Wastewater treatment is a multi-stage process designed to clean water before it is returned to the environment. The initial step, called primary treatment, uses physical processes like screening and settling to remove large solids and grit. Following this mechanical separation, the water moves into the second and most intensive phase, known as secondary treatment. This stage is fundamentally biological, designed to remove the pollutants that remain dissolved or finely suspended in the water.
Targeting Dissolved Organic Pollutants
The primary objective of secondary treatment is the removal of organic matter that could not be settled out in the preceding stage. This material includes dissolved sugars, starches, and other biodegradable compounds from human waste, food scraps, and detergents. The amount of this waste is quantified using a measure called Biological Oxygen Demand, or BOD.
BOD measures the amount of dissolved oxygen that aerobic microorganisms will consume while breaking down the organic contaminants in the water. High levels of organic pollution result in a high BOD value, signaling that a large amount of oxygen would be stripped from a natural water body if the water were discharged untreated. Secondary treatment is specifically engineered to dramatically reduce this load, typically achieving a reduction of 85 to 95% of the BOD.
Biological Processes: The Role of Microbes
The removal of dissolved organic matter is achieved by cultivating a massive, diverse population of microorganisms. These tiny workers, which include aerobic bacteria and protozoa, are directed to consume the pollutants as their food source. In an aerobic environment, which is rich in oxygen, these microbes metabolize the organic compounds, converting the carbon into carbon dioxide, water, and new microbial cell mass.
One common method to facilitate this action is the activated sludge process, where the wastewater is continuously mixed and aerated in large tanks. This constant supply of oxygen allows the beneficial bacteria to thrive and form biological clusters, or flocs, that absorb the organic waste. Another technique, the trickling filter, involves allowing the water to flow over a fixed medium, such as rocks or plastic, where the microbes attach themselves to form a thin film.
In both systems, the goal is to create an ideal habitat where the microorganisms can rapidly consume the biodegradable waste. As the microbes multiply and consume the waste, they transfer the dissolved contamination from the water phase into a solid, manageable form—the biological sludge.
Removal of Remaining Solids and Pathogens
While the primary treatment removes the largest debris, secondary treatment is highly effective at removing the fine suspended solids that remain. Even more significantly, it must also remove the new biological solids, or sludge, created during the microbial digestion of organic matter.
After the biological process is complete, the water moves into a secondary settling tank, or clarifier, where the heavy, microbe-laden flocs settle out by gravity. This physical separation step removes the biomass and the remaining non-biodegradable suspended particles.
Beyond the solids, the biological stage also results in a substantial reduction of pathogens, which are the disease-causing bacteria and viruses present in the wastewater. As the beneficial microbes consume the organic matter, they also break down or otherwise reduce the numbers of these harmful organisms. While secondary treatment significantly reduces pathogens, the final elimination of any remaining infectious agents is typically achieved through a separate disinfection step, often using chlorine or ultraviolet light, which follows the clarification process.
Environmental Necessity of Secondary Treatment
Secondary treatment is necessary for protecting the health of rivers, lakes, and oceans that receive treated water. If organic pollutants were discharged without this biological step, the resulting high BOD would immediately begin to consume the dissolved oxygen in the natural environment.
This rapid oxygen consumption by naturally occurring microorganisms can quickly deplete the oxygen supply, leading to a condition called hypoxia. When oxygen levels drop too low, fish and other aquatic organisms cannot survive, leading to widespread die-offs and disruption of the entire aquatic food web.
The process also helps control the release of nutrients like nitrogen and phosphorus, which are often partially addressed in advanced secondary systems. If these nutrients are not removed, they can cause excessive growth of algae and aquatic plants, a process known as eutrophication. When these algal blooms die and decompose, they contribute more organic material, further lowering the oxygen levels.