Biological Treatment

After an initial round of treatment at the PTF the wastewater moves through a distribution chamber were it can be channeled to one or more, of eight tanks depending on the desired pathway and level of flow. Each of these eight tanks use a suspended growth of small microscopic organisms to clean and treat the incoming wastewater. These suspended microscopic organisms along with the food source, water, and treated mass are collectively called activated sludge.

NOWWTP Treatment Description

The North Olmsted WWTP is a suspended growth (or activated sludge) system utilizing a Vertical Loop Reactor (VLR). A VLR is analogous to an oxidation ditch turned on its side. The VLR is constructed similarly to a conventional plug flow tank, however, the tank is separated into an upper and lower pass with a horizontal divider baffle.

Mixing and aeration is provided by orbal aeration discs. In addition to the orbal discs, a system of coarse bubble diffusers is located at the bottom of each VLR tank. As flow is conveyed around the VLR at a high rate, coarse bubble aeration is applied at one end of the horizontal baffle plate. The bubble and flow travel along the plate and then move to the upper level of the reactor. Air bubbles are retained in the VLR longer than in a conventional activated sludge basin, thereby increasing oxygen transfer in the system.

• VLR Tank: The VLR tanks are rectangular aeration tanks. At NOWWTP the tanks are concrete and consists of four small tanks (1-1,1-2,2-1,2-2) and four large tanks (1-3,1-4, 2-3,2-4). The four large tanks were part of the original Aeration Process and were modified in the 2012 improvements. Collectively all eight tanks provide a total influent detention time of 13.0 hours at a design flow rate of 7.0 MGD.

Each tank contains an intermediate (horizontal) baffle which provides the ability of the flow to “loop” inside each VLR tank. The orbal discs, located at the top of the tank, cause the tank contents to flow over the top of the baffle in one direction, enter a turning vane (located at one end of the tank) that directs the flow down and under the intermediate baffle. The baffle is sloped upward as the flow travels to the end of the tank. The upward slope with the flow path is to provide for air to release at the opposite end of the tank.

• Rotor Assembly with Orbal Discs: The orbal discs are the heart of the VLR process. The discs supply the mechanical mixing and impart the energy that recirculates tank contents at a rate of two feet per second. VLR tanks 1-1, 1-2, 2-1 and 2-2 have one (1) complete rotor assemblies while VLR tanks 1-3, 1-4, 2-3 and 2-4 have two (2) complete rotor assemblies. Each rotor assembly has 40 orbal discs, 4.5 feet in diameter. The discs are split in half so that each disc can be adjusted, removed or attached without requiring any shaft disassembling.

Each rotor has a 25 hp, 1,750 rpm drive motor with a Variable Frequency Drive (VFD) allowing the rotor speed to vary from minimum speed of 29 rpm to a maximum speed of 52 rpm. The aeration disc submergence varies between 11 inches and 17 inches.

• Weatherhoods: 3/16” fiberglass reinforced plastic steel weather and splash hoods are provided with each rotor assembly to prevent the wetting of the bearings and drive units from the disc spray. Also, the hoods are provided to prevent icing of the discs during cold weather. Finally, the weather hoods provide UV protection to the Orbal discs.
• Coarse Bubble Diffusers: Each VLR tank is provided with coarse bubble diffusers. VLR tanks 1-1, 1-2, 2-1 and 2-2 are provided with 32 diffusers (a total of 128) while VLR tanks 1-3, 1-4, 2-3 and 2-4 are provided with 114 diffusers (a total of 456). The diffusers are located near the turning vane where the mixed liquor is first turned down under the intermediate baffle. The release of air at entry point of flow beneath the baffle causes the longest possible “bubble path” in the mixed liquor. The long bubble path results in the coarse bubble diffusers providing extremely high oxygen transfer rates for coarse bubble diffuser while still providing low maintenance and trouble free performance. The optimum air flow rate is 5.0 scfm per diffuser.
• Air Release Plate: An air release plate is provided between the turning vane and the intermediate baffle at the upflow end of each VLR reactor. The air release plate is provided to shear and disperse the air bubbles releasing from beneath the intermediate baffle. The air release plate further improves oxygen transfer and reduces localized turbulence at the end of the tank.
• Blowers and Air Piping: One (1) positive displacement blower is dedicated to supply air to the entire VLR process. Valving is provided in the Blower Building to allow two additional blowers to operate as swing to either the VLR or EQ tanks. The blowers are manufactured by Aerzen Corporation and have a capacity of 700 SCFM. The blowers supply air to the VLR process through a 10 inch common discharge header which services each VLR through a 8 inch butterfly valve and 8 inch drop pipe to a 4 inch distribution manifold for the coarse bubble diffusers.