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Water Pollution Control Treatment Plant Operational Details
In its most basic form, the wastewater treatment plant (WWTP) is a process that removes solids from wastewater. Wastewater is comprised of water and waste, in either or both liquid and solid forms. The waste may be inorganic or organic, and may be soluble or particulate. The organic waste is converted to a solid in the process for removal.
The job of the WWTP and the operators is to remove the waste solids from the water and convert them to a stable, safe and useful material. To accomplish this removal, a number of different processes are used and will be described briefly.
The treatment stages are as follows:
- Sewer System - an assembly of pipes and conduits for the purpose of collecting wastewater and conveying them to the treatment facility. The collection system includes force mains, gravity sewers, lift stations, manholes, septic tanks, and catch basins. There are approximately 150 miles each of sanitary and storm sewers. There are also approximately 4400 catch basins.
- Raw Influent Pumping - accomplished with three 12,500 gallon per minute screw pumps. Each pump is powered by a 150 horsepower electric motor with a total capacity of 36 million gallons per day. Two pumps may run at one time with the third as a back up. The raw wastewater enters the facility from two main trunk lines at the base of the screw pumps. These seven foot diameter pumps lift the wastewater 30 feet so it can flow by gravity through the rest of the treatment process.
- Mechanical Screening - completed with two variable speed nine foot wide bar screens with one-quarter of an inch clearance. The bar screens remove suspended trash such as rags, sticks, sups, and leaves and this material is conveyed to a container for removal to a landfill.
- Primary Clarification - occurs after the screened flow from the screw pumps is diverted by a splitter box into primary clarifiers. Each of the four clarifiers holds 330,000 gallons. All of the settleable solids, about 60% of the suspended solids and roughly 30% of the biological oxygen demand are removed in the primary clarifiers. The settled material is called primary sludge. Floatables, such as grease and scum are also removed by surface skimming. The floatables and primary sludge are removed for further treatment.
- Secondary Treatment - this is a two step process. The first step is secondary treatment where the primary effluent flows through aeration tanks. This secondary process removes soluble organic waste by using activated sludge. Activated sludge is the biomass returned into the aeration tanks from the secondary clarifiers. Each of the four aeration tanks holds 480,000 gallons and is divided into four zones. These sections are used for establishing and mixing what is called in the wastewater field as mixed liquor. Single stage centrifugal blowers supply air to zones three and four through approximately 1200 fine bubble ceramic diffusers in each tank. The second step is secondary clarification. The aeration tank effluent (mixed liquor) flows into four 363,000 gallon secondary clarifiers where the solids formed in the secondary treatment can be separated by gravity. The settled solids are returned to the influent of the aeration tanks and mixed with the primary effluent. The returned solids are called return activated sludge (RAS) and pumped by 4000 gallon per minute pumps back into the aeration tanks. A portion of the mixed liquor is removed and called waste activated sludge (WAS). The WAS prevents over stocking the mixed liquor and forms a portion of the solid waste produced at the WWTP.
- Tertiary Clarification - process where the secondary effluent goes for further settling. Polymer can be added at the tertiary tanks which coagulate finely divided particulates and allows them to settle. The four 461,250 gallon tertiary clarifiers act as back-up to the secondary clarifiers. Tertiary sludge can be returned with the RAS or removed as a waste sludge.
- Chlorination - the disinfection process used on the tertiary effluent. One ton cylinders of chlorine are used to supply chlorine gas/water mixture into the four chlorine contact tanks. Each of the contact tanks has a mechanical mixer that assists in distributing the chlorine solution. There is also a sludge collection system in the tanks that allows removal of any residual solids that settle. Dechlorination of the disinfected effluent is done to meet the residual chlorine limit of the EPA. Sodium bisulfite is applied to the chlorine tank effluent using chemical feed pumps to deliver the solution which removed residual chlorine.
- Solids Handling - processes used to remove the solid portion of the wastewater. Primary sludge is pumped from the primary clarifiers through cyclone degritters to remove grit from the raw sludge. The grit is removed from the system and sent to a landfill. The remaining sludge is sent to the West gravity thickener to reduce the volume of sludge to be treated. WAS is sent to the East thickener along with a polymer for volume reduction. These sludges are thickened and the water is sent back to the head of the plant. The thickened sludge from the thickeners is then pumped by pneumatic pumps to holding tanks where it is blended and stored awaiting dewatering by the belt filter press. Two two-meter belt filter presses are used for dewatering. The belt filter cake is mixed with high calcium quicklime and pumped to the sludge storage pad. The lime sludge mixture is aerated, mixed dried and then stored in the sludge storage building until recycled to agricultural land.