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

Transcription

1 Trickling Filter and Trickling Filter- Suspended Growth Process Design and Operation: A State-of-the-Art Review Glen T. Daiggerl*, Joshua P. Boltz 2 ABSTRACT: The modem trickling filter typically includes the following major components: (1) rotary distributors with speed control; (2) modular plastic media (typically cross-flow media unless the bioreactor is treating high-strength wastewater, which warrants the use of vertical-flow media); (3) a mechanical aeration system (that consists of air distribution piping and low-pressure fans); (4) influent/recirculation pump station; and (5) covers that aid in the uniform distribution of air and foul air containment (for odor control). Covers may be equipped with sprinklers that can spray in-plant washwater to cool the media during emergency shut down periods. Trickling filter mechanics are poorly understood. Consequently, there is a general lack of mechanistic mathematical models and design approaches, and the design and operation of trickling filter and trickling filter/suspended growth (TF/SG) processes is empirical. Some empirical trickling filter design criteria are described in this paper. Benefits inherent to the trickling filter process (when compared with activated sludge processes) include operational simplicity, resistance to toxic and shock loads, and low energy requirements. However, trickling filters are susceptible to nuisance conditions that are primarily caused by macro fauna. Process mechanical components dedicated to minimizing the accumulation of macro fauna such as filter flies, worms, and snail (shells) are now standard. Unfortunately, information on the selection and design of these process components is fragmented and has been poorly documented. The trickling filter/solids contact process is the most common TF/SG process. This paper summarizes state-of-the art design and operational practice for the modem trickling filter. Water Environ. Res., 83, 388 (2011). KEYWORDS: trickling filter, trickling filter/suspended growth, trickling filter/solids contact, biofilm, nitrification, design, operation. doi: / X Introduction Until the 1950s, trickling filter design protocol was scattered and empirical in nature. Then, during the 1950s and 1960s, the Dow Chemical Company began experimentation with modular synthetic media (Bryan, 1955). Numerous trickling filter process studies were conducted during the same period (Eckenfelder, 1961; Galler and Gotaas, 1964; Germain, 1966; Schulze, 1960), which led to the development of generally accepted design criteria. After the U.S. Environmental Protection Agency issued its definition of secondary treatment standards in the early 1970s, the trickling filter process was regarded as being unable to 1, CH2M HILL, 9191 South Jamaica Street, Englewood, CO 80112; Glen.Daigger@CH2M.com. 2 CH2M HILL, Tampa, Florida. 388 consistently produce effluent water quality that met the published standards, in part, because of poor secondary sedimentation tank design (Parker, 1999). Norris et al. (1982) developed the trickling filter-solids contact (TF/SC) process in response. The first fullscale TF/SC process included a rock-media trickling filter followed by a small aeration basin (receiving return sludge) and flocculator clarifier. The researchers demonstrated that wastewater treatment plant (WWTP) effluent water quality could be greatly improved by bioflocculation in the solids contact basin and improved secondary clarifier design. Combined trickling filtersuspended growth (TF/SG) processes preceding the TF/SC process were designed with the suspended growth reactor primarily for oxidation. This paper describes state-of-the art trickling filter and TF/SG process design and operation. General Description A trickling filter is a three-phase system with fixed biofilm carriers. Wastewater enters the bioreactor through a distribution system, trickles downward over the biofilm surface, and air moves upward or downward in the third phase. Trickling filter components typically include a distribution system, containment structure, rock or plastic media, underdrain, and ventilation system. Figure 1 illustrates a trickling filter cross section and typical bioreactor components. Wastewater treatment using the trickling filter results in a net production of total suspended solids. Therefore, liquid-solids separation is required, and is typically achieved with circular or rectangular secondary clarifiers. The trickling filter process typically includes an influent pump station, trickling filter, trickling filter recirculation pump station, and liquid-solids separation unit. Distribution System. Primary effluent (or screened, 3-mm, and degritted wastewater) is either pumped or flows by gravity to a trickling filter distribution system. The distribution system intermittently distributes wastewater over the trickling filter biofilm carriers. The distributors may be hydraulically or electrically driven. The intermittent application allows for resting periods during which aeration occurs. Efficient influent wastewater distribution results in proper media wetting. Poor media wetting may lead to dry media pockets, ineffective treatment zones, and odor. Essentially, there are two types of distribution systems: fixed-nozzle and rotary distributors. Because their efficiency is poor, distribution with fixed nozzles should not be used (Harrison and Timpany, 1988). Hydraulically driven rotary distributors use back-spray orifices, or reverse thrusting jets, to slow rotational speed and maintain the Water Environment Research, Volume 83, Number 5

2 Rotary distributor / FRP Underdrain Grating AirpipeEffluent N Influent Figure 1-Typical trickling filter cross section and bioreactor components. desired instantaneous flushing rate to the trickling filter. Figure 2 depicts both a modem hydraulically driven rotary distributor that uses gates (controlled by variable frequency drive) that either open or close distributor orifices to adjust rotational speed and an electrically driven rotary distributor. Use of a variable-speed drive and electronic controller allow for the more precise conrol of distributor-arm speed. Electrically driven rotary distributors have motorized units that control distributor speed independent of the wastewater pumped flow. Biofirm Carriers. Ideal trickling filter biofilm carriers, or media, provide a high specific surface area, low cost, high durability, and high enough porosity to avoid clogging and promote ventilation (Tchobanoglous et al., 2003). Trickling filter biofilm carriers include rock, random (synthetic), vertical-flow (synthetic), and cross-flow (synthetic). Both vertical-flow and cross-flow media are constructed with smooth and/or corrugated plastic sheets. Another commercially available synthetic media, although not commonly used, are vertically hanging plastic strips. Horizontal redwood or treated wooden slats have also been used, but are typically no longer considered because of their high cost or limited supply. Modules of plastic sheets (i.e., self-supporting vertical-flow or cross-flow modules) are used almost exclusively for new and Figure 2-Hydraulically propelled (left) and electrically driven rotary distributor (right). May

3 Table 1-Properties of some trickling filter media. Rock Media Type Material Nominal Size Bulk Spedfic Surface Void m Density Area Space () kghr 3 (m2/m3) (%) (lbs/) )b River ( ) 1442 (90) 62 (19) 50 Slag ( ) 1600 (100) 46 (14) 60 Plastic' Cross flow 0.61 x 0.61 x 1.22 (2 x 2 x 4) and 223 ( ) (30, 48, and 68) 95 PVC Vertical flow 0.61 x 0.61 x 1.22 (2 x 2 x 4) ( ) 102 and 131 (31 and 40) 95 PVC Random o x H (7.3" o x 2" H) 27 (1.7) 98 (30) 95 polypropylene Notes: 1 Manufacturers of modular plastic media: BF Goodrich (formerly), American Surf-Pac, NSW, Munters, Brentwood Industries (currently), Jaeger Environmental, and SPX Cooling. 2 Manufacturers of random plastic media: NSW Corp. (formerly) and Jaeger Environmental (currently). a ibs/ft 3 x = kg/m 3. b f92/ft3 X = m 2 /M 3. improved trickling filters. However, several trickling filters with rock media exist and are capable of meeting treatment objectives when properly designed and operated. Table I compares the characteristics of various biofilm carrier types. The higher specific surface area and void space in modular synthetic media allow for higher hydraulic loading, enhanced oxygen transfer, and biofilm thickness control in comparison to rock media. 390 Ideally, rock media have a 50-mm diameter, although they may range in size. Rounded (river) rock helps mitigate issues associated with rigid rock (slag) media. The slag rock contains crevices that can retain water and accumulate biomass. Because of structural requirements associated with the large unit weight of the rock media, rock media are shallow in comparison to synthetic media trickling filters and are more susceptible to excessive Water Environment Research, Volume 83, Number 5

4 cooling. Trickling filter performance aside, excessive cooling can subject media to freeze-thaw cycles. Water retained inside slag rock crevices may expand and sever rock fragments. This can result in fine material accumulation which, together with retained biomass, is a primary contributor to rock-media trickling filter clogging (Grady et al., 1999). Generally, rock media are considered to have a low specific surface area, void space, shallow depth, and high unit weight. Although recirculation is common, the low void ratio in rock-media trickling filters results in reduced hydraulic application rates. Excessive hydraulic application can result in ponding, limited oxygen. transfer, and poor bioreactor performance. The performance of existing rock-media trickling filters can be improved by providing forced ventilation, distributor speed control, solids contact channels, and/or deepened secondary clarifiers that include energy dissipating inlets and flocculator-type feed wells. Replacement or deepening of the rock media (with synthetic media) is often requisite in instances where the rock media quality is poor, space is limited, and WWTP expansion (using a trickling filter or TF/SG process) is expected. However, a well-designed and operated rock-media trickling filter can provide high-quality effluent. Grady et al. (1999) suggest that for low organic loads (less than 1 kg 5-day biochemical oxygen demand [BOD 5 ]/dim 3 ), well-designed and operated rock-media trickling filters are capable of providing performance approaching that of synthetic- *media trickling filters. However, as organic load increases, there is likely to be fewer nuisance problems and reduced potential for plugging with the use of synthetic biofilm carriers. Synthetic biofilm carriers (for trickling filters) are generally considered to have a high specific surface area and void space and low unit weight. Due to the reduced unit weight, synthetic media trickling filters can be constructed at depths in excess of 3 times that for a comparably sized rock-media trickling filter. Modular plastic trickling filter media are typically manufactured with the following specific surface areas: 223 m 2 /m 3 (68 ft 2 /ft 3 ) as high density, 138 m 2 /m 3 (42 ft 2 /ft 3 ) as medium density, and 100 m 2 /m 3 (30 ft 2 /ft 3 ) as low density. Both vertical flow and cross-flow media are reported to effectively remove BOD 5 and total suspended solids (TSS) (Aryan and Johnson, 1987; Harrison and Daigger, 1987). Cross-flow modules provide increased treatment efficiency compared to vertical-flow modules of the same specific surface area at low-to-medium volumetric organic loading rates (less than about 2.5 kg BOD 5 /d/m 3 ), but vertical-flow modules may provide advantages at higher volumetric organic loading rates (Harrison and Daigger, 1987). The effects of media type and configuration on trickling filter effluent water quality should be'given careful consideration by the designer. Plastic modules with a specific surface area in the range of 89 to 102 m 2 /m 3 are well suited for carbon oxidation and,combined carbon oxidation and nitrification. Parker et al. (1989) recommended medium-density cross-flow media, and recommended against the use of high-density cross-flow media in nitrifying trickling filters (NTFs). This argument is supported by pilot application data and conclusions of Gujer and Boiler (1983, 1984) and Boller and Gujer (1986), which show higher nitrification rates for lower density modular synthetic media. The researchers claim that lower rates occur with high-density media due to the development of dry spots below the flow interruption points (i.e., higher density media having more interruptions and, therefore, less effective wetting). Using medium-density media also reduces the potential for plugging. These recommendations were developed before the more widespread use of speedcontrolled rotary distributors, which may help to overcome these hydraulic distribution issues. Vertically oriented modular plastic media are generally accepted as being ideally suited for highstrength wastewater (perhaps industrial) or high organic loadings such as with a roughing trickling filter. In some instances, crossflow media have been placed in the top layer of a trickling filter containing vertical-flow media to enhance wastewater distribution, with vertical-flow media comprising the remainder of the trickling filter media. Containment Structure. Rock and random plastic media are not self-supporting and,,therefore, require support from the containment structure. Typically, containment structures are precast or formed concrete tanks. When self-supporting media such as plastic modules are used, materials such as wood, fiberglass, and welded and bolted (coated) steel have also been used as containment structures. The containment structure serves to avoid wastewater splashing and to provide media support, wind protection, and, sometimes, flood containment. Underdrain System and Ventilation. The trickling filter underdrain system is designed to meet two objectives: collect treated wastewater for conveyance to downstream unit processes and create a plenum that allows for the transfer of air throughout the trickling filter media (Grady et al., 1999). Clay or concrete underdrain blocks are commonly used for rock-media trickling filters because of the required structural support. A variety of,support systems including concrete piers and reinforced fiberglass grating are applied for other media types. Figure 3 depicts fieldadjustable plastic stanchions and fiberglass-reinforced plastic grating to support modular plastic media on the concrete floor of a trickling filter containment structure and high-density polyethylene mats used to support random synthetic media. The volume between the concrete slab and media bottom creates the underdrain. Trickling Filter Pump Stations: Influent and Recirculation. A critical unit in the trickling filter system is a pump station that lifts primary effluent and recirculates trickling filter underflow. Generally, trickling filter underflow should be recirculated at a rate required to achieve the hydraulic load (influent plus recirculation) required for proper media wetting and biofilm thickness control (note that distributor speed control may be required if the hydraulic load is insufficient to provide the recommended dosing rate). The intent of recirculating bioreactor effluent is to decouple hydraulic and organic loading. Although effluent from the secondary clarifier can be recirculated, this is not common practice because it may lead to hydraulic overloading of secondary clarifiers. Influent pumping is typically selected to allow trickling filter underflow to flow by gravity to the suspended growth reactor (or solids contact basin), secondary clarifier, or another unit downstream of the trickling filter. Trickling filter recirculation pumps are typically constantspeed, low-head centrifugal units designed to operate with a total head equivalent to the static head, comprised of the trickling filter media depth of approximately 3 to 7 m (depending on media depth), the distance between the distributor outlet and the top of the media, and the distance between the bottom of the media and the water surface in the underdrain, along with associated friction losses (Boltz et al., 2009). Variable frequency drive controlled motors are typical fixtures on process pumps. Submerged or nonsubmerged (dry-pit) vertical pumps have been used exten- May

5 W *'0 P 4 *W_ Pip, W WS '4W 4W g.nrwrq:& * *.0 '0' ý 1W 4* * 4%.Wo.,j:W4:,O P A. 4W V.4 P, -b Z -1b 4k. 41k. Ow 4ýbd. bdb I r Figure 3-Adjustable plastic stanchions and fiberglass-reinforced plastic grating on the concrete floor of a bolted steel containment structure (left), and a high-density polyethylene mat used to support random synthetic media (right). sively. Pump intake screens are usually unnecessary because recirculated flow is typically free of clogging solid materials. Hydraulic computations are always necessary. Computations for minimum flow are necessary to ensure adequate head to drive hydraulically driven distributors; computations for maximum flow indicate the head required to ensure adequate discharge capacity. The net available head at the horizontal center line of the distributor's arm and other points may be calculated by deducting the following applicable losses from the available static head: entrance loss, friction losses in the piping to the distributor, proper allowance for minor head losses, head loss through distributor riser and center port, friction loss in distributor arms, and velocity head of discharge through nozzles necessary to start the hydraulically driven rotary distributor. Trickling filter distribution head requirements are set by the system manufacturer. Despite head loss due to the trickling filter commonly being the greatest in a given WWTP, power requirements for the process (including recirculation pumping and auxiliary powered equipment) are typically significantly less than those for the activated sludge process. Process Flow Sheets and Bloreactor Configuration Trickling filter and combined TF/SG processes typically consist of preliminary treatment (including screening and grit removal), primary clarification, trickling filter, bioreactor, secondary clarification, and disinfection unit processes. Trickling filter recirculation methods influence the process flow. Generally, there are two types of trickling filter recirculation. The first allows for direct recirculation to the trickling filter and the second passes flow through a primary clarifier. Four trickling filter process flow diagrams, including both single- and two-stage trickling filters, are shown in Figure 4. Combined TF/SG process flow sheets are similar, but include a suspended growth reactor and return activated sludge (or return sludge for the TF/SC process) stream that is directed to the head of the suspended growth reactor. Recirculation of trickling filter underflow or settled effluent dilutes influent wastewater, dampens the influent organic loading variability resulting from diurnal fluctuations, and maintains required trickling filter hydraulic application rates. Clarifying trickling filter effluent may enhance the performance of a subsequent trickling filter in two-stage operation, but the designer 392 must ensure that the recirculation flow required for trickling filter wetting and biofilm thickness control does not exceed the limiting hydraulic loading rate for the intermediate clarifier. The design of settling tanks in two-stage trickling filter systems is also affected by the recirculation pattern. Sludge wasting and recirculation streams affect the trickling. filter process. Each of the process flow diagrams illustrated in Figure 4 directs waste biological sludge (which is sometimes referred to as humus in the trickling filter process) to the primary clarifiers where it is co-settled with primary sludge prior to being withdrawn from the system. Many facilities exist that withdraw and thicken primary and biological sludge separately. Bioreactor Classification. Trickling filters can be classified as roughing, carbon oxidation, carbon oxidation and nitrification, and tertiary nitrification. Table 2 summarizes characteristics of each trickling filter. The performance ranges are associated with average design condition. Single-day or average-week observations may be significantly greater. Hydraulics. Recirculation and distributor operation are important to good trickling filter performance and may be used to achieve proper media wetting, flow distribution, biofilm thickness control, and to prevent macro fauna accumulation. Albertson and Eckenfelder (1984) postulated that the active biofilm surface area in a trickling filter is dependent on biofilm thickness and media configuration, and that active biofilm surface area decreases with increasing biofilm thickness. The researchers stated that for medium-density cross-flow media with a 98-m 2 /m 3 specific surface area, a 4-mm increase in biofilm thickness would cause a 12% reduction of active biofilm area (assuming that all the media have been appropriately wetted). Poor trickling filter media wetting results in reduced effluent water quality. In a study of rotary distributor efficiency, Crine et al. (1990) found that the wetted area-to-specific-surface-area ratio ranged from 0.2 to 0.6 with the lowest values for high-density random pack trickling filter media. Many of the design formulations mentioned later in this paper incorporate a term that allows for specific surface area reduction due to distributor inefficiency in trickling filter media wetting. The interrelationship of liquid residence time, dosing, and media configuration on BOD 5 removal kinetics has not been addressed, and additional research is required. Increasing the Water Environment Research, Volume 83, Number 5

6 (A) (B) (c) (D) RS - ~ ~ vwsl I I. - T Figure 4-Typical trickling filter process flow sheets. Legend: (RS)-raw wastewater, (PC) primary clarifier, (PS) primary sludge, (PE) primary effluent, (TFINF) trickling filter influent, (TF) trickling filter, (TFEFF) trickling filter effluent, (TFRCY) trickling filter recycle, (SC) secondary clarifier, (WS) waste sludge, (SE) secondary effluent, (IC) intermediate clarifier, (ICE) intermediate clarifier effluent; (A) and (B) single-stage trickling filter process, (C) two-stage trickling filter process, (D) two-stage trickling filter process with intermediate clarification. WS average hydraulic application rate reduces the liquid residence time, but has been proven to increase wetting efficiency. The recirculation ratio (Q/QR) is typically in the range 0.5 to 4.0. Bryan (1955, 1962) and Bryan and Moeller (1960) demonstrated that vertical-flow media require an average application rate greater than 1.8 m 3 /m 2 /h to maximize BOD 5 removal efficiency. Shallow towers using cross-flow media have used hydraulic rates in the range 0.4 to 1.1 m 3 /m 2 /h. Grady et al. (1999) state that adequate media wetting may be achieved at a total hydraulic load (THL) of 1.8 to 2 m 3 /m 2 /h with rotary distributors. Distributor speed control has the following benefits: controlled flow interruption (periodicity of dosing), increasing. wetting efficiency (percent of media wetted), and biofilm thickness control. The designer should consider recirculation capabilities and the effect of reverse thrusting jets with the use of distributor speed control. Distributor speed control may not be required in all instances provided adequate dosing is applied by recirculation pumps and reverse thrusting jets. A German process control parameter (ATV, 1983), referred to as Spiilkraft, allows for the calculation of a dosing rate (mm/pass) as follows: May

7 Table 2-Trickling filter classification. Carbon Oxidizing Carbon Oxidation Design Parameter Roughing (cbods removal) and Nitrification Nitrification Media Typically Used Vertical flow Rock, cross flow, Rock, cross flow, Cross flow or vertical flow or vertical flow Wastewater Source Primary effluent Primary effluent Primary effluent Secondary effluent Hydraulic Loading m3 (gpm/ft 2 ) ( ) (0.25a-1.5) (0.25a-l.5) ( ) BOD 5 and NH 3 -N Load k 3.gd (lb BOD/d1000 f) ( ) (20-60) (5-15) NA rn2.d d (Ib NH 3 -N/d.1000 ft 2 ) NA NA ( ) ( ) Conversion (%) or Effluent 50 to 75% filtered 20 to 30 mg/l < 10 mg/l as cbod 5 ; 0.5 to 3 mg/l as Concentration (mg/l) cbod 5 conversion cbod 5 and TSSb < 3 mg/l as NH 3 -Nb NH 3 -Nb Macro Fauna No appreciable growth Beneficial Detrimental (nitrifying biofilm) Detrimental Depth, m (feet) (3-20) -s 12.2 (40) -s 12.2 (40) -< 12.2 (40) Notes: "8 Applicable to shallow trickling filters; gpm/ft 2 = gallons per minute per square foot of trickling filter plan area. b Concentration remaining in the clarifier effluent stream gpm/ft 2 x = m 3 /m 2 -d (cubic meter per day per square meter of trickling filter plan area). lb BOD,/d.1000 ft 3 x = kg/d-m 3 (kilograms per day per cubic meter of media). lb NH 3 -N/d-1000 ft 2 x 4.88 = g/d.m 2 (grams per day per square meter of media). Where mm THL" 1,000 - SK= - m.- Na.od 1,440-ay day SK = the Spfilkraft (mrn/pass); THL = the total hydraulic load = (Qi, + QR)/A, (m3/m 2 /d); Na = the number of distributor arms; and coa = the rotational speed (rev/min). Higher dosing rates are recommended for higher organic loading rates to provide biofilm thickness control and controlled sloughing of excess biomass. Besides a normal operating dosing rate, it may be beneficial to periodically use a higher flushing dosing rate for 5 to 10% of a 24-hour operating period. The flushing dose will operate at 6 to 15 times the normal operating dose. Albertson (1995) and Parker et al. (1989) demonstrated that there is benefit to biofilm thickness control in the trickling filter process. These Table 3-Operating and flushing distributors. dosing rates for Total Organic Operating Dosing Flushing Dosing Load kglm 3 /d Rate mm/pass Rate mm/pass (lb BODs/d/1000 ft 3 ) (inches/pass) (inches/pass) <0.4 (< 25) (1-3) 100 (4) 0.8 (50) (2-6) 150 (6) 1.2 (75) (3-9) 225 (9) 1.6(100) (4-12) 300(12) 2.4 (150) (6-18) 450 (18) 3.2 (200) (8-24) 600 (24) Note: Actual values are site-specific and vary with media type. 394 benefits include improved performance, reduced odors, reduced power use for recycling, reduced nuisance organisms, and elimination of heavy sloughing cycles (Albertson, 1995). Parker et al. (1989) described the use of both distributor speed control and variable frequency drive-controlled recirculation pumps to maintain constant trickling filter hydraulic application. However, Parker et al. (1989) also presented evidence that electrically driven distributor speed control did not improve NTF performance. Parker (1999) pointed out that there is little research describing the effect of hydraulic transients on synthetic trickling filter media and their effect on media life. The typical hydraulically driven distributor in North America operates in the range of 2 to 10 mm/pass. Table 3 lists recommended operating and flushing dosing rates for modular synthetic media. Oxygen Requirements and Air Supply Alternatives Trickling filters require oxygen for aerobic biochemical transformation processes. Several researchers have demonstrated that at least some portion (if not the entire bioreactor) of roughing, carbon oxidizing, combined carbon oxidizing and nitrification, and nitrifying trickling filters operates under oxygen-limited conditions (Kuenen et al., 1986; Okey and Albertson, 1989; Schroeder and Tchobanoglous, 1976). Ventilation is essential to maintain aerobic conditions in a trickling filter. The vertical flow of air through trickling filter media can be induced by mechanical ventilation or natural air draft. Mechanical ventilation enhances and controls airflow with low-pressure fans that continuously circulate air throughout the trickling filter. Current design practice requires provision of adequate underdrain and effluent channel sizing to permit free airflow. Passive devices for ventilation include vent stacks on the trickling filter periphery, extensions of underdrains through trickling filter sidewalls, ventilating manholes, louvers on the sidewall of the tower near the underdrain, and discharge of trickling effluent to the subsequent settling basin in an open channel or partially filled pipes. Water Environment Research, Volume 83, Number 5

8 Figure 5-Trickling filter aeration system: distribution pipes (left) and fans (right). Natural Draft. Naturally occurring airflow results from a difference in ambient air temperature and humidity outside and inside the trickling filter. The temperature causes air to expand when warmed or contract when cooled, and humidity differences result in density differences. The result is an air-density gradient throughout the trickling filter and an air front that rises or sinks depending on the differential condition. This rising or sinking action results in a continuous airflow through the bioreactor. If air inside the trickling filter is colder than the ambient air, the air will flow downward. Alternatively, if the ambient air is colder than the air inside the trickling filter, air will flow upward. Schroeder and Tchobanoglous (1976) state that upward airflow is the worst-case scenario from a mass transfer perspective because the dissolved oxygen driving force is lowest in the region of highest oxygen demand (i.e., the top of the trickling filter). Natural ventilation may become unreliable or inadequate in meeting process air requirements when neutral temperature gradients do not produce%air movement. Such conditions may be daily or seasonal, and can lead to the development of anaerobic layers inside the biofilms (near the growth medium) and poor trickling filter performance. Modular plastic media trickling filters that rely on natural draft to provide process oxygen for municipal wastewater treatment should include the following design features: Drains, channels, and pipes should be sufficiently sized to prevent submergence greater than 50% of their cross-sectional area under design hydraulic loading. Ventilating access ports with open-grating covers should be installed at both ends of the central collection channel. Large-diameter trickling filters typically have branch channels (to collect the treated wastewater). These branches should also include ventilating manholes or vent stacks installed at the trickling filter periphery. According to Grady et al. (1999), the open area of the slots in the top of the underdrain blocks should not be less than 15% of the trickling filter area. One square meter gross area of open grating.in ventilating manholes and vent stacks should be provided for each 23 m 2 of trickling filter area. Typically, 0.1 m 2 of ventilating area is provided for every 3 to 4.6 m of trickling filter periphery, and 1 to 2 m 2 of ventilation area in the underdrain area per 1000 m 3 of trickling filter media. Another criterion for rock-media trickling filters is the provision of a vent area at least equal to 15% of the trickling filter cross-sectional area. Mechanical Ventilation. A majority of new and improved trickling filters use low-pressure fans to mechanically induce airflow. The airflow resulting from natural draft will distribute itself. This will not occur with mechanical ventilation. Pressure loss through synthetic trickling filter media is typically low, often less than 1-mm H 2 0 per meter of trickling filter depth (Grady et al., 1999). The low-pressure drop typically results in low fan power requirements (e.g., on the order of 3 to 5 kw for modestsized facilities). The head on the fan is typically less than 20 to 30 mm H 2 0. Unfortunately, the low pressure drop allows air to rise upward through the trickling filter media without distributing itself through the bioreactor section. Therefore, fans are typically connected to distribution pipes. The airflow distribution piping has openings that are sized such that airflow.through each is equal and airflow distribution is uniform. The pipes typically have a velocity in the range 1100 to 2200 m/hr in order to further promote uniform airflow distribution. Airflow requirements are calculated based on process oxygen requirements and characteristic oxygen transfer efficiency, which is typically in the range of 2 to 10%. The mechanically induced airflow may flow upward or downward. Down-flow systems can be designed without covers, but covers are required for upflow systems. Covering trickling filters offers a wintertime benefit of limiting cold airflow and minimizing wastewater cooling. Mechanical ventilation and covered trickling filters may be used to destroy odorous compounds. A trickling filter aeration system is pictured in Figure 5. Trickling Filter Design Models. Numerous investigators have attempted to delineate the fundamentals of the trickling filter process by developing relationships among variables that affect trickling filter operation. Existing trickling filter process models range from simplistic empirical formulations to numerical models. Analyses of operating data have been made to establish equations or curves to fit available data. Results of these data analyses have led to the development of several empirical trickling filter formulas. Unfortunately, numerous models exist and there is lack of an industry standard. Designers need to assess which equation best fits a particular situation when selecting a design model, especially with regard to the confidence level necessary to meet discharge permit requirements. Therefore, many process designers use a forecasting approach and will apply several empirical models to evaluate a system. The following empirical models have been reported by Boltz et al. (2009) and Boltz (2010) as options historically used to describe trickling filter performance in the context of process design: (1) National Research Council (1946), (2) Velz (1948) May

9 kg/looo m 2 * d ' ]' I ' I I L 0 No Redrmatkton Rack utaiion ~ID Z 80 at * D 20 U S I I, I, BODs Load, b/1000 mu Wady 0w ORGANIC LOADING, Ib BO0/1OO0 eqft/day Figure 6-Nitrification efficiency as a function of BODs load in rock-media combined carbon oxidation and nitrification trickling filters (Left: U.S. EPA, 1975; Right: Parker and Richards, 1986). equation, (3) Schulze (1960) equation, (4) Eckenfelder (1961) formula, (5) Galler and Gotaas (1964), (6) Germain (1966) equation, (7) Kincannon and Stover (1982), and (8) the Institution of Water and Environmental Management (1988) formula. A pseudo mechanistic model called the Logan trickling filter model (TRIFL) (Logan et al. 1987a, 1987b) has been used to design modular synthetic media trickling filter processes. There is a general lack of models describing TF/SG systems. Daigger et al. (1993) and Takdcs et al. (1996) presented a mathematical description of TF/SG processes. The model of Daigger et al. (1993) was developed to characterize nitrification in TF/SG processes and was established based on performance observations at the Buck Creek WWTP, Garland, Texas. The model accounts for suspended growth reactor seeding with detached biofilm fragments in the trickling filter effluent stream. The TF/SG process effluent is calculated using the following equation: Where [~~~max~ (M~T kd]. (NH 3.~ 2 [ +kd (NH 3,,-K,) -y,.nh 3 PE,... NH 3 E,, + [( R+kd).(NH3UE-K,)] =0 (2) /iý = the maximum nitrifier growth rate (lid), MCRT = the mean cell residence time (d), kd = the specific decay rate (m/d), Ks = the ammonia-nitrogen half-saturation constant (mg/l), NH3.EFF = the ammonia-nitrogen concentration in the TF/SG process effluent stream (mg1l), NH3.TFE = the ammonia-nitrogen concentration in the trickling filter effluent stream (mg/l), and NH3.pE = the ammonia-nitrogen concentration in the trickling filter process influent stream (mg/l). The model of Daigger et al. (1993) has been independently evaluated and demonstrated to be effective by Biesterfeld et al. 396 (2005). The researchers noted that the model of Daigger et al. (1993) is primarily dependent on nitrification rates in the trickling filter and suspended growth reactor mean cell residence time, or solids residence time. Combined Carbon Oxidation and Nitrification. Combined carbon oxidizing (i.e., carbonaceous 5-day biochemical oxygen demand [cbod 5 ] removal) and nitrification trickling filters may contain rock or synthetic media. The U.S. Environmental Protection Agency (U.S. EPA) (1991) reported survey results of 10 combined carbon oxidation and nitrification facilities. Six of the facilities included the TFISC process. The survey was used to create empirical guides for achieving nitrification in the-secondary treatment process trickling filters. The manual for nitrogen control (U.S. EPA, 1993) presented recommended BOD 5 loading (g/m 2 /d) to achieve both carbon oxidation and nitrification in a single-stage trickling filter. The kinetics of combined BOD 5 removal and nitrification are complex, and the lack of fundamental research supporting combined carbon oxidation and nitrification in the trickling filter process results in the continued use of empirical design procedures. Therefore, the design of combined carbon oxidation and nitrification trickling filters is empirical (Parker 1998). U.S. EPA (1975) summarized full- and pilot-scale rock-media trickling filter data from Lakefield, Minnesota; Allentown, Pennsylvania; Gainesville, Florida; Corvallis, Oregon; Fitchburg, Massachusetts; Ft. Benjamin Harrison, Indiana; Johannesburg, South Africa; and Salford, England. Likewise, significant data are presented for a diverse range of U.S. plants by the Water Environment Federation (2000). Figure 6 illustrates the relationship between BOD 5 volumetric loading and nitrification efficiency using both pilot- and full-scale rock-media combined carbon oxidation and nitrification trickling filters. These observations indicate that an organic loading rate of 0.08 kg BOD,/m 3 /d (5 lb BOD/1000 ft Od) 3 (according to U.S. EPA [1975]) or 2 kg/1000 m 2 /d (0.5 lb/1000 ft 2 / d) (according to Parker and Richards [1986]) is required for rockmedia trickling filters to achieve approximately 90% nitrification. Recirculation typically improves nitrification, particularly for nitrification efficiencies greater than 50%. Daigger et al. (1994) presented an evaluation of three full-scale trickling filters with low-density cross-flow media. The trickling filters were dosed with rotary distributors and designed for Water Environment Research, Volume 83, Number 5

10 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 to 20 Malmo, Sweden Parker et al. (1995) XF to 20 Littleton/Englewood, Colorado Parker et al. (1997) XF to 20 Midland, Michigan Duddles et al. (1974) VF to 13 Lima, Ohio Okey and Albertson (1989) VF to 22 Bloom Township, Illinois Baxter and Woodman (1973) VF to 20 1 Fully corrugated. Note: XF = cross flow and VF vertical flow. combined carbon oxidation and nitrification. Data collected from these studies suggest that an organic load less than 0.2 kg BOD 5 /m 3 /d (13 lbs BOD,/1000 ft 3 /d) is required to achieve 90% nitrification efficiency. Similar to the observations reported by Stenquist et al. (1974), the synthetic media trickling filters studied were able to achieve greater than 90% nitrification efficiency. Biofilm thickness control is recommended to optimize NH 3 -N removal in combined carbon oxidation and nitrification trickling filters (Parker et al. 1995; 1997). Daigger'et al. (1994) proposed the following equation to describe BOD 5 and NH 3 -N removal in modular plastic media carbon oxidation and nitrification trickling filters: Where VOR= [Si SNo.-NJ'( ) ( ý 1 4 -g (3) VOR = the volumetric oxidation rate (kg/m 3 /d), Si = the BOD 5 concentration in the influent stream (g/m 3 ), SNOx-N = the nitrate/nitrite-nitrogen concentration in the effluent stream (g/m 3 ), Q = the flowrate, including recirculation streams (m 3 /d), and VM = the synthetic media volume (mi 3 ). Using eq 3, Daigger et al. (1994) reported the volumetric oxidation rate for three combined carbon oxidation and nitrification trickling filter (with modular plastic media) processes in the range of 0.4 to 1.3 kg/m 3 /d. Nitrifying Trickling Filters. Nitrifying trickling filters are a reliable and cost-effective means for NH 3 -N conversion. The following design practices have been demonstrated in full-scale application: (1)"use medium-density cross-flow media to optimize hydraulic distribution and oxygenation, (2) use mechanical ventilation, (3) periodically alternate the lead NTF to avoid patchy biofilm development in the lower reaches of the second-stage unit, (4) the influent should be secondary effluent to minimize bacterial competition for substrates inside the biofilm, (5) maximize wetting efficiency to avoid the formation of dry spots, (6) dose the NTF at a rate that will minimize the accumulation of macro fauna, and (7) equalize NY 3 -N-laden supernatant from solids processing operations to even out diurnal load variability (Parker et al., 1995; 1997). Benefits to NTFs include low energy consumption, stability, operational simplicity, and reduced sludge yield. The reduced sludge yield and resulting low total suspended solids concentration in the NTF effluent stream has led some units to be constructed without downstream liquid-solids separation units. This is dependent on site-specific treatment objectives arid effluent water quality standards. An operational issue that can be detrimental to process performance is the control of predatory macro fauna. Therefore, the designer must include means for managing solids and macro fauna-laden water resulting from macro fauna control measures. Design and operational features dedicated to macro fauna control are presented in a subsequent section. Nitrifying trickling filters having 6- to 12.2-mr (20- to 40-ft) modular plastic media depths have demonstrated improved performance. Nitrifying trickling filters have been constructed with depths up to 13 m (-42 ft). Shallower units can operate as a two-stage system. Recirculation should be minimized to that required for biofilm thickness control in order to maximize NH 3 -N concentration (i.e., maintain a high driving force) (Parker et al., 1997). The practice of alternating the lead trickling filter in a two-stage trickling system is referred to as alternating double filtration (ADF). Gujer and Boller (1986) and Parker et al. (1989) observed patchy biofilm growth in the lower section of pilot-scale NTFs. The researchers attributed the patchy growth to dry spots. Aspegren and coworkers (1992) observed improved nitrification and reduced biofilm patchiness when operating the NTFs in an ADF system. Use of the ADF approach with trickling filters in series encourages fulldepth biofilm development in both trickling filters. The lead trickling filter should be switched every 3 to 7 days to ensure that both units contain a healthy biofilm developed along the entire bioreactor depth. The primary drawback of ADF is an increase in power requirements, which may be in excess of 50% due to double pumping. In addition to increased operating cost, capital costs associated with pipes and valves will also increase costs. Parker (1998, 1999) described nitrification efficiency in NTFs containing either cross-flow or vertical-flow synthetic media types. Table 4 summarizes his observations, which demonstrate that zero-order ammonia-nitrogen flux rates are greater for crossflow than vertical-flow media. Factors contributing to the enhanced performance may be improved oxygen-transfer efficiency resulting from the increased number of media interruptions and improved oxygenation (Gujer and Boller, 1986; Parker et al., 1989). Autotrophic nitrifying biofilms are thin when compared with the heterotrophic biofilms that are primarily responsible for BOD 5 removal; therefore, medium-density cross-flow media are typically used in NTFs. However, there is a propensity to develop dry pockets when high-density modular plastic media are used (Parker et al., 1989). Gujer and Boiler Nitrifying Trickling Filter Model. Gujer and Bolter (1986) developed the following semi-empirical model that reasonably characterizes NTF performance: JN(S, T) JN, max (T) S I KN +SB, N (4) May

11 Where JN(S, T) = the ammonia-nitrogen flux at SB.N (g/m 2 /d), JN,v, m(t) = the maximum ammonia-nitrogen flux at temperature T (g/m 2 /d) (=Jo 2,m.x(T)/4.3), SB.N = the bulk-liquid ammonia-nitrogen concentration (g/m 3 ), KN = the half-saturation coefficient for ammonianitrogen (g N /m 3 ) (= 1.0 g N /m 3, typical value), and T = the temperature (*C). Based on a "line-fit" relationship, the flux at any depth in the trickling filter can be calculated as JN(Z, T)=JN(0, T)'e-k',. (5) The following two solutions were developed to account for a change in the rate of nitrification with NTF depth (k # 0) (eq 6), and the second assumes no decrease in the rate of nitrification with NTF depth (k = 0) (eq 7): a'jn,a(t) (1 -e-kz) =S i.,n-sb,i+kn In (ŽSi, IV (6) k-vh When k = 0, Where \SB, NI z--nn =T Sin, N -SB,N +KN'tn N (7) An(SB.N a = the specific surface area (m 2 /m 3 ), k = the empirical parameter describing nitrification rate decrease (1/m) (= 0 to 0.16, typical 0.1), vh = the hydraulic load (with or without recirculation) (m 3 / m 2 /d), z = the NTF depth (m), and Siý. N = the ammonia-nitrogen concentration in influent stream (g/m 3 ). These equations can be solved directly to size a NTF for a desired SB,v. When recirculation is used, an iterative solution routine that includes the following equation is required because of the effect recirculation has on both Vh and Si,, N: Where 5 N. I i --- SO, N +-R-SB, N l+r (8) R S,N SM~,N Si., N S8, N So, N = the ammonia-nitrogen concentration in the influent stream prior to being mixed with the recirculation stream. The ammonia-nitrogen concentration in NTF influent stream, Si, N will be less than So. N when recirculation is applied. Parker et al. (1989) proposed a modification of this model to account for oxygen-transfer efficiency variability amongst modular plastic media types and operating conditions. The revised expression is as follows: Where 398 Jo 2,m.(T) SB,N (9) JN (z,t) =EO 4.3 KN+SB, N E02 = the dimensionless NTF media effectiveness factor and Jo 2,max(T) = the maximum dissolved-oxygen flux at temperature T (g/m 2 /d). Based on their experience, Gujer and Boiler (1986) reported an E 0 2 value in the range of 0.93 to 0.96 for Ks,02 = 0.2 g 02 /M 3 and the temperature range of 5 to 25 *C. Parker et al. (1989), on the other hand, observed lower E 02 values (in the range of 0.7 to 1.0) and claimed that a departure from E 0 2 = 1.0 accounts for wetting inefficiency, biofilm grazing by macro fauna, or competition for dissolved, oxygen between autotrophic nitrifiers and heterotrophic bacteria inside the biofilm. The researchers recommended that medium-density cross-flow media are used in NTF applications and that E 0 2 may range from 0.7 to 1.0 for this media. High-density cross-flow media had a corresponding E 0 2 approximately equal to 0.4 (Parker et al., 1995). According to Parker et al. (1995), Eo 2 "Jo 2,,x(T)/4.3 is the zero-order ammonia-nitrogen flux (Parker et al., 1995). The maximum dissolved-oxygen flux reflects the oxygen-transfer efficiency of the selected modular plastic media, and was determined by the researchers using TRIFL (Logan et al., 1987a). The coefficient, Ks. 0 2, determined for the Central Valley WWTP in Utah, was in the range of 1 to 2 mg/l (Parker et al., 1989). Operational Strategies and Facility Improvements for Macro Fauna Control Several strategies have been applied to manage macro fauna accumulation and/or development in trickling filters, including physical, chemical, or a combination of physical and chemical applications. The ideal control strategy is to promote a condition that is either toxic to the macro fauna or creates an environment not conducive to their accumulation. Lee and Welander (1994) demonstrated increased nitrification after predator control using substances toxic to eukaryotic organisms. The toxic substance must either have no effect on or only temporarily inhibit beneficial microorganisms (Parker et al., 1997). Operators have conducted site maintenance that aids in reducing macro fauna presence in trickling filter-based WWTPs. For instance, some operators have observed that the presence of filter flies may be reduced by simply maintaining a short stand of grass on the WWTP site. More specific strategies include periodic high-intensity hydraulic application, trickling filter flooding, ph adjustment with lime or sodium hydroxide, high-concentration aqueous ammonia dosing, trickling filter effluent or secondary clarifier underflow (humus) screening or accelerated gravity separation, gravity separation in low-velocity channels with a dedicated pumping circuit, eliminating dissolved oxygen from the trickling filter feed, adding salt, draining and freezing the infested unit, raising the temperature quickly, adding molluscacide (e.g., copper sulfate), and chlorinating the influent stream. Many of these strategies have proven ineffective in some trickling filters, and others may be detrimental to bioreactor performance. Biochemical reactions are influenced by temperature, ph, and alkalinity; adjusting these parameters may inhibit the biochemical reactions and lower transformation rates. Chemicals such as chlorine are toxic to all organisms in the trickling filter and may result in destruction of sensitive biomass (Parker et al., 1989). A brief summary of the principal control mechanisms in use is provided here. More details are available elsewhere (Boltz et al., 2008). Control mechanisms described here include: Water Environment Research, Volume 83, Number 5

12 Daigger and Bottz "* Periodic high-intensity hydraulic flushing (controlled dosing, or Spalkraft), "* Trickling filter flooding and chemical application, "* Chemical treatment (focus on high-concentration aqueous ammonia dosing and ph adjustment with sodium hydroxide), "* Trickling filter effluent or underflow (humus) screening or accelerated gravity separation (using equipment typically associated with grit removal), and "* Gravity separation in low-velocity channels and removal with a dedicated pumping circuit. Dosing for Macro Fauna Control. Hawkes (1955) demonstrated that high hydraulic loadings and periodically increasing instantaneous dosing rates can control filter fly development. Increased hydraulic loading improves trickling filter media wetting efficiency, thereby reducing dry spots and minimizing ideal spawning areas for filter flies. Gujer and Boiler (1984) reported that filter fly larvae were reduced to quantities that did not have an impact on NTF performance. Andersson et al. (1994) tested three flushing intensities (Spiilkraft values of 5, 40, and 80 mm/pass) and reported that the variable flushing intensity had no apparent effect on filter fly and worms in a pilot-scale NTF. (Note that these Spiilkraft values are below those reported for flushing, as presented in Table 3.) Flooding. Trickling filter flooding requires adequate duty units to isolate a trickling filter for a 3-to-6-hour period. The trickling filters must be designed as water-retaining structures, which is not typical. Variations include (1) saline flooding and (2) flooding and backwashing with an alkaline solution. Parker et al. (1997) reported the use of flooding to control filter flies and an alkaline backwash process to control other macro fauna in two 32- m-diameter, 7.3-m-deep medium-density cross-flow media NTFs at the Littleton-Englewood WWTP in Colorado. Online ph probes and a sodium hydroxide metering system allow for flood water ph adjustment by operator set point. The alkaline flood water is pumped through the NTF bottom, is discharged into an overflow trough, and is then directed to the head of the WWTP for treatment. Alkaline treatment is reported to have removed 76% of larvae at ph 9 and 99% at ph 10 (Parker et al., 1997). Subsequent research trials designed in response to snail development demonstrated that flooding and backwash (4 hours at ph 9) reduced snail quantity by two-thirds and returned the NTFs to high nitrification efficiency (Parker, 1998). Chemical Treatment. Everett et al. (1995) summarized several chemical treatment alternatives including ph adjustment and chlorination, sodium chloride, and molluscicides (e.g., copper sulfate, metaldehyde, niclosamide, and trifenmorph). Factors such as ph, turbidity, and molluscicide dose are key factors in determining chemical application rate. Rotating biological contactors (RBCs) in Lafayette, Louisiana, applied sodium chloride at a dosing concentration of 10 mg/l for a 24-hour period to effectively control the snail accumulation. Calcium hypochlorite in a range of 60 to 70 mg/l was applied during a 2-to-3-day period, and effectively minimized snail accumulation in RBCs at the Deer Creek WWTP, Oklahoma City, Oklahoma. Copper sulfate at low concentrations (0.45 kg of copper sulfate per mi 3 ) may effectively control snail accumulation. Ammonia is toxic to snails. Lacan et al. (2000) conducted a laboratory-scale study and plant-scale application of un-dissociated aqueous ammonia [NH 3 -N(aq)] solutions with elevated ph to control snail growth (P. gyrina) in NTFs. Un-dissociated aqueous NH3-N(aq), not the ammonia ion, is the snail P. gyrina toxophore. The concentration producing 100% mortality is a function of exposure time and the bulk-liquid NH3-N(aq) concentration. The laboratory-scale study demonstrated that an ammonium chloride (NH 4 CI) solution at ph 9.2 [NH 3 -N(ag) = 150 mg N/L] resulted in 100% snail mortality. A much higher concentration of ammonia is required in the trickling filter influent stream (i.e., 1000 to 1500 mg N/L) to maintain the required NH3-N(ag) = 150 mg N/L because of the immediately reduced concentration owing to axial dispersion, biofilm diffusion (both external and internal), and biochemical reaction. Lacan et al. (2000) estimated that an influent ammonia concentration of 1080 mg N/L resulted in an average concentration throughout the NTF of 185 mg N/L. Such a high-concentration NH 3 -N(,q) stream may be readily available in municipal WWTPs as solids processing recycle streams. In some instances, however, it may be necessary to purchase NH3-N(aq). The first full-scale application of this snail control method was reported by Gray et al. (2000) at the Truckee Meadows WWTP, Reno Sparks, Nevada, which uses high-density (215 m 2 /m 3 ) media. Ammonia-rich anaerobic digester centrate was directed to a NTF recirculation pump station. Sodium hydroxide was added to the recirculation stream to raise the ph to 9.05 (range 9.0 to 9.5), which increased the NH3-N(aq) content of the centrate solution. Figure 7 illustrates the (1) normal operating mode, (2) centrate treatment/recirculation mode, and (3) the flushing mode characteristic of the macro fauna control method described by Lacan et al. (2000). This macro fauna control method is typically applied once per month. During the treatment cycle, an NTF is isolated and the solution is recirculated through the trickling filter for approximately 2 hours. The first 20 to 50 minutes of aqueous ammonia dosing is dedicated to reaching a hydrodynamic steady state (i.e., 3 to 4 hydraulic retention times [HRTs]), and the remainder is the minimum recommended exposure time for 100% mortality of both adult snails and their larvae. The treatment solution is returned to the head of the WWTP after dosing is completed and the NTFs are then flushed with secondary effluent in the "recirculation mode" for 10 hours. Mechanical Control. Physical removal techniques include (1) trickling filter effluent or underflow (humus) screening, (2) gravity separation in low-velocity channels and removal with a dedicated pumping circuit, and (3) accelerated gravity separation using equipment typically associated with grit removal. The Central WWTP, Baton Rouge, Louisiana, uses trickling filter secondary clarifier underflow screening to control snail accumulation in, or damage to, solids handling equipment. The City of Lawton, Oklahoma, dnd both the South San Luis Obispo, California, County Sanitation District Oceana Regional Plant and the City of San Luis Obispo Water Reclamation Facility, San Luis Obispo, California, pump secondary clarifier underflow to a free vortex classifier for snail shell removal. The Econchate Water Pollution Control Plant, Montgomery, Alabama, removes snail shells in the chlorine contact basin, which was modified to a twopass channel to serve as a low-velocity sedimentation basin for snail shells escaping secondary clarification. The snail shells deposited in the low-velocity channel are collected in a sump and pumped to a static screen, where they fall by gravity into a collection bin. Tekippe et al. (2006) reported the use of baffles, grit pumps, and classifiers to remove snails from the Ryder Street WWTP, Vallejo, California. The facility treats wastewater with a TF/SG process consisting of two, 32-m-diameter and 7.3-m-deep May

13 I Flshng Mode I SOcnm TF Efft~uwl LEGEND - - Opefralg Flow Path Figure 7-Nitrifying trickling filter operating modes for high-concentration un-dissociated aqueous ammonia dosing (Lacan et al., 2000). cross-flow media trickling filters. Initial zones of the Ryder Street WWTP's aeration basins were improved to provide a zone for the majority of the shells to settle. An automatic mechanism was provided to remove the settled shells (Tekippe et al., 2006). Combined Trickling Filter and Suspended Growth Processes Biological processes including both a trickling filter and suspended growth reactor build on the known performance and operating characteristics of the parent processes. When the suspended growth reactor is used as a flocculating unit it is referred to as the TF/SC process. All other TF/SG processes use the coupled suspended growth reactor as an oxidizing unit. The activated biofilter and biofilter/activated sludge processes, which circulate return activated sludge over the trickling filter (making it a biofilter), are not discussed as these process options are applicable only to wood slat media, which is seldom used these days (Grady et al., 1999). Trickling Filter/Solids Contact. A majority of organic matter in municipal wastewater is colloidal or particulate material (Levine et al., 1985, 1991; and Boltz and La Motta, 2007). Trickling filters are poor bioflocculating reactors (Boltz et al., ). The TF/SC process operates under the premise that trickling filter effluent contains a high concentration of not readily settleable colloidal and particulate organic matter. The material may be removed by bioflocculation, along with the oxidation of residual soluble organics, in a solids contact basin. The TF/SC process includes a trickling filter followed by a small, aerated solids-contact channel. Biomass in the solids contact basin effluent stream flows to a clarifier that has a (1) suction-header sludge withdrawal mechanism and (2) a flocculating feed well (approximately one-third of the clarifier diameter) that promotes gentle mixing and additional bioflocculation of the influent suspended biomass (sludge flocs). LaMotta et al. (2004) indicate the following characteristics for such systems: "* Solids contact basin dissolved oxygen concentration greater than I mg/l, "* Dissolved oxygen uptake rate typically low, and "* Short distance between solids contact basin and clarifier desired (long runs may require aerated channels). There are three modes of operating the TF/SC process: mode I, mode II, and mode III. Mode I relies exclusively on the solids Water Environment Research, Volume 83, Number 5

14 TRICKLING FILTER AERATED SOLIDS CONTACT TANK SECONDARY CLARIFIER FLOCCULATOR CENTER WELL TREATED EFFLUENT Mode I TRICKL ING FILTER SECONDARY CLARIFIER PRIMAR EII1N MIXED LIQUOR PRIMAPV~ FLOCCULATOR ~ R TR L#E T NSLUDGE; Mode II REAERATION TANKS TRICKL NG FILTER AERATED COTC SOLIDS AKFLOCCU SECONDARY CLARIFIER LATOR C8ETER WELL PRIMAR.Y UP I IEMIT WASTEE, ETU RN 'SLUDGE1 ' TREATED EFFLUENT, Mode 1II REAERATION TANKS Figure 8-Three modes of TF/SC process operation (after Parker and Merrill [1984]). contact basin for colloidal and particulate organic matter bioflocculation, and the oxidation of residual soluble organic matter. Mode II relies exclusively on a return sludge aeration chamber. The aerated return sludge is mixed with trickling filter effluent for colloidal and particulate organic matter bioflocculation. Mode III makes use of both the solids contact basin and a return sludge aeration tank. A typical TF/SC process operates as mode I; however, as of 2001, more than one-half of the TF/SCbased WWTPs were operating as mode III (or had the operational flexibility to operate as mode I or I1). It should be noted that mode II is seldom used and is typically not recommended as it does not have a solids contact basin and only a sludge reaeration tank (Parker and Bratby, 2001). These operational modes are illustrated in Figure 8. If the solids contact basin follows a carbon oxidation and nitrification trickling filter(s), autotrophic nitrifiers will detach from the biofilm surface and, essentially, bioaugment the solids contact basin biomass inventory. Despite the short duration solids retention time characteristic of the solids contact basin, the bioaugmentation will cause nitrification, which will exert additional oxygen demand (i.e., increased airflow, blower size, and air piping). In some instances, this may be desirable; however, in instances where increased oxygen demand is not desired and nitrification is inevitable, the designer should seek to maximize nitrification in the carbon oxidation and nitrification trickling filter. This may be achieved with proper air supply system design and process loadings, as discussed above. The solids contact basin is typically 5 to 20% of the volume that would be required with treatment by activated sludge. By combining a trickling filter and solids contact basin, the trickling filter size may be reduced compared to the size typically required if treatment is accomplished with only a trickling filter (Parker and Matasci, 1989). One significant benefit of the TF/SC process is the low power requirements owing to a relatively high dependence on the trickling filter to remove the majority of soluble organic matter BOD 5. Rock- and plastic-media trickling filters can be upgraded with the TF/SC process. Table 5 lists generally accepted design criteria for the TF/SC process. Roughing Filter/Activated Sludge. Roughing trickling filters have been used to expand WWTP treatment capacity. The roughing filter is a highly-loaded trickling filter that uses 10 to 40% of the media volume required if treatment has been accomplished through the use of the trickling filter process alone. Hydraulic retention time in the aeration basin is typically 30 to 50% of that required with the May

15 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 BOD 5 removed) Trickling filter hydraulic load (gpm/ft 2 ) Trickling filter influent total organic load (lbs/1000 ft 3 -day) Solids contact basin side water depth (feet) Solids contact basin HRT at average day flow (min) Solids contact basin HRT at peak flow (min) Solids contact basin solids residence time (d) Solids contact basin MLSS concentration (mg/l) Sedimentation basin overflow rate at average day flow (gpd/ft 2 ) Underflow concentration (% total solids) Note: MLSS = mixed liquor suspended solids. activated sludge process. The TF/SC and roughing filter/activated Summary sludge (RF/AS) processes have the same process flow sheet. The modem trickling filter typically includes the following However, 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 the contaminant oxidation. This differs from the TF/SC process, where bioreactor is treating high-strength wastewater, which warrants the trickling filter is larger and provides the majority of the BOD 5 the use of vertical-flow media); (3) a mechanical aeration system removal, 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 odor vated sludge (TF/AS) process is designed at high organic loads, control). Covers may be equipped with sprinklers that can spray However, a unique feature of TF/AS is the intermediate clarifier. washwater to cool the media during emergency shut down The 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 mathematical growth reactor. A benefit of using the TF/AS combined process is models and design approaches, and the design and operation of that solids generated in the trickling filter can be removed before trickling filter and TF/SG processes is empirical. Some empirical second-stage activated sludge treatment. This is often a preferred trickling filter design criteria and semi-empirical NTF models mode of operation where NH 3 -N removal is required. The reduced have been described in this paper. Benefits inherent to the oxygen demand afforded by intermediate clarification is typically trickling filter process (when compared to activated sludge considered less significant than the savings in capital and processes) include operational simplicity, resistance to toxic and operating costs gained by eliminating intermediate clarification, shock loads, and low energy requirements. However, trickling Therefore, cost-to-benefit evaluations typically guide designers to filters are susceptible to nuisance conditions that are primarily use 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 filter TF/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 BOD 5 removed) Trickling filter hydraulic load (gpm/ft 2 ) TF influent total organic load (lbs/1000 ft 3 -day) Aeration basin side water depth (feet) Aeration basin hydraulic retention time at average day flow (min) Aeration basin hydraulic retention time at peak flow (min) Aeration basin solids residence time (d) Aeration basin MLSS concentration (mg/l) Sedimentation basin overflow rate at average day flow (gpd/ft2) Underflow concentration (% total solids) Note: MLSS = mixed liquor suspended solids. 402 Water Environment Research, Volume 83, Number 5

16 information on the selection and design of these process components is fragmented and has been poorly documented. The TF/SC process is the most common TF/SG process. State-ofthe art design and operational practice for the trickling filter process has been reviewed and described in this paper. Acknowledgments A preliminary version of this paper was prepared as supplemental information for the "Trickling Filter and Combined Trickling Filter-Suspended Growth Process Design and Operation" presentation in Workshop W213, Biofilm Reactors: Application to Today's Global Wastewater Challenges, presented at the 82nd Water Environment Federation Technical Exhibition and Conference (WEFTEC) in Orlando, Florida, in October, Submitted for publication December 22, 2009; revised manuscript submitted March 14, 2010; accepted for publication June 21, References Albertson, 0. E. (1995) Excess Biofilm Control by Distributor-Speed Modulation. J. Environ. Eng., 121 (4), 330. Albertson, 0. 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18 COPYRIGHT INFORMATION Author: Title: Trickling Filter and Trickling Filter -- Suspended Growth Process Design and Operation: A State-of-the-Art Review Source: Water Environ Res 83 no5 My 2011 p ISSN: DOI: / X Publisher: Daigger, Glen T.; Boltz, Joshua P. Water Environment Federation 601 Wythe Street, Alexandria, Va The magazine publisher is the copyright holder of this article and it is reproduced with permission. Further reproduction of this article in violation of the copyright is prohibited. This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sublicensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.