Sewage treatment, also known as wastewater treatment, is a critical component of building services and maintenance. It involves a series of physical, chemical, and biological processes designed to remove contaminants from wastewater and sewage, primarily from domestic, commercial, and industrial sources. The primary goal of sewage treatment is to produce treated effluent that is safe for discharge into the environment or for reuse and to manage and dispose of sewage sludge appropriately. The principles of sewage treatment are based on several stages, each performing specific functions to ensure the efficient and effective purification of wastewater.

### 1. Collection and Conveyance

The first principle of sewage treatment is the proper collection and conveyance of wastewater from buildings. In a building services system, sewage is collected from bathrooms, kitchens, and laundry areas through a network of sanitary plumbing and drainage pipes. These pipes are connected to the municipal sewer lines or to an on-site treatment system like a septic tank. Proper slope, pipe sizing, ventilation, and access points for cleaning are critical in this phase to ensure smooth flow and avoid blockages.

### 2. Preliminary Treatment

Preliminary treatment involves the removal of large and coarse materials that can damage equipment or hinder later treatment processes. This stage includes:

* Screening: Wastewater passes through screens to remove large objects such as sticks, rags, plastics, and other debris.
* Grit Removal: Grit chambers allow sand, gravel, and other heavy particles to settle, protecting pumps and mechanical equipment.
* Flow Equalization: In some systems, flow equalization tanks are used to manage the fluctuations in sewage flow, providing a steady flow to the subsequent treatment stages.

This stage focuses on protecting downstream equipment and enhancing the overall efficiency of the treatment process.

### 3. Primary Treatment

Primary treatment is a physical process that removes suspended solids and organic matter through sedimentation. In this stage:

* Sedimentation Tanks or Primary Clarifiers: Wastewater is held in large tanks where heavy solids settle to the bottom as primary sludge, and lighter materials like oils and grease float to the surface and are skimmed off.
* This process typically removes about 60% of suspended solids and 30–40% of the Biological Oxygen Demand (BOD).

The settled sludge is collected and sent for further treatment, while the partially clarified water moves to the secondary treatment stage.

### 4. Secondary Treatment

Secondary treatment is a biological process that uses microorganisms to break down and remove dissolved and suspended organic matter. This stage is essential for significantly reducing the BOD, total suspended solids (TSS), and pathogens. The common methods include:

#### a. Activated Sludge Process (ASP)

* In this method, air or oxygen is pumped into aeration tanks to promote the growth of aerobic bacteria that feed on organic pollutants.
* The mixture of wastewater and biological mass (mixed liquor) is then sent to a secondary clarifier where the biological flocs settle out.
* A portion of this settled sludge (return activated sludge) is returned to the aeration