After entering the Waste Water Treatment Plant (WWTP) in coming wastewater is directed to the Preliminary Treatment Facility (PTF). The purpose of the PTF is to remove un-treatable items, such as coffee grounds, sand, plastics, and flushed fibrous cleaning rags. This material is removed from the waste stream as it cannot be naturally removed or treated by downstream processes.
The first unit process at NOWWTP are the Andritz Perforated Plate Screens. The principal role of screening is to remove coarse materials from the flow stream that could (1) damage subsequent process equipment or (2) reduce overall treatment process reliability and effectiveness.
One quarter inch fine screens were selected at North Olmsted to provide superior screenings removal and to prevent maintenance issues downstream. Material removed by the screen is dewatered, bagged and placed in a dumpster for conveyance to landfill. A bypass channel with a manually cleaned bar screen is provided as redundancy.
Two Andritz Perforated Plate Screen rated at 20 MGD each, uses close fitting perforated plates to remove floating and suspended solids. The panels are shaped to form steps which are carried on heavy duty chains. The captured screenings are elevated from the channel on the stepped plates to the top of the screen and ejected through an enclosed discharge chute. High pressure spray water and a rotary brush ensure debris is removed from the panels. A dedicated screw compactor is provided for each screen.
After screening, inorganic solids such as sand and gravel, and non-putrescible organics (coffee grounds, fruit rinds and seeds) are removed. These solids are referred to as grit. Grit is removed from the wastewater to (1) protect mechanical equipment from abrasion, (2) reduce deposits in pipes and tanks and (3) reduce digester cleaning caused by excess accumulation of grit.
The Grit Removal Process at NOWWTP consists of:
- Grit collection (Headcells)
- Grit separation (Slurry Cups)
- Grit washing and dewatering (Grit Snail)
- Grit pumps, and
- Grit storage (Disposal for landfill)
A stacked tray system, manufactured by Eutek, provides grit separation from the flow stream, through vortex movement. Flow is introduced tangentially and establishes a vortex flow pattern causing solids to settle to the center of the tray. Degritted effluent flows out of the trays, over a weir and into an effluent trough.
Grit concentrates in the center of the tank and is pumped out by a Fairbanks Nijhuis recessed impeller pump to grit separation also called a Slurry Cup. Grit is washed and discharged to the Grit Snail where it is dewatered and discharged to a 8-cubic yard roll-off dumpster. The system is designed to handle a peak flow of 50 MGD through two individual treatment trains. The system provides 95% removal of 110 micron and larger grit.