Wastewater treatment is an essential process that removes contaminants from used water before it returns to the environment. This multi-stage process involves primary, secondary, and sometimes tertiary phases. Secondary treatment plays a central role in purifying water.
The Role of Secondary Treatment
Secondary wastewater treatment follows primary treatment, which physically removes larger solids and floating materials from the wastewater. After this initial physical separation, the water still contains significant dissolved and suspended organic matter. The primary objective of secondary treatment is to substantially reduce these compounds.
This stage is crucial for reducing the biochemical oxygen demand (BOD) of the water, which measures the oxygen microorganisms consume to break down organic pollutants. High BOD in discharged water can deplete oxygen levels in natural water bodies, harming aquatic life. Secondary treatment prepares the water for further advanced treatment or safe discharge into receiving waters.
Biological Principles at Work
Secondary wastewater treatment relies on the activity of microorganisms. These microscopic organisms, primarily bacteria and protozoa, consume organic pollutants as their food source. They metabolize these compounds, converting them into more stable and less harmful byproducts like carbon dioxide, water, and new microbial cells.
Most secondary treatment systems operate under aerobic conditions, requiring oxygen. Aeration, introducing air into the wastewater, supplies the oxygen needed for these beneficial microorganisms to thrive and break down organic matter. This process promotes the growth of a biological floc, often called activated sludge, a community of microorganisms that aids in the purification.
Common Secondary Treatment Methods
Various systems employ these biological principles for secondary treatment.
Activated Sludge: This widely used method mixes wastewater with microorganisms in aeration tanks. Air or oxygen is continuously introduced, supporting microbes as they break down organic matter and form biological flocs. After aeration, the mixture flows into a secondary clarifier where solids settle, separating treated water.
Trickling Filters: Wastewater flows downward over a fixed bed of media (e.g., rocks, plastic). A microbial slime, or biofilm, grows on the media’s surface. These attached microorganisms consume the organic compounds in the wastewater. Aerobic conditions are maintained through natural air circulation or forced aeration.
Rotating Biological Contactors (RBCs): Parallel discs mounted on a rotating shaft are partially submerged in wastewater. Microorganisms grow on the disc surfaces, forming a biofilm. As discs rotate, the biofilm is alternately exposed to wastewater and the atmosphere, allowing microbes to degrade pollutants and receive oxygen. This continuous rotation and exposure facilitates the biological breakdown of organic matter.
Stabilization Ponds (Lagoons): These large, man-made basins use natural processes to treat wastewater over longer periods. They rely on sunlight, wind, microorganisms, and algae to reduce organic content and pathogens. Ponds can be designed as a series of anaerobic, facultative, and aerobic stages, each contributing to the breakdown of wastes through different microbial activities.
What Secondary Treatment Achieves
Secondary wastewater treatment significantly improves water quality, making it suitable for subsequent discharge or further advanced treatment. It substantially reduces the organic content, measured as biochemical oxygen demand (BOD), and the suspended solids (TSS) in the wastewater. Typically, secondary treatment removes approximately 85% or more of these pollutants.
This reduction prevents the depletion of oxygen in receiving water bodies, protecting aquatic ecosystems. By breaking down harmful organic compounds, secondary treatment also safeguards public health. The treated effluent poses a reduced risk to environmental health, allowing for safer return to natural water cycles.