Wastewater treatment is a fundamental process for safeguarding public health and environmental integrity. This multi-stage process removes a wide array of pollutants, transforming contaminated water into a quality suitable for safe discharge or further beneficial use. This ensures aquatic ecosystems are protected from harmful substances and communities have access to cleaner water.
The Role of Secondary Treatment
Secondary treatment is an important stage in wastewater purification, following primary treatment which removes large solids through physical separation. This phase specifically targets dissolved and suspended organic matter that remains after initial settling. Its main objective is to significantly reduce Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) in the wastewater. This reduction is achieved through the controlled activity of microorganisms, which break down organic pollutants. Secondary treatment helps prevent oxygen depletion in natural water bodies, which can harm aquatic life.
Biological Principles of Secondary Treatment
Secondary wastewater treatment relies on biological processes driven by diverse microorganisms. These microscopic organisms, primarily bacteria, but also protozoa and fungi, consume and break down organic pollutants present in the wastewater. Most secondary treatment systems employ aerobic processes, requiring oxygen for these microbes to thrive and metabolize organic compounds. The microorganisms convert organic matter into simpler substances like carbon dioxide, water, and new microbial cell mass. This biological activity reduces organic content and can also help in removing nutrients such as nitrogen and phosphorus.
Key Technologies for Secondary Treatment
Several technologies facilitate the biological processes in secondary wastewater treatment, each designed to optimize microbial activity.
Activated Sludge Process
This process involves an aeration tank where wastewater is mixed with a biological floc, or “activated sludge,” rich in microorganisms. Air or oxygen is continuously supplied to this mixture, providing the necessary conditions for aerobic microbes to consume organic pollutants. After aeration, the mixture flows into a clarifier where the microbial floc settles, and a portion of this settled sludge is recycled back to the aeration tank to maintain a healthy microbial population.
Trickling Filter
This system consists of a bed of media, such as rocks or plastic, over which wastewater trickles downwards. As the wastewater flows, a microbial slime, or biofilm, grows on the surface of the media. These attached microorganisms break down the organic matter in the wastewater as it passes over them. Air circulates through the filter bed, providing the oxygen needed for the aerobic biological processes.
Rotating Biological Contactors (RBCs)
RBCs are a fixed-film biological treatment technology. They feature a series of closely spaced, parallel discs mounted on a rotating shaft. These discs are partially submerged in the wastewater and slowly rotate, alternately exposing the microbial film growing on their surfaces to the wastewater and then to the air. This rotation allows microorganisms to absorb organic pollutants from the water and then receive oxygen from the atmosphere, facilitating the breakdown of contaminants.
The Outcome of Secondary Treatment
Upon completion of secondary treatment, the wastewater, now referred to as effluent, has significantly reduced levels of organic pollutants and suspended solids. Secondary treatment lowers Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) by up to 85% or more. The treated water is substantially cleaner and clearer, having undergone the biological degradation of most dissolved and suspended organic matter. This improved water quality is important for environmental protection, as it minimizes the impact on receiving water bodies. After secondary treatment, the effluent is often suitable for direct discharge into rivers, lakes, or oceans, or it can proceed to further advanced treatment stages, such as tertiary treatment, if higher purity is required for specific reuse applications.