several processes that address specific needs for wastewater treatment.
The treatment system treats the sewage from domestic use, rainwater, runoff, and other pollutants that go down the street gutters. The sewage also includes water from agricultural and industrial sources.
This article explains the working of a wastewater treatment plant in brief.
Stages of Wastewater Treatment Plant
There are three main stages involved in the treatment of sewage:
- Primary Treatment
- Secondary Treatment
- Tertiary Treatment
Some wastewater treatment plants only employ primary and secondary treatment, while others engage all three treatment stages.
Based on the region and standards, the order of treatment can vary. Figure-1 below shows the general layout of a wastewater treatment plant:
1. Primary Treatment of Wastewater
The sewage water is first allowed to pass through a primary wastewater treatment plant. At this stage, the treatment uses screens and settling tanks to remove most of the floating materials from the wastewater.
Solid materials account for around 35% of the wastewater. Hence, removing the solid waste at the primary stage makes the subsequent treatment procedures less tedious.
The screens that are used have openings of 10 mm that can collect sticks, garbage, and other larger materials from the wastewater. After the screening process, solid waste is collected and disposed of.
The sewage then passes through the grit chamber, where sand, cinders, and small stones settle at the bottom. The communities with combined sewer systems mostly contain gravel or sand due to rainwater runoff. These materials are removed easily by a grit chamber.
Once the settled grit is removed, the wastewater is passed to the sedimentation tanks, settling tanks, or clarifiers. This step removes organic and inorganic matter and suspended solids.
By properly adjusting the water flow in the sedimentation tank, the suspended particles start to sink to the bottom and form a solid mass. The solid mass is called raw primary biosolids or sludge. Scum is formed on the top of the wastewater and is skimmed off from the top.
The primary treatment process removes around 90% of suspended solids, 55% of fecal coliforms, and 50% of biological oxygen demand (BOD). For the complete removal of harmful substances, the waste must go through secondary treatment.
2. Secondary Treatment of Wastewater
Around 85% of organic matter from sewage is removed during secondary wastewater treatment. The process involves forcefully mixing the wastewater with bacteria and oxygen, bacteria digest organic matter with the help of oxygen. These processes are performed by the trickling filter and activated sludge tank.
2.1. Trickling Filter
A trickling filter forms a bed of stones placed at a height of 6 feet. The sewage is allowed to pass through this stone layer once it is out of the sedimentation tank. The bacteria gather on these stones and start to multiply and develop until they completely consume organic matter in the sewage. After the process, the clean water trickles out through pipes and moves to another sedimentation tank.
2.2. Activated Sludge Process
Nowadays, the activated sludge process is used more than the trickling filter. In this process, the air and sludge are allowed to come in close contact with the bacteria. After this, it is allowed to pass through the settling tank.
The sewage coming out of the settling tank is first taken to an aeration tank, where air and sludge are fully loaded with bacteria. The whole arrangement is left as such for several hours and the bacteria decompose organic matter into harmful by-products during this time.
The sludge, activated with billions of bacteria, again moves to the aeration tank to treat the new sewage. The before-treated wastewater flows to the sedimentation tank to remove excess bacteria.
The secondary treatment is completed by finally disinfecting the effluents using chlorine, ultraviolet light, or ozone and then discharged. The secondary treatment removes around 85 to 90% of BOD, suspended solids, and 90 to 99% of coliform bacteria.
3. Tertiary Treatment of Wastewater
This treatment is employed when the water from the secondary treatment has an undesirable amount of dissolved substances like metals, color, nutrients, and organic chemicals. Several physical, chemical, and biological treatment processes are performed for tertiary treatment.
There are mainly three stages of wastewater treatment plants:
1. Primary Treatment
2. Secondary Treatment
3. Tertiary Treatment
The sewage water is first allowed to pass through a primary wastewater treatment plant. At this stage, the treatment uses screens and settling tanks to remove most of the floating materials from the wastewater. The primary treatment process removes around 90% of suspended solids, 55% of fecal coliforms, and 50% of biological oxygen demand (BOD). For the complete removal of harmful substances, the waste must go through secondary treatment.