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Aeration Basins and Blowers Overview
Purpose
Biological treatment removes biochemical oxygen demand (BOD), solids, and nutrients from wastewater.  Biological treatment is accomplished with aeration basins and return activated sludge (RAS) pumping. The aeration basin blowers provide oxygen to aerobic zones to promote biological treatment. RAS pumps return settled solids to the aeration basin anoxic zone and WAS pumps maintain the microbial population by wasting excess biomass. During nitrification, mixed liquor recycle pumps are provided to recycle nitrate rich mixed liquor to the anoxic zones to reduce the oxygen requirement in the aerobic zones and restore alkalinity to the mixed liquor due to denitrification.
Theory of Operation
Carbonaceous BOD Removal
Primary effluent biochemical oxygen demand (BOD) is biologically oxidized to carbon dioxide and water in the aerobic zones. The aeration basin mixed liquor flows to the secondary clarifiers where it settles to provide liquid/solids separation. The clarified secondary effluent flows to the UV disinfection system. Return activated sludge from the bottom of the secondary clarifiers is pumped back to the aeration basins. Waste activated sludge is pumped to the gravity belt thickener and anaerobically digested. 
Nitrification/Partial Denitrification Mode
The biological treatment system uses the Modified Ludzack Ettinger (MLE) variation of the activated sludge process. The MLE process includes a sequence of anoxic and aerobic biological reactors to provide BOD, ammonia, and nitrogen reduction. BOD oxidation and nitrification is achieved in the aerobic basin. Nitrate-rich MLSS is recycled back from the end of the aeration basins to the front of the anoxic basins, where the nitrate is denitrified to nitrogen gas. The combination of nitrification and denitrification thus accomplishes ammonia removal and partial nitrogen removal from the influent wastewater.
Nitrification
Influent wastewater nitrogen is primarily in the form of ammonia and organic nitrogen. These two nitrogen forms are collectively measured as Total Kjeldahl Nitrogen (TKN). The majority of the organic nitrogen degrades to ammonia in the treatment process. The subsequent biological oxidation of ammonia to nitrate in the aeration basin aerobic zones is called nitrification and it occurs in two steps. The ammonia is first oxidized to nitrite by Nitrosomonas bacteria. Then the nitrite is oxidized to nitrate by Nitrobacter bacteria. Conditions typically necessary for nitrification to occur include a mixed liquor dissolved oxygen concentration > 2.0 mg/L, a long sludge age (>8 days), and a mixed liquor pH near neutral. The minimum SRT required depends upon the wastewater temperature.  The lower the temperature, the longer the SRT needs to be.  Alkalinity is consumed in the process.
Denitrification 
Denitrification is the biological reduction of nitrate to nitrogen gas and is accomplished by several groups of denitrifying bacteria. This reaction occurs in the anoxic zones in the absence of oxygen. The rate of denitrification is significantly higher in the presence of organic carbon, which provides an energy source for the denitrifying bacteria. The primary effluent flows into the anoxic zones to provide the carbon source for denitrification. The harmless nitrogen gas that is produced escapes from the mixed liquor and is released to the surrounding atmosphere. A portion of the alkalinity lost in the nitrification step is recovered in the denitrification step. Ammonia passes through the anoxic zones and is nitrified in the aerobic zones to nitrates.  Nitrates are recycled back to the front of the anoxic zone to be denitrified.
Description
The biological treatment system consists of an influent channel, aeration basins with anoxic and aerobic zones, and process blowers. A plan view of the biological treatment system is shown in Figure 1.

Flow splitting occurs in the common influent channel of the aeration basins. The aeration basins are equipped for step feed or plug flow operation, using downward acting slide gates (weir gate) that allow a proportion of primary effluent to be diverted to Zones A, B, C, and D. When operated in step feed, a proportion of primary effluent is delivered to each aeration zone (B, C, and D) The split of RAS between basins is accomplished using magnetic flow meters and throttling valves at the south end of the aeration basin.

Influent Channel
Primary effluent from the primary clarifiers flows into a common channel between the basins where the flow is divided between the two aeration basins. The reconfiguration of the aeration basins from complete mix to step feed configuration required the reconfiguration of this channel from an effluent channel to an influent channel. The downward acting weir gates at each of four zones of the aeration basin can be raised to hydraulically isolate an entire basin. These same gates control flow to each zone of each aeration basin (with flow rate controlled by the PCS using a measured water surface elevation and a weir equation). A manual staff gauge can be used at each gate to calculate flow rate to each zone. A self-contained butterfly gate immediately downstream from the Zone A (anoxic zone) weir gate, can be closed to hydraulically isolate the influent channel for maintenance, and diverts all primary effluent into Zone A for plug flow operation.

RAS Piping
RAS is withdrawn from the secondary clarifiers using a downward acting weir gate in the RAS wet well. RAS is pumped to the anoxic zone of each aeration basin. The partitioning of the basin to include an anoxic zone can be used to operational advantage during high flow events. If RAS is continually pumped to the anoxic zone and primary effluent is partially diverted away from the anoxic zone, (to the downstream aerobic zone(s)) solids are effectively “stored” in the anoxic zone, reducing MLSS concentrations in the mixed liquor going to the secondary clarifiers and prevents solids overloading of the secondary clarifiers. A concurrent effect is possible reduction in substrate removal in the aeration basin, so careful operation of the step feed process is required. The RAS flow control is summarized in the RAS Pumping section.

Mixed Liquor Junction Box
Mixed liquor (ML) exits the aeration basins over a new stationary steel weir and flows into a common center channel (which is hydraulically separated from the common influent channel by a new concrete bulkhead) and flows through an existing common 42” pipe at the north end of the basins. The pipe is routed to the northeast side of aeration basin #2 and discharges into the mixed liquor junction box containing two adjustable slide gates that distribute flow to the two secondary clarifiers. ML is conveyed to each clarifier by gravity flow through 24-inch pipes. The slide gates at the mixed liquor junction box can be closed to isolate an individual clarifier.

Aeration Basins
The biological treatment system has two step feed aeration basins with a depth of 15 feet and volume of 0.54 million gallons per basin. Each basin has an anoxic selector that is 12 percent of the overall volume of the basin. The anoxic zone serves as a selector zone, providing an environment that limits the growth of filamentous organisms, resulting in a mixed liquor with good settling characteristics.
Aeration basin influent is mixed with RAS pumped from the secondary clarifiers in the anoxic zone. Mixing is performed by an axial flow, submersible mixers. The anoxic zone also contains coarse-bubble diffusers for aeration and mixing. When operating in the anoxic mode, the diffusers will not be in use. Flow proceeds to the aerobic zone over a baffle wall weir. This baffle wall is constructed of a painted carbon steel frame with fiberglass panels and is designed only for moderate hydraulic differential across the wall (i.e. 30 pound per square foot lateral load which is less than 6 inch differential). For this reason, 24-inch slide gates are provided at the bottom of each baffle wall and must be in OPEN position during basin filling and draining to prevent damage to the FRP baffle walls.

The aerobic zone contains fine-bubble membrane diffusers in a tapered aeration configuration. Diffusers are separated into three aeration zones along the length of the basin where the number of diffusers in each zone decrease with each progressing zone. Each aeration zone has a drop leg with a pneumatically actuated butterfly valve to adjust aeration in the zone. Air is supplied by the process air blowers located in the blower building.

A dissolved oxygen (DO) sensor monitors dissolved oxygen in each zone of the basin. The system consists of a DO sensor and transmitter located in each of the four zones of each aeration basin. The sensors are mounted to the handrail on the east and west side of the central walkway above the influent channel that separates the two basins. The DO transmitters are generally located adjacent to each sensor.

Process Blowers
The aeration blower system consists of three existing multistage centrifugal type blowers, inlet filter silencers, inlet valves, check valves, isolation valves, and expansion joints. Each blower is rated for 50 percent of the overall maximum required air flow. Air flow from the blowers is varied by the modulation of the inlet valves of the blowers. All three blowers discharge into a common header. Air flow to the Headworks and aeration is balanced by control valves and manual valves in the low pressure air system.

Electrical power for the aeration blowers is supplied from MCC-2 located in the blower building electrical room.
Design Criteria
Click on the link to obtain information on the Aeration Basin Design Criteria.
Equipment
The aeration basin/aeration blower system includes the following components:
  • Aeration basin inlet weir gates
  • Anoxic zone mixers
  • Mixed liquor recycle pumps
  • Mixed liquor effluent junction box
  • Aeration blowers  
  • Aeration basin diffusers
  • Dissolved oxygen meters
Aeration Basin Inlet Weir Gates6
The downward acting weir gates at each of four zones of the aeration basin can be raised to hydraulically isolate an entire basin. These same gates control flow to each zone of each aeration basin (with flow rate controlled by the PCS using a measured water surface elevation and a weir equation). A manual staff gauge can be used at each gate to calculate flow rate to each zone. A self-contained butterfly gate immediately downstream from the Zone A (anoxic zone) weir gate, can be closed to hydraulically isolate the influent channel for maintenance, and diverts all primary effluent into Zone A for plug flow operation.
Anoxic Zone Mixers6
 
Aeration basin influent is mixed with RAS pumped from the secondary clarifiers in the anoxic zone. Mixing is performed by an axial flow, submersible mixers.

The anoxic zone also contains coarse-bubble diffusers for aeration and mixing. When operating in the anoxic mode, the diffusers will not be in use. Flow proceeds to the aerobic zone over a baffle wall weir.
Mixed Liquor Recycle Pumps6
 
Each aeration basin is equipped with a submersible pump at the outlet end of the basin in zone D. The pumps slide up and down on a guide bar and can be lifted out of the basin for maintenance using a manual hoist.

The pump recycles the nitrate-rich mixed liquor from aerobic zone D into the anoxic zone. The denitrifying bacteria in the anoxic zone use the nitrate to oxidize BOD and in the process reduce the nitrate to nitrogen gas.
Mixed Liquor Junction Box 6
Mixed liquor (ML) exits the aeration basins over a new stationary steel weir and flows into a common center channel (which is hydraulically separated from the common influent channel by a new concrete bulkhead) and flows through an existing common 42” pipe at the north end of the basins.

The pipe is routed to the northeast side of aeration basin #2 and discharges into the mixed liquor junction box containing two adjustable slide gates that distribute flow to the two secondary clarifiers. ML is conveyed to each clarifier by gravity flow through 24-inch pipes. The slide gates at the mixed liquor junction box can be closed to isolate an individual clarifier.
Aeration Blowers6
The aeration blower system consists of three existing multistage centrifugal type blowers, inlet filter silencers, inlet valves, check valves, isolation valves, and expansion joints.( Each blower is rated for 50 percent of the overall maximum required air flow. Air flow from the blowers is varied by the modulation of the inlet valves of the blowers. All three blowers discharge into a common header. Air flow to the Headworks and aeration is balanced by control valves and manual valves in the low pressure air system.
Electrical power for the aeration blowers is supplied from MCC-2 located in the blower building electrical room.
Aeration Basin Diffusers6
The aerobic zone contains fine-bubble membrane diffusers in a tapered aeration configuration. Diffusers are separated into three aeration zones along the length of the basin where the number of diffusers in each zone decrease with each progressing zone. Each aeration zone has a drop leg with a pneumatically actuated butterfly valve to adjust aeration in the zone. Air is supplied by the process air blowers located in the blower building.
Dissolved Oxygen Meters6
A dissolved oxygen (DO) sensor monitors dissolved oxygen in each zone of the basin. The system consists of a DO sensor and transmitter located in each of the four zones of each aeration basin. The sensors are mounted to the handrail on the east and west side of the central walkway above the influent channel that separates the two basins. The DO transmitters are generally located adjacent to each sensor.  

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Last Updated: 9/16/2013 9:13:50 AM
Version 4.0.1