Applied Engineering Technology: Rotamix testing
OPERATION PROCESS
The patented Treatment Shaft (Pat. No. 6,503,404, 7,341,670 plus other patents pending) process incorporates a vertical capture facility that can be constructed in a variety of ways, depending on local ground conditions. It is equipped with systems that provide complete treatment for CSO or SSO events, including capture, settling, skimming, screening and disinfection (see Attachment B). The shaft includes a vertical baffle wall at the mid-section, extending from the roof to an elevation above the base of the shaft. This layout streamlines U-shaped flow from the influent sewers through the shaft, thus avoiding short-circuiting of flow, and ensuring sufficient disinfection contact time. The upward flow velocities are very low, which reduces head loss (only few inches at peak flow rates) and promotes settling of solids at the bottom of the shaft.
During most storm events, the shaft will capture the entire overflow volume and will not require discharge to the receiving waters. During very large and less frequent storms, the shaft’s design capacity may be exceeded, but overflows will be settled, skimmed and disinfected before passing through fine horizontal raked bar screens and subsequent discharge to the receiving water body. These screens are compact, easy to maintain, and do not require a screening building.
Screenings are raked using hydraulically activated combs when the water level reaches the screens. Wastewater remaining in the shaft after storm events is dewatered to the sewer interceptor using submersible chopper pumps. All floatables and screenings will follow the receding water to the frustum-shaped bottom of the shaft. When the water level in the shaft reaches a certain level above the bottom, the flushing system is automatically activated. High-pressure nozzles will re-suspend accumulated solids and transport the floatables and screenings into the chopper dewatering pumps located at the bottom of the shaft. An optional rinsing system may be incorporated at minimal cost.
This innovative technology provides treatment for all overflow volumes to address health concerns and to satisfy permit requirements associated with raw sewerage discharges. The key feature of this advanced technology is its superior hydraulic performance manifested by low flow velocities and associated head losses verified by physical modeling. Since the head losses introduced by the Treatment Shaft design are very low, minor modifications to the existing sewer system are often the only change needed to eliminate expensive pump stations. Eliminating a pump station can result in significant saving on capital and O&M costs. In addition, these low flow velocities promote settling of suspended solids at the bottom of the shaft and improve discharge water quality.
The innovative horizontal bar screens are installed within the shaft without the need for a screening building, thus reducing the above ground building size by about half. In addition, the screen arrangement does not require storage, handling or disposal of screenings, which simplifies operations and minimizes odors.
The Treatment Shaft design also greatly reduces groundwater infiltration and eliminates the need for costly surge controls that are commonly required with tunnel systems, thus further reducing upfront capital and treatment costs. The relatively small footprint associated with the Treatment Shaft (about one-sixth to one-eighth compared to a retention treatment basin) is a big advantage for congested metropolitan areas.