Wastewater Treatment
Using IFAS for improved WWTP capacity and
performance By Valera Saknenko, Vincent Nazareth, Roberson Gibb, Patrick Devlin and Krista Thomas
T
he Peterborough Wastewater
Treatment Plant (WWTP) in
southern Ontario is a Class
IV plant discharging to the
Otonabee River. As a result of municipal growth, average capacity had to
be increased from 60,000 m3/day to
68,200 m3/day, with a simultaneous improvement in effluent quality.
This plant re-rating was achieved by
full conversion of the existing aeration
tanks to a hybrid integrated fixed film
activated sludge (IFAS) media system.
Concurrent upgrades included improvements to the inlet works, construction
of four new primary clarifiers, a sludge
dewatering facility and a new septage
receiving station.
Since September 2011, it has been
operating as an IFAS plant, resulting in
a number of process benefits, including
lower average total ammonia nitrogen
(TAN), non-toxic effluent, and greater
resiliency of the nitrification process to
both organic and hydraulic shock loads.
Nitrogen transformation
To effectively evaluate the impacts of
the IFAS system, it is important to first
revisit some chemistry theory to appreciate the transformation process that occurs inside one.
The majority of nitrogen enters
wastewater in the form of urea and fecal
matter, and is then converted through
hydrolysis to TAN. TAN (or more correctly TAN-N, which measures only
the mass of nitrogen) is the sum of two
molecules: NH3 (ammonia or un-ionized ammonia) and NH4+ (ammonium
or ionized ammonia). In essentially all
solutions, including wastewater, both
the ionized (NH4) and un-ionized (NH3)
forms are present due to the principles
of acid dissociation.
The un-ionized form is the compound that is most toxic to fish. Fortunately, for aquatic species, at pH levels
in typical wastewater, NH4 is present in
much greater concentrations than NH3
(100:1 ratio at pH of 7.4). However, the
NH3 percentage rises with increasing
38 | January/February 2015
The Peterborough WWTP. (1) Raw sewage pumping station. (2) Grit tanks. (3)
Screen building. (4) Primary clarifiers. (5) Secondary treatment Plant One. (6)
Secondary treatment Plant Two. (7) UV disinfection. (8) Digesters.
pH and/or sewage temperature. This
means that, for identical TAN measurements, warmer effluent results in greater
toxicity due to the higher NH3 levels.
To limit un-ionized ammonia effluent concentration, TAN must be reduced
through a two-step bacteriological conversion process (known as nitrification)
during secondary treatment. Ammonia
is first converted to nitrite (N02) using
Nitrosomonas bacteria. Subsequently,
the nitrite is converted to nitrate using
Nitrobacter bacteria. The first process
(conversion to nitrite) is the rate limiting
process, meaning that nitrate is usually
present in much greater concentrations.
For aquatic species, the rate limiting reaction is critical, since nitrite is toxic at
significantly lower concentrations.
IFAS overview
An IFAS system is capable of improving the resiliency of conversion of
TAN to nitrate within a WWTP. The
IFAS system comprises fixed or floating media, placed within an activated
sludge tank to augment the number of
nitrifying bacteria. The IFAS system
takes a hybrid approach to nitrification.
It uses traditional mixed liquor suspended solids (MLSS) within the aeration
tank, as well as biofilm attached to plastic media, to promote the development
of additional nitrifying bacteria.
A primary advantage of IFAS, versus
a traditional activated sludge system, is
the ability to create a higher biomass in
the aeration tank by providing a surface
for bacteria growth to aid the process.
This additional biomass is retained in
the tanks, thereby maintaining acceptable solids loading to the secondary
clarifiers. A significant percentage of
the nitrifying bacteria are kept within
the aeration tank attached to the biofilm.
This contributes to the conversion of
ammonia to nitrate, without a proportional increase in MLSS concentrations.
The solids retention time (SRT),
and, therefore, aeration tank size, can
be reduced for a given level of nutrient
removal. This makes the IFAS technology attractive for the following retrofit
applications:
% Increasing plant capacity, while
maintaining existing effluent requirements;
% Meeting more stringent effluent criteria imposed on a WWTP within the
confines of the existing tankage;
% Providing biological nitrogen removal (denitrification) by partitioning an
existing tank to contain both an IFAS
aerobic zone and a separate anaerobic/anoxic zone.
The fixed biofilm also provides other
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Wastewater Treatment
inherent benefits to increase the resiliency of the nitrification process. Fixed media provides better nitrification in cold
climates, and the biomass population is
more resistant to both organic and hydraulic shock loads. Fixed biomass attached to free floating media provides a
seeding source to re-populate biomass
after a flow surge through secondary
treatment.
Preliminary and primary treatment
at the Peterborough WWTP consists of
mechanical screening, aerated grit tanks
and primary clarification. Secondary
treatment is provided through two parallel IFAS treatment trains and horizontal secondary clarifiers, referred to as
Plants One and Two. Prior to discharge
into the Otonabee River, secondary effluent is disinfected using UV irradiation. Unit process sizes (bioreactors and
secondary clarifiers) in the two plants
are marginally different. Primary and
waste activated sludge are stabilized in
two primary anaerobic digesters (2,444
m3 each) and two secondary digesters
(1,445 m3 each) and dewatered using
centrifuges.
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Table 1: Effluent objectives and limits.
Effluent Parameter
Total Phosphorus
Total Ammonia Nitrogen
Un-ionized Ammonia
Effluent Objective
0.35 mg/l
5 mg/l Winter
2.98 mg/l Summer
0.1 mg/l
Effluent Limits
0.39 mg/l
n/a
Acute Lethality to Rainbow
Trout and Daphnia Magna
Total Suspended Solids
Non-Acute Lethality
10 mg/l
17.5 mg/l
CBOD5
10 mg/l
17.5 mg/l
Discharge effluent requirements for
the Peterborough WWTP are provided in Table 1. TAN reduction is a prescribed effluent objective. There are no
total nitrogen or nitrate limits imposed
on the effluent, negating any denitrification requirements.
Each aeration tank was retrofitted
with a non-proprietary IFAS system.
One metal cage per plant was installed
with a fine perforated metal mesh elevated above the diffusers. This prevents
media from settling when the tank is
taken out of service. The cages were installed in the front half of the first pass
of the three-pass aeration tanks. Air
lances were added to assist with media
circulation.
Approximately 190 m3 of media was
added in each aeration tank. The plastic
media used was not engineered specifically for IFAS systems, but rather regular
packing media with total specific surface area of approximately 210 m2/m3.
Although this media may not be as effective as engineered media specifically
designed for this type of application, it
has proved to be sufficiently effective.
Operational results
To compare the impact of IFAS on
continued overleaf...
January/February 2015 | 39
Wastewater Treatment
Table 2. Maximum recorded daily peak TAN levels.
nitrification at the WWTP, monthly average TAN effluent concentrations were
compared for time periods when the
plant was operating with and without
IFAS. Data indicates that full implementation of the IFAS system at Plants
One and Two has had a significant impact on stabilization of the nitrification
process.
As can be seen from Figure 1, after installation of IFAS, fluctuations in
flows did not produce the TAN concentration exceedances observed prior
to the retrofit. For example, high flow
events in 2005, 2008 and 2011 prior to
IFAS resulted in significant wash out
of MLSS, including nitrifiers. This resulted in long recovery periods (about
six months) before TAN effluent concentration dropped below the effluent
objective. After IFAS implementation,
similar high flow events in March 2012
and April 2013 did not cause significant
effluent TAN spikes.
It is noted that a minor spike in TAN
occurred in the late summer of 2013,
corresponding to the time period when
one aeration tank had to be taken out of
service.
Other effluent indicators also validated performance of the IFAS system.
Cross referencing effluent un-ionized
ammonia concentrations showed a parallel decrease and stabilization of toxic
un-ionized ammonia. This is to be expected since un-ionized ammonia is a direct function of TAN concentration, pH
and temperature. Further validation was
completed by confirming the increase
in effluent nitrate levels after the IFAS
implementation. As per the nitrification
chemistry described earlier, nitrate levels
are expected to increase as nitrification
improves.
Seasonal aggregates of the average
TAN concentrations shown in Figure 1
substantiate the fact that IFAS provides
considerable advantages over a conventional suspended biomass process
during the coldest seasons. For the period of January through March, effluent
total ammonia levels were 30% of the
comparative period when the WWTP
was operating without IFAS. The impact of IFAS was less during the warmer
months, with effluent TAN levels being
70% of pre-IFAS conditions.
A review of the annual maximum
40 | January/February 2015
Date
Ammonia – N – Effluent
Concentration mg/l
Full IFAS Maximum Daily TAN Value
11-Sep-13
6.0 mg/l
04-Apr-12
5.4 mg/l
Non-IFAS Maximum Daily TAN Value
10-Feb-04
16.0 mg/L
15-Mar-05
17.5 mg/l
20-Feb-08
10.2 mg/l
Daily Flow Rate
52.7 MLD
48.5 MLD
50.2 MLD
49.0 MLD
42.9 MLD
Figure 1. Seasonal TAN effluent levels.
Elevated metal cage.
peak daily TAN effluent concentrations
gathered from daily samples shows a
similar ability of the IFAS system to attenuate peak TAN levels. As shown in
Table 2, the maximum recorded TAN
levels dropped by 60% after implementation of the full-scale IFAS.
Summary
Upgrading the Peterborough WWTP
with an IFAS system allowed maximum
use of the existing infrastructure and
increased efficiency by augmenting the
total amount of biomass in the system.
This approach eliminated the need to
build new aeration tanks and secondary
clarifiers, saving some $13 million in
construction costs.
Converting the secondary treatment
train to IFAS has lowered average TAN
to 1.25 mg/l (37% of original levels)
and resulted in a corresponding reduction in un-ionized ammonia. It achieves
its greatest efficiency during the coldest
winter months.
Operators have noted that the ammonia reduction in the “IFAS Zone” has
been approximately 80%. The food to
micro-organism ratio (F:M Ratio) has
changed from 0.4 to 0.28-0.30, leading to a decrease in return activated
sludge. Effluent total suspended solids
and carbonaceous biochemical oxygen
demand have continued to be below effluent limits. Long-term effluent results
demonstrate the potential for IFAS to be
successfully used in retrofits to increase
plant capacity, meet more stringent effluent criteria, or to increase rated capacity to maximize existing tankage
volume use within the secondary treatment train.
Valera Saknenko, P. Eng., Vincent
Nazareth, P. Eng., and Roberson Gibb
are with R.V. Anderson Associates Limited. Patrick Devlin, HBSc., and Krista
Thomas are with the City of Peterborough. For more information, Email:
[email protected]
Environmental Science & Engineering Magazine