A Review of Operational Control Strategies for Snail and
Other Macrofauna Infestations in Trickling Filters
Joshua P. Boltz*, Steven J. Goodwin, Dana Rippon, Glen T. Daigger
1
CH2M HILL, Inc. 4350 W. Cypress Street No. 600 Tampa, Florida 33607-4155.
*To whom correspondence should be addressed. Email: [email protected].
ABSTRACT
Trickling filters (TFs) are an ideal habitat for a diverse microbial community enriched with animals, or
fauna. Fauna may have a beneficial impact on carbon-oxidizing TF performance when in proper balance,
but an infestation can be also detrimental in several ways. State-of-the-art TF-process designs must
incorporate mechanisms to manage macro fauna. Snail infestation is a common TF operational issue that
can degrade effluent quality, adversely impact biosolids handling infrastructure, and be detrimental to
costly process mechanical equipment. The most significant detrimental performance impacts include (1)
nitrifying biofilm grazing, (2) process mechanical equipment damage due to snail shells, (3) excess
biochemical oxygen demand exerted by snail bodies, and (4) increased suspended solids measurements
owing to snail shells and bodies. Little information exists in the environmental engineering literature
detailing the production rate of higher life-form predators, such as snails, in the TF process and a
corresponding lack of information on the effectiveness of snail control techniques. Only case specific
studies related to substrate transformation reaction rates or fauna mass weight measurement methods have
been used to quantify macro fauna production. This review (1) describes macro fauna that are commonly
found in TFs treating municipal wastewater, (2) surveys state-of-the-art snail removal technologies and
their reported effectiveness, (3) identifies protocol for implementing various snail removal technologies,
and (4) presents means for creating a database that can be used to establish snail production and removal in
TF-based wastewater treatment plants.
KEYWORDS: Trickling filter, snails, macro fauna, biofilm grazing, predation, nitrification, nitrifying
trickling filter, municipal wastewater treatment, biofilm reactor.
INTRODUCTION
Trickling filters (TFs) are an ideal habitat for a diverse microbial community, including both macro fauna
and micro fauna. Macro fauna are relatively large, visible, motile, animals that are predatory in nature. As
they pertain to the present study, micro fauna are biological films, or biofilms, that are responsible for the
majority of physico-chemical and biochemical reactions. The control of both macro- and micro fauna is
important to TF performance. The present study addresses macro fauna control, with a particular focus on
snails but with selective comments on other macro-fauna. Their effective control is based on an
understanding of their common phylum, class, and family. The presence of macro fauna can be both
beneficial and harmful to the TF process (Hawkes, 1955a). Beneficial impact on TF bioreactor performance
exists when fauna are in proper balance, but an infestation can be detrimental to several aspects of TFbased wastewater treatment plants (WWTPs).
TF inhabitance by macro fauna such as snails, filter flies, and worms have been documented since the early
20th century. Brown (1937) reported their troublesome production in a rock-media TF as being “twenty-five
to thirty bushels” of snails requiring removal “at least once a year and usually twice.” Ingram et al. (1958)
reported that TFs treating 150,000 m3/day (40 mgd) in Dayton, OH, produced an average of 544 kg/day
(1,200 lb/d) of snails. The snail accumulation plugged sludge lines. Metz (2007) reported that, during the
years 2003 through 2005, the Central WWTP, Baton Rouge, LA, treating an average flow less than 38,000
m3/day (10 mgd), produced snail shells at an average rate of 635 kg/day (1,400 lb/d). Filter fly larvae
Water Practice™ • Vol. 2 • No. 4 © 2008 Water Environment Federation • doi: 10.2175/193317708X335181
Boltz et al.
biofilm grazing negatively impacted performance in a pilot-scale nitrifying trickling filter (NTF) (Gujer and
Boller, 1984). Designers must be aware of a variety of predatory macro fauna that may occur in the TF
process.
In the extreme, snail infestations have been known to completely strip the TF media of biofilm, so state-ofthe-art TF-process designs incorporate mechanisms to control macro fauna accumulation and/or
development. Snail infestation is a common TF operational issue that can degrade effluent quality,
adversely impact biosolids handling infrastructure, and be detrimental to costly process mechanical
equipment. A paucity of information exists in the environmental engineering literature detailing the
production rate of higher life-form predators, such as snails, in the TF process, resulting in an absence of
data on the effectiveness of snail minimization, or removal, techniques. Generally, only case specific
studies related to substrate transformation reaction rates or fauna mass weight measurement methods have
been used to quantify macro fauna production. The present study (a) describes macro fauna that are
commonly found in TFs treating municipal wastewater, (b) surveys state-of-the-art snail removal
technologies and their reported effectiveness, (c) identifies protocol for implementing various snail removal
techniques, and (d) presents means for creating a snail production and removal database in TF-based
WWTPs.
IDENTIFICATION: PREDATOR CLASSIFICATION AND PROCESS IMPACT
Three types of organisms affect TF process performance. All are members of the kingdom Animalia, and
are generically known in practice as snails, filter flies, and worms. Snails typically found in TFs are
members of the phylum Mollusca, class Gastropoda, subclass Orthogastropoda, superorder
Heterobranchia, order Pulmonata, superfamily Planorboidea, family Physidae, and genus Physa.
Commonly known as “pouch snails” or “pond snails”, they range in size from 3 to 15 mm for young to
adult snails, respectively. Wettlaufer (1992) identified the Physa species P. gyrina and P. integra in
rotating biological contactors (RBCs) at the Skaneateles WWTP, NY. Palsdottir and Bishop (1997)
identified P. gyrina in the NTFs at the North Regional WWTP, Dayton, OH. Lacan et al. (2000) identified
P. gyrina in the NTFs at the Truckee Meadows Water Reclamation Facility (WRF), Reno, NV. Figure 1
illustrates the Physa species P. gyrina.
Figure 1. Photograph of the Common TF Snail (P. gyrina).
P. gyrina is lung breathing, metabolizes in a facultatively anaerobic manner, is relatively tolerant to anoxia,
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A Review of Operational Control Strategies for Snail…
and is resistant to desiccation (Thorp and Covich, 1991). Tolerable seasonal temperature variations for P.
gyrina range from 0 to 33°C (Hunter and Russell-Hunter, 1983). Physa and Lymnaeidae are the least
tolerant Pulmonata, and can generally withstand less than 16 to 24 hours of anoxic conditions. P. gyrina
are good scavengers of organic material, dislike saline conditions, require a minimum 2 mg/L of dissolved
oxygen, feed on nitrifiers, require dissolved calcium carbonate for shell development, and have a life span
of 15 to 18 months (Wettlaufer, 1992). Andersson et al. (1994) observed P. Fontinalis in a pilot-scale NTF
in Malmö, Sweden. Palsdottir and Bishop (1997) claimed this fresh-water Pulmonata species is resistant to
saline environments and have been known to exist at salinities of 11% in the Baltic. Thus, salt dosing may
be an effective control mechanism in some instances, but ineffective in others. Pulmonata are all
monoecious (possess both male and female reproductive systems) for a portion of their life. Eggs can be
fertilized by the same snail’s sperm or by another, but Pulmonata outcross when possible. Their ability to
move about in a TF macro environment is poorly understood, but the snails are presumed able to traverse
the biofilm covered TF media at will. Their reproductive byproduct is a gelatinous mass containing eight to
twenty-five eggs, none of which commonly hatch at the same time. The multiple egg mass is difficult to
penetrate and affixes well to most commercially available biofilm growth medium (Lacan et al. 2000).
Snails are most vulnerable directly after hatching. The number of eggs, egg development time, and
reproductive cycle is temperature dependent (Wettlaufer, 1992). Egg development occurs principally in
spring, but population spikes have been observed during the winter, which is also consistent with
observation of nematode development. Pulmonata are primarily ammonotelic (secrete ammonia) and
uroetelic (secrete urea) organisms.
Flies typically found in TFs are members of the phylum Arthropada, class Insecta, order Diptera, and
suborder Nematocera. Perhaps the most common fly includes infraorder Physcodomorpha and family
Psychodidae. However, another common filter fly includes infraorder Culicomorpha, superfamily
Chironomoidea, and family Chironomidae. Learner (1979) evaluated 67 TF facilities in the United
Kingdom and found that the most abundant fly was Psychodidae species P. alternate, which are commonly
known as “moth flies” and “sand flies”. Andersson et al. (1994) identified P. alternata in pilot-scale NTFs
in Malmö, Sweden. Gujer and Boller (1984) observed a pilot-scale NTF upset in Zurich, Switzerland, cause
by filter fly larvae (Psychodidae) grazing nitrifying biofilms. Figure 2 presents ammonia-nitrogen (NH3-N)
concentration profiles in the pilot-scale NTF during the “massive” invasion of fly larvae.
Ammonia-Nitrogen, NH3-N (mg/L)
0
2
4
6
8
10
0
100
NTF Depth (cm)
200
300
400
Day 127; 07-03-82
500
Day 69; 08-01-82
600
Day 65; 04-01-82
700
Figure 2. NH3-N Concentration Profiles Observed in a Pilot-Scale NTF During a “Massive” Filter Fly
Larvae Infestation, Day 65 and 69 Compared to Optimum Operation on the 127th Day (after Gujer
and Boller, 1984).
Psychodidae are small flies with hairy bodies and wings, which give them a “furry” appearance. The adults
have long antennae and leaf-shaped wings, either slender or broad, that have a series of parallel veins
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Boltz et al.
(without cross veins). The adult body length can range from approximately 1.5 to 4 mm. Larvae length can
range from approximately 3 to 10 mm. Adult Psychodidae are mostly nocturnal. The larvae live in aquatic
habitats (often with low oxygen). Chironomidae superficially resemble mosquitoes, but lack the wing
scales and elongated mouthparts. Adult body length can range from approximately 2 to 3 mm. Males are
recognized by their plumose antennae. Adults are known as "lake flies" or "blind mosquitoes" in North
America. Chironomidae larvae can be found in almost any aquatic or semiaquatic habitat. Larvae of some
species are bright red in color due to hemoglobin; these are often known as “bloodworms” (Coffman and
Ferrington, 1996). These filter flies are pictured in Figure 3.
Numerous worms may exist in a TF, but nematodes and annelids are discussed here. Nematodes are one of
the most common phyla of animals with over 20,000 different described species. Amongst the simplest
animal groups to have a complete digestive system, they are ubiquitous in freshwater, marine, and
terrestrial environments. Many nematodes are parasitic. Nematodes exist as members of the phylum
Nematoda. Major classes of Nematoda include Adenophorea, Chromadoria, Secernentea, Rhabditia, and
Diplogasteria. Annelids exist as members of the superphylum Lophotrochozoa and phylum Annelida.
Major classes of Annelida include Oligochaeta, Polychaeta, Clitellata, and Acanthobdellida, Rhabditia.
Andersson et al. (1994) identified the Oligochaeta families Aelosomatidae (species Aelosomatidae
hemprichi), Naididae (species Nais barbata, Nais communis, Nais elinguis), and Enchytraeidae (species
Lumbricillus lineatus) in pilot-scale NTFs in Malmö, Sweden. Depending upon the species, annelids can
reproduce both sexually and asexually. Laybourne-Perry and Woombs (1988) studied the seasonal and
spatial distribution of nematodes in full-scale rock-media TFs located in Lancaster, United Kingdom. They
observed that the majority of nematodes existed near the upper planar surface of the two TFs studied
(Figure 4). The nematode species were also stratified with length, suggesting that a relationship exists
between macro fauna concentration, biofilm composition, and fauna inoculation species (i.e., species found
in raw waters). Two peaks were also observed annually; the first during May associated with spring biofilm
sloughing cycles and the second during the winter months.
Psychodidae adult
Psychodidae larvae
Chironomidae adult
Chironomidae larvae
Figure 3. Photographs of the Common Filter Flies Psychodidae, Chironomidae, and their larvae
(www.wikipedia.org, 2007).
Macro fauna, and their larvae, graze biofilm. Curds and Hawkes (1975) reported that grazing activity in
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A Review of Operational Control Strategies for Snail…
full-scale TFs treating municipal and industrial wastewaters was affected by the nature of the biofilm and
operational variables. Possible benefits associated with the presence of macro fauna include reduced sludge
production, improved sludge settlability, and biofilm thickness control. Williams and Taylor (1968)
conducted laboratory-scale experiments demonstrating that macro fauna free systems transformed only
40% of the available organic carbon, with almost a complete absence of NH3-N transformation. In contrast,
macro fauna containing systems converted approximately 90% of the organic carbon and substantial
nitrification occurred. Fauna respiration explains the reduced biomass yield in the TF process, suggesting
that approximately 10% of carbon dioxide produced in the TF process results from macro fauna respiration
(Curds and Hawkes, 1975). Solbe et al. (1967) found that the absence of macro fauna adversely affected TF
sludge settleability. During an hour settling period, 34% of the solids settled compared to 68% when the
macro fauna were present. On the other hand, TF macro fauna may have the following detrimental impacts:
(1) grazing of nitrifying biofilm, (2) plugging of process piping, (3) damaging pumps, (4) damaging belts
on gravity-belt sludge thickening and belt-press dewatering equipment (5) organic snail bodies remaining
in the effluent stream may exert BOD5, (6) shells remaining in the effluent stream may increase effluent
fecal counts by shielding bacteria from disinfection processes, (7) may exert additional solids loading on
secondary sedimentation tanks, (8) may accumulate in aeration basins, thereby reducing aeration capacity
and/or efficiency in combined trickling filter-suspended growth (TF-SG) processes
Figure 4. Nematode Production Observed in Two WWTPs, and Their Seasonal and Spatial
Distribution with Trickling Filter Depth (Laybourne-Perry and Woombs, 1988).
CONTROL: OPERATIONAL STRATEGIES AND FACILITY IMPROVEMENTS
Several strategies have been applied to manage macro fauna accumulation and/or development in TFs,
including physical, chemical, or a combination of physical and chemical applications. The key is
application of a condition that is either toxic to the animals 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 TF-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, TF
flooding, pH adjustment with lime or sodium hydroxide, high-concentration ammonia dosing, TF humus
screening or accelerated gravity separation, gravity separation in low-velocity channels with a dedicated
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Boltz et al.
pumping circuit, eliminating dissolved oxygen from the TF feed, adding salt, draining and freezing the
infested unit, raising the temperature quickly, adding molluskacide (e.g., copper sulfate), and chlorinating
the TF’s influent stream. Many of these strategies have proven ineffective in some TFs, and others may be
detrimental to TF performance. Biochemical reactions are to some degree 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 TF and may result in the destruction of
sensitive biomass (Parker et al., 1989).
Control mechanisms reviewed here include:
• periodic high-intensity hydraulic flushing (Spülkraft –SK)
• TF flooding and chemical application
• chemical treatment (focus on high concentration NH3-N dosage and pH adjustment with sodium
hydroxide)
• TF effluent or underflow (humus) screening or accelerated gravity separation using equipment usually
associated with grit removal
• gravity separation in low-velocity channels and removal with a dedicated pumping circuit
Spülkraft. Spülkraft (SK) is the instantaneous dosing intensity as a function of distributor speed (ATV,
1983). Slowing distributor rotation increases the instantaneous dosing intensity and increases flushing
which removes excess biomass and helps to control macro fauna while increased distributor rotation
increases wetting efficiency and improves treatment efficiency. Means for controlling distributor speed
include electric drives, recirculation pumping, and reverse thrusting jets. SK (mm/pass) is calculated using
Eq. (1).
millimeter s
meter
minutes
N a ⋅ ω d ⋅ 60
hour
THL ⋅ 1,000
SK =
(1)
Na is the number of arms on the distributor, ωa is the distributor rotational speed in revolutions per minute
(rev min-1), the total hydraulic load (THL) is Q/A where Q is the influent to the TF (influent + recycle,
m3/hr), and A is the TF plan area (m2). When mechanical distributor speed control is used the definition of
SK may be inverted to calculate the rotational speed required to achieve flushing intensity. Figure 5 depicts
an electrically driven TF wastewater distribution mechanism, which is one method of controlling rotary
distributor speed. The optimum SK is still to be defined and, to a degree, may be site- and applicationspecific. Current suggestions for SK are listed in Table 1 (WEF, 1998). If applied, the flushing SK period is
typically 5 to 10% of the 24-hour period and will operate at 6 to 15 times the routine operating SK, which
emphasizes the need for a significant range in rotational speed control. WWTPs retro-fitted with distributor
speed modulation may initially observe (2 to 10 weeks) higher settled TF effluent BOD5 and TSS as these
sloughed solids are not readily settleable. This is similar to sloughing cycle effects common to many TF
facilities.
Hawkes and co-workers (1955; 1963) demonstrated that high hydraulic loadings and instantaneous dosing
rates can control filter fly development, as illustrated in Figure 6. Increased hydraulic loading improves TF
media wetting efficiency, reducing TF media dry spots and eliminating ideal spawning areas for filter flies.
Gujer and Boller (1984) reported that filter fly larvae were reduced to quantities that did not have NTF
process impact by hydraulic application. The THL must be sufficient to guarantee complete media wetting.
Pilot-scale NTFs, with a medium-density cross-flow media (XFM) and fixed distributors, required a THL
of 3 m/hr. Grady et al. (1999) later reported that adequate media wetting may be achieved at THLs of 1.8 to
2 m h-1 with rotary distributors. In contrast, Andersson et al. (1994) tested three flushing intensities (SK
values of 5, 40, and 80 mm/pass) and reported that the variable flushing intensity had no apparent affect on
filter fly and worms in a pilot-scale NTF. Note that these values are below those reportedly needed for
flushing, as presented in Table 1.
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A Review of Operational Control Strategies for Snail…
TABLE 1
Recommendation for Rotary Distributor SK Rates (WEF, 1998)
Total Organic Load
Operating SK
Minimum SK
Flushing SK
(kg/d.m3)
(mm/pass)
(mm/pass)
(mm/pass)
0.25
20-60
18
270
0.50
20-60
20
300
1.00
30-80
24
365
2.00
40-100
30
490
3.00
50-150
40
615
60-150
50
740
4.00
3
3
kg/d.m x 62.4 = lbs/d.1,000 ft
flushing SK = 240 + 125 BOD5 loading
Figure 5. Typical Electrically Driven Rotary Distributor.
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Boltz et al.
Figure 6. Biofilm and Filter Fly Control with Low-Frequency Dosing (Hawkes, 1963).
Flooding
TF flooding requires adequate duty units to isolate a TF for a period of 3 and 6 hours. The TFs must be
designed as water-retaining structures, which is not typical and represents a small fraction of existing TFs.
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 32-m diameter, 7.3-m deep medium-density XFM NTFs at the LittletonEnglewood WWTP, CO. On-line pH probes and a sodium hydroxide metering system allow for flood water
pH adjustment by set point. The alkaline flood water is pumped in through the bottom of the NTFs,
discharges into an overflow trough, and 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 full-grown snail development showed 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 the
chemicals’ application rate. RBCs in Lafayette, LA, applied sodium chloride at a dosing concentration of
10 mg/L for a 24-hour period to effectively control the snail population. Calcium hypochlorite at 60 to 70
mg/L for a 2 to 3-day period effectively controlled snails in RBCs at the Deer Creek WWTP, Oklahoma
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A Review of Operational Control Strategies for Snail…
City, OK. Copper sulfate at low concentrations (0.45 kg of copper sulfate per 3.785 m3) may effectively
control snail accumulation.
Ammonia is toxic to snails (Arthur et al. 1987). Lacan et al. (2000) conducted a laboratory-scale study and
plant-scale application of undissociated aqueous ammonia (NH3-N(aq)) solutions with elevated pH to control
snail growth (P. gyrina) in NTFs. Undissociated aqueous NH3-N(aq), not the ammonium ion, is the snail P.
gyrina toxophore. The concentration producing 100% mortality is a function of exposure time and the bulkliquid NH3-N(aq) concentration. The laboratory-scale study demonstrated that an ammonium chloride
(NH4Cl) solution at pH 9.2 (NH3-N(ag) = 150 mg N/L) resulted in 100% snail mortality. A much higher
concentration of ammonia in a TF’s influent stream (1,000 to 1,500 mg N/L) is necessary in practice 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 (according to TF
bioreactor hydrodynamics). Lacan et al. (2000) estimated that an influent ammonia concentration of 1,080
mg N/L resulted in an average concentration throughout the NTF of 185 mg N/L. Such a highconcentration NH3-N(aq) stream may be readily available in municipal WWTPs as solids processing recycle
streams. In some cases, 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, NV, which uses high density (215 m2/m3) XFM. 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 between 9.0 and 9.5), which increased the NH3N(aq) content of the centrate solution, as illustrated in Figure 7. Applied once per month, during application
a NTF is isolated and the solution recirculated through the TF for approximately 2 hours. The first 20 to 50
minutes is dedicated to reaching hydrodynamic steady-state, 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 “Flushing Mode” for ten hours. Periodic grab samples or on-line monitoring
during the flushing cycle is necessary to insure the effluent NH3-N concentration is less than or equal to the
influent concentration before the NTFs were returned to service. Lucero et al. (2002) observed that a
similar procedure on carbon-oxidizing TFs required less than 8 hours.
Lacan et al. (2000), who reported that the use of ammonia for snail control has also been applied to carbonoxidizing TFs, suggested that the above-described procedure will not produce deleterious effects in the
carbon-oxidizing TFs because of the lower sensitivity of the heterotrophic bacteria to pH and NH3-N, and
more expedient regeneration of the heterotrophic biofilm. Rather than a reduced flushing period, it may be
possible to eliminate the 10-hour backwashing step if the carbon-oxidizing TF-based WWTP is not subject
to NH3-N permit limitations and has typical pH discharge standards of “6 to 9” (Lacan et al. 2000). Lucero
et al. (2002) applied this snail control operation to combined carbon oxidizing and nitrification TFs in a
trickling filter/solids contact (TF/SC) process at the Duck Creek Wastewater Treatment Center, Garland,
TX. The TF/SC process returned to full nitrification capacity after a period of declining performance,
subsequent to snail treatment. Diminishing nitrification performance because of an inhibitory substance in
the raw sewage was later reported.
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Figure 7. Nitrifying Trickling Filter Operating Modes for High-Concentration Undissociated
Aqueous Ammonia Dosing (Lacan et al. 2000)
Mechanical Control.
Physical removal techniques described here include: (a) TF effluent or underflow (humus) screening, (b)
gravity separation in low-velocity channels and removal with a dedicated pumping circuit, and (c)
accelerated gravity separation using equipment usually associated with grit removal.
The Central WWTP, Baton Rouge, LA, utilizes TF secondary clarifier underflow screening to control snail
accumulation in, or damage to, solids handling equipment. Through a series of statistical analyses Lin and
Sansalone (2001) found that snail infestation in another TF-based WWTP in Baton Rouge, LA, does not
affect BOD5 removal efficiency in carbon-oxidizing TFs. However, snail shells fill gravity thickening
tanks, plug sludge process piping, and damage belts in sludge thickening and dewatering equipment. A
structure containing parallel static screens was subsequently erected with underflow pumps directing the TF
humus through them. Snail shells retained by the screens fall by gravity into a collection bin, and the waste
sludge flows through the screens to sludge thickening. Although effective in protecting biosolids handling
equipment, the system illustrated in Figure 8 is reported to be a source of odor.
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Water Practice™ • Vol. 2 • No. 4 © 2008 Water Environment Federation • doi: 10.2175/193317708X335181
A Review of Operational Control Strategies for Snail…
Figure 8. Underflow Snail Screening Device (Metz, 2007).
The City of Lawton, OK, (49,000 m3/day) and both the South San Luis Obispo, CA, County Sanitation
District Oceana Regional Plant (19,000 m3/day) and the City of San Luis Obispo WRF, San Luis Obispo,
CA, (9,000 m3/day) pump TF secondary clarifier underflow to a free vortex classifier for snail shell
removal. Each of these systems is reported to prevent excessive snail shell accumulation in digesters. The
units remove approximately 0.69-, 0.076-, and 0.23 cubic meters of snails per day, respectively (Neumayer
2002).
The Econchate Water Pollution Control Plant (WPCP), Montgomery, AL, removes snail shells remaining
in the secondary effluent which were shielding pathogens from chlorine disinfection. The chlorine contact
basin was modified to a two-pass 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, pumped through a static screen where they fall by gravity into a collection bin. The wastewater is
returned to the chlorine contact basin for disinfection and final disposal.
Tekippe et al. (2006) reported the use of baffles, grit pumps and classifiers to remove snails from the Ryder
Street WWTP, Vallejo, California, USA. The facility treats wastewater with a TF-SG process consisting of
two 32-m diameter and 7.3-m deep XFM TFs. The TFs were reported to produce snail shells in quantities
that hindered aeration basin diffuser performance, requiring both the aeration basins and the secondary
clarifiers to be taken out of service for cleaning and maintenance. SK was ineffective to control snail
accumulation in the TFs. The properties of the snail shells were found to be similar to grit; essentially light
enough to be pumped by conventional centrifugal pumps. A small portion of the snails have air-entrained in
their shells and others may be disconnected from their shell, so a fraction of the snails in a TF effluent may
float or be neutrally buoyant. The influent of the Ryder Street WWTP’s aeration basins was subsequently
improved to provide a zone for the majority of the shells to settle and an automatic mechanism to remove
the settled shells (Tekippe et al., 2006). Redwood baffles were inserted to the basin entrance to distribute
inlet velocity, and others downstream to contain the snail shells in the first 6 meters of the rectangular
aeration basins. The basin floor was sloped to prevent snail shell accumulation in the corners and direct the
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snails to a sump, and the aeration system in this region of the basins was modified to promote settling with
a rolling flow pattern similar to that in an aerated grit removal chamber. The shells are then conveyed, via
grit pump, to a grit cyclone and classifier system adjacent to the aeration basin influent pump station. The
classifier discharges to a bin, and the shells are transported to final disposal. Tekippe et al. (2006) reported
that, while the system removed approximately 49,000 m3/day during the first few days of operations, since
start up the quantity of shells stabilized to approximately 1.53 m3/day.
OBSERVED SNAIL SHELL YIELD
While several methodologies for determining the spatial distribution, concentration, and mass of snails
produced in TF bioreactors have been reported, these efforts are largely crude and labor intensive. This
section addresses appropriate means for recording snail production, and understanding the applicability of
the measurements. The discussion is designed to encourage the creation of an industry-wide database
describing snail production, which will require frequent measurement of TF snail production. Two
procedures are described: (1) a sampling regime and (2) a mass measurement of snail shells and debris
removed from the wastewater treatment process.
Macro fauna production in a TF is similar to biomass production. A TF influent stream has sufficient larvae
to inoculate the bioreactor, but from a mass production perspective negligible macro fauna enter the
bioreactor through the influent stream. While transformation processes, mediated by substrate availability,
result in a net production of snails, the growth rate and relative influence of governing parameters (e.g., SK,
temperature, substrate availability, environmental conditions, biofilm thickness, bacterial distribution and
type) are poorly understood. A snail-shell yield (Ysnail shell) can be calculated, however, and used to quantify
production, used by operators to project future disposal costs and storage requirements, and by engineers
when designing future WWTP expansion(s). The TF effluent stream can be sampled to determine snail
shell concentration. Data obtained from the effluent stream is most conveniently analyzed if the sample is
taken down stream of recirculation, but accumulation in the process such as a recirculation pump station
must be considered. If the sample must be collected up-stream of the recirculation pump station, the snailshell load imparted by the recirculation stream must be accounted for by subtracting the concentration in
the TF influent stream from the concentration in the effluent stream. TF flow streams are illustrated in
Figure 9.
Qe, Xe
Q0, X0
Qi, Xi
QR
Figure 9. Typical Trickling Filter Flow Scheme. Q0, X0 = influent parameter prior to receiving the
recirculation stream; Qi, Xi = influent parameter after receiving the recirculation stream; Qe, Xe = effluent
parameter.
The following Equation 2 may eliminate the need to collect influent snail-shell concentrations as it
expresses the influent snail shell concentration as a function of the effluent concentration and the
12
Water Practice™ • Vol. 2 • No. 4 © 2008 Water Environment Federation • doi: 10.2175/193317708X335181
A Review of Operational Control Strategies for Snail…
dimensionless recirculation ratio, R, or
X
snail shells
TFi
=
QR
.
Q
snail shells
snail shells
X TF
+ R ⋅ X TF
0
e
1+ R
snail shells
⇒ X TF
≈0
0
(2)
snail shells
X TF
is the snail shell concentration in the TF influent stream prior to recirculation (mg/L),
0
snail shells
X TF
is the snail shell concentration in the TF influent stream after recirculation (mg/L), and
i
snail shells
X TF
is the snail shell concentration in the TF effluent stream (mg/L). When coupled with the flow
e
rate, Q (m3/day), the snail load, L (kg/day), can be calculated with Equation 3.
snail shells
Lsnail shells = X TF
⋅ Q / 1,000
e
(3)
The potential for the addition of snail shells from other recycle streams must also be considered, especially
for streams other than the TF effluent.
Neumayer (2002) also observed that the snail shells may range from 3 to 15 mm. Therefore, a wire mesh
equivalent to a standard sieve number 30 (0.5 mm) can be used to separate the snail shells from other solid
material in the sample. Properly sized degritting systems will generally remove particles greater than 0.1- to
0.3-mm equivalent spherical diameter, and particles greater than a 0.1-mm equivalent spherical diameter
are generally removed by discrete settling in properly sized primary sedimentation basins. If debris is
observed amongst the snail shells, however, it may be necessary to wash the sample with distilled water
prior to weight measurement. Both wet and dry snail shell mass measurements are to be made. It is
imperative that the sample be collected from a well mixed flow to ensure that they are representative. The
snail shell mass measurements will consist of empty snail shells, shells inhabited by snails, and snails that
have been separated from their shells. Therefore, these measurements are used primarily to estimate future
mass requiring disposal and to develop snail shell peaking factor similar to those created for influent flow
and loading conditions. These peaking factors will only provide an estimate for similar operating
conditions, but the snail shell yield (Ysnail shell) will fluctuate with a variety of conditions. Until a broad data
base exists, generalizations cannot be made with these measurements.
The determination of snail shell yield (Ysnail shell) can be described mathematically by Equation 4.
Ysnail shell =
snail shells
X TF
kg snail shells
effluent
=
kg BOD5 removed TBOD5in − TBOD5eff
(4)
TBOD5 is the total biochemical oxygen demand (mg/L). Applying this analysis to the data collected during
2003 at the Central WWTP, Baton Rouge, LA, the average snail shell yield is 0.028 kg snails/kg TBOD5
removed, with a standard deviation is 0.043 kg snails/kg TBOD5 removed and variance is 0.00188. Figure
10 presents long-term data for this WWTP. Similar to the observations of nematodes presented by
Laybourne-Perry and Woombs (1988), the Central WWTP is subject to peak snail shell loading during the
early spring months. This may be due to spring sloughing and macro fauna spawning cycles. Figure 11
plots the daily mass of underflow (predominantly snail shells) retained on static screens (2.54-mm
openings) that receive TF clarifier underflow as a function of WWTP influent BOD5 and total suspended
solids (TSS) loading, respectively.
13
Water Practice™ • Vol. 2 • No. 4 © 2008 Water Environment Federation • doi: 10.2175/193317708X335181
Boltz et al.
25,000
BOD5
TSS
20,000
Snails (Screened from underflow)
Load
(lbs/d)
15,000
10,000
5,000
Ja
n
Fe - 0 3
b
M -03
ar
Ap -03
M r-0
ay 3
Ju -03
n
J u -0 3
Au l-0
3
Seg-0
p- 3
O 03
c
N t-03
ov
D -0
ec 3
J a -0 3
n
Fe - 0 4
bM 04
a
Apr-04
M r-0
ay 4
Ju -04
n
J u -0 4
Au l-0
4
Seg-0
p- 4
O 04
c
No t-04
D v-0
ec 4
J a -0 4
n
Fe - 0 5
bM 05
a
Apr-05
M r-0
ay 5
Ju -05
n
J u -0 5
Au l-0
5
Seg-0
p- 5
O 05
c
N t-05
ov
D -0
ec 5
-0
5
0
Date (month-yy)
Snail Screened from Underflow
(lbs/d)
Figure 10. BOD5, TSS, and Underflow Snail Shell Masses Observed at the Central WWTP.
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
14000
16000
18000
Snail Screened from Underflow
(lbs/d)
Five-Day Biochemical Oxygen Demand (lbs/d)
4,000
3,500
3,000
2,500
2,000
1,500
1,000
500
0
0
2000
4000
6000
8000
10000
12000
Total Suspended Solids (lbs/d)
Figure 11. Snail Mass Screened from Trickling Filter Underflow at the Central WWTP as a Function
of Five-Day Biochemical Oxygen Demand and Total Suspended Solids Loading Influent to the
Facility.
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Water Practice™ • Vol. 2 • No. 4 © 2008 Water Environment Federation • doi: 10.2175/193317708X335181
A Review of Operational Control Strategies for Snail…
CONCLUSIONS
In summary, the following can be concluded:
1. Macro fauna are not necessarily nuisance animals, but infestations may lead to over grazing of
nitrifying biofilm, plugging of process piping, damaging pumps, damaging belts on gravity-belt sludge
thickening and belt-press dewatering equipment, organic snail bodies remaining in the effluent stream
may exert BOD5, shells remaining in the effluent stream may increase effluent fecal counts by
shielding bacteria from disinfection processes, may exert additional solids loading on secondary
sedimentation tanks, and may accumulate in aeration basins; thereby reducing aeration capacity and/or
efficiency in TF-SG processes.
2. Macro fauna control, particularly snail control, is an integral facet of modern TF design. State-of-theart control strategies include: periodic high-intensity hydraulic flushing, TF flooding and chemical
application, chemical treatment (focus on high concentration NH3-N dosage and pH adjustment with
sodium hydroxide), TF effluent or underflow (humus) screening, gravity separation in low-velocity
channels and removal with a dedicated pumping circuit, and accelerated gravity separation using
equipment traditionally associated with grit removal.
3. When operational control strategies are implemented, a database should be established describing snail
shell production.
Snail shell production can be quantified, as illustrated at the Central WWTP, Baton Rouge, LA. A data
analysis technique describing snail production in terms of yield (similar to biomass yield) was presented.
The average snail shell yield observed in TF clarifier underflow at the Central WWTP was 0.028 kg
snails/kg TBOD5 removed.
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