Assessing watershed-scale impacts of wastewater treatment

Wastewater systems reduce contaminants known to cause environmental impacts in receiving streams, but they are not designed to remove all chemicals.  For example, domestic wastewater treatment plants remove carbonaceous material known to reduce oxygen and harm aquatic life; however, many do not remove ammonia or other related nitrogen-containing materials.  Similarly, industrial wastewater plants are designed to remove select chemicals and may not remove simple salts (e.g., bromide, iodide). The residual chemicals in the discharges from these wastewater treatment facilities may cause ecological effects (e.g., eutrophication from nitrogen) or they may be diluted to such an extent that their presence is benign (as is generally true for monovalent ions).

The objective of wastewater treatment is to reduce negative impacts on the aquatic environment that caused by discharging chemicals found in the wastewater. Unintended consequences to other environmental compartments (e.g., air) or systems (e.g., climate) or downstream effects on the next water user (e.g., drinking water plants) are not routinely considered in the design or operation of wastewater facilities.  However, such impacts have been widely observed.  For example, many organic compounds volatilize during wastewater treatment, and the greenhouse gases methane and carbon dioxide are expected airborne releases associated with domestic wastewater treatment.  Similarly, increased formation of disinfection by-products during drinking water treatment at plants downstream of domestic and industrial discharges has been reported as have taste-and-odor and toxicity problems associated with nutrient-inducted algal blooms.

While wastewater treatment facilities are not designed to consider these unintended impacts, significant prior work has demonstrated that operational decisions alter the magnitude of these effects, and that costs associated with specific upstream choices (in wastewater and nonpoint source control) can be estimated.  Understanding the effect of operational conditions on multiple intended and unintended effects on the environment is critical to optimizing the value of wastewater treatment across multiple objectives. The present work focuses on unintended effects associated with discharge of treated wastewaters that contain nitrogen or bromide and the role of different treatment options at the wastewater plant on these effects.