?/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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