How often do you experience heavy rainfall in your city – seldom, sometimes or often? These “wet weather” events are sporadic and increasingly difficult to predict. When they happen, increased flow could seep into combined sanitary sewer systems and affect treatment quality, reliability or create a system backup.
Increased flow from wet weather events could exceed your plant capacity and send untreated wastewater into the environment, creating concern for public health and wildlife. One option to handle the additional flow is to increase capacity in the collection system or biological treatment facility or reduce infiltration; however, these options can be costly. Instead, an alternative to consider for excess wet weather flow is Chemically Enhanced Primary Treatment (CEPT).
The Science of CEPT
CEPT is a wastewater treatment solution that removes total suspended solids and biochemical oxygen demand. CEPT uses coagulants to combine solids and force them to settle more quickly, resulting in treatment of higher flows compared to conventional primary treatment. Once solids are settled and removed, the wastewater is disinfected with chlorine, if required by permit limits, and any residual chlorine is neutralized. This treatment process allows you to handle increased, unexpected wet weather flows.
Applying CEPT to Reduce Cost
Currently, B&N is on a team to design a CEPT treatment facility for Southerly Wastewater Treatment Plant (SWWTP) in Columbus, Ohio, to increase hydraulic capacity from 330 to 440 million gallons per day (MGD). During wet weather events, up to 110 MGD will be directed and treated through the CEPT facility. The CEPT facility will only be used during storm events that require the additional hydraulic capacity.
B&N’s Project Manager Vui Chung in the 96-inch diameter pipe during construction.
The B&N team designed the disinfection chamber and 4,300-foot pipe at 96 inches in diameter to convey the CEPT flow to the plant effluent. The time it takes for the wastewater to travel through the long pipe at SWWTP provides the adequate amount of contact time for disinfection. Before the flow is discharged to the Scioto River, any residual chlorine is neutralized to safeguard the aquatic ecosystems.
CEPT allows wet weather treatment without increasing biological treatment capacity.
When a wet weather event occurs, a plant operator activates the new CEPT system to direct up to 110 MGD into the CEPT facility. Addition of ferric chloride and polymer enhances the primary clarification process by promoting coagulation and settling. The flow is disinfected, dechlorinated and combined with the plant flow prior to discharge.
The 4,300-foot pipe provides contact time for disinfection.
SWWTP intends to activate this process an average of two times per year; however, this may deviate due to additional collection system modifications. Also, the frequency and severity of wet weather events are becoming harder to predict due to irregular weather patterns in the last century. CEPT is a cost-effective solution for the City to manage these intermittent, uncertain wet weather events without increasing biological treatment capacity.