nutrient removal and power savings in wastewater treatment systems todd l. steinbach, pe aero-mod ®...

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)



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 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®


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


Temperature

Colder temperatures require an older sludge age because reproduction slows down

Colder temperatures cause more of the ammonia to be ionized (NH4+)


pH

Nitrifiers are sensitive to changes in pH

As pH decreases, ionization increases and less NH3-N is available


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


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


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


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


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®


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



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)


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

nutrient removal and power savings in wastewater treatment systems todd l. steinbach, pe aero-mod ®...

Documents