After leaving the Secondary Clarifiers the water has been treated but still contains a small amount of suspended material and microscopic organism. The water next travels to the plant’s final effluent building for advance filtration, disinfection, and a final inspection.
The Tertiary filters receive secondary effluent from Final Clarifiers No. 1, 2, and 3. The filters operate by collecting flow into the center of the drum, discharging it to the exterior of the tank. Solids are retained on the inside of the filter panels, which consist of a series of 20 micron-panels. Material captured on the screens effectively reduces suspended solids, BOD and phosphorus. As solids accumulate, the water level in the disc begin to rise, triggering a level sensor to backwash the filters. Backwash is discharged to the Plant Drain Pump Station. The filter equipment is manufactured by Kruger.
Tertiary filtration has been used at North Olmsted WWTP since 1970 to polish secondary effluent. Previously the plant had five cloth tertiary filters (roughly 100-200 microns), which were replaced in 2012 with three (3) (20 micron) units. Each unit is sized for 15 MGD firm capacity and 22.5 MGD total capacity.
The filters can handle a max flow of 22.5 MGD. A downward opening motorized slide gate, opens to limit the flow to the tertiary filters and protects them from rupture. During a high flow event, filter effluent and bypass flow is blended prior to disinfection.
After filtration, flow proceeds to UV disinfection.
To prevent the spread of waterborne diseases and minimize public health problems, the NOWWTP disinfects by ultraviolet (UV) irradiation from April 1 through October 31 prior to discharge to Rocky River. The UV Disinfection system is located in the Filter Effluent Building and is the last step in the treatment process. The UV system is a 3000 plus manufactured by Trojan Technologies.
Disinfection by ultraviolet light has become the most common alternative to chlorination for wastewater disinfection. Unlike chlorine, ultraviolet disinfection does not produce toxic byproducts, and operation of UV disinfection facilities is safer than operation of a chlorination system. UV disinfection also requires shorter contact times than chlorination systems. At the plant’s maximum flow, the approximate contact time for ultraviolet treatment is approximately 4 seconds, compared to the typical contact time for chlorination of 30 minutes.
Ultraviolet irradiation is a physical disinfection process, and has several fundamental characteristics that distinguish it from typical chemical disinfection processes. UV treatment achieves disinfection by inducing photobiochemical changes within the microorganisms. The UV light does not necessarily kill the targeted microorganisms. The light passes through the cell membrane of the organism and alters its DNA, which renders it unable to reproduce, or inactivates it, and eventually the organism dies. This process does not alter the treated wastewater other than by destruction of the microorganisms. UV treatment does not add chemicals to the water, and, unlike chlorine, it does not leave a residual.
Extensive research has shown that nucleic acids (DNA and RNA) and proteins are effective absorbers of UV radiation, particularly between the wavelength range from 240 to 260 nanometers (nm). The wavelength of the UV light that is used for disinfection is 254 nanometers. Although the UV lamps produce a blue light, the UV portion of the light spectrum is actually not visible to the human eye.
During wastewater’s journey through the North Olmsted Wastewater Treatment Plant each step is controlled and monitored by a Supervisory Control and Data Acquisition computer control program called SCADA. The SCADA system also monitors and samples, through a flow pace automatic sampler, the quality of the final treated water before discharge to the Rocky River.
The platform for the North Olmsted Wastewater Treatment Plant is Visio-Citect. The developer and primary support for Citect is Schneider Electric. Installation of the SCADA system, HMI screens, and CPUs was performed under supervision of Dmytryka Jacobs Engineering. Local installer and support of the SCADA system, HMI, and CPUs is provided by Systems Group Technologies.