Advanced wastewater treatment (AWT) is the final, sophisticated phase in purifying used water. This process removes contaminants that remain after conventional cleaning methods have been applied. The primary goal of AWT is to improve water quality to a standard that protects sensitive water bodies from pollution and supports water reuse initiatives. AWT ensures the water returned to the environment or recycled for other uses is safe and meets strict regulatory requirements. It utilizes specialized physical, chemical, and biological technologies tailored to target specific pollutants.
Defining the Three Stages of Treatment
Wastewater treatment is traditionally structured in a sequence of three major phases. Primary treatment begins by physically separating the largest solids and floating materials using screens and large settling tanks. This stage is effective at removing heavy debris and a significant portion of suspended matter through gravity-based sedimentation.
Following primary clarification, secondary treatment focuses on removing dissolved organic matter that remains in the water. This is achieved through biological processes, where microorganisms consume and break down the organic carbon compounds. These conventional stages successfully remove most solids and biodegradable material, but they are limited in their ability to address small, dissolved substances.
Advanced treatment, often referred to as tertiary treatment, is the step required when the desired water quality exceeds what secondary processes can achieve. This advanced phase targets specific pollutants, such as nutrients and pathogens, that can harm aquatic ecosystems or pose risks to public health. Implementing AWT becomes mandatory when the receiving body of water has a low capacity to naturally assimilate the remaining contaminants.
Specialized Nutrient Elimination Processes
A major driver for implementing AWT is the need to remove excess nitrogen and phosphorus, known as nutrients, which fuel excessive algae growth leading to eutrophication in natural waters. These nutrients are not fully captured by primary and secondary processes and must be addressed through dedicated biological and chemical pathways. Nitrogen removal relies on a two-step biological process involving specialized microorganisms.
The first step, called nitrification, is an aerobic process where ammonia, a form of nitrogen toxic to aquatic life, is converted into nitrate by bacteria. Next, in a chamber with no oxygen, denitrification occurs, where other bacteria utilize the nitrate for respiration. This action transforms the nitrate into harmless nitrogen gas, which then vents into the atmosphere.
Phosphorus removal can be achieved biologically using enhanced biological phosphorus removal (EBPR). This process selects for specific microbes, called Polyphosphate-Accumulating Organisms (PAOs), which are encouraged to over-consume and store phosphorus inside their cell structure. The phosphorus is then removed from the water when the PAO-rich sludge is separated from the treated effluent.
Another method for phosphorus elimination is chemical precipitation, where metal salts are introduced into the water stream. Common chemicals like aluminum sulfate or ferric chloride are dosed to react with the soluble phosphate. This reaction forms insoluble precipitates, which are then easily removed from the water through physical settling or filtration.
High-Level Polishing and Pathogen Control
Once nutrients are largely removed, the water undergoes a high-level polishing step to eliminate residual suspended solids and ensure pathogen control. This polishing typically involves membrane filtration, a physical separation process that acts as a highly effective barrier. Microfiltration and ultrafiltration membranes utilize fine pores to physically screen out particles, including bacteria, protozoa, and fine colloidal matter.
For applications requiring the highest purity, such as water reuse, reverse osmosis (RO) is employed as a final physical separation barrier. RO uses high pressure to force water molecules through a semipermeable membrane, effectively rejecting nearly all dissolved salts, viruses, and the smallest particles. This process yields water of near-distilled quality, but it is energy-intensive due to the high operating pressures.
Pathogen control is a mandated component of high-level AWT, ensuring the destruction of any remaining disease-causing microorganisms. Ultraviolet (UV) light is one of the most common high-level disinfection methods, as it is a chemical-free process. The high-energy UV light penetrates the cell wall of bacteria and viruses, damaging their DNA or RNA and rendering them incapable of reproduction and infection.
Ozonation is an alternative or complementary method, introducing ozone gas, a powerful oxidant, into the water. The ozone quickly oxidizes and destroys the cell structures of pathogens, effectively inactivating them. Both UV and ozone are preferred for advanced applications because they reduce the need for traditional chlorine disinfection, minimizing the formation of potentially harmful byproducts.
Addressing Micropollutants and Emerging Contaminants
The newest challenge for AWT involves the removal of trace organic compounds, often referred to as Contaminants of Emerging Concern (CECs). These substances include trace amounts of pharmaceuticals, endocrine-disrupting chemicals, and personal care products that are not fully removed by conventional treatment. CECs are typically present at very low concentrations, making them difficult to target.
One primary technology for addressing these dissolved micropollutants is adsorption, commonly executed using Granular Activated Carbon (GAC). GAC has a vast internal surface area, acting like a molecular sponge that physically traps and holds the organic contaminant molecules to its porous structure. This method is highly effective for a wide range of non-polar or moderately polar organic compounds.
For the most resilient or chemically stable molecules, Advanced Oxidation Processes (AOPs) are necessary to break them down chemically. AOPs typically involve combining strong oxidants, such as ozone or hydrogen peroxide, with energy sources like UV light. This combination generates highly reactive free radicals, particularly hydroxyl radicals, which rapidly attack and mineralize the complex organic molecules into simpler, harmless compounds. This chemical destruction method ensures that trace contaminants are effectively neutralized before the water is returned to the environment or prepared for reuse.