Wastewater treatment separates used water into two main products: a cleaned liquid stream, known as effluent, and a solid or semi-solid material, referred to as biosolids. These products are managed through strictly controlled pathways to be safely reintroduced into the environment or reused. The destination of the treated effluent is determined by its final level of purification and local water needs, while the solids follow a separate management path.
Discharge: Returning Treated Water to Receiving Bodies
The most frequent destination for treated wastewater is direct release back into natural water sources, known as discharge. Before release, the effluent must comply with quality standards set by regulatory agencies, such as the National Pollutant Discharge Elimination System (NPDES) in the United States. Permits specify maximum allowable concentrations for various pollutants. Key parameters monitored include Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS).
The receiving bodies commonly include rivers, lakes, estuaries, and oceans. Discharging into a river or lake allows the treated water to re-enter the natural hydrologic cycle, supplementing the flow. For coastal facilities, effluent is often released into the ocean through deep outfalls, where natural dilution and dispersion rapidly integrate the water.
Discharge standards also address nutrients like nitrogen and phosphorus, which can lead to eutrophication (excessive algae growth). Many advanced treatment plants employ tertiary processes, such as biological nutrient removal, to reduce these compounds before discharge. Another controlled destination is managed aquifer recharge, where highly purified effluent is injected or allowed to percolate into underground aquifers. This practice helps replenish groundwater supplies.
Reclamation: Recycling Water for Non-Environmental Uses
Water reclamation, or reuse, is the use of treated effluent for specific human utility. This practice requires treatment beyond standard environmental discharge levels to ensure appropriate water quality. Agricultural irrigation is one of the largest global uses, providing water for non-food crops, fodder, and some food crops (with stricter quality requirements for those consumed raw). Reclaimed water is also used in industrial settings for non-contact cooling processes, boiler feed water, and manufacturing operations.
Municipal applications for recycled water are expanding, particularly for non-potable uses in urban environments. This includes the irrigation of public parks, golf courses, green spaces, and highway medians, often delivered through a separate distribution system of “purple pipes.” Within buildings, reclaimed water can be used for toilet flushing and air conditioning systems, offsetting the demand for drinking-quality water.
The concept of potable reuse involves treating wastewater to a standard safe for human consumption, achieved through two main methods. Indirect Potable Reuse (IPR) involves blending the highly treated effluent with a natural water source, such as a reservoir or groundwater basin, before it is withdrawn and treated again at a drinking water plant. Direct Potable Reuse (DPR) bypasses the environmental buffer, sending the purified water directly into the public water supply system or the raw water supply line for the drinking water treatment plant.
Biosolids: Management of Solid Waste
The solid material separated during treatment, initially called sludge, is further treated to become biosolids. This material is rich in organic matter and nutrients like nitrogen and phosphorus. To be classified as biosolids, the material must meet specific federal regulations, such as those in 40 CFR Part 503, which govern pathogen reduction and heavy metal limits.
Land application is a primary management method, where treated biosolids are used as a fertilizer and soil amendment on agricultural fields, forests, and reclaimed land. Biosolids are categorized into Class A and Class B based on their level of pathogen reduction. Class A biosolids have virtually no detectable pathogens and can be used in public contact areas. Class B biosolids have fewer restrictions on their application but require site-specific management practices to limit public access.
Another path for biosolids is disposal in sanitary landfills, either in a dedicated area or mixed with municipal solid waste. This method is used when the material does not meet the standards for land application or when local regulations prohibit it. A third pathway is incineration, which involves burning the dewatered biosolids at high temperatures, significantly reducing their volume and mass. This process requires strict air quality controls and results in an ash that is then sent to a landfill for final disposal.