?/EPA
United States
Environmental Protection
Agency
United States Environmental Protection Agency
August 2013
Wastewater Treatment Fact Sheet:
External Carbon Sources for Nitrogen Removal
DESCRIPTION
Discharge permits for publicly owned treatment
works (POTWs) and industries often include
effluent limits for nutrients, including nitrogen.
Total maximum daily loads (TMDLs) for
nutrients have and are being developed for many
water bodies throughout the United States. The
TMDLs and resultant waste load allocations to
protect impaired water bodies have resulted in
more stringent effluent limits for total nitrogen.
In order to achieve very low total nitrogen limits
of less than 6 mg/1 through biological
denitrification, a readily biodegradable carbon
source must be available for the denitrifying
organisms to use. A supplemental external
carbon source is often required when organic
material in the wastewater has been oxidized.
This is especially true in denitrification processes
that are located after the aeration process such
as post or second anoxic zone and denitrifying
filters.
This fact sheet will provide information
on external supplemental carbon sources to utility
managers and operators of wastewater treatment
facilities that have existing nitrogen permit
limits or will be required to remove nitrogen in
the future.
OVERVIEW
Bacteria utilize carbon as an energy source to
drive metabolism as well as for the synthesis
of new cellular material. Microorganisms obtain
their carbon needs from organic compounds or
from carbon dioxide. Heterotrophic
microorganisms are able to utilize organic carbon
sources while autotrophic organisms utilize
carbon dioxide as a carbon source. When
microorganisms utilize organic carbon as a
substrate, energy is produced by the biochemical
oxidation of organic carbon to carbon dioxide.
There are two major sources of organic carbon
utilized in wastewater treatment operations.
The sources are defined with respect to
whether they originate within the influent
wastewater entering the treatment facility or
are provided as an external supplemental
carbon source added to the treatment system.
Carbon sources are termed external when the
carbon substrate is sourced from outside the
wastewater treatment process i.e., it is not
derived from the influent wastewater or any
onsite treatment processes at the treatment
facility. External supplemental carbon sources
are brought into the wastewater treatment
process usually as pure compounds or high
strength waste materials where concentrations
can be as high as 1.5 g/L chemical oxygen
demand (COD) to facilitate nutrient removal.
Internal carbon sources refer to organic
carbon substrates obtained either within
the influent wastewater (as an organic
wastewater load entering into the plant from
the influent) or from accumulated materials
stored within the cells also referred to as
endogenous carbon sources.
CARBON AUGMENTATION FOR
NITROGEN REMOVAL
Nitrogen removal involves the initial
transformation of ammonia and organic
nitrogen to nitrates via nitrification, and the
subsequent elimination of nitrogen through
denitrification. Because nitrification typically
only occurs following carbonaceous biological
oxygen demand (BOD) removal, the limiting
factor for effective ddenitrification is often
the absence of a readily biodegradable carbon
source that can be used as an effective
substrate by denitrifying bacteria during the
denitrification process. Without the
availability of a ready source of biodegradable
carbon, denitrification will not occur, or will
occur too slowly for sufficient nitrogen
removal to occur.
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Denitrification with
Supplemental
Carbon Sources
4-Stage System Example
Supplemental
Carbon
Addition
Point
Supplemental
Carbon
Addition - , ., . ,,,
Point Return Activated Sludge
| Physical Process
I Biological Process
; Primary Sludge
MMMIMMMIMMMIMMMI
.•..•Solid Flow
oidFlow
Waste Activated Sludge;
Figure 1: Illustration of Wastewater Treatment Process and Supplemental Carbon Feed Points
By using recycle schemes and step-feed
processes, nitrates are brought into contact with
sources of readily biodegradable carbon in the
plant. Processes such as the Modified
Ludzak-Ettinger (MLE) process enable contact
between the nitrates formed at the back end of
the wastewater treatment process and soluble
COD generally found in the influent wastewater
streams by recycling nitrate laden process flows
to the head of the treatment system. However, in
the MLE, step-feed and sequencing batch reactor
(SBR) processes, a supplemental external carbon
source might still be required to facilitate nitrate
removal when internal carbon is not available in
high enough concentrations (see Figure 1 for
MLE and post anoxic zone processes common
supplemental carbon feed points).
Denitrification processes that are located after
the aeration process such as post or second
anoxic zones and denitrifying filters will
generally always need an external
supplemental carbon source to be added, as
almost all of the internal carbon sources have
been utilized in the aeration process and only
a very limited amount endogenous carbon is
available for denitrification.
A wide range of carbon sources can be
used to meet the soluble COD needs for
denitrification. Commonly used sources
of external carbon include methanol,
ethanol, acetate, acetic acid, glycerol,
molasses sugar water and proprietary
formulations like MicroC™, a suite of carbon
sources manufactured by Environmental
Operating Solutions, Inc.
The choice of carbon source typically will
depend on the evaluation of a number of
product attributes, including: safety, cost,
handling requirements, ease of use, materials
compatibility, as well as kinetics and yield
dynamics. The choice of a carbon source can
have profound implications not just on the
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efficacy of nutrient removal, but also on plant
and personnel safety, sludge yields, aeration
adequacy, environmental sustainability,
overall effluent quality and other factors.
Tables 1 and 2 provide an overview of
different carbon sources as well as a
qualitative assessment of their ratings on a
variety of attributes and product
characteristics.
IMPORTANT FACTORS TO CONSIDER
WHEN SELECTING AN EXTERNAL
CARBON SOURCE
Safety
Safety is a very important consideration for
carbon source selection. The use of external
carbon sources such as methanol, acetic acid
and ethanol for enhanced nutrient removal has
expanded the scope of safety concerns in
wastewater treatment facilities. The most
significant concerns are that of flammability
and explosion hazards associated with
methanol and ethanol.
The levels of flammability associated with the
carbon source selected will have an impact on
the cost of the systems that will need to be put
in place to ensure that the risk of fires and
explosions are mitigated through compliance
with applicable National Fire Protection
Association (NFPA) fire and safety codes.
Also, the hazards associated with handling
these products will require Occupational
Safety and Health Administration (OSHA)
specific initial and annual recertification
training of the plant operations staff. In 2006,
a serious incident involving fatalities was
recorded at a wastewater treatment plant using
methanol as an external carbon source. This
tragic incident brought to the forefront the
need for safety considerations for flammable
carbon sources, which include foam
suppression systems, explosion proof storage,
and spill containment.
Price Fluctuations
Many external carbon sources are derived
from fossil fuel based raw materials.
Significant price fluctuations in the methanol,
ethanol, and acetic acid markets can have a
huge impact on the prices of these carbon
sources.
Agriculturally derived carbon sources such as
molasses, glycerol, corn syrup, sucrose and
MicroC™, tend to have more predictable and
less volatile price profiles. In order to ensure
that lifecycle costs associated with the external
carbon system reflect the potential for
significant price movements, especially with
regards to fossil fuel based carbon sources,
evaluations for carbon source selection should
incorporate an analysis that incorporates
historical price changes into the evaluative
framework.
DOSAGE AND KINETIC
CONSIDERATIONS
The dosage requirement refers to the amount
of COD that is required to remove each unit of
nitrate (i.e., the COD:N ratio, which is usually
expressed as Ibs COD/lbs NOs-N removed).
This ratio is affected by factors such as the
nature of the carbon source, the species of
biomass supported, the electron donor
capacity of the carbon source, the solids
retention time (SRT) of the treatment system
and the sludge yields associated with bacterial
species supported by the carbon source.
Kinetic considerations typically focus on the
specific denitrification rates and the
biomass growth rates associated with the
carbon source. This is generally a function of
the species of biomass that are selected for
use in the treatment process when a carbon
source is utilized. When methanol is used as a
carbon source, methylotrophic denitrifying
bacteria are selected for, resulting in a slower
overall growth. Non methanol carbon sources
such as glycerol, acetic acid, and the
MicroC™ suite of products can be
metabolized by the general heterotrophic
bacterial populations found in the wastewater
treatment process.
Methylotrophic biomasses are known to have
very low growth rates. In colder
temperatures, the growth rate is even lower,
potentially leading to a reduced capacity for
denitrification in the winter period
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COD mg/L
Bulk Density Ibs/gal.
Yield gCOD/g COD
Total COD/ N
Total dose gal
substrate/ IbNOSN
Methanol
1,200,000
6,6
0.41
4.82
0.48
Ethanol
1,650,000
6.6
0.55
6.36
0.46
MicroCg™ MicroCglycerin™
670,000
10.2
0.6
6.45
1.15
1,016,000
9.92
0.55
6.36
0.77
56% Acetic
Acid
577,000
9.09
0.53
6.09
1.19
30% Sodium
Acetate
222,480
9.8
0.53
6.09
3.09
Table 1: Product Characterization for Some External Carbon Sources
QUALITY CONTROL
Carbon sources are generally pure products
(e.g., methanol, ethanol, MicroC™), unrefined
wastes, or purified waste materials derived
from a variety of industrial and agricultural
processes. Some typical sources of external
carbon include spent sugars from food and
beverage manufacturing and glycerol from
bio-diesel production. Generally, the costs of
carbon sources derived from waste products
tend to vary with the level of purity. However,
given the processes from which these materials
are derived, waste materials can contain
impurities that could be problematic to the
wastewater treatment process, pumping and
handling and process kinetics. Such unrefined
waste materials tend to have variable
compositions, and this can have significant
effects on the safety as well as the efficient
functioning of the nutrient removal process.
When external carbon sources are applied for
nutrient removal, especially in tertiary deep
bed biological filters, it is important for the
carbon source to have a consistent COD
loading, given the fact that tertiary effluent
systems have few downstream processes that
are capable of handling significant COD
breakthrough. Furthermore, variability in
product quality can have a significant effect
on the temperature vs. viscosity relationship,
gelling and freezing point temperatures, phase
separation, and presence of suspended solids
and foreign material; all of which could impact
the handling properties of the carbon source.
To further supplement this fact sheet, The
Water Environment Research Foundation has
published a detailed protocol to evaluate
alternative external carbon sources for
denitrification at full-scale wastewater
treatment plants that provides in-depth
information in evaluating carbon source (see
the reference section below for more details).
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Alcohols
Product Attribute
Safety / Fiarrimability
Price Volatility
Raw of D«nitrificaiKm
Viscosity / Handfmg
Fre€ttog Po^nt
Product Stability
Supply Availability
Quality Control
LOK
Large foody of technical
literature
M«to«
1
2
2
4
4
4
4
4
4
4
4
3
2
1
1 Ethsnoi
1
2
4
4
4
i 4
4
4
3
4
Very Good
Good
Fair
Pooif
Acetate
Acid"
2
2
4
4
4
4
4
4
1
4
Sodium
Acetate* *
4
2
4
3
1
4
4
4
1
4
Carbohydrates
torn
Sytup
4
3
3
I
2
2
4
4
1
2
Mola««
4
3
3
1
2
2
4
3
2
2
Sucrose
4
3
3
1
2
1
4
3
2
2
•
Co-
products
«v«™
4
2
3
2
3
3
2
1
3
2
«*»,*»,
EOSi Products
M«™CS M,
4
3
3
4
3
4 [
4
4
2
3
* * §0% solution &e
MCtlyc«m
4
3
4
4
4
4
3
4
3
3
**««««.
Table 2: External Carbon Sources and Evaluation of Some Common Attributes
REFERENCES
1. Chemical Safety Board, Bethune Point
Wastewater Treatment Plant, Explosion Report. 2007.
1. Cherchi, C., Onnis-Hayden, A., El-Shawabkeh, I.,
Gu, A.Z. Implication of Using Different Carbon
Sources for Denitrification in Wastewater Treatments.
Wat. Env. Res. Vol 81, No 8. 2009.
S.Eckenfelder, W. W & Musterman, J. L. Activated
Sludge Treatment of Industrial Wastewater. Technomic
Publishing, 1995.
4.Fabiyi, M. E., Ledwell, S., & Stoermer, E. A
Framework for Integrating Hedging Strategies in the
Evaluative Process for the Selection of Enhanced
Nutrient Removal Solutions: Factoring Volatility &
Safety. Pennsylvania Water Environment Association,
Penntec2010.
S.Kang, S. J., Olmstead, K. P., Takacs, K. M., Collins,
J., Wheeler, J., Zharaddine, P. Sustainability of Full-
Scale Nutrient Removal Technologies. WEF Nutrient
Conference, 2009.
6.Metcalf & Eddy, 4th ed. Wastewater Engineering:
Treatment and Reuse. Revised by Tchobanoglous,
G., Burton, F. L., Stensel, H. D. McGrawHill, 2003.
7. Protocol to Evaluate Alternative External Carbon
Sources for Denitrification at Full Scale Wastewater
Treatment Plants by April Z. Gu Annalisa Onnis-
Hayden Department of Civil and Environmental
Engineering Northeastern University, Water
Environment Research Foundation, 207 0.
Some of the information presented
in this fact sheet was provided by
the manufacturer or vendor and
could not be verified by the EPA.
The mention of trade names,
specific vendors, or products does
not represent an actual or presumed
endorsement, preference, or
acceptance by the EPA or federal
government.
Stated results, conclusions, usage,
or practices do not necessarily
represent the views or policies of
the EPA.
Environmental Protection Agency
Office of Wastewater Management
EPA 832-F-13-016
August 2013
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