Trickling Filter and Trickling FilterSuspended Growth Process Design and
Operation: A State-of-the-Art Review
2
Glen T. Daiggerl*, Joshua P. Boltz
ABSTRACT: The modem trickling filter typically includes the following major components: (1) rotary distributors with speed control; (2)
modular plastic media (typically cross-flow media unless the bioreactor is
treating high-strength wastewater, which warrants the use of vertical-flow
media); (3) a mechanical aeration system (that consists of air distribution
piping and low-pressure fans); (4) influent/recirculation pump station; and
(5) covers that aid in the uniform distribution of air and foul air
containment (for odor control). Covers may be equipped with sprinklers
that can spray in-plant washwater to cool the media during emergency shut
down periods. Trickling filter mechanics are poorly understood. Consequently, there is a general lack of mechanistic mathematical models and
design approaches, and the design and operation of trickling filter and
trickling filter/suspended growth (TF/SG) processes is empirical. Some
empirical trickling filter design criteria are described in this paper.
Benefits inherent to the trickling filter process (when compared with
activated sludge processes) include operational simplicity, resistance to
toxic and shock loads, and low energy requirements. However, trickling
filters are susceptible to nuisance conditions that are primarily caused by
macro fauna. Process mechanical components dedicated to minimizing the
accumulation of macro fauna such as filter flies, worms, and snail (shells)
are now standard. Unfortunately, information on the selection and design
of these process components is fragmented and has been poorly
documented. The trickling filter/solids contact process is the most
common TF/SG process. This paper summarizes state-of-the art design
and operational practice for the modem trickling filter. Water Environ.
Res., 83, 388 (2011).
KEYWORDS: trickling filter, trickling filter/suspended growth, trickling filter/solids contact, biofilm, nitrification, design, operation.
doi: 10.2175/106143010X12681059117210
Introduction
Until the 1950s, trickling filter design protocol was scattered
and empirical in nature. Then, during the 1950s and 1960s, the
Dow Chemical Company began experimentation with modular
synthetic media (Bryan, 1955). Numerous trickling filter process
studies were conducted during the same period (Eckenfelder,
1961; Galler and Gotaas, 1964; Germain, 1966; Schulze, 1960),
which led to the development of generally accepted design
criteria. After the U.S. Environmental Protection Agency issued
its definition of secondary treatment standards in the early 1970s,
the trickling filter process was regarded as being unable to
1, CH2M HILL, 9191 South Jamaica Street, Englewood, CO 80112; e-mail:
Glen.Daigger@CH2M.com.
2 CH2M HILL, Tampa, Florida.
388
consistently produce effluent water quality that met the published
standards, in part, because of poor secondary sedimentation tank
design (Parker, 1999). Norris et al. (1982) developed the trickling
filter-solids contact (TF/SC) process in response. The first fullscale TF/SC process included a rock-media trickling filter
followed by a small aeration basin (receiving return sludge) and
flocculator clarifier. The researchers demonstrated that wastewater treatment plant (WWTP) effluent water quality could be
greatly improved by bioflocculation in the solids contact basin and
improved secondary clarifier design. Combined trickling filtersuspended growth (TF/SG) processes preceding the TF/SC
process were designed with the suspended growth reactor
primarily for oxidation. This paper describes state-of-the art
trickling filter and TF/SG process design and operation.
General Description
A trickling filter is a three-phase system with fixed biofilm
carriers. Wastewater enters the bioreactor through a distribution
system, trickles downward over the biofilm surface, and air moves
upward or downward in the third phase. Trickling filter
components typically include a distribution system, containment
structure, rock or plastic media, underdrain, and ventilation
system. Figure 1 illustrates a trickling filter cross section and
typical bioreactor components. Wastewater treatment using the
trickling filter results in a net production of total suspended solids.
Therefore, liquid-solids separation is required, and is typically
achieved with circular or rectangular secondary clarifiers. The
trickling filter process typically includes an influent pump station,
trickling filter, trickling filter recirculation pump station, and
liquid-solids separation unit.
Distribution System. Primary effluent (or screened, 3-mm,
and degritted wastewater) is either pumped or flows by gravity to
a trickling filter distribution system. The distribution system
intermittently distributes wastewater over the trickling filter
biofilm carriers. The distributors may be hydraulically or
electrically driven. The intermittent application allows for resting
periods during which aeration occurs. Efficient influent wastewater distribution results in proper media wetting. Poor media
wetting may lead to dry media pockets, ineffective treatment
zones, and odor. Essentially, there are two types of distribution
systems: fixed-nozzle and rotary distributors. Because their
efficiency is poor, distribution with fixed nozzles should not be
used (Harrison and Timpany, 1988).
Hydraulically driven rotary distributors use back-spray orifices,
or reverse thrusting jets, to slow rotational speed and maintain the
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
Rotary
distributor
/
Underdrain
FRP
Grating
AirpipeEffluent
N Influent
Figure 1-Typical trickling filter cross section and bioreactor components.
desired instantaneous flushing rate to the trickling filter. Figure 2
depicts both a modem hydraulically driven rotary distributor that
uses gates (controlled by variable frequency drive) that either
open or close distributor orifices to adjust rotational speed and an
electrically driven rotary distributor. Use of a variable-speed drive
and electronic controller allow for the more precise conrol of
distributor-arm speed. Electrically driven rotary distributors have
motorized units that control distributor speed independent of the
wastewater pumped flow.
Biofirm Carriers. Ideal trickling filter biofilm carriers, or
media, provide a high specific surface area, low cost, high durability,
and high enough porosity to avoid clogging and promote ventilation
(Tchobanoglous et al., 2003). Trickling filter biofilm carriers include
rock, random (synthetic), vertical-flow (synthetic), and cross-flow
(synthetic). Both vertical-flow and cross-flow media are constructed
with smooth and/or corrugated plastic sheets. Another commercially
available synthetic media, although not commonly used, are
vertically hanging plastic strips. Horizontal redwood or treated
wooden slats have also been used, but are typically no longer
considered because of their high cost or limited supply.
Modules of plastic sheets (i.e., self-supporting vertical-flow or
cross-flow modules) are used almost exclusively for new and
Figure 2-Hydraulically propelled (left) and electrically driven rotary distributor (right).
May 2011
389
Daigger and Boltz
Table 1-Properties of some trickling filter media.
Media Type
Material
Nominal Size
m
()
Bulk
Density
kghr 3
Spedfic Surface
Area
(m2/m3)
Void
Space
(%)
(lbs/) )b
Rock
1442
(90)
0.024 - 0.076
River
(0.08 -0.25)
Slag
62
(19)
50
0.076 -0.128
(0.25 - 0.42)
1600 (100)
0.61 x 0.61 x 1.22
(2 x 2 x 4)
24-45
(1.5 -2.8)
100 and 223
(30, 48, and 68)
95
0.61 x 0.61 x 1.22
(2 x 2 x 4)
24-45
(1.5 -2.8)
102 and 131
(31 and 40)
95
0.185 o x 0.051 H
(7.3" o x 2" H)
27
(1.7)
98
(30)
95
46
(14)
60
Plastic'
Cross flow
PVC
Vertical flow
PVC
Random2
polypropylene
Notes:
1 Manufacturers of modular plastic media: BF Goodrich (formerly), American Surf-Pac, NSW, Munters, Brentwood Industries (currently),
Jaeger Environmental, and SPX Cooling.
2 Manufacturers of random plastic media: NSW Corp. (formerly) and Jaeger Environmental (currently).
a ibs/ft 3 x 16.02 = kg/m 3 .
2
3
b f92/ft3 X 3.281 = m /M .
improved trickling filters. However, several trickling filters with
rock media exist and are capable of meeting treatment objectives
when properly designed and operated. Table I compares the
characteristics of various biofilm carrier types. The higher specific
surface area and void space in modular synthetic media allow for
higher hydraulic loading, enhanced oxygen transfer, and biofilm
thickness control in comparison to rock media.
390
Ideally, rock media have a 50-mm diameter, although they may
range in size. Rounded (river) rock helps mitigate issues
associated with rigid rock (slag) media. The slag rock contains
crevices that can retain water and accumulate biomass. Because of
structural requirements associated with the large unit weight of the
rock media, rock media are shallow in comparison to synthetic
media trickling filters and are more susceptible to excessive
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
cooling. Trickling filter performance aside, excessive cooling can
subject media to freeze-thaw cycles. Water retained inside slag
rock crevices may expand and sever rock fragments. This can
result in fine material accumulation which, together with retained
biomass, is a primary contributor to rock-media trickling filter
clogging (Grady et al., 1999). Generally, rock media are
considered to have a low specific surface area, void space,
shallow depth, and high unit weight. Although recirculation is
common, the low void ratio in rock-media trickling filters results
in reduced hydraulic application rates.
Excessive hydraulic application can result in ponding, limited
oxygen. transfer, and poor bioreactor performance. The performance of existing rock-media trickling filters can be improved by
providing forced ventilation, distributor speed control, solids
contact channels, and/or deepened secondary clarifiers that
include energy dissipating inlets and flocculator-type feed wells.
Replacement or deepening of the rock media (with synthetic
media) is often requisite in instances where the rock media quality
is poor, space is limited, and WWTP expansion (using a trickling
filter or TF/SG process) is expected. However, a well-designed
and operated rock-media trickling filter can provide high-quality
effluent. Grady et al. (1999) suggest that for low organic loads
(less than 1 kg 5-day biochemical oxygen demand [BOD 5]/dim 3 ),
well-designed and operated rock-media trickling filters are
capable of providing performance approaching that of synthetic*mediatrickling filters. However, as organic load increases, there is
likely to be fewer nuisance problems and reduced potential for
plugging with the use of synthetic biofilm carriers.
Synthetic biofilm carriers (for trickling filters) are generally
considered to have a high specific surface area and void space and
low unit weight. Due to the reduced unit weight, synthetic media
trickling filters can be constructed at depths in excess of 3 times
that for a comparably sized rock-media trickling filter. Modular
plastic trickling filter media are typically manufactured with the
following specific surface areas: 223 m2/m 3 (68 ft 2/ft 3) as high
2 3
2 3
2 3
density, 138 m /m (42 ft /ft ) as medium density, and 100 m /m
2 3
(30 ft /ft ) as low density. Both vertical flow and cross-flow media
are reported to effectively remove BOD 5 and total suspended
solids (TSS) (Aryan and Johnson, 1987; Harrison and Daigger,
1987). Cross-flow modules provide increased treatment efficiency
compared to vertical-flow modules of the same specific surface
area at low-to-medium volumetric organic loading rates (less than
about 2.5 kg BOD 5/d/m 3), but vertical-flow modules may provide
advantages at higher volumetric organic loading rates (Harrison
and Daigger, 1987). The effects of media type and configuration
on trickling filter effluent water quality should be'given careful
consideration by the designer.
Plastic modules with a specific surface area in the range of 89 to
102 m2 /m3 are well suited for carbon oxidation and,combined
carbon oxidation and nitrification. Parker et al. (1989) recommended medium-density cross-flow media, and recommended
against the use of high-density cross-flow media in nitrifying
trickling filters (NTFs). This argument is supported by pilot
application data and conclusions of Gujer and Boiler (1983, 1984)
and Boller and Gujer (1986), which show higher nitrification rates
for lower density modular synthetic media. The researchers claim
that lower rates occur with high-density media due to the
development of dry spots below the flow interruption points
(i.e., higher density media having more interruptions and,
therefore, less effective wetting). Using medium-density media
May 2011
also reduces the potential for plugging. These recommendations
were developed before the more widespread use of speedcontrolled rotary distributors, which may help to overcome these
hydraulic distribution issues. Vertically oriented modular plastic
media are generally accepted as being ideally suited for highstrength wastewater (perhaps industrial) or high organic loadings
such as with a roughing trickling filter. In some instances, crossflow media have been placed in the top layer of a trickling filter
containing vertical-flow media to enhance wastewater distribution, with vertical-flow media comprising the remainder of the
trickling filter media.
Containment Structure. Rock and random plastic media are
not self-supporting and, ,therefore, require support from the
containment structure. Typically, containment structures are
precast or formed concrete tanks. When self-supporting media
such as plastic modules are used, materials such as wood,
fiberglass, and welded and bolted (coated) steel have also been
used as containment structures. The containment structure serves
to avoid wastewater splashing and to provide media support, wind
protection, and, sometimes, flood containment.
Underdrain System and Ventilation. The trickling filter
underdrain system is designed to meet two objectives: collect
treated wastewater for conveyance to downstream unit processes
and create a plenum that allows for the transfer of air throughout the
trickling filter media (Grady et al., 1999). Clay or concrete
underdrain blocks are commonly used for rock-media trickling
filters because of the required structural support. A variety of
,support systems including concrete piers and reinforced fiberglass
grating are applied for other media types. Figure 3 depicts fieldadjustable plastic stanchions and fiberglass-reinforced plastic
grating to support modular plastic media on the concrete floor of a
trickling filter containment structure and high-density polyethylene
mats used to support random synthetic media. The volume between
the concrete slab and media bottom creates the underdrain.
Trickling Filter Pump Stations: Influent and Recirculation. A critical unit in the trickling filter system is a pump station
that lifts primary effluent and recirculates trickling filter underflow.
Generally, trickling filter underflow should be recirculated at a rate
required to achieve the hydraulic load (influent plus recirculation)
required for proper media wetting and biofilm thickness control
(note that distributor speed control may be required if the hydraulic
load is insufficient to provide the recommended dosing rate). The
intent of recirculating bioreactor effluent is to decouple hydraulic
and organic loading. Although effluent from the secondary clarifier
can be recirculated, this is not common practice because it may
lead to hydraulic overloading of secondary clarifiers. Influent
pumping is typically selected to allow trickling filter underflow to
flow by gravity to the suspended growth reactor (or solids contact
basin), secondary clarifier, or another unit downstream of the
trickling filter.
Trickling filter recirculation pumps are typically constantspeed, low-head centrifugal units designed to operate with a total
head equivalent to the static head, comprised of the trickling filter
media depth of approximately 3 to 7 m (depending on media
depth), the distance between the distributor outlet and the top of
the media, and the distance between the bottom of the media and
the water surface in the underdrain, along with associated friction
losses (Boltz et al., 2009). Variable frequency drive controlled
motors are typical fixtures on process pumps. Submerged or
nonsubmerged (dry-pit) vertical pumps have been used exten391
Daigger and Boltz
------
...........
*'0
P4
Pip,
W WS
*W_
4W
* * .0
'4W 4W 410
6 g.NrWrQ:&
4*
1W
'0'ý
*
4%.Wo.
,j:W4:,O P
-b
A.
4W
V .4 P,
Z
-1b
bd.
4k. 41k. Ow 4ý-
bdb I
r
Figure 3-Adjustable plastic stanchions and fiberglass-reinforced plastic grating on the concrete floor of a bolted
steel containment structure (left), and a high-density polyethylene mat used to support random synthetic
media (right).
sively. Pump intake screens are usually unnecessary because
recirculated flow is typically free of clogging solid materials.
Hydraulic computations are always necessary. Computations for
minimum flow are necessary to ensure adequate head to drive
hydraulically driven distributors; computations for maximum flow
indicate the head required to ensure adequate discharge capacity.
The net available head at the horizontal center line of the
distributor's arm and other points may be calculated by deducting
the following applicable losses from the available static head:
entrance loss, friction losses in the piping to the distributor, proper
allowance for minor head losses, head loss through distributor riser
and center port, friction loss in distributor arms, and velocity head of
discharge through nozzles necessary to start the hydraulically driven
rotary distributor. Trickling filter distribution head requirements are
set by the system manufacturer. Despite head loss due to the
trickling filter commonly being the greatest in a given WWTP,
power requirements for the process (including recirculation
pumping and auxiliary powered equipment) are typically significantly less than those for the activated sludge process.
Process Flow Sheets and Bloreactor Configuration
Trickling filter and combined TF/SG processes typically consist
of preliminary treatment (including screening and grit removal),
primary clarification, trickling filter, bioreactor, secondary
clarification, and disinfection unit processes. Trickling filter
recirculation methods influence the process flow. Generally, there
are two types of trickling filter recirculation. The first allows for
direct recirculation to the trickling filter and the second passes
flow through a primary clarifier. Four trickling filter process flow
diagrams, including both single- and two-stage trickling filters, are
shown in Figure 4. Combined TF/SG process flow sheets are
similar, but include a suspended growth reactor and return
activated sludge (or return sludge for the TF/SC process) stream
that is directed to the head of the suspended growth reactor.
Recirculation of trickling filter underflow or settled effluent
dilutes influent wastewater, dampens the influent organic loading
variability resulting from diurnal fluctuations, and maintains
required trickling filter hydraulic application rates. Clarifying
trickling filter effluent may enhance the performance of a
subsequent trickling filter in two-stage operation, but the designer
392
must ensure that the recirculation flow required for trickling filter
wetting and biofilm thickness control does not exceed the limiting
hydraulic loading rate for the intermediate clarifier. The design of
settling tanks in two-stage trickling filter systems is also affected
by the recirculation pattern.
Sludge wasting and recirculation streams affect the trickling.
filter process. Each of the process flow diagrams illustrated in
Figure 4 directs waste biological sludge (which is sometimes
referred to as humus in the trickling filter process) to the primary
clarifiers where it is co-settled with primary sludge prior to being
withdrawn from the system. Many facilities exist that withdraw
and thicken primary and biological sludge separately.
Bioreactor Classification. Trickling filters can be classified
as roughing, carbon oxidation, carbon oxidation and nitrification,
and tertiary nitrification. Table 2 summarizes characteristics of
each trickling filter. The performance ranges are associated with
average design condition. Single-day or average-week observations may be significantly greater.
Hydraulics. Recirculation and distributor operation are
important to good trickling filter performance and may be used
to achieve proper media wetting, flow distribution, biofilm
thickness control, and to prevent macro fauna accumulation.
Albertson and Eckenfelder (1984) postulated that the active
biofilm surface area in a trickling filter is dependent on biofilm
thickness and media configuration, and that active biofilm surface
area decreases with increasing biofilm thickness. The researchers
2 3
stated that for medium-density cross-flow media with a 98-m /m
specific surface area, a 4-mm increase in biofilm thickness would
cause a 12% reduction of active biofilm area (assuming that all the
media have been appropriately wetted). Poor trickling filter media
wetting results in reduced effluent water quality. In a study of
rotary distributor efficiency, Crine et al. (1990) found that the
wetted area-to-specific-surface-area ratio ranged from 0.2 to 0.6
with the lowest values for high-density random pack trickling
filter media. Many of the design formulations mentioned later in
this paper incorporate a term that allows for specific surface area
reduction due to distributor inefficiency in trickling filter media
wetting. The interrelationship of liquid residence time, dosing, and
media configuration on BOD 5 removal kinetics has not been
addressed, and additional research is required. Increasing the
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
(A)
(B)
(c)
(D)
RS
-
~
~
vWSL
WS
I
I.
-
T
Figure 4-Typical trickling filter process flow sheets.
Legend: (RS)-raw wastewater, (PC) primary clarifier, (PS) primary sludge, (PE) primary effluent, (TFINF) trickling
filter influent, (TF) trickling filter, (TFEFF) trickling filter effluent, (TFRCY) trickling filter recycle, (SC) secondary
clarifier, (WS) waste sludge, (SE) secondary effluent, (IC) intermediate clarifier, (ICE) intermediate clarifier effluent;
(A) and (B) single-stage trickling filter process, (C) two-stage trickling filter process, (D) two-stage trickling filter
process with intermediate clarification.
average hydraulic application rate reduces the liquid residence
time, but has been proven to increase wetting efficiency. The
recirculation ratio (Q/QR) is typically in the range 0.5 to 4.0.
Bryan (1955, 1962) and Bryan and Moeller (1960) demonstrated
that vertical-flow media require an average application rate
greater than 1.8 m 3/m 2 /h to maximize BOD 5 removal efficiency.
Shallow towers using cross-flow media have used hydraulic rates
in the range 0.4 to 1.1 m 3/m 2 /h. Grady et al. (1999) state that
adequate media wetting may be achieved at a total hydraulic load
(THL) of 1.8 to 2 m 3/m 2 /h with rotary distributors.
May 2011
Distributor speed control has the following benefits: controlled
flow interruption (periodicity of dosing), increasing. wetting
efficiency (percent of media wetted), and biofilm thickness
control. The designer should consider recirculation capabilities
and the effect of reverse thrusting jets with the use of distributor
speed control. Distributor speed control may not be required in all
instances provided adequate dosing is applied by recirculation
pumps and reverse thrusting jets. A German process control
parameter (ATV, 1983), referred to as Spiilkraft, allows for the
calculation of a dosing rate (mm/pass) as follows:
393
Daigger and Boltz
Table 2-Trickling filter classification.
Design Parameter
Carbon Oxidizing
(cBODs removal)
Roughing
Media Typically Used
Vertical flow
Wastewater Source
Hydraulic Loading
m3 (gpM/ft2)
BOD5 and NH3 -N Load
k3.gd (lb BOD/d1000 f)
2
d (Ib NH3-N/d.1000 ft )
rn2.d
Conversion (%) or Effluent
Concentration (mg/L)
Macro Fauna
Depth, m (feet)
Carbon Oxidation
and Nitrification
Nitrification
Rock, cross flow,
or vertical flow
Primary effluent
Cross flow
Primary effluent
Rock, cross flow,
or vertical flow
Primary effluent
52.8-178.2 (0.9-2.9)
14.7-88.0 (0.25a-1.5)
14.7-88.0 (0.25a-l.5)
35.2-88.0 (0.6-1.5)
1.6-3.52 (100-220)
0.32-0.96 (20-60)
0.08-0.24 (5-15)
NA
NA
NA
0.2-1.0 (0.04-0.2)
0.5-2.4 (0.1-0.5)
50 to 75% filtered
cBOD5 conversion
No appreciable growth
0.91-6.10 (3-20)
20 to 30 mg/L
cBOD5 and TSSb
Beneficial
-s 12.2 (40)
< 10 mg/L as cBOD5 ;
< 3 mg/L as NH3-Nb
Detrimental (nitrifying biofilm)
-s 12.2 (40)
0.5 to 3 mg/L as
NH3-Nb
Detrimental
-<12.2 (40)
Secondary effluent
Notes:
"8Applicable to shallow trickling filters; gpm/ft2 = gallons per minute per square foot of trickling filter plan area.
b Concentration remaining in the clarifier effluent stream
gpm/ft2 x 58.674 = m3/m2 -d (cubic meter per day per square meter of trickling filter plan area).
lb BOD,/d.1000 ft3 x 0.0160 = kg/d-m 3 (kilograms per day per cubic meter of media).
lb NH3-N/d-1000 ft2 x 4.88 = g/d.m 2 (grams per day per square meter of media).
mm
THL" 1,000 SK=
m.Na.od 1,440-ay
day
Where
SK = the Spfilkraft (mrn/pass);
THL = the total hydraulic load = (Qi, + QR)/A,
(m3/m 2/d);
Na = the number of distributor arms; and
coa = the rotational speed (rev/min).
Higher dosing rates are recommended for higher organic loading
rates to provide biofilm thickness control and controlled sloughing
of excess biomass. Besides a normal operating dosing rate, it may
be beneficial to periodically use a higher flushing dosing rate for 5
to 10% of a 24-hour operating period. The flushing dose will
operate at 6 to 15 times the normal operating dose. Albertson
(1995) and Parker et al. (1989) demonstrated that there is benefit
to biofilm thickness control in the trickling filter process. These
Table 3-Operating and flushing dosing rates for
distributors.
Total Organic
Load kglm 3/d
(lb BODs/d/1000 ft3)
<0.4 (< 25)
0.8 (50)
1.2 (75)
1.6(100)
2.4 (150)
3.2 (200)
Operating Dosing
Rate mm/pass
(inches/pass)
25-75
50-150
75-225
100-300
150-450
200-600
(1-3)
(2-6)
(3-9)
(4-12)
(6-18)
(8-24)
Flushing Dosing
Rate mm/pass
(inches/pass)
100 (4)
150 (6)
225 (9)
300(12)
450 (18)
600 (24)
Note: Actual values are site-specific and vary with media type.
394
benefits include improved performance, reduced odors, reduced
power use for recycling, reduced nuisance organisms, and
elimination of heavy sloughing cycles (Albertson, 1995). Parker
et al. (1989) described the use of both distributor speed control
and variable frequency drive-controlled recirculation pumps to
maintain constant trickling filter hydraulic application. However,
Parker et al. (1989) also presented evidence that electrically
driven distributor speed control did not improve NTF performance. Parker (1999) pointed out that there is little research
describing the effect of hydraulic transients on synthetic trickling
filter media and their effect on media life. The typical
hydraulically driven distributor in North America operates in the
range of 2 to 10 mm/pass. Table 3 lists recommended operating
and flushing dosing rates for modular synthetic media.
Oxygen Requirements and Air Supply Alternatives
Trickling filters require oxygen for aerobic biochemical
transformation processes. Several researchers have demonstrated
that at least some portion (if not the entire bioreactor) of roughing,
carbon oxidizing, combined carbon oxidizing and nitrification,
and nitrifying trickling filters operates under oxygen-limited
conditions (Kuenen et al., 1986; Okey and Albertson, 1989;
Schroeder and Tchobanoglous, 1976). Ventilation is essential to
maintain aerobic conditions in a trickling filter. The vertical flow
of air through trickling filter media can be induced by mechanical
ventilation or natural air draft. Mechanical ventilation enhances
and controls airflow with low-pressure fans that continuously
circulate air throughout the trickling filter. Current design practice
requires provision of adequate underdrain and effluent channel
sizing to permit free airflow. Passive devices for ventilation
include vent stacks on the trickling filter periphery, extensions of
underdrains through trickling filter sidewalls, ventilating manholes, louvers on the sidewall of the tower near the underdrain,
and discharge of trickling effluent to the subsequent settling basin
in an open channel or partially filled pipes.
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
Figure 5-Trickling filter aeration system: distribution pipes (left) and fans (right).
Natural Draft. Naturally occurring airflow results from a
difference in ambient air temperature and humidity outside and
inside the trickling filter. The temperature causes air to expand
when warmed or contract when cooled, and humidity differences
result in density differences. The result is an air-density gradient
throughout the trickling filter and an air front that rises or sinks
depending on the differential condition. This rising or sinking
action results in a continuous airflow through the bioreactor. If air
inside the trickling filter is colder than the ambient air, the air will
flow downward. Alternatively, if the ambient air is colder than the
air inside the trickling filter, air will flow upward. Schroeder and
Tchobanoglous (1976) state that upward airflow is the worst-case
scenario from a mass transfer perspective because the dissolved
oxygen driving force is lowest in the region of highest oxygen
demand (i.e., the top of the trickling filter).
Natural ventilation may become unreliable or inadequate in
meeting process air requirements when neutral temperature
gradients do not produce%air movement. Such conditions may be
daily or seasonal, and can lead to the development of anaerobic
layers inside the biofilms (near the growth medium) and poor
trickling filter performance. Modular plastic media trickling filters
that rely on natural draft to provide process oxygen for municipal
wastewater treatment should include the following design features:
Drains, channels, and pipes should be sufficiently sized to prevent
submergence greater than 50% of their cross-sectional area under
design hydraulic loading. Ventilating access ports with open-grating
covers should be installed at both ends of the central collection
channel. Large-diameter trickling filters typically have branch
channels (to collect the treated wastewater). These branches should
also include ventilating manholes or vent stacks installed at the
trickling filter periphery. According to Grady et al. (1999), the open
area of the slots in the top of the underdrain blocks should not be less
than 15% of the trickling filter area. One square meter gross area of
open grating .in ventilating manholes and vent stacks should be
provided for each 23 m2 of trickling filter area. Typically, 0.1 m2 of
ventilating area is provided for every 3 to 4.6 m of trickling filter
periphery, and 1 to 2 m2 of ventilation area in the underdrain area per
1000 m3 of trickling filter media. Another criterion for rock-media
trickling filters is the provision of a vent area at least equal to 15% of
the trickling filter cross-sectional area.
Mechanical Ventilation. A majority of new and improved
trickling filters use low-pressure fans to mechanically induce
May 2011
airflow. The airflow resulting from natural draft will distribute
itself. This will not occur with mechanical ventilation. Pressure
loss through synthetic trickling filter media is typically low, often
less than 1-mm H2 0 per meter of trickling filter depth (Grady et
al., 1999). The low-pressure drop typically results in low fan
power requirements (e.g., on the order of 3 to 5 kW for modestsized facilities). The head on the fan is typically less than 20 to
30 mm H2 0. Unfortunately, the low pressure drop allows air to
rise upward through the trickling filter media without distributing
itself through the bioreactor section. Therefore, fans are typically
connected to distribution pipes. The airflow distribution piping
has openings that are sized such that airflow.through each is equal
and airflow distribution is uniform. The pipes typically have a
velocity in the range 1100 to 2200 m/hr in order to further
promote uniform airflow distribution. Airflow requirements are
calculated based on process oxygen requirements and characteristic oxygen transfer efficiency, which is typically in the range of 2
to 10%. The mechanically induced airflow may flow upward or
downward. Down-flow systems can be designed without covers, but
covers are required for upflow systems. Covering trickling filters
offers a wintertime benefit of limiting cold airflow and minimizing
wastewater cooling. Mechanical ventilation and covered trickling
filters may be used to destroy odorous compounds. A trickling filter
aeration system is pictured in Figure 5.
Trickling Filter Design Models. Numerous investigators
have attempted to delineate the fundamentals of the trickling filter
process by developing relationships among variables that affect
trickling filter operation. Existing trickling filter process models
range from simplistic empirical formulations to numerical models.
Analyses of operating data have been made to establish equations or
curves to fit available data. Results of these data analyses have led
to the development of several empirical trickling filter formulas.
Unfortunately, numerous models exist and there is lack of an
industry standard. Designers need to assess which equation best fits
a particular situation when selecting a design model, especially with
regard to the confidence level necessary to meet discharge permit
requirements. Therefore, many process designers use a forecasting
approach and will apply several empirical models to evaluate a
system. The following empirical models have been reported by
Boltz et al. (2009) and Boltz (2010) as options historically used to
describe trickling filter performance in the context of process
design: (1) National Research Council (1946), (2) Velz (1948)
395
Daigger and Boltz
2
kg/lOOO m * d
0
' ]'
I
'
I
100
I
0 No Redrmatkton
~ID
RackutaIion
L
Z
10
18
2.0
3.0
20
80
at
60
*0
40
40
0
20
2D
S I
I
,
I
,
0
w
U•
0
10
20
30
0
40
BODsLoad,b/1000 muWady
1.0
4.0
ORGANIC LOADING, Ib BO0/1OO0 eqft/day
Figure 6-Nitrification efficiency as a function of BODs load in rock-media combined carbon oxidation and
nitrification trickling filters (Left: U.S. EPA, 1975; Right: Parker and Richards, 1986).
equation, (3) Schulze (1960) equation, (4) Eckenfelder (1961)
formula, (5) Galler and Gotaas (1964), (6) Germain (1966) equation,
(7) Kincannon and Stover (1982), and (8) the Institution of Water
and Environmental Management (1988) formula. A pseudo
mechanistic model called the Logan trickling filter model (TRIFL)
(Logan et al. 1987a, 1987b) has been used to design modular
synthetic media trickling filter processes.
There is a general lack of models describing TF/SG systems.
Daigger et al. (1993) and Takdcs et al. (1996) presented a
mathematical description of TF/SG processes. The model of
Daigger et al. (1993) was developed to characterize nitrification
in TF/SG processes and was established based on performance
observations at the Buck Creek WWTP, Garland, Texas. The model
accounts for suspended growth reactor seeding with detached
biofilm fragments in the trickling filter effluent stream. The TF/SG
process effluent is calculated using the following equation:
(M~T
[~~~max~
[
+kd
... NH3 E,,
+ [(
kd] . (NH3 .~
2
(NH 3,,-K,) -y,.NH 3PE,
R+kd).(NH3UE-K,)]
=0
(2)
Where
/iý = the maximum nitrifier growth rate (lid),
MCRT = the mean cell residence time (d),
kd = the specific decay rate (m/d),
Ks = the ammonia-nitrogen half-saturation constant (mg/L),
NH3.EFF = the ammonia-nitrogen concentration in the
TF/SG process effluent stream (mg1L),
NH3.TFE = the ammonia-nitrogen concentration in the
trickling filter effluent stream (mg/L), and
NH3.pE = the ammonia-nitrogen concentration in the
trickling filter process influent stream
(mg/L).
The model of Daigger et al. (1993) has been independently
evaluated and demonstrated to be effective by Biesterfeld et al.
396
(2005). The researchers noted that the model of Daigger et al.
(1993) is primarily dependent on nitrification rates in the trickling
filter and suspended growth reactor mean cell residence time, or
solids residence time.
Combined Carbon Oxidation and Nitrification. Combined
carbon oxidizing (i.e., carbonaceous 5-day biochemical oxygen
demand [cBOD 5 ] removal) and nitrification trickling filters may
contain rock or synthetic media. The U.S. Environmental Protection
Agency (U.S. EPA) (1991) reported survey results of 10 combined
carbon oxidation and nitrification facilities. Six of the facilities
included the TFISC process. The survey was used to create empirical
guides for achieving nitrification in the-secondary treatment process
trickling filters. The manual for nitrogen control (U.S. EPA, 1993)
presented recommended BOD 5 loading (g/m2/d) to achieve both
carbon oxidation and nitrification in a single-stage trickling filter.
The kinetics of combined BOD 5 removal and nitrification are
complex, and the lack of fundamental research supporting combined
carbon oxidation and nitrification in the trickling filter process
results in the continued use of empirical design procedures.
Therefore, the design of combined carbon oxidation and nitrification
trickling filters is empirical (Parker 1998).
U.S. EPA (1975) summarized full- and pilot-scale rock-media
trickling filter data from Lakefield, Minnesota; Allentown, Pennsylvania; Gainesville, Florida; Corvallis, Oregon; Fitchburg,
Massachusetts; Ft. Benjamin Harrison, Indiana; Johannesburg,
South Africa; and Salford, England. Likewise, significant data are
presented for a diverse range of U.S. plants by the Water
Environment Federation (2000). Figure 6 illustrates the relationship
between BOD 5 volumetric loading and nitrification efficiency using
both pilot- and full-scale rock-media combined carbon oxidation and
nitrification trickling filters. These observations indicate that an
3
organic loading rate of 0.08 kg BOD,/m3 /d (5 lb BOD/1000 ftOd)
(according to U.S. EPA [1975]) or 2 kg/1000 m2/d (0.5 lb/1000 ft2 /
d) (according to Parker and Richards [1986]) is required for rockmedia trickling filters to achieve approximately 90% nitrification.
Recirculation typically improves nitrification, particularly for
nitrification efficiencies greater than 50%.
Daigger et al. (1994) presented an evaluation of three full-scale
trickling filters with low-density cross-flow media. The trickling
filters were dosed with rotary distributors and designed for
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
Table 4-Reported zero-order nitrification rates for vertical and cross-flow media (after Parker [1998; 1999]).
Location
Reference
Central Valley, Utah
Malmo, Sweden
Littleton/Englewood, Colorado
Midland, Michigan
Lima, Ohio
Bloom Township, Illinois
Media Type
Parker et al. (1989)
Parker et al. (1995)
Parker et al. (1997)
Duddles et al. (1974)
Okey and Albertson (1989)
Baxter and Woodman (1973)
XF
XF
XF
VF
VF
VF
JN°(g/m 2/d)
140
140
140
891
891
891
Temperature Range (°C)
2.3-3.2
1.6-2.8
1.7-2.3
0.9-1.2
1.2-1.8
1.1-1.2
11
13
15
7
18
17
to
to
to
to
to
to
20
20
20
13
22
20
1 Fully corrugated.
Note: XF = cross flow and VF
vertical flow.
combined carbon oxidation and nitrification. Data collected
from these studies suggest that an organic load less than 0.2 kg
BOD 5 /m 3/d (13 lbs BOD,/1000 ft 3/d) is required to achieve
90% nitrification efficiency. Similar to the observations reported
by Stenquist et al. (1974), the synthetic media trickling filters
studied were able to achieve greater than 90% nitrification
efficiency. Biofilm thickness control is recommended to optimize
NH 3-N removal in combined carbon oxidation and nitrification
trickling filters (Parker et al. 1995; 1997). Daigger'et al. (1994)
proposed the following equation to describe BOD 5 and NH 3 -N
removal in modular plastic media carbon oxidation and nitrification trickling filters:
(
VOR= [Si+ 4 .6 -SNo.-NJ'
)
ý(14 -g
(3)
Where
VOR = the volumetric oxidation rate (kg/m 3/d),
Si = the BOD 5 concentration in the influent stream
(g/m 3),
SNOx-N = the nitrate/nitrite-nitrogen concentration in the
effluent stream (g/m 3),
Q = the flowrate, including recirculation streams
(m 3/d), and
VM = the synthetic media volume (mi3 ).
Using eq 3, Daigger et al. (1994) reported the volumetric
oxidation rate for three combined carbon oxidation and nitrification trickling filter (with modular plastic media) processes in the
range of 0.4 to 1.3 kg/m 3/d.
Nitrifying Trickling Filters. Nitrifying trickling filters are a
reliable and cost-effective means for NH 3-N conversion. The
following design practices have been demonstrated in full-scale
application: (1)"use medium-density cross-flow media to optimize
hydraulic distribution and oxygenation, (2) use mechanical ventilation, (3) periodically alternate the lead NTF to avoid patchy biofilm
development in the lower reaches of the second-stage unit, (4) the
influent should be secondary effluent to minimize bacterial
competition for substrates inside the biofilm, (5) maximize wetting
efficiency to avoid the formation of dry spots, (6) dose the NTF at a
rate that will minimize the accumulation of macro fauna, and (7)
equalize NY 3 -N-laden supernatant from solids processing operations
to even out diurnal load variability (Parker et al., 1995; 1997). Benefits
to NTFs include low energy consumption, stability, operational
simplicity, and reduced sludge yield. The reduced sludge yield and
resulting low total suspended solids concentration in the NTF effluent
stream has led some units to be constructed without downstream
liquid-solids separation units. This is dependent on site-specific
May 2011
treatment objectives arid effluent water quality standards. An
operational issue that can be detrimental to process performance is
the control of predatory macro fauna. Therefore, the designer must
include means for managing solids and macro fauna-laden water
resulting from macro fauna control measures. Design and operational
features dedicated to macro fauna control are presented in a
subsequent section. Nitrifying trickling filters having 6- to 12.2-mr
(20- to 40-ft) modular plastic media depths have demonstrated
improved performance. Nitrifying trickling filters have been constructed with depths up to 13 m (-42 ft). Shallower units can operate
as a two-stage system. Recirculation should be minimized to that
required for biofilm thickness control in order to maximize NH 3 -N
concentration (i.e., maintain a high driving force) (Parker et al., 1997).
The practice of alternating the lead trickling filter in a two-stage
trickling system is referred to as alternating double filtration
(ADF). Gujer and Boller (1986) and Parker et al. (1989) observed
patchy biofilm growth in the lower section of pilot-scale NTFs. The
researchers attributed the patchy growth to dry spots. Aspegren and
coworkers (1992) observed improved nitrification and reduced
biofilm patchiness when operating the NTFs in an ADF system. Use
of the ADF approach with trickling filters in series encourages fulldepth biofilm development in both trickling filters. The lead
trickling filter should be switched every 3 to 7 days to ensure that
both units contain a healthy biofilm developed along the entire
bioreactor depth. The primary drawback of ADF is an increase in
power requirements, which may be in excess of 50% due to double
pumping. In addition to increased operating cost, capital costs
associated with pipes and valves will also increase costs.
Parker (1998, 1999) described nitrification efficiency in NTFs
containing either cross-flow or vertical-flow synthetic media
types. Table 4 summarizes his observations, which demonstrate
that zero-order ammonia-nitrogen flux rates are greater for crossflow than vertical-flow media. Factors contributing to the
enhanced performance may be improved oxygen-transfer efficiency resulting from the increased number of media interruptions
and improved oxygenation (Gujer and Boller, 1986; Parker et al.,
1989). Autotrophic nitrifying biofilms are thin when compared
with the heterotrophic biofilms that are primarily responsible for
BOD5 removal; therefore, medium-density cross-flow media are
typically used in NTFs. However, there is a propensity to develop
dry pockets when high-density modular plastic media are used
(Parker et al., 1989).
Gujer and Boiler Nitrifying Trickling Filter Model. Gujer
and Bolter (1986) developed the following semi-empirical model
that reasonably characterizes NTF performance:
JN(S, T)
JN, max (T)
I N
KN S+SB,
(4)
397
Daigger and Boltz
Where
JN(S, T) = the ammonia-nitrogen flux at SB.N (g/m 2/d),
JN,v,
m(T) = the maximum ammonia-nitrogen flux at
temperature T (g/m 2/d) (=Jo
2 ,m.x(T)/4.3),
SB.N = the bulk-liquid ammonia-nitrogen concentration (g/m3 ),
KN = the half-saturation coefficient for ammonianitrogen (g N /m3) (= 1.0 g N /m 3, typical
value), and
T = the temperature (*C).
Based on a "line-fit" relationship, the flux at any depth in the
trickling filter can be calculated as
JN(Z, T)=JN(0, T)'e-k',.
(5)
The following two solutions were developed to account for a
change in the rate of nitrification with NTF depth (k # 0) (eq 6),
and the second assumes no decrease in the rate of nitrification
with NTF depth (k = 0) (eq 7):
a'JN,a(T) (1
-e-kz) =Si.,N-SB,I+KN
In (ŽSi, IV (6)
\SB, NI
k-vh
When k = 0,
z--Nn
=TSin, N -SB,N +KN'tn
N
An(SB.N•
(7)
Where
a = the specific surface area (m2/m 3),
k = the empirical parameter describing nitrification
rate decrease (1/m) (= 0 to 0.16, typical 0.1),
vh = the hydraulic load (with or without recirculation) (m 3/ m 2 /d),
z = the NTF depth (m), and
Siý. N = the ammonia-nitrogen concentration in influent
stream (g/m 3 ).
These equations can be solved directly to size a NTF for a
desired SB,v. When recirculation is used, an iterative solution
routine that includes the following equation is required because of
the effect recirculation has on both Vh and Si,, N:
5I
N
N. i ---SO, N +-R-SB,
l+R
RS,N
(8)
SM~,N
Si., N S8, N
Where
So, N = the ammonia-nitrogen concentration in the
influent stream prior to being mixed with the
recirculation stream.
The ammonia-nitrogen concentration in NTF influent stream,
Si, N• will be less than So. N when recirculation is applied. Parker
et al. (1989) proposed a modification of this model to account for
oxygen-transfer efficiency variability amongst modular plastic media
types and operating conditions. The revised expression is as follows:
JN (z,T) =EO
Where
398
the dimensionless NTF media effectiveness factor and
Jo2 ,max(T) = the maximum dissolved-oxygen flux at
E02 =
Jo 2,m.(T)
4.3
SB,N
KN+SB, N
(9)
temperature T (g/m 2 /d).
Based on their experience, Gujer and Boiler (1986) reported an3
E0 2 value in the range of 0.93 to 0.96 for Ks,02 = 0.2 g 02 /M
and the temperature range of 5 to 25 *C. Parker et al. (1989), on
the other hand, observed lower E02 values (in the range of 0.7 to
1.0) and claimed that a departure from E0 2 = 1.0 accounts for
wetting inefficiency, biofilm grazing by macro fauna, or
competition for dissolved, oxygen between autotrophic nitrifiers
and heterotrophic bacteria inside the biofilm. The researchers
recommended that medium-density cross-flow media are used in
NTF applications and that E0 2 may range from 0.7 to 1.0 for this
media. High-density cross-flow media had a corresponding E0 2
approximately equal to 0.4 (Parker et al., 1995). According to
Parker et al. (1995), Eo2"Jo2,,x(T)/4.3 is the zero-order
ammonia-nitrogen flux (Parker et al., 1995). The maximum
dissolved-oxygen flux reflects the oxygen-transfer efficiency of
the selected modular plastic media, and was determined by the
researchers using TRIFL (Logan et al., 1987a). The coefficient,
Ks.0 2 , determined for the Central Valley WWTP in Utah, was in
the range of 1 to 2 mg/L (Parker et al., 1989).
Operational Strategies and Facility Improvements for
Macro Fauna Control
Several strategies have been applied to manage macro fauna
accumulation and/or development in trickling filters, including
physical, chemical, or a combination of physical and chemical
applications. The ideal control strategy is to promote a condition
that is either toxic to the macro fauna or creates an environment
not conducive to their accumulation. Lee and Welander (1994)
demonstrated increased nitrification after predator control using
substances toxic to eukaryotic organisms. The toxic substance
must either have no effect on or only temporarily inhibit beneficial
microorganisms (Parker et al., 1997). Operators have conducted
site maintenance that aids in reducing macro fauna presence in
trickling filter-based WWTPs. For instance, some operators have
observed that the presence of filter flies may be reduced by simply
maintaining a short stand of grass on the WWTP site. More
specific strategies include periodic high-intensity hydraulic
application, trickling filter flooding, pH adjustment with lime or
sodium hydroxide, high-concentration aqueous ammonia dosing,
trickling filter effluent or secondary clarifier underflow (humus)
screening or accelerated gravity separation, gravity separation in
low-velocity channels with a dedicated pumping circuit, eliminating dissolved oxygen from the trickling filter feed, adding salt,
draining and freezing the infested unit, raising the temperature
quickly, adding molluscacide (e.g., copper sulfate), and chlorinating the influent stream. Many of these strategies have proven
ineffective in some trickling filters, and others may be detrimental
to bioreactor performance. Biochemical reactions are influenced
by temperature, pH, and alkalinity; adjusting these parameters
may inhibit the biochemical reactions and lower transformation
rates. Chemicals such as chlorine are toxic to all organisms in the
trickling filter and may result in destruction of sensitive biomass
(Parker et al., 1989). A brief summary of the principal control
mechanisms in use is provided here. More details are available
elsewhere (Boltz et al., 2008). Control mechanisms described here
include:
Water Environment Research, Volume 83, Number 5
Daigger and Bottz
"* Periodic high-intensity
hydraulic flushing (controlled dosing,
or Spalkraft),
"* Trickling filter flooding and chemical application,
"* Chemical treatment (focus on high-concentration aqueous
ammonia dosing and pH adjustment with sodium hydroxide),
"* Trickling filter effluent or underflow (humus) screening or
accelerated gravity separation (using equipment typically
associated with grit removal), and
"* Gravity separation in low-velocity channels and removal
with a dedicated pumping circuit.
Dosing for Macro Fauna Control. Hawkes (1955) demonstrated that high hydraulic loadings and periodically increasing
instantaneous dosing rates can control filter fly development.
Increased hydraulic loading improves trickling filter media
wetting efficiency, thereby reducing dry spots and minimizing
ideal spawning areas for filter flies. Gujer and Boiler (1984)
reported that filter fly larvae were reduced to quantities that did
not have an impact on NTF performance. Andersson et al. (1994)
tested three flushing intensities (Spiilkraft values of 5, 40, and
80 mm/pass) and reported that the variable flushing intensity had
no apparent effect on filter fly and worms in a pilot-scale NTF.
(Note that these Spiilkraft values are below those reported for
flushing, as presented in Table 3.)
Flooding. Trickling filter flooding requires adequate duty
units to isolate a trickling filter for a 3-to-6-hour period. The
trickling filters must be designed as water-retaining structures,
which is not typical. Variations include (1) saline flooding and (2)
flooding and backwashing with an alkaline solution. Parker et al.
(1997) reported the use of flooding to control filter flies and an
alkaline backwash process to control other macro fauna in two 32m-diameter, 7.3-m-deep medium-density cross-flow media NTFs
at the Littleton-Englewood WWTP in Colorado. Online pH probes
and a sodium hydroxide metering system allow for flood water pH
adjustment by operator set point. The alkaline flood water is
pumped through the NTF bottom, is discharged into an overflow
trough, and is then directed to the head of the WWTP for
treatment. Alkaline treatment is reported to have removed 76% of
larvae at pH 9 and 99% at pH 10 (Parker et al., 1997). Subsequent
research trials designed in response to snail development
demonstrated that flooding and backwash (4 hours at pH 9)
reduced snail quantity by two-thirds and returned the NTFs to high
nitrification efficiency (Parker, 1998).
Chemical Treatment. Everett et al. (1995) summarized
several chemical treatment alternatives including pH adjustment
and chlorination, sodium chloride, and molluscicides (e.g., copper
sulfate, metaldehyde, niclosamide, and trifenmorph). Factors such
as pH, turbidity, and molluscicide dose are key factors in
determining chemical application rate. Rotating biological contactors (RBCs) in Lafayette, Louisiana, applied sodium chloride at
a dosing concentration of 10 mg/L for a 24-hour period to
effectively control the snail accumulation. Calcium hypochlorite
in a range of 60 to 70 mg/L was applied during a 2-to-3-day
period, and effectively minimized snail accumulation in RBCs at
the Deer Creek WWTP, Oklahoma City, Oklahoma. Copper
sulfate at low concentrations (0.45 kg of copper sulfate per
3.785 mi3 ) may effectively control snail accumulation.
Ammonia is toxic to snails. Lacan et al. (2000) conducted a
laboratory-scale study and plant-scale application of un-dissociated aqueous ammonia [NH 3 -N(aq)] solutions with elevated pH to
control snail growth (P. gyrina) in NTFs. Un-dissociated aqueous
May 2011
NH3-N(aq), not the ammonia ion, is the snail P. gyrina toxophore.
The concentration producing 100% mortality is a function of
exposure time and the bulk-liquid NH3-N(aq) concentration. The
laboratory-scale study demonstrated that an ammonium chloride
(NH 4CI) solution at pH 9.2 [NH 3 -N(ag) = 150 mg N/L] resulted in
100% snail mortality. A much higher concentration of ammonia is
required in the trickling filter influent stream (i.e., 1000 to
1500 mg N/L) to maintain the required NH3-N(ag) = 150 mg N/L
because of the immediately reduced concentration owing to axial
dispersion, biofilm diffusion (both external and internal), and
biochemical reaction. Lacan et al. (2000) estimated that an
influent ammonia concentration of 1080 mg N/L resulted in an
average concentration throughout the NTF of 185 mg N/L. Such a
high-concentration NH 3 -N(,q) stream may be readily available in
municipal WWTPs as solids processing recycle streams. In some
instances, however, it may be necessary to purchase NH3-N(aq).
The first full-scale application of this snail control method was
reported by Gray et al. (2000) at the Truckee Meadows WWTP,
Reno Sparks, Nevada, which uses high-density (215 m2 /m 3)
media. Ammonia-rich anaerobic digester centrate was directed to
a NTF recirculation pump station. Sodium hydroxide was added to
the recirculation stream to raise the pH to 9.05 (range 9.0 to 9.5),
which increased the NH3-N(aq) content of the centrate solution.
Figure 7 illustrates the (1) normal operating mode, (2) centrate
treatment/recirculation mode, and (3) the flushing mode characteristic of the macro fauna control method described by Lacan et
al. (2000). This macro fauna control method is typically applied
once per month. During the treatment cycle, an NTF is isolated
and the solution is recirculated through the trickling filter for
approximately 2 hours. The first 20 to 50 minutes of aqueous
ammonia dosing is dedicated to reaching a hydrodynamic steady
state (i.e., 3 to 4 hydraulic retention times [HRTs]), and the
remainder is the minimum recommended exposure time for 100%
mortality of both adult snails and their larvae. The treatment
solution is returned to the head of the WWTP after dosing is
completed and the NTFs are then flushed with secondary effluent
in the "recirculation mode" for 10 hours.
Mechanical Control. Physical removal techniques include
(1) trickling filter effluent or underflow (humus) screening, (2)
gravity separation in low-velocity channels and removal with a
dedicated pumping circuit, and (3) accelerated gravity separation
using equipment typically associated with grit removal. The
Central WWTP, Baton Rouge, Louisiana, uses trickling filter
secondary clarifier underflow screening to control snail accumulation in, or damage to, solids handling equipment. The City of
Lawton, Oklahoma, dnd both the South San Luis Obispo,
California, County Sanitation District Oceana Regional Plant
and the City of San Luis Obispo Water Reclamation Facility, San
Luis Obispo, California, pump secondary clarifier underflow to a
free vortex classifier for snail shell removal. The Econchate Water
Pollution Control Plant, Montgomery, Alabama, removes snail
shells in the chlorine contact basin, which was modified to a twopass channel to serve as a low-velocity sedimentation basin for
snail shells escaping secondary clarification. The snail shells
deposited in the low-velocity channel are collected in a sump and
pumped to a static screen, where they fall by gravity into a
collection bin. Tekippe et al. (2006) reported the use of baffles,
grit pumps, and classifiers to remove snails from the Ryder Street
WWTP, Vallejo, California. The facility treats wastewater with a
TF/SG process consisting of two, 32-m-diameter and 7.3-m-deep
399
Daigger and Boltz
I
Flshng Mode
I
SOcnm
TF
Efft~uwl
LEGEND
-
-
Opefralg Flow Path
Figure 7-Nitrifying trickling filter operating modes for high-concentration un-dissociated aqueous ammonia dosing
(Lacan et al., 2000).
cross-flow media trickling filters. Initial zones of the Ryder Street
WWTP's aeration basins were improved to provide a zone for the
majority of the shells to settle. An automatic mechanism was
provided to remove the settled shells (Tekippe et al., 2006).
Combined Trickling Filter and Suspended
Growth Processes
Biological processes including both a trickling filter and
suspended growth reactor build on the known performance and
operating characteristics of the parent processes. When the
suspended growth reactor is used as a flocculating unit it is referred
to as the TF/SC process. All other TF/SG processes use the coupled
suspended growth reactor as an oxidizing unit. The activated biofilter
and biofilter/activated sludge processes, which circulate return
activated sludge over the trickling filter (making it a biofilter), are
not discussed as these process options are applicable only to wood
slat media, which is seldom used these days (Grady et al., 1999).
Trickling Filter/Solids Contact. A majority of organic
matter in municipal wastewater is colloidal or particulate material
(Levine et al., 1985, 1991; and Boltz and La Motta, 2007).
Trickling filters are poor bioflocculating reactors (Boltz et al.,
400
2006). The TF/SC process operates under the premise that
trickling filter effluent contains a high concentration of not readily
settleable colloidal and particulate organic matter. The material
may be removed by bioflocculation, along with the oxidation of
residual soluble organics, in a solids contact basin. The TF/SC
process includes a trickling filter followed by a small, aerated
solids-contact channel. Biomass in the solids contact basin
effluent stream flows to a clarifier that has a (1) suction-header
sludge withdrawal mechanism and (2) a flocculating feed well
(approximately one-third of the clarifier diameter) that promotes
gentle mixing and additional bioflocculation of the influent
suspended biomass (sludge flocs). LaMotta et al. (2004) indicate
the following characteristics for such systems:
"* Solids contact
basin dissolved oxygen concentration greater
than I mg/L,
"* Dissolved oxygen uptake rate typically low, and
"* Short distance between solids contact basin and clarifier
desired (long runs may require aerated channels).
There are three modes of operating the TF/SC process: mode I,
mode II, and mode III. Mode I relies exclusively on the solids
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
TRICKLING
FILTER
AERATED SOLIDS
CONTACT TANK
SECONDARY CLARIFIER
FLOCCULATOR
CENTER WELL
TREATED
EFFLUENT
Mode I
TRICKL ING
FILTER
PRIMAR EII1N
SECONDARY CLARIFIER
MIXED LIQUOR
PRIMAPV~
•L#E
••T
__
~
R TR
FLOCCULATOR____
NSLUDGE;
REAERATION TANKS
Mode II
TRICKL NG
FILTER
AERATED
AKFLOCCU
COTC SOLIDS
SECONDARY CLARIFIER
LATOR
•C8ETER WELL
' TREATED
EFFLUENT,
PRIMAR.Y
UP I IEMIT______
WASTEE
,
•
ETU RN 'SLUDGE1
REAERATION TANKS
Mode 1II
Figure 8-Three modes of TF/SC process operation (after Parker and Merrill [1984]).
contact basin for colloidal and particulate organic matter
bioflocculation, and the oxidation of residual soluble organic
matter. Mode II relies exclusively on a return sludge aeration
chamber. The aerated return sludge is mixed with trickling filter
effluent for colloidal and particulate organic matter bioflocculation. Mode III makes use of both the solids contact basin and a
return sludge aeration tank. A typical TF/SC process operates as
mode I; however, as of 2001, more than one-half of the TF/SCbased WWTPs were operating as mode III (or had the operational
flexibility to operate as mode I or I1). It should be noted that
mode II is seldom used and is typically not recommended as it
does not have a solids contact basin and only a sludge reaeration
tank (Parker and Bratby, 2001). These operational modes are
illustrated in Figure 8.
If the solids contact basin follows a carbon oxidation and
nitrification trickling filter(s), autotrophic nitrifiers will detach
from the biofilm surface and, essentially, bioaugment the solids
contact basin biomass inventory. Despite the short duration solids
retention time characteristic of the solids contact basin, the
bioaugmentation will cause nitrification, which will exert
additional oxygen demand (i.e., increased airflow, blower size,
and air piping). In some instances, this may be desirable; however,
May 2011
in instances where increased oxygen demand is not desired and
nitrification is inevitable, the designer should seek to maximize
nitrification in the carbon oxidation and nitrification trickling
filter. This may be achieved with proper air supply system design
and process loadings, as discussed above.
The solids contact basin is typically 5 to 20% of the volume that
would be required with treatment by activated sludge. By
combining a trickling filter and solids contact basin, the trickling
filter size may be reduced compared to the size typically required
if treatment is accomplished with only a trickling filter (Parker
and Matasci, 1989). One significant benefit of the TF/SC process
is the low power requirements owing to a relatively high
dependence on the trickling filter to remove the majority of
soluble organic matter BOD5 . Rock- and plastic-media trickling
filters can be upgraded with the TF/SC process. Table 5 lists
generally accepted design criteria for the TF/SC process.
Roughing Filter/Activated Sludge. Roughing trickling filters
have been used to expand WWTP treatment capacity. The roughing
filter is a highly-loaded trickling filter that uses 10 to 40% of the
media volume required if treatment has been accomplished through
the use of the trickling filter process alone. Hydraulic retention time
in the aeration basin is typically 30 to 50% of that required with the
401
Daigger and Boltz
Table 5-Typical design criteria for TFISC processes.
Design Criteria
Parameter
Trickling Filter/Solids Contact (modularsynthetic media)
Solids production (mg volatile suspended solids in waste/mg BOD5 removed)
Trickling filter hydraulic load (gpm/ft2 )
Trickling filter influent total organic load (lbs/1000 ft3-day)
Solids contact basin side water depth (feet)
Solids contact basin HRT at average day flow (min)
Solids contact basin HRT at peak flow (min)
Solids contact basin solids residence time (d)
Solids contact basin MLSS concentration (mg/L)
Sedimentation basin overflow rate at average day flow (gpd/ft2 )
Underflow concentration (% total solids)
Range
Common
0.7-0.9
0.1-2.0
20-75
18-22
45-120
15-30
1.0-2.0
1500-3000
500-1000
0.6-1.2
0.7
1.0
50
20
60
30
1.0
2000
800
0.8
Note: MLSS = mixed liquor suspended solids.
activated sludge process. The TF/SC and roughing filter/activated
sludge (RF/AS) processes have the same process flow sheet.
However, with RF/AS, a smaller trickling filter is used so that the
aeration basin is depended on to provide a significant portion of
contaminant oxidation. This differs from the TF/SC process, where
the trickling filter is larger and provides the majority of the BOD5
removal, leaving the contact channel to provide enhanced colloidal
and suspended solids removal by bioflocculation.
Trickling Filter/Activated Sludge. The trickling filter/activated sludge (TF/AS) process is designed at high organic loads,
However, a unique feature of TF/AS is the intermediate clarifier.
The intermediate clarifier removes solids produced in the trickling
filter before partially treated wastewater enters the suspended
growth reactor. A benefit of using the TF/AS combined process is
that solids generated in the trickling filter can be removed before
second-stage activated sludge treatment. This is often a preferred
mode of operation where NH 3 -N removal is required. The reduced
oxygen demand afforded by intermediate clarification is typically
considered less significant than the savings in capital and
operating costs gained by eliminating intermediate clarification,
Therefore, cost-to-benefit evaluations typically guide designers to
use the RF/AS or TF/SC processes rather than the TF/AS process.
Table 6 lists generally accepted design criteria for the RF/AS and
TF/AS processes.
Summary
The modem trickling filter typically includes the following
major components: (I) rotary distributors with speed control; (2)
modular plastic media (typically cross-flow media unless the
bioreactor is treating high-strength wastewater, which warrants
the use of vertical-flow media); (3) a mechanical aeration system
(that consists of air distribution piping and low-pressure fans); (4)
influent/recirculation pump station; and (5) covers that aid in the
uniform distribution of air and foul air containment (for odor
control). Covers may be equipped with sprinklers that can spray
washwater to cool the media during emergency shut down
periods. Trickling filter mechanics are poorly understood.
Consequently, there is a general lack of mechanistic mathematical
models and design approaches, and the design and operation of
trickling filter and TF/SG processes is empirical. Some empirical
trickling filter design criteria and semi-empirical NTF models
have been described in this paper. Benefits inherent to the
trickling filter process (when compared to activated sludge
processes) include operational simplicity, resistance to toxic and
shock loads, and low energy requirements. However, trickling
filters are susceptible to nuisance conditions that are primarily
caused by macro fauna. Process mechanical components dedicated to minimizing the accumulation of macro fauna such as filter
flies, worms, and snail (shells) are now standard. Unfortunately,
Table 6-Typical design criteria for RFIAS and AF/AS processes.
Design Criteria
Parameter
Roughing or Trickling Filter/ActivatedSludge (modularsynthetic media)
Solids production (mg volatile suspended solids in waste/mg BOD5 removed)
Trickling filter hydraulic load (gpm/ft2 )
TF influent total organic load (lbs/1000 ft3-day)
Aeration basin side water depth (feet)
Aeration basin hydraulic retention time at average day flow (min)
Aeration basin hydraulic retention time at peak flow (min)
Aeration basin solids residence time (d)
Aeration basin MLSS concentration (mg/L)
Sedimentation basin overflow rate at average day flow (gpd/ft2)
Underflow concentration (% total solids)
Range
Common
0.8-1.2
0.8-5.0
75-300
12-24
120-480
40-120
1.0-12.0
1500-6000
500-1000
0.6-1.2
1.0
1.0
150
18
240
90
8.0
3000
800
0.8
Note: MLSS = mixed liquor suspended solids.
402
Water Environment Research, Volume 83, Number 5
Daigger and Boltz
information on the selection and design of these process
components is fragmented and has been poorly documented.
The TF/SC process is the most common TF/SG process. State-ofthe art design and operational practice for the trickling filter
process has been reviewed and described in this paper.
Acknowledgments
A preliminary version of this paper was prepared as
supplemental information for the "Trickling Filter and Combined
Trickling Filter-Suspended Growth Process Design and Operation" presentation in Workshop W213, Biofilm Reactors:
Application to Today's Global Wastewater Challenges, presented
at the 82nd Water Environment Federation Technical Exhibition
and Conference (WEFTEC) in Orlando, Florida, in October, 2009.
Submitted for publication December 22, 2009; revised
manuscript submitted March 14, 2010; acceptedfor publication
June 21, 2010.
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Water Environment Research, Volume 83, Number 5
COPYRIGHT INFORMATION
Author: Daigger, Glen T.; Boltz, Joshua P.
Title: Trickling Filter and Trickling Filter -- Suspended Growth Process Design and
Operation: A State-of-the-Art Review
Source: Water Environ Res 83 no5 My 2011 p. 388-404
ISSN: 1061-4303
DOI:10.2175/106143010X12681059117210
Publisher: Water Environment Federation
601 Wythe Street, Alexandria, Va 22314-1994
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