Nutrient Removal and Power Savings in Wastewater Treatment

Report
Aero-Mod
®
Wastewater Process Solutions
Nutrient Removal and Power Savings
in Wastewater Treatment Systems
Todd L. Steinbach, PE
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?
Nutrient Discharge Limits
 Ammonia toxicity to aquatic organisms
 Nitrite toxicity to aquatic organisms
 Nitrate toxicity to humans
 Methemoglobinemia (blue baby syndrome)
 Eutrophication
 Fertilization
Aero-Mod
®
Wastewater Process Solutions
Ammonia
Ammonia Reduction
 Oxidation of Ammonia
 Urea (CH4N2O) => NH3 => NO3-
 Protein => Amino Acid => NH3 => NO3-
Nitrification
 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
Nitrifying Bacteria
 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
Wastewater Characteristics that Impact
Nitrification
 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
Problems Caused by 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
Aero-Mod
®
Wastewater Process Solutions
Nitrogen Removal
Denitrification
 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 Methods
 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 Designs
 A2O and Bardenpho
 SBR
 Oxidation Ditch w/ Mixed Anoxic Zone
 MBBR (Moving Bed BioReactor)
 Step feed aeration
 SEQUOX
Denitrification Designs
Denitrification Issues
 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
Aero-Mod SEQUOX Solution
2nd Stage Aeration
Supernatant
(Air-off)
Effluent
Influent
Clarification
1st Stage Aeration
RAS
(Air on)
Aerobic
Digestion
WAS
Bio-Selector
WAS
Effluent
RAS
1st Stage Aeration
Clarification
(Air off)
2nd Stage Aeration
(Air on)
Supernatant
Aerobic
Digestion
Aero-Mod SEQUOX Solution
2 hours later
2nd Stage Aeration
Supernatant
(Air on)
Effluent
Influent
Clarification
1st Stage Aeration
RAS
(Air off)
Aerobic
Digestion
WAS
Bio-Selector
WAS
Effluent
RAS
1st Stage Aeration
Clarification
(Air on)
2nd Stage Aeration
(Air Off)
Supernatant
Aerobic
Digestion
SEQUOX Nitrogen Removal Process
 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 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, Kansas
0.528 MGD Bio-P
Ammonia Removal - Nitrification
 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-
Nitrogen Removal - Denitrification
 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
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

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