Wastewater Processes at the Grand Island WWTP
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Liquids Processes Treatment Steps
Below are descriptions of the liquid treatment process listed in sequence, and following the flow of the schematic. The liquid and solid treatments processes are inter-related. Solids are derived from and separated from the liquids treatment process.
1. Sewer Collection System
Sewage is collected through a network of pipes and manholes from residences, businesses and from industries. Most sewage flows by gravity through the pipes. Manholes are used to allow access for cleaning. A significant amount of the sewage is from stormwater (rainwater) or from groundwater that seeps into the sewage collection system.
2. Lift Station(s) (Influent Pumping)
Lift stations provide pumping to lift the sewage from the deep sewer to ground level where it can flow into the treatment process. The lift station uses centrifugal pumps to lift the sewage. There is a large lift station, also known as the headworks pump, at the head of the wastewater plant.
3. Bar Screens
Bar Screens remove solids in the influent sewer that are larger than one inch (like sticks, plastic gloves, tampons or other items). Solids are raked off the screen and put into a dumpster. The dumpster is hauled to the landfill.
4. Grit Basins
Heavy solids drop to the bottom of the basins and are directed toward a low spot using an auger. From the low spot a pump is used to suck out the solids and pump it through a hydro-cyclone and grit classifier to spin out the solids and deposit them into a dumpster. Solids removed include sand, corn, metal shavings and other items. The solids removed from this process are taken to a landfill.
5. Primary Clarifiers (Sedimentation)
Clarifiers provide a quiescent area where the solids are settled out and collected at the bottom. These “primary” clarifiers remove solids such as soil, coffee grounds or other waste matter. Flow enters the center of the clarifier then radiates out toward a weir. As it travels outward, the flow moves progressively slower and more solids are removed as it passes through the process. Additionally, scum (floatable solids) is collected at the top of the primary clarifier. The scum is pumped to the secondary sludge holding tanks, also referred to as Waste Activated Sludge (WAS) tanks. Solids removed from the primary clarifier, called primary solids, are pumped and discharged to holding tanks for dewatering.
6. Screw Pumps (in-plant lift station)
Screw pumps provide additional lift for the sewage to move it to the secondary treatment process. Screw pumps (Archimedes screws, which look like an auger) are used because of the large flow and low lift required.
7. Aeration Basin (Secondary Treatment)
Flow from the screw pumps is aerated and mixed in these long rectangular basins. These basins are designed to allow commonly available micro-organisms (bacteria) to come into contact with the sewage and to literally, eat their way to clean the sewage. The bacteria is the real “treatment” process. Carbon source, ammonia and many other items are removed in this way. Aeration basins are used to maximize the growth of the bacteria. Flow is mixed to keep sludge from settling. At the beginning of the basin, additional micro-organisms are added to the process stream to allow the bacteria a “head-start” in their growth. These bacteria are simply recycled from the end of the basin into the beginning. This allows increased treatment efficiency.
8. Secondary Clarifiers (Sedimentation)
Flow from the aeration basins is settled in the secondary clarifiers. Clarifiers provide a quiescent area where the solids are settled out and collected at the bottom. The solids from the secondary clarifiers consist mostly of bacteria which has eaten the waste from the sewage. A portion of the solids (bacteria) are recycled to the aeration basin to provide additional bacteria growth. The remainder of these secondary solids is pumped into an aerated holding tank before dewatering. Flow enters the center of the circular clarifier then radiates out toward a weir. Flow moves progressively slower and solids drop to the bottom and are removed.
Any remaining micro-organisms in the water are treated using ultra-violet light. The flow passes near a specially-designed light-bulb and the bacteria are sterilized rendering them inert and unable to reproduce. This is the final treatment step. Flow is measured as it exits the treatment plant using a Parshall flume.
10. Effluent Parshall Flume (Effluent Flow Measurement)
This structure measures the effluent flow to direct equal amounts of flow treated by the WWTP. Many plants use this flume because of its accuracy and ease of operation. This type of flume was invented in the 1900's by a man named Parshall.
The treated flow, now in compliance with regulatory requirements, flows by gravity to a utility ditch adjacent to the WWTP and then flows to the Wood River.