Aeration Basics – the Bug’s Eye View Leonard E. Ripley, Ph.D., P.E., BCEE Senior Environmental Engineer Freese and Nichols, Inc. WEAT Electrical & Instrumentation Seminar March 20, 2019
Why Do We Aerate? Supply process oxygen: 1. Oxidation of organics (BOD) 2. Endogenous respiration Suspend mixed liquor solids
Biochemical Oxygen Demand (“BOD”) BOD has two components: 1. Carbonaceous BOD (“CBOD”) is oxygen required for oxidation of carbon: C x H y O z + O 2 → CO 2 + H 2 O Carried out by heterotrophic bacteria … relatively rapid process 2. Nitrogenous BOD is oxygen required for oxidation of ammonia to nitrate: NH 3 + O 2 → NO 2 + O 2 → NO 3 + H 2 O Carried out by “nitrifiers” … slow growing, relatively sensitive bacteria
Endogenous Respiration Without wastewater organics for food: Bacteria coast and respire “endogenously” (resting rate) Bacteria eventually die, rupture (lysis), and provide food for their relatives This is the main process in aerobic digestion, but it also is important in aeration basins, especially if they are organically underloaded
TCEQ Chapter 217 Design Criteria for DO Oxygen Requirements (O 2 R) of wastewater: An aeration system must be designed to provide a minimum dissolved oxygen concentration in the aeration basin of 2.0 milligrams per liter (mg/L). Note: This is at the max design loading in the future.
Mixing Keep mixed liquor solids in suspension: Air flow rate must be > 20 scfm/1000 cu ft for course bubble diffusers, > 0.12 scfm/sq ft for fine bubble diffusers Mechanical mixing must provide > 0.75 hp/1000 cu ft Swing zone can be aerated or just mixed
How Much Oxygen is Necessary? Depends mainly on: Wastewater flow rate, cBOD & ammonia concentrations → organic loading rates Other factors: Characteristics of BOD: degrades readily or slowly? Solids retention time (sludge age) Basin configuration -- selectors?
Why Aeration is Expensive 1. Even highly efficient aeration is not very efficient in actually transferring oxygen into solution. 2. Besides pushing oxygen into the aeration basin, we also have to pressurize the accompanying nitrogen. Example: with 33% O 2 transfer efficiency: 1 lb O 2 transferred requires 3 lb O 2 applied 3 lb O 2 applied carries 11 lb nitrogen Total air required to transfer 1 lb O 2 = 14 lb
How Much Air is Necessary? Depends mainly on: Wastewater flow rate, BOD & ammonia concentraGons → loadings Other factors: Characteristics of BOD: readily or slowly degradable Solids retention time (sludge age) Transfer efficiency of diffusers WHAT CAN DO concentrations YOU Wastewater temperature CONTROL? Presence of surfactants and/or grease Basin configuration (selectors?) and AB volume Air temperature and humidity
Aeration Control Overview (simplified) FLOW RATE (MGD) OXYGEN UPTAKE RATE (mg/L-hr) BOD & NH 3 OXYGEN If supplied (mg/L) REQUIRED > required (lbs/hr) DO ↑ O 2 TRANSFER DISSOLVED EFFICIENCY OXYGEN (%) (mg/L) If supplied < required AIR SUPPLIED DO ↓ (scfm)
Aeration Control Overview (simplified) FLOW RATE (MGD) OXYGEN UPTAKE RATE (mg/L-hr) OXYGEN BOD & NH 3 REQUIRED (mg/L) (lbs/hr) ? ? ? O 2 TRANSFER DISSOLVED Is DO EFFICIENCY OXYGEN increasing or (%) (mg/L) decreasing? AIR SUPPLIED (scfm)
Possible Game-Changing Technology FLOW RATE Floating hood collects off-gas and (MGD) OXYGEN analyzes residual O 2 and CO 2 UPTAKE RATE content. Calculates: (mg/L-hr) BOD & NH 3 (mg/L) Oxygen Uptake Rate (OUR) O2 TRANSFER Oxygen Transfer Efficiency EFFICIENCY (%) Expensive … no units in Texas at this time.
Example AB Oxygen Uptake Patterns (Dallas Water Utilities Central Plant – B Complex) 80 Oxygen Uptake Rate (mg/L-hr) 70 BOD & Ammonia Oxidation 60 50 40 Endogenous Respiration (Digestion) 30 20 10 0 0 25 50 75 100 Influent ← Basin PosiGon (%) → Effluent
More Typical AB Oxygen Uptake Pattern 80 Oxygen Uptake Rate (mg/L-hr) Much higher demand 70 at front of basin. 60 50 40 Endogenous Respiration (Digestion) 30 20 BOD & Ammonia Oxidation 10 0 0 25 50 75 100 ← Basin PosiGon (%) →
Tapered Aeration Install diffusers in zones to match oxygen uptake pattern – higher density at influent end of basin.
Example Air Flow Distribution: Leon Creek 2,500 2,058 Annual Average 2,000 Air Flow Rate (scfm) Max Month Peak Load 1,500 1,235 1,008 1,000 823 605 50% 403 500 30% 20% 0 A B C Aeration Basin Zone
Leon Creek Minimum Air Flow Rates 2,500 Annual Average 2,000 Air Flow Rate (scfm) Max Month Peak Load 1,500 1,000 435 500 361 361 0 A B C Aeration Basin Zone
Non-Aeration: Anoxic and Anaerobic Zones “Anoxic” – with very little, if any, oxygen present. Heterotrophic bacteria substitute nitrate for oxygen in degrading BOD ... Can reduce aeration by 15-20%. “Anaerobic” – with no oxygen and no nitrate present. Phosphorus Accumulating Bacteria (PAO’s) release phosphorus, then take up extra phosphorus in the aerobic zone … biological phosphorus removal.
Anoxic/Anaerobic Zones for BNR Denitrifiers use Heterotrophs oxidize BOD, nitrate to nitrifiers oxidize ammonia, PAO’s release replace oxygen PAO’s uptake phosphorus phosphorus N 2 Gas Aerobic RAW INFLUENT CLARIFIER 8 8 Anaerobic Anoxic Nitrified Recycle Return Activated Sludge
Anoxic/Anaerobic (BNR) Effects on Aeration Can reduce oxygen, and aeration, demand by 15-20%. Recycle will even out uptake rate along length of basin. Important to minimize returning dissolved oxygen to an anoxic zone. Critically important to avoid returning dissolved oxygen to an anaerobic zone.
The “Perfect” Aeration Strategy? Supply just enough air to meet process requirements – no air “wasted” on mixing. Supply enough air to meet process requirements by mid-basin, use last zone(s) as a safety cushion.
Final (Process) Thoughts … 1. Every plant has large aeration fluctuations – hourly, daily, seasonally – you’ll never reach “perfection”. 2. Compliance is priority #1, even if you have to waste some air. 3. Be diligent about monitoring/maintaining the DO probes. 4. Make aeration changes gradually. 5. Turndown may require taking AB’s out of service. 6. DO control may be more important for BNR than for saving energy.
Thank You! Questions / comments: Leonard E. Ripley, Ph.D., P.E. BCEE Freese and Nichols, Inc. LER@Freese.com
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