Stormwater and Wastewater: What's the Difference?

Stormwater is water from rain or melting snow that flows over streets, lawns, and other surfaces, eventually entering our waterways. Wastewater, on the other hand, comes from household and industrial activities—everything from showering and washing dishes to industrial processes. Both types of water need to be managed carefully to protect public health and the environment.

Combined Sewer Systems
In a combined sewer system, stormwater and wastewater are collected in the same pipes and sent to a treatment facility. This system is efficient in areas with limited space for separate infrastructure. However, during heavy rainfall or snowmelt, the combined flow can overwhelm the system's capacity, leading to the discharge of untreated water into nearby rivers and lakes. These overflow events, known as Combined Sewer Overflows (CSOs), can pose significant environmental risks, contaminating water bodies with pollutants from untreated sewage and stormwater runoff.

Separate Sewer Systems
Separate sewer systems address this issue by using two distinct networks of pipes: one for stormwater and one for wastewater. Stormwater is typically directed to nearby water bodies without treatment, while wastewater is sent to a treatment facility. This separation reduces the risk of overflows and ensures that wastewater is properly treated before being released into the environment. However, because stormwater is often untreated in this system, pollutants from urban runoff can still enter waterways.

Benefits and Capacity of Combined Sewer Systems
Combined sewer systems offer several advantages, particularly in older urban areas. They require less infrastructure, making them more cost-effective to install and maintain. Additionally, during dry weather, all water is directed to treatment facilities, ensuring that both stormwater and wastewater are treated together, which can be beneficial for overall water quality.

However, the capacity of combined sewer systems is a critical factor. These systems are designed to handle a specific volume of water, and when that capacity is exceeded during heavy storms, the risk of CSOs increases. Managing and upgrading combined sewer systems to handle larger volumes can help mitigate the risks, but it also involves significant costs and logistical challenges.

Balancing the Systems
The choice between combined and separate sewer systems depends on various factors, including local geography, climate, and urban development patterns. Both systems have their advantages and drawbacks, and understanding these can help communities make informed decisions about managing stormwater and wastewater effectively.