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Primary Treatment Overview
The primary treatment process removes readily settleable suspended solids and floating materials such as fats, oils, and greases from the screened and degritted influent wastewater. Primary treatment typically removes approximately 33% to 50% of the influent BOD and 60% - 75% of the influent settleable solids, greatly reducing the load on the secondary treatment system.
Theory of Operation
Primary treatment separates settleable and floating solids by reducing the wastewater velocity to a fraction of a foot per second. Primary treatment is the “first line of defense” for the biological treatment process by reducing suspended solids and BOD associated with these solids.
Reducing the load on the secondary treatment system lowers the oxygen demand, decreasing energy consumption for oxidation of organic wastes, and reducing the volume of waste activated sludge generated. Primary treatment removes floatable material, decreasing build-up of secondary scum and filaments, which feed on greases. Thickening the primary sludge before sludge draw-off decreases dewatering requirements and is a secondary function of the primary treatment process.
Primary clarifiers are sized mainly on the basis of surface overflow rate. Other design factors such as tank depth, detention time, and sludge scraper conveyance capacity are considered to ensure that the design provides acceptable process performance. 
The primary clarifier design operating conditions are as follows:
Primary Clarifier Design Criteria (2 clarifiers)
Criteria Units Value
Type - Circular
Number of Units each 2
Diameter (PC-1) ft 80
Diameter (PC-2) ft 65
Side Water Depth (PC-1) ft 14
Side Water Depth (PC-2) ft 10
Clarifier Surface Area (PC-1) sf 5,024
Clarifier Surface Area (PC-2) sf 3,317
Surface overflow rate w/both in service (average) gal/sq ft/day  600
Surface overflow rate w/both in service (peak) gal/sq ft/day 1,320
The primary clarification system includes the following equipment: 
  • Headworks Splitter Box
  • Primary Clarifiers & Collectors 
  • Primary Sludge/Scum Pumps 
Degritted effluent exits the headworks channel through two manually operated gates, one for each primary clarifier. The primary influent flows up the center column of each clarifier and spills into the clarifier tank. The heavier solids settle to the bottom of the tank while the lighter solids float to the surface. Primary effluent flows over the effluent weir and travels via gravity to the aeration basins for additional treatment.

Under normal operating conditions, primary clarifier #1 is in use and #2 is off-line.  Both primary clarifiers are used during periods of higher flows.

Settled sludge is moved to a hopper in the bottom of each clarifier by a continuously rotating, sludge scraper mechanism. The primary sludge pumps draw from these hoppers. Attached to the same drive unit as the sludge scrapers is a scum skimmer arm. This collector removes scum (floatables) from the surface of the tank between the influent distribution well and the peripheral scum baffle and deposit it in a scum collection hopper near the inside wall of the clarifier. The scum then flows to the scum box located on the outside perimeter of the clarifier tank. 

Each primary clarifier has a dedicated sludge/scum pump. Scum and sludge from the primary clarifiers is pumped to the digester for additional treatment.

Primary clarifier #1 is drained by using the H&H pump.  Primary clarifier #2 is drained by gravity through a 6" drain line connected to the raw sewage wet well.
Headworks Splitter Box and Gates6
The Primary clarifier splitter box is located on the West side of the headworks channel.  Effluent flow from the grit removal system flows into a common channel upstream of the splitter box. By opening one or both clarifier isolation gates, degritted flow travels to the designated primary clarifier.
Primary Clarifiers & Collectors6
Clarifier 1 is 80 feet in diameter, average side water depth of 14 feet, and feed well diameter of 20 feet.  Clarifier 2 is 65 feet in diameter, average side water depth of 10 feet, and feed well diameter of 14 feet. Each clarifier consists of a 24-inch influent column,  drive unit, sludge collector and scum collector arms, effluent v-notch weirs and scum baffle, scum box, and scum sump.
Primary effluent flows out of each clarifier through a 30-inch pipe to the aeration basin.
Primary Sludge/Scum Pumps6

Primary sludge is thickened in the primary clarifier and pumped directly to the anaerobic digester for stabilization. Primary scum is pumped to the digester by the primary sludge pump.  Modification to the inlet valves of the system is necessary to configure the system to pump from the scum pits.

Two rotary lobe primary sludge/scum pumps withdraw sludge from the sludge hopper at the bottom of each clarifier or scum pit and pump the sludge/scum to the digester.

One sludge pump is designated for each clarifier but each pump can be configured to pump scum from the other clarifier if only one clarifier is in use.

To protect the pumps and piping, pressure switches are installed on the discharge side of each pump. These switches will cause an alarm and also stop the associated pump whether in the Auto or Hand mode of operation.

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Last Updated: 9/16/2013 9:06:43 AM
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