What are the main stages of wastewater treatment?

The main stages of wastewater treatment are critical processes designed to remove contaminants and purify water before it is released back into the environment or reused. The stages outlined in the correct choice include:

1. Preliminary Treatment: This initial stage involves the removal of large debris and grit that could damage equipment or hinder subsequent treatment processes. Screens and grit chambers are commonly used to accomplish this task.

2. Primary Treatment: During this stage, wastewater is held in tanks where heavier solids settle to the bottom, forming sludge, while lighter materials like fats and oils rise to the surface and are skimmed off. This step significantly reduces the suspended solids and some organic matter from the wastewater.

3. Secondary Treatment: This biological stage further breaks down organic matter. Microorganisms consume the remaining organic pollutants, leading to a significant reduction in biochemical oxygen demand (BOD). This treatment can take place in various configurations such as activated sludge systems or trickling filters.

4. Tertiary Treatment: While not explicitly mentioned in the answer choice, it is often critical in modern processes. This stage involves additional treatment steps, such as filtration and disinfection, to remove remaining contaminants and pathogens, ensuring the treated water meets regulatory standards for discharge or