trickling filter and trickling filter-suspended growth process design and operation

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TricklingFilterandTricklingFilter-SuspendedGrowthProcessDesignandOperation:AState-of-the-ArtReview

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Trickling Filter and Trickling Filter-Suspended Growth Process Design and

Operation: A State-of-the-Art ReviewGlen T. Daiggerl*, Joshua P. Boltz 2

ABSTRACT: The modem trickling filter typically includes the follow-ing major components: (1) rotary distributors with speed control; (2)modular plastic media (typically cross-flow media unless the bioreactor istreating high-strength wastewater, which warrants the use of vertical-flowmedia); (3) a mechanical aeration system (that consists of air distributionpiping and low-pressure fans); (4) influent/recirculation pump station; and(5) covers that aid in the uniform distribution of air and foul aircontainment (for odor control). Covers may be equipped with sprinklersthat can spray in-plant washwater to cool the media during emergency shutdown periods. Trickling filter mechanics are poorly understood. Conse-quently, there is a general lack of mechanistic mathematical models anddesign approaches, and the design and operation of trickling filter andtrickling filter/suspended growth (TF/SG) processes is empirical. Someempirical trickling filter design criteria are described in this paper.Benefits inherent to the trickling filter process (when compared withactivated sludge processes) include operational simplicity, resistance totoxic and shock loads, and low energy requirements. However, tricklingfilters are susceptible to nuisance conditions that are primarily caused bymacro fauna. Process mechanical components dedicated to minimizing theaccumulation of macro fauna such as filter flies, worms, and snail (shells)are now standard. Unfortunately, information on the selection and designof these process components is fragmented and has been poorlydocumented. The trickling filter/solids contact process is the mostcommon TF/SG process. This paper summarizes state-of-the art designand operational practice for the modem trickling filter. Water Environ.Res., 83, 388 (2011).

KEYWORDS: trickling filter, trickling filter/suspended growth, trick-ling filter/solids contact, biofilm, nitrification, design, operation.

doi: 10.2175/106143010X12681059117210

IntroductionUntil the 1950s, trickling filter design protocol was scattered

and empirical in nature. Then, during the 1950s and 1960s, theDow Chemical Company began experimentation with modularsynthetic media (Bryan, 1955). Numerous trickling filter processstudies were conducted during the same period (Eckenfelder,1961; Galler and Gotaas, 1964; Germain, 1966; Schulze, 1960),which led to the development of generally accepted designcriteria. After the U.S. Environmental Protection Agency issuedits 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:[email protected] CH2M HILL, Tampa, Florida.

388

consistently produce effluent water quality that met the publishedstandards, in part, because of poor secondary sedimentation tankdesign (Parker, 1999). Norris et al. (1982) developed the tricklingfilter-solids contact (TF/SC) process in response. The first full-scale TF/SC process included a rock-media trickling filterfollowed by a small aeration basin (receiving return sludge) andflocculator clarifier. The researchers demonstrated that wastewa-ter treatment plant (WWTP) effluent water quality could begreatly improved by bioflocculation in the solids contact basin andimproved secondary clarifier design. Combined trickling filter-suspended growth (TF/SG) processes preceding the TF/SCprocess were designed with the suspended growth reactorprimarily for oxidation. This paper describes state-of-the arttrickling filter and TF/SG process design and operation.

General DescriptionA trickling filter is a three-phase system with fixed biofilm

carriers. Wastewater enters the bioreactor through a distributionsystem, trickles downward over the biofilm surface, and air movesupward or downward in the third phase. Trickling filtercomponents typically include a distribution system, containmentstructure, rock or plastic media, underdrain, and ventilationsystem. Figure 1 illustrates a trickling filter cross section andtypical bioreactor components. Wastewater treatment using thetrickling filter results in a net production of total suspended solids.Therefore, liquid-solids separation is required, and is typicallyachieved with circular or rectangular secondary clarifiers. Thetrickling filter process typically includes an influent pump station,trickling filter, trickling filter recirculation pump station, andliquid-solids separation unit.

Distribution System. Primary effluent (or screened, 3-mm,and degritted wastewater) is either pumped or flows by gravity toa trickling filter distribution system. The distribution systemintermittently distributes wastewater over the trickling filterbiofilm carriers. The distributors may be hydraulically orelectrically driven. The intermittent application allows for restingperiods during which aeration occurs. Efficient influent wastewa-ter distribution results in proper media wetting. Poor mediawetting may lead to dry media pockets, ineffective treatmentzones, and odor. Essentially, there are two types of distributionsystems: fixed-nozzle and rotary distributors. Because theirefficiency is poor, distribution with fixed nozzles should not beused (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

Rotarydistributor

/ FRP GratingUnderdrain

AirpipeEffluent

N Influent

Figure 1-Typical trickling filter cross section and bioreactor components.

desired instantaneous flushing rate to the trickling filter. Figure 2depicts both a modem hydraulically driven rotary distributor thatuses gates (controlled by variable frequency drive) that eitheropen or close distributor orifices to adjust rotational speed and anelectrically driven rotary distributor. Use of a variable-speed driveand electronic controller allow for the more precise conrol ofdistributor-arm speed. Electrically driven rotary distributors havemotorized units that control distributor speed independent of thewastewater pumped flow.

Biofirm Carriers. Ideal trickling filter biofilm carriers, ormedia, 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 includerock, random (synthetic), vertical-flow (synthetic), and cross-flow(synthetic). Both vertical-flow and cross-flow media are constructedwith smooth and/or corrugated plastic sheets. Another commerciallyavailable synthetic media, although not commonly used, arevertically hanging plastic strips. Horizontal redwood or treatedwooden slats have also been used, but are typically no longerconsidered because of their high cost or limited supply.

Modules of plastic sheets (i.e., self-supporting vertical-flow orcross-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 Bulk Spedfic Surface Voidm Density Area Space() kghr3 (m2/m3) (%)

(lbs/) )b

Rock

0.024 - 0.076

(0.08 -0.25)

0.076 -0.128(0.25 - 0.42)

0.61 x 0.61 x 1.22

(2 x 2 x 4)

0.61 x 0.61 x 1.22(2 x 2 x 4)

0.185 o x 0.051 H(7.3" o x 2" H)

24-45 100 and 223(1.5 -2.8) (30, 48, and 68)

24-45(1.5 -2.8)

27(1.7)

102 and 131(31 and 40)

98(30)

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/ft3 x 16.02 = kg/m3.b f92/ft3 X 3.281 = m2

/M 3 .

improved trickling filters. However, several trickling filters withrock media exist and are capable of meeting treatment objectiveswhen properly designed and operated. Table I compares thecharacteristics of various biofilm carrier types. The higher specificsurface area and void space in modular synthetic media allow forhigher hydraulic loading, enhanced oxygen transfer, and biofilmthickness control in comparison to rock media.

390

Ideally, rock media have a 50-mm diameter, although they mayrange in size. Rounded (river) rock helps mitigate issuesassociated with rigid rock (slag) media. The slag rock containscrevices that can retain water and accumulate biomass. Because ofstructural requirements associated with the large unit weight of therock media, rock media are shallow in comparison to syntheticmedia trickling filters and are more susceptible to excessive


River

Slag

Plastic'

1442

(90)

1600 (100)

62

(19)

46

(14)

50

60

Cross flow

PVC

95

Vertical flow

Random2

PVC

95

95

Daigger and Boltz

cooling. Trickling filter performance aside, excessive cooling cansubject media to freeze-thaw cycles. Water retained inside slagrock crevices may expand and sever rock fragments. This canresult in fine material accumulation which, together with retainedbiomass, is a primary contributor to rock-media trickling filterclogging (Grady et al., 1999). Generally, rock media areconsidered to have a low specific surface area, void space,shallow depth, and high unit weight. Although recirculation iscommon, the low void ratio in rock-media trickling filters resultsin reduced hydraulic application rates.

Excessive hydraulic application can result in ponding, limitedoxygen. transfer, and poor bioreactor performance. The perfor-mance of existing rock-media trickling filters can be improved byproviding forced ventilation, distributor speed control, solidscontact channels, and/or deepened secondary clarifiers thatinclude energy dissipating inlets and flocculator-type feed wells.Replacement or deepening of the rock media (with syntheticmedia) is often requisite in instances where the rock media qualityis poor, space is limited, and WWTP expansion (using a tricklingfilter or TF/SG process) is expected. However, a well-designedand operated rock-media trickling filter can provide high-qualityeffluent. Grady et al. (1999) suggest that for low organic loads(less than 1 kg 5-day biochemical oxygen demand [BOD5]/dim 3),well-designed and operated rock-media trickling filters arecapable of providing performance approaching that of synthetic-*media trickling filters. However, as organic load increases, there islikely to be fewer nuisance problems and reduced potential forplugging with the use of synthetic biofilm carriers.

Synthetic biofilm carriers (for trickling filters) are generallyconsidered to have a high specific surface area and void space andlow unit weight. Due to the reduced unit weight, synthetic mediatrickling filters can be constructed at depths in excess of 3 timesthat for a comparably sized rock-media trickling filter. Modularplastic trickling filter media are typically manufactured with thefollowing specific surface areas: 223 m2/m3 (68 ft2/ft3) as highdensity, 138 m2/m3 (42 ft2/ft3) as medium density, and 100 m2/m3

(30 ft2/ft3) as low density. Both vertical flow and cross-flow mediaare reported to effectively remove BOD 5 and total suspendedsolids (TSS) (Aryan and Johnson, 1987; Harrison and Daigger,1987). Cross-flow modules provide increased treatment efficiencycompared to vertical-flow modules of the same specific surfacearea at low-to-medium volumetric organic loading rates (less thanabout 2.5 kg BOD5/d/m 3), but vertical-flow modules may provideadvantages at higher volumetric organic loading rates (Harrisonand Daigger, 1987). The effects of media type and configurationon trickling filter effluent water quality should be'given carefulconsideration by the designer.

Plastic modules with a specific surface area in the range of 89 to102 m2/m3 are well suited for carbon oxidation and,combinedcarbon oxidation and nitrification. Parker et al. (1989) recom-mended medium-density cross-flow media, and recommendedagainst the use of high-density cross-flow media in nitrifyingtrickling filters (NTFs). This argument is supported by pilotapplication data and conclusions of Gujer and Boiler (1983, 1984)and Boller and Gujer (1986), which show higher nitrification ratesfor lower density modular synthetic media. The researchers claimthat lower rates occur with high-density media due to thedevelopment 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

also reduces the potential for plugging. These recommendationswere developed before the more widespread use of speed-controlled rotary distributors, which may help to overcome thesehydraulic distribution issues. Vertically oriented modular plasticmedia are generally accepted as being ideally suited for high-strength wastewater (perhaps industrial) or high organic loadingssuch as with a roughing trickling filter. In some instances, cross-

flow media have been placed in the top layer of a trickling filtercontaining vertical-flow media to enhance wastewater distribu-tion, with vertical-flow media comprising the remainder of thetrickling filter media.

Containment Structure. Rock and random plastic media arenot self-supporting and, ,therefore, require support from thecontainment structure. Typically, containment structures areprecast or formed concrete tanks. When self-supporting mediasuch as plastic modules are used, materials such as wood,fiberglass, and welded and bolted (coated) steel have also beenused as containment structures. The containment structure servesto avoid wastewater splashing and to provide media support, windprotection, and, sometimes, flood containment.

Underdrain System and Ventilation. The trickling filterunderdrain system is designed to meet two objectives: collecttreated wastewater for conveyance to downstream unit processesand create a plenum that allows for the transfer of air throughout thetrickling filter media (Grady et al., 1999). Clay or concreteunderdrain blocks are commonly used for rock-media tricklingfilters because of the required structural support. A variety of

,support systems including concrete piers and reinforced fiberglassgrating are applied for other media types. Figure 3 depicts field-adjustable plastic stanchions and fiberglass-reinforced plasticgrating to support modular plastic media on the concrete floor of atrickling filter containment structure and high-density polyethylenemats used to support random synthetic media. The volume betweenthe concrete slab and media bottom creates the underdrain.

Trickling Filter Pump Stations: Influent and Recircula-tion. A critical unit in the trickling filter system is a pump stationthat lifts primary effluent and recirculates trickling filter underflow.Generally, trickling filter underflow should be recirculated at a raterequired 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 hydraulicload is insufficient to provide the recommended dosing rate). The

intent of recirculating bioreactor effluent is to decouple hydraulicand organic loading. Although effluent from the secondary clarifiercan be recirculated, this is not common practice because it maylead to hydraulic overloading of secondary clarifiers. Influentpumping is typically selected to allow trickling filter underflow toflow by gravity to the suspended growth reactor (or solids contactbasin), secondary clarifier, or another unit downstream of thetrickling filter.

Trickling filter recirculation pumps are typically constant-speed, low-head centrifugal units designed to operate with a total

head equivalent to the static head, comprised of the trickling filtermedia depth of approximately 3 to 7 m (depending on mediadepth), the distance between the distributor outlet and the top ofthe media, and the distance between the bottom of the media andthe water surface in the underdrain, along with associated frictionlosses (Boltz et al., 2009). Variable frequency drive controlledmotors are typical fixtures on process pumps. Submerged ornonsubmerged (dry-pit) vertical pumps have been used exten-

May 2011 391

Daigger and Boltz

Figure 3-Adjustable plastic stanchions and fiberglass-reinforced plastic grating on the concrete floor of a boltedsteel containment structure (left), and a high-densitymedia (right).

sively. Pump intake screens are usually unnecessary becauserecirculated flow is typically free of clogging solid materials.Hydraulic computations are always necessary. Computations forminimum flow are necessary to ensure adequate head to drivehydraulically driven distributors; computations for maximum flowindicate the head required to ensure adequate discharge capacity.The net available head at the horizontal center line of thedistributor's arm and other points may be calculated by deductingthe following applicable losses from the available static head:entrance loss, friction losses in the piping to the distributor, properallowance for minor head losses, head loss through distributor riserand center port, friction loss in distributor arms, and velocity head ofdischarge through nozzles necessary to start the hydraulically drivenrotary distributor. Trickling filter distribution head requirements areset by the system manufacturer. Despite head loss due to thetrickling filter commonly being the greatest in a given WWTP,power requirements for the process (including recirculationpumping and auxiliary powered equipment) are typically signifi-cantly less than those for the activated sludge process.

Process Flow Sheets and Bloreactor ConfigurationTrickling filter and combined TF/SG processes typically consist

of preliminary treatment (including screening and grit removal),primary clarification, trickling filter, bioreactor, secondaryclarification, and disinfection unit processes. Trickling filterrecirculation methods influence the process flow. Generally, thereare two types of trickling filter recirculation. The first allows fordirect recirculation to the trickling filter and the second passesflow through a primary clarifier. Four trickling filter process flowdiagrams, including both single- and two-stage trickling filters, areshown in Figure 4. Combined TF/SG process flow sheets aresimilar, but include a suspended growth reactor and returnactivated sludge (or return sludge for the TF/SC process) streamthat is directed to the head of the suspended growth reactor.Recirculation of trickling filter underflow or settled effluentdilutes influent wastewater, dampens the influent organic loadingvariability resulting from diurnal fluctuations, and maintainsrequired trickling filter hydraulic application rates. Clarifyingtrickling filter effluent may enhance the performance of asubsequent trickling filter in two-stage operation, but the designer

392

polyethylene mat used to support random synthetic

must ensure that the recirculation flow required for trickling filterwetting and biofilm thickness control does not exceed the limitinghydraulic loading rate for the intermediate clarifier. The design ofsettling tanks in two-stage trickling filter systems is also affectedby the recirculation pattern.

Sludge wasting and recirculation streams affect the trickling.filter process. Each of the process flow diagrams illustrated inFigure 4 directs waste biological sludge (which is sometimesreferred to as humus in the trickling filter process) to the primaryclarifiers where it is co-settled with primary sludge prior to beingwithdrawn from the system. Many facilities exist that withdrawand thicken primary and biological sludge separately.

Bioreactor Classification. Trickling filters can be classifiedas roughing, carbon oxidation, carbon oxidation and nitrification,and tertiary nitrification. Table 2 summarizes characteristics ofeach trickling filter. The performance ranges are associated withaverage design condition. Single-day or average-week observa-tions may be significantly greater.

Hydraulics. Recirculation and distributor operation areimportant to good trickling filter performance and may be usedto achieve proper media wetting, flow distribution, biofilmthickness control, and to prevent macro fauna accumulation.Albertson and Eckenfelder (1984) postulated that the activebiofilm surface area in a trickling filter is dependent on biofilmthickness and media configuration, and that active biofilm surfacearea decreases with increasing biofilm thickness. The researchersstated that for medium-density cross-flow media with a 98-m 2/m3

specific surface area, a 4-mm increase in biofilm thickness wouldcause a 12% reduction of active biofilm area (assuming that all themedia have been appropriately wetted). Poor trickling filter mediawetting results in reduced effluent water quality. In a study ofrotary distributor efficiency, Crine et al. (1990) found that thewetted area-to-specific-surface-area ratio ranged from 0.2 to 0.6with the lowest values for high-density random pack tricklingfilter media. Many of the design formulations mentioned later inthis paper incorporate a term that allows for specific surface areareduction due to distributor inefficiency in trickling filter mediawetting. The interrelationship of liquid residence time, dosing, andmedia configuration on BOD 5 removal kinetics has not beenaddressed, and additional research is required. Increasing the


........... ------

P 4 Pip,*'0 W WS*W_

4W

'4W 4W 410 6 g.NrWrQ:&* * .0 '0' ý1W 4* *

4%.Wo.

,j:W4:,O P A. 4W V .4 P,

-b Z

-1b 4k. 41k. Ow 4ý-

bd. bdb I

r

Daigger and Boltz

(A)

(B)

(c)

(D)

RS

- ~ ~ vWSL I

I. - T

WS

Figure 4-Typical trickling filter process flow sheets.Legend: (RS)-raw wastewater, (PC) primary clarifier, (PS) primary sludge, (PE) primary effluent, (TFINF) tricklingfilter influent, (TF) trickling filter, (TFEFF) trickling filter effluent, (TFRCY) trickling filter recycle, (SC) secondaryclarifier, (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 filterprocess with intermediate clarification.

average hydraulic application rate reduces the liquid residencetime, but has been proven to increase wetting efficiency. Therecirculation ratio (Q/QR) is typically in the range 0.5 to 4.0.Bryan (1955, 1962) and Bryan and Moeller (1960) demonstratedthat vertical-flow media require an average application rategreater than 1.8 m3/m2 /h to maximize BOD 5 removal efficiency.Shallow towers using cross-flow media have used hydraulic ratesin the range 0.4 to 1.1 m3/m2 /h. Grady et al. (1999) state that

adequate media wetting may be achieved at a total hydraulic load(THL) of 1.8 to 2 m3/m2 /h with rotary distributors.

Distributor speed control has the following benefits: controlledflow interruption (periodicity of dosing), increasing. wettingefficiency (percent of media wetted), and biofilm thicknesscontrol. The designer should consider recirculation capabilitiesand the effect of reverse thrusting jets with the use of distributorspeed control. Distributor speed control may not be required in allinstances provided adequate dosing is applied by recirculationpumps and reverse thrusting jets. A German process controlparameter (ATV, 1983), referred to as Spiilkraft, allows for thecalculation of a dosing rate (mm/pass) as follows:

May 2011 393

Daigger and Boltz

Table 2-Trickling filter classification.

Carbon Oxidizing Carbon OxidationDesign Parameter Roughing (cBODs removal) and Nitrification Nitrification

Media Typically Used Vertical flow Rock, cross flow, Rock, cross flow, Cross flowor vertical flow or vertical flow

Wastewater Source Primary effluent Primary effluent Primary effluent Secondary effluent

Hydraulic Loading

m3 (gpM/ft2) 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)

BOD5 and NH3-N Loadk3.gd (lb BOD/d1000 f) 1.6-3.52 (100-220) 0.32-0.96 (20-60) 0.08-0.24 (5-15) NA

d (Ib NH3-N/d.1000 ft2) NA NA 0.2-1.0 (0.04-0.2) 0.5-2.4 (0.1-0.5)rn2.dConversion (%) or Effluent 50 to 75% filtered 20 to 30 mg/L < 10 mg/L as cBOD5 ; 0.5 to 3 mg/L as

Concentration (mg/L) cBOD5 conversion cBOD5 and TSSb < 3 mg/L as NH3-Nb NH3-NbMacro Fauna No appreciable growth Beneficial Detrimental (nitrifying biofilm) DetrimentalDepth, m (feet) 0.91-6.10 (3-20) -s 12.2 (40) -s 12.2 (40) -< 12.2 (40)

Notes:"8 Applicable to shallow trickling filters; gpm/ft2 = gallons per minute per square foot of trickling filter plan area.b Concentration remaining in the clarifier effluent streamgpm/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-m3 (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).

mmTHL" 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; andcoa = the rotational speed (rev/min).

Higher dosing rates are recommended for higher organic loadingrates to provide biofilm thickness control and controlled sloughingof excess biomass. Besides a normal operating dosing rate, it maybe beneficial to periodically use a higher flushing dosing rate for 5to 10% of a 24-hour operating period. The flushing dose willoperate at 6 to 15 times the normal operating dose. Albertson(1995) and Parker et al. (1989) demonstrated that there is benefitto biofilm thickness control in the trickling filter process. These

Table 3-Operating and flushingdistributors.

dosing rates for

Total Organic Operating Dosing Flushing DosingLoad kglm 3/d Rate mm/pass Rate mm/pass(lb BODs/d/1000 ft3) (inches/pass) (inches/pass)

<0.4 (< 25) 25-75 (1-3) 100 (4)0.8 (50) 50-150 (2-6) 150 (6)1.2 (75) 75-225 (3-9) 225 (9)1.6(100) 100-300 (4-12) 300(12)2.4 (150) 150-450 (6-18) 450 (18)3.2 (200) 200-600 (8-24) 600 (24)

Note: Actual values are site-specific and vary with media type.

benefits include improved performance, reduced odors, reducedpower use for recycling, reduced nuisance organisms, andelimination of heavy sloughing cycles (Albertson, 1995). Parkeret al. (1989) described the use of both distributor speed controland variable frequency drive-controlled recirculation pumps tomaintain constant trickling filter hydraulic application. However,Parker et al. (1989) also presented evidence that electricallydriven distributor speed control did not improve NTF perfor-mance. Parker (1999) pointed out that there is little researchdescribing the effect of hydraulic transients on synthetic tricklingfilter media and their effect on media life. The typicalhydraulically driven distributor in North America operates in therange of 2 to 10 mm/pass. Table 3 lists recommended operatingand flushing dosing rates for modular synthetic media.

Oxygen Requirements and Air Supply AlternativesTrickling filters require oxygen for aerobic biochemical

transformation processes. Several researchers have demonstratedthat 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-limitedconditions (Kuenen et al., 1986; Okey and Albertson, 1989;Schroeder and Tchobanoglous, 1976). Ventilation is essential tomaintain aerobic conditions in a trickling filter. The vertical flowof air through trickling filter media can be induced by mechanicalventilation or natural air draft. Mechanical ventilation enhancesand controls airflow with low-pressure fans that continuouslycirculate air throughout the trickling filter. Current design practicerequires provision of adequate underdrain and effluent channelsizing to permit free airflow. Passive devices for ventilationinclude vent stacks on the trickling filter periphery, extensions ofunderdrains through trickling filter sidewalls, ventilating man-holes, louvers on the sidewall of the tower near the underdrain,and discharge of trickling effluent to the subsequent settling basinin an open channel or partially filled pipes.


Daigger and Boltz

Figure 5-Trickling filter aeration system: distribution pipes (left) and fans (right).

Natural Draft. Naturally occurring airflow results from adifference in ambient air temperature and humidity outside andinside the trickling filter. The temperature causes air to expandwhen warmed or contract when cooled, and humidity differencesresult in density differences. The result is an air-density gradientthroughout the trickling filter and an air front that rises or sinksdepending on the differential condition. This rising or sinkingaction results in a continuous airflow through the bioreactor. If airinside the trickling filter is colder than the ambient air, the air willflow downward. Alternatively, if the ambient air is colder than theair inside the trickling filter, air will flow upward. Schroeder andTchobanoglous (1976) state that upward airflow is the worst-casescenario from a mass transfer perspective because the dissolvedoxygen driving force is lowest in the region of highest oxygendemand (i.e., the top of the trickling filter).

Natural ventilation may become unreliable or inadequate inmeeting process air requirements when neutral temperaturegradients do not produce%air movement. Such conditions may bedaily or seasonal, and can lead to the development of anaerobiclayers inside the biofilms (near the growth medium) and poortrickling filter performance. Modular plastic media trickling filtersthat rely on natural draft to provide process oxygen for municipalwastewater treatment should include the following design features:Drains, channels, and pipes should be sufficiently sized to preventsubmergence greater than 50% of their cross-sectional area underdesign hydraulic loading. Ventilating access ports with open-gratingcovers should be installed at both ends of the central collectionchannel. Large-diameter trickling filters typically have branchchannels (to collect the treated wastewater). These branches shouldalso include ventilating manholes or vent stacks installed at thetrickling filter periphery. According to Grady et al. (1999), the openarea of the slots in the top of the underdrain blocks should not be lessthan 15% of the trickling filter area. One square meter gross area ofopen grating .in ventilating manholes and vent stacks should beprovided for each 23 m2 of trickling filter area. Typically, 0.1 m2 ofventilating area is provided for every 3 to 4.6 m of trickling filterperiphery, and 1 to 2 m2 of ventilation area in the underdrain area per1000 m3 of trickling filter media. Another criterion for rock-mediatrickling filters is the provision of a vent area at least equal to 15% ofthe trickling filter cross-sectional area.

Mechanical Ventilation. A majority of new and improvedtrickling filters use low-pressure fans to mechanically induce

airflow. The airflow resulting from natural draft will distributeitself. This will not occur with mechanical ventilation. Pressureloss through synthetic trickling filter media is typically low, oftenless than 1-mm H20 per meter of trickling filter depth (Grady etal., 1999). The low-pressure drop typically results in low fanpower requirements (e.g., on the order of 3 to 5 kW for modest-sized facilities). The head on the fan is typically less than 20 to30 mm H20. Unfortunately, the low pressure drop allows air torise upward through the trickling filter media without distributingitself through the bioreactor section. Therefore, fans are typicallyconnected to distribution pipes. The airflow distribution pipinghas openings that are sized such that airflow.through each is equaland airflow distribution is uniform. The pipes typically have avelocity in the range 1100 to 2200 m/hr in order to furtherpromote uniform airflow distribution. Airflow requirements arecalculated based on process oxygen requirements and character-istic oxygen transfer efficiency, which is typically in the range of 2to 10%. The mechanically induced airflow may flow upward ordownward. Down-flow systems can be designed without covers, butcovers are required for upflow systems. Covering trickling filtersoffers a wintertime benefit of limiting cold airflow and minimizingwastewater cooling. Mechanical ventilation and covered tricklingfilters may be used to destroy odorous compounds. A trickling filteraeration system is pictured in Figure 5.

Trickling Filter Design Models. Numerous investigatorshave attempted to delineate the fundamentals of the trickling filterprocess by developing relationships among variables that affecttrickling filter operation. Existing trickling filter process modelsrange from simplistic empirical formulations to numerical models.Analyses of operating data have been made to establish equations orcurves to fit available data. Results of these data analyses have ledto the development of several empirical trickling filter formulas.Unfortunately, numerous models exist and there is lack of anindustry standard. Designers need to assess which equation best fitsa particular situation when selecting a design model, especially withregard to the confidence level necessary to meet discharge permitrequirements. Therefore, many process designers use a forecastingapproach and will apply several empirical models to evaluate asystem. The following empirical models have been reported byBoltz et al. (2009) and Boltz (2010) as options historically used todescribe trickling filter performance in the context of processdesign: (1) National Research Council (1946), (2) Velz (1948)

May 2011 395

Daigger and Boltz

' ] ' I ' I I

0 No RedrmatktonL ~ID Rack utaIion

0

S I I , I ,

Z

100

80

60

40

20

0

0 10 20 30 40

BODs Load, b/1000 mu Wady

kg/lOOO m2* d

100 18 20

0 1.0 2.0 3.0 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 andnitrification 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 Waterand Environmental Management (1988) formula. A pseudomechanistic model called the Logan trickling filter model (TRIFL)(Logan et al. 1987a, 1987b) has been used to design modularsynthetic 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 amathematical description of TF/SG processes. The model ofDaigger et al. (1993) was developed to characterize nitrificationin TF/SG processes and was established based on performanceobservations at the Buck Creek WWTP, Garland, Texas. The modelaccounts for suspended growth reactor seeding with detachedbiofilm fragments in the trickling filter effluent stream. The TF/SGprocess effluent is calculated using the following equation:

[~~~max~ (M~T kd] . (NH3.~2

[ +kd (NH 3,,-K,) -y,.NH 3PE,

... NH3E,, + [( 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 con-

stant (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), andNH3.pE = the ammonia-nitrogen concentration in the

trickling filter process influent stream(mg/L).

The model of Daigger et al. (1993) has been independentlyevaluated and demonstrated to be effective by Biesterfeld et al.

(2005). The researchers noted that the model of Daigger et al.(1993) is primarily dependent on nitrification rates in the tricklingfilter and suspended growth reactor mean cell residence time, orsolids residence time.

Combined Carbon Oxidation and Nitrification. Combinedcarbon oxidizing (i.e., carbonaceous 5-day biochemical oxygendemand [cBOD5] removal) and nitrification trickling filters maycontain rock or synthetic media. The U.S. Environmental ProtectionAgency (U.S. EPA) (1991) reported survey results of 10 combinedcarbon oxidation and nitrification facilities. Six of the facilitiesincluded the TFISC process. The survey was used to create empiricalguides for achieving nitrification in the-secondary treatment processtrickling filters. The manual for nitrogen control (U.S. EPA, 1993)presented recommended BOD 5 loading (g/m2/d) to achieve bothcarbon oxidation and nitrification in a single-stage trickling filter.The kinetics of combined BOD5 removal and nitrification arecomplex, and the lack of fundamental research supporting combinedcarbon oxidation and nitrification in the trickling filter processresults in the continued use of empirical design procedures.Therefore, the design of combined carbon oxidation and nitrificationtrickling filters is empirical (Parker 1998).

U.S. EPA (1975) summarized full- and pilot-scale rock-mediatrickling filter data from Lakefield, Minnesota; Allentown, Penn-sylvania; Gainesville, Florida; Corvallis, Oregon; Fitchburg,Massachusetts; Ft. Benjamin Harrison, Indiana; Johannesburg,South Africa; and Salford, England. Likewise, significant data arepresented for a diverse range of U.S. plants by the WaterEnvironment Federation (2000). Figure 6 illustrates the relationshipbetween BOD 5 volumetric loading and nitrification efficiency usingboth pilot- and full-scale rock-media combined carbon oxidation andnitrification trickling filters. These observations indicate that anorganic loading rate of 0.08 kg BOD,/m3/d (5 lb BOD/1000 ft3Od)(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 rock-media trickling filters to achieve approximately 90% nitrification.Recirculation typically improves nitrification, particularly fornitrification efficiencies greater than 50%.

Daigger et al. (1994) presented an evaluation of three full-scaletrickling filters with low-density cross-flow media. The tricklingfilters were dosed with rotary distributors and designed for


at

*0

40

2D

U•w

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Daigger and Boltz

Table 4-Reported zero-order nitrification rates for vertical and cross-flow media (after Parker [1998; 1999]).

Location Reference Media Type JN° (g/m 2/d) Temperature Range (°C)

Central Valley, Utah Parker et al. (1989) XF 140 2.3-3.2 11 to 20Malmo, Sweden Parker et al. (1995) XF 140 1.6-2.8 13 to 20Littleton/Englewood, Colorado Parker et al. (1997) XF 140 1.7-2.3 15 to 20Midland, Michigan Duddles et al. (1974) VF 891 0.9-1.2 7 to 13Lima, Ohio Okey and Albertson (1989) VF 891 1.2-1.8 18 to 22Bloom Township, Illinois Baxter and Woodman (1973) VF 891 1.1-1.2 17 to 20

1 Fully corrugated.

Note: XF = cross flow and VF vertical flow.

combined carbon oxidation and nitrification. Data collectedfrom these studies suggest that an organic load less than 0.2 kgBOD5/m3/d (13 lbs BOD,/1000 ft 3/d) is required to achieve90% nitrification efficiency. Similar to the observations reportedby Stenquist et al. (1974), the synthetic media trickling filtersstudied were able to achieve greater than 90% nitrificationefficiency. Biofilm thickness control is recommended to optimizeNH 3-N removal in combined carbon oxidation and nitrificationtrickling filters (Parker et al. 1995; 1997). Daigger'et al. (1994)proposed the following equation to describe BOD 5 and NH3 -Nremoval in modular plastic media carbon oxidation and nitrifica-tion trickling filters:

VOR= [Si+ 4 .6 -SNo.-NJ' ( ) (14 ý -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), andVM = the synthetic media volume (mi3 ).

Using eq 3, Daigger et al. (1994) reported the volumetricoxidation rate for three combined carbon oxidation and nitrifica-tion trickling filter (with modular plastic media) processes in therange of 0.4 to 1.3 kg/m 3/d.

Nitrifying Trickling Filters. Nitrifying trickling filters are areliable and cost-effective means for NH 3-N conversion. Thefollowing design practices have been demonstrated in full-scaleapplication: (1)"use medium-density cross-flow media to optimizehydraulic distribution and oxygenation, (2) use mechanical ventila-tion, (3) periodically alternate the lead NTF to avoid patchy biofilmdevelopment in the lower reaches of the second-stage unit, (4) theinfluent should be secondary effluent to minimize bacterialcompetition for substrates inside the biofilm, (5) maximize wettingefficiency to avoid the formation of dry spots, (6) dose the NTF at arate that will minimize the accumulation of macro fauna, and (7)equalize NY 3 -N-laden supernatant from solids processing operationsto even out diurnal load variability (Parker et al., 1995; 1997). Benefitsto NTFs include low energy consumption, stability, operationalsimplicity, and reduced sludge yield. The reduced sludge yield andresulting low total suspended solids concentration in the NTF effluentstream has led some units to be constructed without downstreamliquid-solids separation units. This is dependent on site-specific

treatment objectives arid effluent water quality standards. Anoperational issue that can be detrimental to process performance isthe control of predatory macro fauna. Therefore, the designer mustinclude means for managing solids and macro fauna-laden waterresulting from macro fauna control measures. Design and operationalfeatures dedicated to macro fauna control are presented in asubsequent section. Nitrifying trickling filters having 6- to 12.2-mr(20- to 40-ft) modular plastic media depths have demonstratedimproved performance. Nitrifying trickling filters have been con-structed with depths up to 13 m (-42 ft). Shallower units can operateas a two-stage system. Recirculation should be minimized to thatrequired for biofilm thickness control in order to maximize NH3 -Nconcentration (i.e., maintain a high driving force) (Parker et al., 1997).

The practice of alternating the lead trickling filter in a two-stagetrickling system is referred to as alternating double filtration(ADF). Gujer and Boller (1986) and Parker et al. (1989) observedpatchy biofilm growth in the lower section of pilot-scale NTFs. Theresearchers attributed the patchy growth to dry spots. Aspegren andcoworkers (1992) observed improved nitrification and reducedbiofilm patchiness when operating the NTFs in an ADF system. Useof the ADF approach with trickling filters in series encourages full-depth biofilm development in both trickling filters. The leadtrickling filter should be switched every 3 to 7 days to ensure thatboth units contain a healthy biofilm developed along the entirebioreactor depth. The primary drawback of ADF is an increase inpower requirements, which may be in excess of 50% due to doublepumping. In addition to increased operating cost, capital costsassociated with pipes and valves will also increase costs.

Parker (1998, 1999) described nitrification efficiency in NTFscontaining either cross-flow or vertical-flow synthetic mediatypes. Table 4 summarizes his observations, which demonstratethat zero-order ammonia-nitrogen flux rates are greater for cross-flow than vertical-flow media. Factors contributing to theenhanced performance may be improved oxygen-transfer effi-ciency resulting from the increased number of media interruptionsand improved oxygenation (Gujer and Boller, 1986; Parker et al.,1989). Autotrophic nitrifying biofilms are thin when comparedwith the heterotrophic biofilms that are primarily responsible forBOD5 removal; therefore, medium-density cross-flow media aretypically used in NTFs. However, there is a propensity to developdry pockets when high-density modular plastic media are used(Parker et al., 1989).

Gujer and Boiler Nitrifying Trickling Filter Model. Gujerand Bolter (1986) developed the following semi-empirical modelthat reasonably characterizes NTF performance:

JN(S, T) JN, max (T) S IKN +SB, N

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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/m2/d) (=Jo2 ,m.x(T)/4.3),SB.N = the bulk-liquid ammonia-nitrogen concentra-

tion (g/m3 ),KN = the half-saturation coefficient for ammonia-

nitrogen (g N /m3) (= 1.0 g N /m3, typicalvalue), and

T = the temperature (*C).

Based on a "line-fit" relationship, the flux at any depth in thetrickling filter can be calculated as

JN(Z, T)=JN(0, T)'e-k',. (5)

The following two solutions were developed to account for achange in the rate of nitrification with NTF depth (k # 0) (eq 6),and the second assumes no decrease in the rate of nitrificationwith NTF depth (k = 0) (eq 7):

a'JN,a(T) (1 -e-kz) =S i.,N-SB,I+KN In (ŽSi, IV (6)k-vh \SB, NI

When k = 0,

z--Nn =T Sin, N -SB,N +KN'tn N (7)An(SB.N•

Where

a = the specific surface area (m2/m3),k = the empirical parameter describing nitrification

rate decrease (1/m) (= 0 to 0.16, typical 0.1),vh = the hydraulic load (with or without recircula-

tion) (m 3/ m2 /d),

z = the NTF depth (m), andSiý. N = the ammonia-nitrogen concentration in influent

stream (g/m3).

These equations can be solved directly to size a NTF for adesired SB,v. When recirculation is used, an iterative solutionroutine that includes the following equation is required because ofthe effect recirculation has on both Vh and Si,, N:

I SO, N +-R-SB, N5N. i ---

l+R (8)R S,N SM~,NSi., N S8, N

Where

So, N = the ammonia-nitrogen concentration in theinfluent stream prior to being mixed with therecirculation stream.

The ammonia-nitrogen concentration in NTF influent stream,Si, N• will be less than So. N when recirculation is applied. Parkeret al. (1989) proposed a modification of this model to account foroxygen-transfer efficiency variability amongst modular plastic mediatypes and operating conditions. The revised expression is as follows:

Jo 2,m.(T) SB,N (9)JN (z,T) =EO 4.3 KN+SB, N

Where

E02 = the dimensionless NTF media effective-ness factor and

Jo2 ,max(T) = the maximum dissolved-oxygen flux attemperature T (g/m 2 /d).

Based on their experience, Gujer and Boiler (1986) reported anE0 2 value in the range of 0.93 to 0.96 for Ks,02 = 0.2 g 02 /M3

and the temperature range of 5 to 25 *C. Parker et al. (1989), onthe other hand, observed lower E02 values (in the range of 0.7 to1.0) and claimed that a departure from E0 2 = 1.0 accounts forwetting inefficiency, biofilm grazing by macro fauna, orcompetition for dissolved, oxygen between autotrophic nitrifiersand heterotrophic bacteria inside the biofilm. The researchersrecommended that medium-density cross-flow media are used inNTF applications and that E0 2 may range from 0.7 to 1.0 for thismedia. High-density cross-flow media had a corresponding E0 2approximately equal to 0.4 (Parker et al., 1995). According toParker et al. (1995), Eo2"Jo2,,x(T)/4.3 is the zero-orderammonia-nitrogen flux (Parker et al., 1995). The maximumdissolved-oxygen flux reflects the oxygen-transfer efficiency ofthe selected modular plastic media, and was determined by theresearchers using TRIFL (Logan et al., 1987a). The coefficient,Ks.0 2, determined for the Central Valley WWTP in Utah, was inthe range of 1 to 2 mg/L (Parker et al., 1989).

Operational Strategies and Facility Improvements forMacro Fauna Control

Several strategies have been applied to manage macro faunaaccumulation and/or development in trickling filters, includingphysical, chemical, or a combination of physical and chemicalapplications. The ideal control strategy is to promote a conditionthat is either toxic to the macro fauna or creates an environmentnot conducive to their accumulation. Lee and Welander (1994)demonstrated increased nitrification after predator control usingsubstances toxic to eukaryotic organisms. The toxic substancemust either have no effect on or only temporarily inhibit beneficialmicroorganisms (Parker et al., 1997). Operators have conductedsite maintenance that aids in reducing macro fauna presence intrickling filter-based WWTPs. For instance, some operators haveobserved that the presence of filter flies may be reduced by simplymaintaining a short stand of grass on the WWTP site. Morespecific strategies include periodic high-intensity hydraulicapplication, trickling filter flooding, pH adjustment with lime orsodium hydroxide, high-concentration aqueous ammonia dosing,trickling filter effluent or secondary clarifier underflow (humus)screening or accelerated gravity separation, gravity separation inlow-velocity channels with a dedicated pumping circuit, elimi-nating dissolved oxygen from the trickling filter feed, adding salt,draining and freezing the infested unit, raising the temperaturequickly, adding molluscacide (e.g., copper sulfate), and chlori-nating the influent stream. Many of these strategies have provenineffective in some trickling filters, and others may be detrimentalto bioreactor performance. Biochemical reactions are influencedby temperature, pH, and alkalinity; adjusting these parametersmay inhibit the biochemical reactions and lower transformationrates. Chemicals such as chlorine are toxic to all organisms in thetrickling filter and may result in destruction of sensitive biomass(Parker et al., 1989). A brief summary of the principal controlmechanisms in use is provided here. More details are availableelsewhere (Boltz et al., 2008). Control mechanisms described hereinclude:


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 typicallyassociated with grit removal), and

"* Gravity separation in low-velocity channels and removalwith a dedicated pumping circuit.

Dosing for Macro Fauna Control. Hawkes (1955) demon-strated that high hydraulic loadings and periodically increasinginstantaneous dosing rates can control filter fly development.Increased hydraulic loading improves trickling filter mediawetting efficiency, thereby reducing dry spots and minimizingideal spawning areas for filter flies. Gujer and Boiler (1984)reported that filter fly larvae were reduced to quantities that didnot have an impact on NTF performance. Andersson et al. (1994)tested three flushing intensities (Spiilkraft values of 5, 40, and80 mm/pass) and reported that the variable flushing intensity hadno apparent effect on filter fly and worms in a pilot-scale NTF.(Note that these Spiilkraft values are below those reported forflushing, as presented in Table 3.)

Flooding. Trickling filter flooding requires adequate dutyunits to isolate a trickling filter for a 3-to-6-hour period. Thetrickling 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 analkaline backwash process to control other macro fauna in two 32-m-diameter, 7.3-m-deep medium-density cross-flow media NTFsat the Littleton-Englewood WWTP in Colorado. Online pH probesand a sodium hydroxide metering system allow for flood water pHadjustment by operator set point. The alkaline flood water ispumped through the NTF bottom, is discharged into an overflowtrough, and is then directed to the head of the WWTP fortreatment. Alkaline treatment is reported to have removed 76% oflarvae at pH 9 and 99% at pH 10 (Parker et al., 1997). Subsequentresearch trials designed in response to snail developmentdemonstrated that flooding and backwash (4 hours at pH 9)reduced snail quantity by two-thirds and returned the NTFs to highnitrification efficiency (Parker, 1998).

Chemical Treatment. Everett et al. (1995) summarizedseveral chemical treatment alternatives including pH adjustmentand chlorination, sodium chloride, and molluscicides (e.g., coppersulfate, metaldehyde, niclosamide, and trifenmorph). Factors suchas pH, turbidity, and molluscicide dose are key factors indetermining chemical application rate. Rotating biological con-tactors (RBCs) in Lafayette, Louisiana, applied sodium chloride ata dosing concentration of 10 mg/L for a 24-hour period toeffectively control the snail accumulation. Calcium hypochloritein a range of 60 to 70 mg/L was applied during a 2-to-3-dayperiod, and effectively minimized snail accumulation in RBCs atthe Deer Creek WWTP, Oklahoma City, Oklahoma. Coppersulfate at low concentrations (0.45 kg of copper sulfate per3.785 mi3) may effectively control snail accumulation.

Ammonia is toxic to snails. Lacan et al. (2000) conducted alaboratory-scale study and plant-scale application of un-dissoci-ated aqueous ammonia [NH3 -N(aq)] solutions with elevated pH tocontrol snail growth (P. gyrina) in NTFs. Un-dissociated aqueous

NH3-N(aq), not the ammonia ion, is the snail P. gyrina toxophore.The concentration producing 100% mortality is a function ofexposure time and the bulk-liquid NH3-N(aq) concentration. Thelaboratory-scale study demonstrated that an ammonium chloride(NH4CI) solution at pH 9.2 [NH 3 -N(ag) = 150 mg N/L] resulted in100% snail mortality. A much higher concentration of ammonia isrequired in the trickling filter influent stream (i.e., 1000 to1500 mg N/L) to maintain the required NH3-N(ag) = 150 mg N/Lbecause of the immediately reduced concentration owing to axialdispersion, biofilm diffusion (both external and internal), andbiochemical reaction. Lacan et al. (2000) estimated that aninfluent ammonia concentration of 1080 mg N/L resulted in anaverage concentration throughout the NTF of 185 mg N/L. Such ahigh-concentration NH3-N(,q) stream may be readily available inmunicipal WWTPs as solids processing recycle streams. In someinstances, however, it may be necessary to purchase NH3-N(aq).The first full-scale application of this snail control method wasreported by Gray et al. (2000) at the Truckee Meadows WWTP,Reno Sparks, Nevada, which uses high-density (215 m2

/m3)

media. Ammonia-rich anaerobic digester centrate was directed toa NTF recirculation pump station. Sodium hydroxide was added tothe 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) centratetreatment/recirculation mode, and (3) the flushing mode charac-teristic of the macro fauna control method described by Lacan etal. (2000). This macro fauna control method is typically appliedonce per month. During the treatment cycle, an NTF is isolatedand the solution is recirculated through the trickling filter forapproximately 2 hours. The first 20 to 50 minutes of aqueousammonia dosing is dedicated to reaching a hydrodynamic steadystate (i.e., 3 to 4 hydraulic retention times [HRTs]), and theremainder is the minimum recommended exposure time for 100%mortality of both adult snails and their larvae. The treatmentsolution is returned to the head of the WWTP after dosing iscompleted and the NTFs are then flushed with secondary effluentin 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 adedicated pumping circuit, and (3) accelerated gravity separationusing equipment typically associated with grit removal. TheCentral WWTP, Baton Rouge, Louisiana, uses trickling filtersecondary clarifier underflow screening to control snail accumu-lation in, or damage to, solids handling equipment. The City ofLawton, Oklahoma, dnd both the South San Luis Obispo,California, County Sanitation District Oceana Regional Plantand the City of San Luis Obispo Water Reclamation Facility, SanLuis Obispo, California, pump secondary clarifier underflow to afree vortex classifier for snail shell removal. The Econchate WaterPollution Control Plant, Montgomery, Alabama, removes snailshells in the chlorine contact basin, which was modified to a two-pass channel to serve as a low-velocity sedimentation basin forsnail shells escaping secondary clarification. The snail shellsdeposited in the low-velocity channel are collected in a sump andpumped to a static screen, where they fall by gravity into acollection bin. Tekippe et al. (2006) reported the use of baffles,grit pumps, and classifiers to remove snails from the Ryder StreetWWTP, Vallejo, California. The facility treats wastewater with aTF/SG process consisting of two, 32-m-diameter and 7.3-m-deep

May 2011 399

Daigger and Boltz

SOcnm

TFEfft~uwl

I Flshng Mode I

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 StreetWWTP's aeration basins were improved to provide a zone for themajority of the shells to settle. An automatic mechanism wasprovided to remove the settled shells (Tekippe et al., 2006).

Combined Trickling Filter and SuspendedGrowth Processes

Biological processes including both a trickling filter andsuspended growth reactor build on the known performance andoperating characteristics of the parent processes. When thesuspended growth reactor is used as a flocculating unit it is referredto as the TF/SC process. All other TF/SG processes use the coupledsuspended growth reactor as an oxidizing unit. The activated biofilterand biofilter/activated sludge processes, which circulate returnactivated sludge over the trickling filter (making it a biofilter), arenot discussed as these process options are applicable only to woodslat media, which is seldom used these days (Grady et al., 1999).

Trickling Filter/Solids Contact. A majority of organicmatter 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 thattrickling filter effluent contains a high concentration of not readilysettleable colloidal and particulate organic matter. The materialmay be removed by bioflocculation, along with the oxidation ofresidual soluble organics, in a solids contact basin. The TF/SCprocess includes a trickling filter followed by a small, aeratedsolids-contact channel. Biomass in the solids contact basineffluent stream flows to a clarifier that has a (1) suction-headersludge withdrawal mechanism and (2) a flocculating feed well(approximately one-third of the clarifier diameter) that promotesgentle mixing and additional bioflocculation of the influentsuspended biomass (sludge flocs). LaMotta et al. (2004) indicatethe following characteristics for such systems:

"* Solids contact basin dissolved oxygen concentration greaterthan 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


Daigger and Boltz

TRICKLINGFILTER

AERATED SOLIDSCONTACT TANK SECONDARY CLARIFIER

FLOCCULATORCENTER WELL

TREATEDEFFLUENT

Mode I

TRICKL INGFILTER SECONDARY CLARIFIER

PRIMAPV~ ~ FLOCCULATOR____ __MIXED LIQUOR R TRPRIMAR EII1N

•L#E ••T NSLUDGE;

REAERATION TANKS

Mode II

TRICKL NGFILTER AERATED SOLIDS SECONDARY CLARIFIERCOTC AKFLOCCU LATOR

•C8ETER WELL

' TREATEDEFFLUENT,

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 matterbioflocculation, and the oxidation of residual soluble organicmatter. Mode II relies exclusively on a return sludge aerationchamber. The aerated return sludge is mixed with trickling filtereffluent for colloidal and particulate organic matter biofloccula-tion. Mode III makes use of both the solids contact basin and areturn sludge aeration tank. A typical TF/SC process operates asmode I; however, as of 2001, more than one-half of the TF/SC-based WWTPs were operating as mode III (or had the operationalflexibility to operate as mode I or I1). It should be noted thatmode II is seldom used and is typically not recommended as itdoes not have a solids contact basin and only a sludge reaerationtank (Parker and Bratby, 2001). These operational modes areillustrated in Figure 8.

If the solids contact basin follows a carbon oxidation andnitrification trickling filter(s), autotrophic nitrifiers will detachfrom the biofilm surface and, essentially, bioaugment the solidscontact basin biomass inventory. Despite the short duration solidsretention time characteristic of the solids contact basin, thebioaugmentation will cause nitrification, which will exertadditional oxygen demand (i.e., increased airflow, blower size,and air piping). In some instances, this may be desirable; however,

in instances where increased oxygen demand is not desired andnitrification is inevitable, the designer should seek to maximizenitrification in the carbon oxidation and nitrification tricklingfilter. This may be achieved with proper air supply system designand process loadings, as discussed above.

The solids contact basin is typically 5 to 20% of the volume thatwould be required with treatment by activated sludge. Bycombining a trickling filter and solids contact basin, the tricklingfilter size may be reduced compared to the size typically requiredif treatment is accomplished with only a trickling filter (Parkerand Matasci, 1989). One significant benefit of the TF/SC processis the low power requirements owing to a relatively highdependence on the trickling filter to remove the majority ofsoluble organic matter BOD5 . Rock- and plastic-media tricklingfilters can be upgraded with the TF/SC process. Table 5 listsgenerally accepted design criteria for the TF/SC process.

Roughing Filter/Activated Sludge. Roughing trickling filtershave been used to expand WWTP treatment capacity. The roughingfilter is a highly-loaded trickling filter that uses 10 to 40% of themedia volume required if treatment has been accomplished throughthe use of the trickling filter process alone. Hydraulic retention timein the aeration basin is typically 30 to 50% of that required with the

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Table 5-Typical design criteria for TFISC processes.

Design Criteria

Parameter Range Common

Trickling Filter/Solids Contact (modular synthetic media)

Solids production (mg volatile suspended solids in waste/mg BOD5 removed) 0.7-0.9 0.7Trickling filter hydraulic load (gpm/ft2 ) 0.1-2.0 1.0Trickling filter influent total organic load (lbs/1000 ft3-day) 20-75 50Solids contact basin side water depth (feet) 18-22 20Solids contact basin HRT at average day flow (min) 45-120 60Solids contact basin HRT at peak flow (min) 15-30 30Solids contact basin solids residence time (d) 1.0-2.0 1.0Solids contact basin MLSS concentration (mg/L) 1500-3000 2000Sedimentation basin overflow rate at average day flow (gpd/ft2 ) 500-1000 800Underflow concentration (% total solids) 0.6-1.2 0.8

Note: MLSS = mixed liquor suspended solids.

activated sludge process. The TF/SC and roughing filter/activated Summarysludge (RF/AS) processes have the same process flow sheet. The modem trickling filter typically includes the followingHowever, with RF/AS, a smaller trickling filter is used so that the major components: (I) rotary distributors with speed control; (2)aeration basin is depended on to provide a significant portion of modular plastic media (typically cross-flow media unless thecontaminant oxidation. This differs from the TF/SC process, where bioreactor is treating high-strength wastewater, which warrantsthe trickling filter is larger and provides the majority of the BOD5 the use of vertical-flow media); (3) a mechanical aeration systemremoval, leaving the contact channel to provide enhanced colloidal (that consists of air distribution piping and low-pressure fans); (4)and suspended solids removal by bioflocculation. influent/recirculation pump station; and (5) covers that aid in the

Trickling Filter/Activated Sludge. The trickling filter/acti- uniform distribution of air and foul air containment (for odorvated sludge (TF/AS) process is designed at high organic loads, control). Covers may be equipped with sprinklers that can sprayHowever, a unique feature of TF/AS is the intermediate clarifier. washwater to cool the media during emergency shut downThe intermediate clarifier removes solids produced in the trickling periods. Trickling filter mechanics are poorly understood.filter before partially treated wastewater enters the suspended Consequently, there is a general lack of mechanistic mathematicalgrowth reactor. A benefit of using the TF/AS combined process is models and design approaches, and the design and operation ofthat solids generated in the trickling filter can be removed before trickling filter and TF/SG processes is empirical. Some empiricalsecond-stage activated sludge treatment. This is often a preferred trickling filter design criteria and semi-empirical NTF modelsmode of operation where NH3 -N removal is required. The reduced have been described in this paper. Benefits inherent to theoxygen demand afforded by intermediate clarification is typically trickling filter process (when compared to activated sludgeconsidered less significant than the savings in capital and processes) include operational simplicity, resistance to toxic andoperating costs gained by eliminating intermediate clarification, shock loads, and low energy requirements. However, tricklingTherefore, cost-to-benefit evaluations typically guide designers to filters are susceptible to nuisance conditions that are primarilyuse the RF/AS or TF/SC processes rather than the TF/AS process. caused by macro fauna. Process mechanical components dedicat-Table 6 lists generally accepted design criteria for the RF/AS and ed to minimizing the accumulation of macro fauna such as filterTF/AS processes. flies, worms, and snail (shells) are now standard. Unfortunately,

Table 6-Typical design criteria for RFIAS and AF/AS processes.

Design Criteria

Parameter Range Common

Roughing or Trickling Filter/Activated Sludge (modular synthetic media)

Solids production (mg volatile suspended solids in waste/mg BOD5 removed) 0.8-1.2 1.0Trickling filter hydraulic load (gpm/ft2 ) 0.8-5.0 1.0TF influent total organic load (lbs/1000 ft3-day) 75-300 150Aeration basin side water depth (feet) 12-24 18Aeration basin hydraulic retention time at average day flow (min) 120-480 240Aeration basin hydraulic retention time at peak flow (min) 40-120 90Aeration basin solids residence time (d) 1.0-12.0 8.0Aeration basin MLSS concentration (mg/L) 1500-6000 3000Sedimentation basin overflow rate at average day flow (gpd/ft2) 500-1000 800Underflow concentration (% total solids) 0.6-1.2 0.8

Note: MLSS = mixed liquor suspended solids.


Daigger and Boltz

information on the selection and design of these processcomponents is fragmented and has been poorly documented.The TF/SC process is the most common TF/SG process. State-of-the art design and operational practice for the trickling filterprocess has been reviewed and described in this paper.

Acknowledgments

A preliminary version of this paper was prepared assupplemental information for the "Trickling Filter and CombinedTrickling Filter-Suspended Growth Process Design and Opera-tion" presentation in Workshop W213, Biofilm Reactors:Application to Today's Global Wastewater Challenges, presentedat the 82nd Water Environment Federation Technical Exhibitionand Conference (WEFTEC) in Orlando, Florida, in October, 2009.

Submitted for publication December 22, 2009; revisedmanuscript submitted March 14, 2010; accepted for publicationJune 21, 2010.

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Trickling Filter and Trickling Filter -- Suspended Growth Process Design andOperation: A State-of-the-Art Review

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