Nutrient Removal and Power Savings in Wastewater Treatment Systems
Todd L. Steinbach, PE
Aero-Mod®
Wastewater Process Solutions
Energy Consumption• What determines the amount of aeration required in an
activated sludge plant?
It can be the organic loading (Organic Requirement)…
but it is often the amount of energy required to keep the basin(s) in suspension (Mixing Requirement).
How does an under-loaded plant operate energy-efficiently?
How does this relate to Nitrogen Removal?
Organic Requirement• Oxygen required by the bacteria to break down BOD and
ammonia.
For Extended Aeration:
1 lb of BOD requires from 1.33 to 1.5 lbs of O2.
1 lb of ammonia requires 4.6 lbs of O2.
Organic Requirement• 1.0 MGD Typical Example:
BOD: 240 mg/l, NH3-N: 35 mg/l, 1.5 lbs O2/lb BOD, 24 hr HRT,
11’ water depth, fine bubble efficiency of 2.0%/ft of subm.,
5.5 psi, 1,000 FASL, summer temp.
O2 for BOD would be 325 lbs/hr,
…or 1,409 scfm (1,656 icfm) of blower air.
O2 for NH3-N would be 145 lbs/hr,
…or 630 scfm (741 icfm) of blower air.
Organic Requirement• 1.0 MGD Typical Example:
Blower Power Required, assuming pd blower @ 70% efficiency
BHP for BOD = (icfm) * (psi) / (229 * eff%)
= (1,656 icfm) * (5.5 psi) / (229 * 70%)
= 57 HP
BHP for NH3-N = (icfm) * (psi) / (229 * eff%)
= (741 icfm) * (5.5 psi) / (229 * 70%)
= 25 HP
82 HP Total (sizing program gave me 79 HP)
Mixing Requirement• 1.0 MGD Typical Example:
Side-roll aeration, 20 cfm/1,000 cf, 24 hr HRT, 11’ water depth,
5.5 psi, 1,000 FASL, summer temp.
Air required for mixing would be:
cfm = (1 Mgal) / 7.48 cf/gal / 1,000 cf * 20 cfm
= 2,674 cfm
BHP = (2,674 cfm) * (5.5 psi) / (229 * 70%)
= 92 HP (sizing program gave me 89 HP)
Energy Consumption• What determines the amount of aeration required in an
activated sludge plant?
It can be the organic loading (Organic Requirement)…
but it is often the amount of energy required to keep the basin(s) in suspension (Mixing Requirement).
How does an under-loaded plant operate energy-efficiently?
How does this relate to Nitrogen Removal?
Ammonia toxicity to aquatic organisms
Nitrite toxicity to aquatic organisms
Nitrate toxicity to humans
Methemoglobinemia (blue baby syndrome)
Eutrophication
Fertilization
Nutrient Discharge Limits
Ammonia
Aero-Mod®
Wastewater Process Solutions
Oxidation of Ammonia
Urea (CH4N2O) => NH3 => NO3-
Protein => Amino Acid => NH3 => NO3-
Ammonia Reduction
Nitrification is accomplished by two unrelated
groups of autotrophic microorganisms
Ammonia-oxidizing bacteria such as Nitrosomonas
Nitrite-oxidizing bacteria such as Nitrobacter
Nitrification
Consumes 4.6 grams of O2 per gram of NH3-N oxidized
Consumes 7.1 grams of alkalinity per gram of NH3-N oxidized
Forms 0.15 grams of new cells per gram of NH3-N oxidized
Nitrification
Nitrite oxidizers cannot proliferate until the ammonia oxidizers have produced enough nitrite for the nitrite oxidizers
Different species nitrify at different D.O. levels
Clusters of ammonia oxidizers and nitrite oxidizers appear to grow close together within the floc
Nitrifiers need NH3-N, not NH4+-N
Nitrifying Bacteria
SRT
Temperature
pH
Alkalinity
D.O.
Wastewater Characteristics that Impact Nitrification
SRT
Typically, at least 5 days will be required for stable nitrification
Wastewater Characteristics that Impact Nitrification
Temperature
Colder temperatures require an older sludge age because reproduction slows down
Colder temperatures cause more of the ammonia to be ionized (NH4+)
Wastewater Characteristics that Impact Nitrification
pH
Nitrifiers are sensitive to changes in pH
As pH decreases, ionization increases and less NH3-N is available
Wastewater Characteristics that Impact Nitrification
pH vs. Alkalinity
pH is a measure of hydrogen ion concentration
Alkalinity is a measure of a water’s ability to neutralize acid
Water with high alkalinity will always have an elevated pH, but a water with elevated pH does not always have a high alkalinity
Both measurements are needed
Wastewater Characteristics that Impact Nitrification
Why Low Alkalinity Affects Nitrifiers
pH Alkalinity neutralizes acid
Inadequate alkalinity results in low pH
Carbon Source Nitrifiers cannot use organic compounds for synthesis
and growth
Bicarbonate/carbonate alkalinity may satisfy their need for an inorganic carbon source
Wastewater Characteristics that Impact Nitrification
Chemical Sources of Alkalinity
For every mg of _______ added, _______ mg of alkalinity as CaCO3 is gained
CaO Quick Lime 1.8
Ca(OH)2 Hydrated Lime 1.4
Mg(OH)2 Magnesium Hydroxide 1.4
NaOH Caustic 1.2
Na2CO3 Soda Ash 0.9
Wastewater Characteristics that Impact Nitrification
Dissolved Oxygen
Nitrification is an aerobic process and elemental oxygen (O2) is required
Nitrifiers may not compete as well for oxygen as heterotrophic bacteria
If not enough oxygen is present, the heterotrophs may get most of it first
Wastewater Characteristics that Impact Nitrification
Large oxygen requirement
Potential low pH (if alkalinity is low)
If pH is low, fungi can develop
Discharge of nitrogen as Nitrate
Potential for clarifier denitrification
Sludge age range where filaments can develop
Problems Caused by Nitrification
Nitrogen Removal
Aero-Mod®
Wastewater Process Solutions
The other half of biological nitrogen removal
Accomplished by many different kinds of
facultative bacteria
Facultative bacteria can use oxygen or nitrate
Denitrifiers are facultative heterotrophs and
must have an organic carbon food source
Bacteria forced to use the oxygen in Nitrate
Denitrification
Bacteria reuse about 60% of nitrification O2
Produces 3.6 grams of alkalinity per gram of
Nitrate reduced (about 50%)
Forms about 0.5 grams of new cells per gram
of Nitrate reduced
Consumes about 2.9 grams of BOD per gram
of Nitrate reduced
Denitrification
Anoxic zone with nitrate recycle from aeration tank
High recycle rate of 2Q to 4Q
Sequenced aeration
Low D.O. operation
D.O. Probes & Controller
VFD Motor Drives
PLC Process Controller
Denitrification Methods
A2O and Bardenpho
SBR
Oxidation Ditch w/ Mixed Anoxic Zone
MBBR (Moving Bed BioReactor)
Step feed aeration
SEQUOX
Denitrification Designs
Denitrification Designs
D.O. level too high will prevent bacteria from using NO3-
Lack of carbon source available for bacteria
Recycle rate too low will not bring back enough Nitrate
Recycle rate too high will shorten detention time of
aeration basin
High peak flows in an SBR reduces allowed time for
aeration on and aeration off
High fluctuations of BOD/ammonia disrupt D.O. level
Denitrification Issues
RAS
Clarification1st Stage Aeration
(Air off) AerobicDigestion
WAS
Supernatant
Bio-Selector
2nd Stage Aeration(Air-off)
AerobicDigestion1st Stage Aeration
(Air on)
2nd Stage Aeration(Air on)
Clarification
RAS
WAS
Supernatant
Influent
Effluent
Effluent
Aero-Mod SEQUOX Solution
RAS
Clarification1st Stage Aeration
(Air on) AerobicDigestion
WAS
Supernatant
Bio-Selector
2nd Stage Aeration(Air on)
AerobicDigestion1st Stage Aeration
(Air off)
2nd Stage Aeration(Air Off)
Clarification
RAS
WAS
Supernatant
Influent
Effluent
Effluent
Aero-Mod SEQUOX Solution2 hours later
Denitrification without mixers
Sequenced aeration with continuous clarification
Reclaim portion of oxygen & alkalinity consumed in nitrification
Concentrated settled biomass consumes D.O. quickly
Oxygen-starved biomass uses nitrates quickly when basin is re-aerated
Plug flow pattern ensures several cycles of sequenced aeration
Common-wall construction provides small footprint
SEQUOX Nitrogen Removal Process
SEQUOX Features
SEQUOX controls:
1. Where we the air is placed (only 50% of basins aerated at a time)
2. When we aerate basins (simple timer control on typical 2-hour cycle)
3. How much air we provide via VFD control on the aeration blowers
4. How fast we allow the D.O. to rise in the Aeration Basins using a PLC-based D.O. control system to control each blower VFD
SEQUOX with DO2ptimizer Benefits
1. Energy Savings a. When D.O. is below low set point, blower output increases.
(Organic Requirement) b. When in-between low and high set points, blower output
decreases to mixing requirement.
(Mixing Requirement)c. When above high set point, blowers can be turned off.
(Rest)
2. Flexibility when organic loading is high, plant can automatically switch to SEQUOX (both 1st Stage Aeration Basins aerating) and when the organic loading subsides – go back to SEQUOX-Plus.
3. Nitrogen Removal levels to Total N of 3 mg/l achieved.
NEYCSA - Mt. Wolf, PA
1.70 MGD
Neligh, NE
210,000 gpd municipal facility
One 30 HP blower for process and
aerobic digester
Blower operated with manual control of
VFD for nine years
PLC-based D.O. control placed
into operation in Fall of 2011
Average of 5,000 kWh reduction per month ≈ $500 savings per month
Along with the power savings, plant is also achieving TN reduction
Holton, Kansas0.528 MGD Bio-P
Ammonia is oxidized by nitrifying bacteria
Bacteria use oxygen to strip carbon from
alkalinity and hydrogen from ammonia
Bacteria use 7.1 mg alkalinity per mg ammonia
reduced
Bacteria use 4.6 mg oxygen per mg ammonia
reduced
Nitrate is reduced product – NO3-
Ammonia Removal - Nitrification
Nitrate is reduced by heterotrophic bacteria
Bacteria use the oxygen from nitrate
DO must be controlled to force the bacteria to
use the nitrate
Alkalinity is reclaimed – about 3.6 mg per mg of
nitrate
A carbon source must be available for the
bacteria to use
Nitrogen Removal - Denitrification
Energy Consumption• What determines the amount of aeration required in an
activated sludge plant?
It can be the organic loading (Organic Requirement),…
but it is often the amount of energy required to keep the basin(s) in suspension (Mixing Requirement)
How does an under-loaded plant operate energy-efficiently?
USING SEQUOX &AERO-MOD’S DO2PTIMIZER
1 • SEQUOX BNR
• DO2ptimizer D.O. Control
• Sliderail Diffuser Access System
• ClarAtor Clarifier
• Tritan Belt Filter Press
Custom Designed Wastewater Treatment Solutions
www.aeromod.com