Fixed Film Secondary Treatment Flashcards
What is a rotating biological contactor?
Fixed film biological treatment where biological slime growth is rotated through the waste stream and air to remove BOD and dissolved solids
Components of an RBC
Biological slime growth on the media Rotating shaft Tanks - sits about 40% submerged Baffles Covers
Why are RBCs almost always preceded by primary treatment?
If settlable solids arent removed the filter can plug, the basin can fill which decreases detent time, and the basin can go septic
What is sloughing?
When extra biological growth detaches from the media as it becomes heavier. It is then carried to the secondary clarifier
How does RBC sludge compare to waste activated sludge?
Generally much thicker/denser because it is typically older
What is the purpose of the voids in the media?
Allow growth of bacteria
Evenly distribute wastewater
Allow airflow for the aerobic process
How is biological growth accomplished on RBC media?
Via adsorption absorption
What is the typical surface area of standard density RBC media?
90,000-110,000 square feet
What is the surface area of high density RBC media?
120,000 to 160,000 square feet
What is the typical hydraulic detention time of an RBC?
1-4 hours
Can RBCs have stages?
Yes, typically separated by baffles. BOD levels reduce from stage to stage and late stages can be used for nitrification
RBC shaft rotation speed
About 1.5 RPM while 40% immersed
What is the typical length if an RBC shaft?
25 feet
How are RBC shafts driven?
Mechanical drives or air drives
Advantages of mechanical drive RBC
Lower energy consumption
Uniform rotation speed
Speed adjustments are easier
Disadvantages of mechanical drive RBC
Supplemental aeration may be required
Air drive RBC advantages
Adds dissolved oxygen Less maintenance Aids in nitrification Strips off extra zoogleal mass Less torque on the drive Minimizes solids accumulation in the tank
Disadvantages of air drive RBC
Speed control is not exact
Prone to growth imbalance
Can strip off too much zoogleal mass
Higher power requirements
What is the purpose of an RBC cover?
- Protects from weather
- Prevent hard rain from washing slime
- Prevents algae growth
- Protects media from becoming brittle from sunlight exposure
- Contains odors
Factors affecting RBC treatment
F:M ratio Dissolved oxygen Temp Nutrients Toxicity Flow Rotational speed Disc submergence
pH requirements for nitrifying
pH needs to be higher than typical domestic wastewater, around 8.4
Typical domestic wastewater pH
6.5-8.5
How much alkalinity is required for nitrifying?
7.14 times the concentration of the influent NH3
Dissolved oxygen for an RBC designed for primary effluent
0.5-1.0 mg/L in the first stage
1-3 mg/L at the end of the first stage
Dissolved oxygen in an RBC designed for nitrification
1-3 mg/L in the first stage
4-6 mg/L by the final stage
Temperature impact on RBC
More sensitive to temp than suspended growth
<55 degrees F will result in decreased BOD removal
Nitrification will be affected first
RBC Hydraulic loading for BOD removal
1.5-6 GPD/square foot
RBC hydraulic loading rate for nitrogen removal
1.5 - 1.8 GPD/square foot
Organic loading rate for RBC
Soluble BOD: 2.5-4lbs BOD/Day/1,000 sq feet
Total BOD 6-8lbs BOD/Day/1,000 sq feet
How are RBCs affected by toxicity?
Generally less vulnerable than suspended growth
Toxic impact is directly related to dose and duration of exposure
Examples of submerged fixed film
Moving bed biological reactors
Fixed film biological reactors
Integrated fixed film activated sludge
Advantages of submerged fixed film
More efficient, more biomass, longer detent times, similar footprint
What is an MBBR?
A tank partially filled with free floating plastic media that the biomass attaches to.
Advantages of an MBBR
Smaller footprint No return Biomass is self regulated Less sensitive to toxics Nitrification is possible Increase capacity with more media
Disadvantages of MBBR
Higher energy costs
Cant remove phosphorus biologically
Proprietary media
Need additional screening
What is Integrated Fixed Film Activated Sludge?
Media is submerged in mixed liquor and a return is set up on the clarifier
Advantages of IFAS?
Higher level of treatment in smaller footprint
Biological phosphorous removal
Nitrification
What should biological growth look like during normal operations?
Brown to grey
No algae
Shaggy and fairly uniform in coverage
What would black slime growth imply?
Solid or BOD overload with probable low dissolved oxygen
What would white appearance of slime imply?
Sulfer loving bacteria are present (filaments)
Industrial discharges containing sulfer
Sludge deposits accumulating in tank
What colors are present on media when Nitrifying In an RBC?
1st stage is usually light brown, later stages can be gold/rusty/reddish
What are characteristics of an RBC designed for BOD removal?
Standard density media
Thicker biofilm
What are characteristics of an RBC designed for nitrification?
High density media
Thinner biofilm
What is plug flow in an RBc?
When the waste is introduced to a portion of the RBC rather than the entire surface area at once, and the waste is polished in later stages
What is completely mixed flow in an RBC?
The flow is introduced to the entire reactor volume simultaneously
What are the two primary types of trickling filters?
Fixed nozzle
Rotary distributor
Pros of a fixed nozzle trickling filter
Even flow distribution
Cons of a fixed nozzle trickling filter
Maintenance is difficult
Benefits of random media in a trickling filter?
Increases detention time. Ideal in low organic matter applications with few particles
Benefits of vertical flow media in trickling filter
Less prone to clogging, efficient in removing excess biomass.
Useful in high BOD loading applications
Cross flow media benefits in a trickling filter
Improved pollution removal
Effective with low to medium BOD loading
Purpose of recirculation in a trickling filter
Increases removal efficiency Increase ventilation Promotes uniform sloughing Decreased snails and flies (psychoda) Reduces strength of wastewater applied
Purpose of the underdrain system if a trickling filter
Collect effluent
Supports the media
Permits airflow
How does oxygen affect trickling filter treatment?
Recirculation increases dissolved oxgen
Maintain a 1.5-2.0mg/L DO in effluent
How does temperature affect trickling filter operation?
Lower temperatures lower removal efficiencies
Recirculation lowers temp
What is the appropriate pH range for a trickling filter?
5.5-8.5 (stay higher if you want to nitrify)
What are the classifications of trickling filters based on organic-loading rate?
Low-rate <25 lbs/day/1000ft
Intermediate-rate 25-40lbs/day/1000ft
High-rate 40-100 lbs/day/1000ft
Roughing filter 100-300 lbs/day/1000ft
What is the purpose of a low-rate trickling filter?
Achieve high BOD removal and may perform nitrification
What is the purpose of intermediate rate trickling filters?
Removal of BOD without complete nitrification
What is the purpose if high-rate trickling filters?
Need to treat high incoming BOD; they likely need to be combined with another process
What is the purpose of a roughing trickling filter?
To reduce the required size of the second stage treatment process; often in industrial pretreatment
Organic loading rate for a nitrifying trickling filter
3-10lbs BOD/day/1000ft with rock
6-18lbs BOD/Day/1000ft with synthetic
What conditions are required to nitrify in a trickling filter?
Greater surface area
More DO
Adequate alkalinity
Warmer temps
What are causes of psychoda and snails in a trickling filter?
Poor distribution and insufficient hydraulic loading
