Wastewater Technology Fact Sheet
 Side Stream Nutrient Removal
     United States
     Environmental Protection

A significant nutrient load can be  generated
internally by  publicly owned  treatment works
(POTWs). These nutrients can be found in the
reject water,  also known as  side  streams, of
many municipal wastewater and sewage sludge
treatment processes. These side streams include
the reject stream from membranes, supernatant
liquid   from   sludge   digesters,    and  the
centrate/filtrate  return  stream from  sewage
sludge dewatering processes,  among others.
Most of these side streams are conventionally
returned to the headworks of the POTW, where
they are  combined with  the  normal influent.
Estimates of  the nitrogen load from  this side
stream return  are between 15 and 30 percent of
the total nitrogen load on the process (Solley, D.

Recently,  there  has  been  research   into the
separate treatment  of these high-nutrient side
streams.  It is reported to be more efficient and
cost effective than the conventional method of
returning these side  streams  untreated  to the
headworks of the plant. The use of side stream
treatment is intended to decrease the loading on
the main nutrient removal process, resulting in
lower effluent nutrient concentrations (Vandaele
et al. 2000).

Several relatively  new  processes  have been
developed  to  remove   nitrogen   in  high-
concentration  side   streams  from   biosolids
processing—SHARON®,  ANAMMOX®,  and
InNitri® (Warakomski et al. 2006),  and BABE®
(STOWA  2006). Each procedure  has unique
characteristics that  remove nitrogen more cost
effectively. SHARON® incorporates a different
metabolic pathway than is usually implemented
in wastewater treatment. InNitri® and BABE®
use bioaugmentation  (the  seeding of specific
strains of organisms to achieve  desirable results)
in a  new way, and  ANAMMOX®  uses  an
                   entirely  new  group  of bacteria  to  oxidize
                   ammonia anaerobically.

                   SHARON PROCESS

                   1. Description and working principle
                   The SHARON (Single reactor for High activity
                   Ammonia Removal Over Nitrite) process takes
                   place in a completely mixed  reactor  without
                   biomass  retention.  It has been developed  to
                   treat high strength ammonia  side streams from
                   sludge digestion.   The conventional means  of
                   converting ammonia  to nitrogen  gas is  by
                   utilizing  the nitrification/denitrification  process.
                   In  the   SHARON®  process,   ammonia   is
                   converted directly  to nitrite (as opposed  to
                   nitrate in  conventional methods),  and then
                   directly to nitrogen gas (Solley, D. 2000).  The
                   conversion  from  ammonium  to  nitrite   is
                   described by the following formula:

                   NH4+ + 1.5 O2 ^> NO2" + H2O + 2 H+

                   (STOWA 2006)

                   The ammonia oxidation is stopped at the nitrite
                   step by operating the SHARON® process at an
                   elevated  temperature.  At higher temperatures,
                   the ammonia oxidizers grow  significantly faster
                   than  the nitrite-oxidizing   bacteria.  In  this
                   process the  hydraulic retention time (HRT)  is
                   equal to  solids retention time (SRT).  Therefore
                   the slow-growing nitrite oxidizers are  washed
                   out of the system and the ammonia oxidization
                   is stopped at nitrite.

                   This is an exothermic process which operates at
                   process temperatures between 30 and 40 degrees
                   Celcius (°C) (86 to 104 degrees Fahrenheit (°F).
                   Depending on the ammonia concentrations  in
                   the side  stream being treated and final  effluent
                   limitations, hydraulic retention times may be  in
                   the range of 1 to 2 days (Solley, D. 2000).

Temperatures  of  side   stream  water  from
digesters can be expected to be generally around
25 to 30 °C. The  exothermic  microbiological
activity  in the SHARON® reactor produces a
temperature  rise of approximately 5 to 8  °C.
Depending  on  the local climate,  additional
heating might be required only in wintertime.

2. Design guidelines/Technical data
Two wastewater treatment plants (WWTPs) in
the Netherlands utilizing the SHARON® process
have   been   studied   to   determine  their
performance and costs. The SHARON® process
takes  place in a single  reactor, therefore,  the
amount  of land required  can be much less than
expanding      conventional     nitrification/
denitrifi cation    systems.       The    design
specifications for the SHARON® process at the
two Dutch WWTPs (Utrecht and Dokhaven) is
presented below.

                  Table 1.
       Process design specifications
Tank volume
Design flow
Maximum flow
Design N-load
Maximum N-load
Influent NH4 cone.
Aerobic retention time
Anoxic retention time
The   Dokhaven  WWTP  operates  in  an
aerobic/anoxic  cycle.    The  Utrecht  WWTP
utilizes a two reactor continuous system (Solley,
D. 2006).  For the Utrecht facility, the length of
one aerated  period can be dependent  on inlet
flow and pH set points. During aerobic periods,
the pH will decrease; during anoxic periods, the
pH will increase. This results in a discernible
pattern.    The   fluctuations  in  pH  can  be
addressed with process controls.

3. Performance
Performance at  WWTP Dokhaven (Rotterdam,
The Netherlands).

The SHARON® process was first introduced at
the Dokhaven  WWTP in  1997.   As shown
below, the  ammonium  concentration  in  the
effluent  from  has  continued  to  decrease
(STOWA 2006).
                  Table 2.
 Average Ammonium effluent concentration
            - Dokhaven WWTP
Average NH4+ effluent.
cone, [mg/l]

Removal efficiencies of ammonia during this
time period have  averaged  in  the  70 to 90
percent range, with the process becoming more
efficient in more recent years.


1. Description and working principle
The    ANaerobic   AMMonium    Oxidation
(ANAMMOX®) process is  a variation on the
SHARON® process described previously. In the
ANAMMOX® process, ammonium is converted
to dinitrogen gas (TSk). The combination of the
SHARON®  process  with  the  ANAMMOX®
provides an efficient and cost effective means to
treat nutrient rich side streams (STOWA 2006).

Treatment step 1: SHARON® process.

The SHARON® process is used to produce an
ammonium-nitrite mixture.  For the SHARON®/
ANAMMOX® process however, the  goal  is to
convert  only  50  percent of the  ammonium  to
nitrite  so the difference  in the SHARON®
portion of this process is the conversion of only
50 percent  of the ammonium.  To ensure that
only 50 percent of the ammonium is converted
to nitrite, the oxygen supply is limited.

Treatment step 2: ANAMMO^1 process.

In this  treatment  step, the  ammonium-nitrite
mixture produced in the SHARON® reactor is
converted to nitrogen gas.  Ammonium is  used
as an electron donor under anoxic conditions.
The  conversion  of ammonium  and nitrite  to
nitrogen  gas is  described  by  the  following
formula (STOWA 2006):

NO2" + NH4 => N2 + 2 H20

The  bacteria  involved  in the  reaction  are
autotrophic  do not  need  the  addition of  an
external carbon source.

Since the ANAMMOX® bacteria have  a  slow
growth  rate  (doubling  in  10  days at 30°C)
(STOWA 2006),  sufficient volume  must  be
available to prevent wash  out  of the bacteria.
Sequential  batch  reactors have been  used for
this purpose in pilot tests (STOWA 2006).

The  treatment  sequence  of  the  combined
SHARON®/ANAMMOX®  process  is depicted
in Figure 1.
Figure 1.  SHARON/ANAMMOX process

2. Design and Application
When sludge thickening and dewatering occur
before digestion, higher ammonia concentration
and lower overflow rates result.

WWTP  Dokhaven  (Rotterdam, The Netherlands):
In 2002, the Dokhaven  WWTP's  SHARON®
reactor  was combined with an  ANAMMOX®
system.   The installation  began operating in
June 2002 (STOWA 2006).

3. Performance
It is estimated  that the combined  SHARON®/
ANAMMOX® process can achieve an  overall
nitrogen removal rate of 90 to 95  percent. Since
the  process  does not  need an external organic
source  and  operates  under  low  oxygen
concentrations, cost savings can be realized over
the    traditional    nitrification/denitrification
systems (Solley, D. 2000).
1. Description and working principle
InNitri®  (Inexpensive Nitrification) is a new,
side stream  nitrification  process offered by
Mixing & Mass Transfer  (M2T) Technologies,
Inc. It  allows nitrification  at short SRT values,
even at low winter temperatures and provides
nitrification in  a  substantially smaller aeration
tank   than   is  required  for  conventional
nitrification design. The InNitri® process was
developed to provide an inexpensive alternative
for  plants  in  northern  climates  that need  to
upgrade  their  air  or  pure  oxygen  activated
sludge  process for year-round nitrification  or
nitrogen removal (M2T 2002).

In  general,  the InNitri®  process consists  of
supplemental nitrifiers being added constantly
to the main activated sludge process to replenish
nitrifiers removed  with the wasted  activated
sludge. The supplemental nitrifiers are grown in
a separate,  small,  side-stream aeration  tank
using either ammonia available in the digested
sludge  dewatering  liquid  and  in the digester
supernatant or commercial  ammonia.

A conventional secondary  treatment plant might
consist of  primary  sedimentation, an aeration
tank,   secondary   clarification,  and  sludge
thickening, followed by anaerobic digestion and
sludge  dewatering.  Upgrading  such a plant to
provide  year-round  nitrification  using   the
InNitri®  (short Solids Retention Time  (SRT)
nitrification) process requires the addition of a
small  aeration  tank and  clarifier for growing
nitrifiers. In the process, the warm (typically 30
to 35 °C) dewatering liquid containing  a  high
ammonia content (between 300 and 900 mg/L)
is mixed with a  small  portion of primary
effluent (to adjust the temperature and provide
organic matter), and it  is nitrified in the side
stream  nitrification  aeration tank. A portion of
the  resulting biological sludge—containing  a
high percentage of nitrifiers—is discharged into
the  main aeration tank  and provides  the main
activated  sludge process with  supplemental
nitrifiers. This results in the plant being able to
provide year-round nitrification.

The same process can also be applied in plants
without  digesters.  In  this  case, commercial

ammonia is  used  instead  of the  dewatering
return stream.

The treatment sequence of the InNitri® system is
schematically depicted in Figure2.
Figure 2.  InNitri® system

2. Design Guidelines/Technical Data
The conventional nitrification process  typically
consists of a complete mix-steady-state aeration
tank  with  6  hr.  hydraulic  detention  time,
operating at 10 °C, and receiving an influent
containing 25 mg/L of TKN (with 25 percent of
the   TKN   contained   in   the  side-stream
dewatering   liquid).    To   demonstrate   the
difference between the conventional nitrification
process  and  nitrification  with supplemental
nitrifiers   (InNitri    process),    KOS    (1998)
presented theoretical  equations and results  of
modeling for  a typical WWTP. Mathematical
modeling results showed that, for conventional
nitrification at  10 °C, as the operating SRT is
decreased, the  concentration  of nitrifiers also
decreased,  while   ammonia  nitrogen  in  the
effluent  increased.  For the InNitri® approach,
results indicate that nitrifiers are present in the
main aeration tank at all SRT values.

Nitrifiers cannot  be washed  out from  the
aeration  tank  even if operated  at lower  SRT
values; therefore, partial nitrification takes place
even at  extremely  low  SRT values.  In other
words, the  InNitri®  process does  not have  a
minimum SRT under which nitrification would
not  occur.  Therefore, it  will  be  much more
stable  and may not  require as  high  a safety
factor   as   conventional   nitrification.   The
modeling was repeated at temperatures from 7.5
to 20 °C. Results  showed the InNitri® process
allowed for significantly lower design SRT than
the conventional  nitrification  to achieve  the
same  effluent ammonia concentration. Where
the design effluent  ammonia concentration is
2.0 mg/1, the minimum required design SRT for
the InNitri® process  is about 60 percent of that
required   for  conventional  nitrification.   A
comparison   of   the   SRT  necessary   for
nitrification using  InNitri® versus conventional
nitrification  showed  significant  reductions  in
costs for low temperature wastewaters using the
InNitri® process.

Research  by  the  University   of  Manitoba
indicated that the transport  of nitrifying sludge
from  a warm side  stream  reactor to a  cold
mainstream  reactor   should pose  no  process
problems. Also, the evaluation of the process to
upgrade  an existing  facility showed significant
cost savings using the  InNitri®  system versus
using   conventional    and   other   advanced
nitrification processes.

Brinjac,   Kambic,   and  Associates  (2000)
completed  a feasibility analysis  for upgrading
the Harrisburg  City  Advanced  Wastewater
Treatment  Facility (Harrisburg,  Pennsylvania)
for nutrient control. This facility was considered
to be  typical of many of the plants designed to
meet  the effluent  requirements  of the federal
Clean Water Act.  The  facility is in the colder
climate in  northeastern United States  and is a
principal point source contributor of nitrogen to
the Susquehanna River.  The river flows to the
Chesapeake Bay where efforts are underway to
improve  water quality by reducing the nutrient
load to the bay.  The  facility is site-constrained
with little room  for flow or process expansion.
Due to  the  results  of the feasibility study,
Brinjac,  et al. recommended that the facility
implement the InNitri® process. (As of this date,
the city has not yet initiated the project.)

3. Performance
Currently,  there  are  no  full-scale  InNitri®
installations.   Capital   and  operations   and
maintenance  (O&M)  costs for the system  vary

by the type and size of facility. For site-specific
unit   design   and   costs,   contacting   the
manufacturer directly is recommended.

1. Description and working principle
The   BioAugmentation   Batch   Enhanced
(BABE®) process is comprised of a single batch
reactor.   Side  stream waters high in ammonia
content and return activated sludge (RAS) from
the  main  biological  treatment process  are
combined  (STOWA  2006)  with  previously
settled sludge in the batch reactor. The RAS is
used to  augment  the bacteria  in  the  settled
sludge.  By utilizing a batch reactor, the long
residence  times necessary to grow both  the
nitrifying and denitrifying bacteria are possible.
There are five phases to the BABE® process:  1)
filling, 2) mixing and aeration,  3)  mixing,  4)
settling,  and 5) settling and  decant (STOWA

The  first  two  steps are done  under aerobic
conditions. The third involves mixing without
aeration to achieve anoxic  conditions.   This
condition is conducive to denitrification.  Steps
four and five complete the process.

2. Design and Performance
As with  the  SHARON® process, testing  has
shown that higher concentrations of ammonia in
the  influent   to   the  BABE®  process  are
preferable  (STOWA  2006).    The BABE®
process operates at temperatures between 20 and
25  degrees  C, which  is   lower  than  the
SHARON® process.

If the process temperature in the BABE® reactor
is less than 20°C, the  reactor  volume must
increase dramatically.   However, temperatures
greater than the normal  operating temperature
range have minimal impact on the process.

The  SHARON® process has  been shown to be
able  to tolerate  suspended solids in the influent
to the process.  However, pH control is vital to
the proper operation of the process and robust
process controls must be in place to respond
quickly to changes in process temperatures and

The ANAMMOX® process can respond well to
biomass that is washed out of the  SHARON®
process in a combined system. Due to the slow
growth of the denitrifying bacteria, long start-up
times are required. Again, pH and temperature
control are imperative for proper operation of
the system.

The  InNitri® System's advantages are a  low
capital cost and small  footprint at a facility. It
also appears that process control  is  not as vital
(M2T 2002) when compared to other side stream
nitrification processes.

The  SHARON®  process has been compared
with other techniques for nitrogen removal from
reject water and was  found to be the  least
expensive under Dutch circumstances. A cost
estimate of 1.5 Euro/kg Nremoved (approximately
$2/kg Nremoved) was given.

The   investment   costs  for  a  SHARON®/
ANAMMOX® installation with  a capacity of
1.200 kg NH/t-N/day are estimated at 2 million
Euros (approximately 2.75 million U.S. dollars).
The operating costs are linked to the costs for
energy,   methanol,  and  caustic   chemicals
(STOWA 2006).

Actual costs for  the InNitri®  and  BABE®
processes are not available; there is no full scale
implementation  of these systems.

Plug-flow, activated-sludge with
denitrification filters
The   Central  Johnston  County Wastewater
Treatment Plant in Smithfield, North Carolina
achieves  biological phosphorus  removal  and
nitrogen  removal  in  a plug-flow,  activated-
sludge process and separate-stage denitrification

The plug-flow, activated-sludge process utilizes
anoxic and aerobic basins in  series. This  was a
retrofit   design   implemented   by  facility

The  two-stage biological  processes  in  series
offer  high efficiency  in nutrient  removal  at
minimal  costs. The source  of wastewater  is
typical residential customers in the  suburb  of a
large,  metropolitan  area.  The  BOD  to  total
phosphorus  (TP)  ratio  averages  55:1.   The
retrofitted, activated-sludge process consists  of
an anoxic stage with a 4.8-hour residence time,
followed by an aerobic stage in two tanks with a
residence time of  11.5  hours.  The  operating
strategy  developed  at this facility is unique
because the sludge blanket at the clarifiers is 3-
4 feet deep,  and  the  return  activated sludge
(RAS) flow  rate is maintained at a  low (10-25
percent)  portion of the plant flow. The second-
stage  denitrification filters then  remove  the
remaining nitrogen with a methanol feed.

The design and operation result in a high level
of removals with an effluent TN concentration
of 2.14 mg/L  and  an effluent TP concentration

The costs of removal  were very low  for both
capital and O&M.  The  life-cycle cost for TP
removal was $2.21/lb of  TP removed, while the
life-cycle cost for  TN  removal  was $0.98/lb  of
TN removed,  including the cost for methanol.
The  capital  cost  for  the  flow  capacity  was
$0.58/gallon per day  (gpd) capacity.

A/O Process with Alum  Feed
The Clark County Water Reclamation Facility
(WRF) is in Las  Vegas, Nevada. Three major
factors   contribute  to  reliable   phosphorus
removal  and  nitrification  at this facility:  (1)
multiple  chemical  feeding,  (2)  good biological
phosphorus removal  with in-plant volatile fatty
acid (VFA) generation  and full nitrification, and
(3)  good tertiary  filters for suspended  solids
removal.  This   combination   of  chemical,
biological, and physical  processes  in  series  is
effective  in   providing  exceptionally   low
phosphorus (0.09  mg/L)  and ammonia-nitrogen
(0.05 mg/L) concentrations.
The  capital cost for  phosphorus removal  and
complete   nitrification  is  estimated  to   be
$2.01/gpd.  The unit costs for capital and O&M
were  $5.43/lb for phosphorus and  $1.38/lb of
nitrogen removed. The unit costs for O&M were
$1.84/lb of phosphorus removed  and $0.51/lb of
nitrogen removed.

The  Clark County plant operation  has been
successful in reducing effluent phosphorus  to a
level  that is  considered to  be  the  state-of-the
science at the existing plant using a combination
of biological and chemical treatment processes
in series with good  reliability.  The  plant is
almost at capacity and yet has produced effluent
far below the discharge limits. The technique of
using several different technologies in series can
achieve the treatment  objective especially when
the operation is  computer  controlled and the
system has been designed  with a  reasonable
amount of redundancies to allow for repairs and
routine  maintenance.  Operational   costs  are
reasonable, with life-cycle costs of $5.24/lb and
$0.98/lb for phosphorus and nitrogen removal,

In  addition  to  the  technologies   mentioned
above,  there  are   other   treatment   options
available.  For their  applicability in  removing
nutrients  at  wastewater  treatment  facilities,

1.  Bioaugmentation with recycle treatment

-  MAUREEN (Mainstream Autotrophic
   Recycle Enabling Enhanced N-removal)

2.  Fixed-film nitrification and
   de-ammonification processes
-  OLAND (Oxygen Limited Aerobic

-  CANON (Completely Autotrophic Nitrogen
   Removal Over Nitrite) (Stensel 2006)

1.  The treating of recycle side streams can
   provide more stable and effective

2. Recycle side stream treatment can increase
  nitrification capacity of existing systems
  (BABE®, InNitri®, and MAUREEN).

3. Recycle side stream treatment can help
  reduce carbon demand for nitrogen removal
  CANON) (Stensel 2006).
OLAND, and
                        Impe. 2000. Ammonia removal from
                        centrate of anaerobically digested sludge:
                        State of the art biological methods.
                        1st World Water Congress of the IWA
The authors  of this report are grateful for the
information provided  by the  Central Johnston
County Wastewater Treatment Plant-Smithfield,
North Carolina  and  the Clark County Water
Reclamation Facility - Clark County, Nevada.

M2T (Mixing & Mass Transfer Technologies,
    Inc.) 2002. The InNitri® Nitrification
    System: An Innovative Wastewater
    Treatment Process.

Solley, D. 2000.  Upgrading of Large
    Wastewater Treatment Plant for Nutrient
    Removal. Churchill Fellowship 2000
    Report. The Winston Churchill Memorial
    Trust of Australia.

Stensel, H.D. 2006. Sidestream Treatment for
    Nitrogen Removal.

STOWA (Dutch Foundation for Applied Water
    Research). 2006. SHARON and
    SHARON/ANAMOXProcess Sheets.

Warakomski, A., R. van Kempen, and P. Kos.
    2006. Microbiology/Biochemistry of the
    Nitrogen Cycle Innovative Process
    Applications. International Water
    Association Proceedings, Moncton, New
    Brunswick, Canada.

Vandaele, S., F. Bollen, C. Thoeye, E.
    November, H. Verachtert, and J.F. van
                             United States
                             Environmental Protection
                             EPA 832-F-07-017
                             Office of Water
                             September 2007