The modern trickling filter is quite advanced from the rock filters of old. These new filters are engineered systems that provide a very cost-effective process for treatment of both domestic and industrial wastewater. Trickling filters are routinely designed to treat wastewater to NPDES standards including ammonia removal and/or they can be designed to provide low-cost roughing of high-strength wastewater. Trickling filters are often teamed up with activated sludge systems to reduce the overall cost of wastewater treatment
PVC Trickling Filter Media
The introduction of thermoformed PVC sheetmedia
is largely responsible for the success of the modern trickling filter.
This advancement allows construction of modules of superior compressive
strength and higher void-volumes necessary for stacking to heights not
achievable with rock filters. In addition, greater specific surface area
makes higher organic loadings possible and makes more efficient nitrification
towers possible. Deeper bio-towers are easily ventilated because of the
higher void volumes.
New
Trickling Filter Installations
New trickling filter installations take advantage of the benefits
offered by PVC sheet media:
Low Power Requirements
Trickling filters only require power for pumping and do not need large
power-hungry aeration blowers like suspended growth systems such as Activated
Sludge and Sequencing Batch Reactors. For this reason, the trickling filter
is frequently used as a roughing process in tandem with activated sludge
systems
Simple Operation
The operational requirements of trickling filters are less demanding than
those of activated sludge systems. However, there is enough flexibility
to allow the operator to optimize performance. For example, recycle rates,
flushing rates, and wetting rates are important variables that can be
adjusted to accommodate changing organic and hydraulic loadings. Far less
control data must be acquired andmonitored for tricklingfilter operation
than for an activated-sludge system or sequencing-batch system.
Lower Sludge Production
Trickling filters produce less sludge than suspended-growth systems. The
sludge that is produced tends to settle well because it is compact and
heavy.
The Elements of the Modern Trickling Filter
• High Specific Surface Area
• Wide Flow Passages
• Superior Ventilation (Open Plenum and High Void Volume)
• Heights up to 30 feet
Old rock trickling filters are being upgraded and rehabilitated with plastic sheet media. The greater surface area and higher void volume of structured-sheet media provides improved treatment efficiency, even at the very shallow depths used in old rock filters (typically 3 ft. to 7ft.). In some cases, the walls of the filter beds may be extended upward a few feet for additional increases in the rated capacity of the retrofitted plant.
Compared with rock, plastic sheet media has 2-3 times the specific surface area, which provides proportionally more area for biomass attachment. Also, the increase in the void volume from 50% to 95% improves the airflow and hydraulic loading capacity, decreases the tendency of the system to clog with biomass, and reduces odors associated with anaerobic pockets caused by silting.
Structured-Sheet
Plastic Media
Structured-sheet plastic media is the heart of the trickling filter. The
specific type of media to be used in a given system is based on the organic
loading and wastewater treatment objectives: roughing, complete treatment,
or nitrification. The media’s specific surface area, void volume,
and distribution characteristics are important to the specific application
and system performance.
The wetting rate, organic load, ammonia load, temperature of the wastewater, and desired effluent quality determine the volume of media required.
A typical media installation layout consists of modules 2 ft. wide x 2 ft. high x 4 ft. or 6 ft. long placed in layers, each layer placed at a right angle to the layer below. The media is cut to fill the tank at the periphery.
Media Support System
In
newer towers, the media is supported well above the concrete floor of
the filter tower. This creates a plenum that allows air to move freely
through the vent windows and under the media/lintel structure. The air
moves up through the tower in summer (when the air is warmer than the
wastewater) or down through the tower in winter (when the air is colder
than the wastewater), providing oxygen to the bacteria throughout the
tower.
Domes & Forced-Draft Ventilation
Domes and forced-draft ventilation systems are often used in new trickling
filter systems. Older, open systems relied strictly on natural draft for
ventilation. The dome at the top of the trickling filter serves to reduce
temperature losses in the winter and shield the system from strong winds
that could interfere with ventilation. In some systems, the dome is also
used to collect trickling filter vent gasses that are then channeled to
scrubbers.
Rotating-Arm Distributor
A rotating arm distributes the mixed wastewater/recycled water over the
top of the media. The distribution arm can be driven by hydraulic reaction
or by mechanical means. Typically, the speed of rotation can be adjusted
to effect higher media flushing intensity. Speed change in the distributor
mechanism is particularly valuable in systems that have high organic loads.
In nitrification towers, speed change is used to flush predators, such
as snails, from the tower.
Recycle Pump
A collection trough at the bottom of the tower collects the treated wastewater
and channels it to a sump, where it can be recycled as wastewater or discharged
to a secondary clarifier.
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Cross
Flow Media Cross flow media is made of sheets formed with alternating corrugations at 60° to vertical. The sheets are solvent-welded to each other to form modules for easy stacking in the biofilter vessel. The down-flowing liquid is split at each cross point creating 180 redistribution points per foot of depth for Brentwood CFS-3000 (31 ft2/ft3) media and up to 720 mixing points per foot of depth for the CF-1900 (48 ft2/ft3). |
 CF-1900 |
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Vertical
Flow Media Vertical flow media has vertical channels with contact points at one foot intervals. Lacking the cross-mixing points of cross flow media, vertical flow media redistributes the flow only at module interface. As a result, vertical flow media has superior bio-solids flushing action. |
 VF-5000 |
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Mixed
Media The optimal configuration of media in the modern bio-tower over 16 ft. deep is the combination of cross flow media in the upper two layers with vertical media in the lower layers. This configuration combines the superior distribution properties of cross flow media with the reduced potential for clogging of vertical flow media, to give consistent and efficient biological wastewater treatment.
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BOD Roughing and Secondary Treatment
For pre-treatment of high-strength wastes or BOD reduction prior to further
treatment for nitrification, CFS-3000 alone (for shallow rock filter retrofits)
or CFS-3000 in combination with VF-5000 (30 ft2/ft3) in deep bio-towers
are the usual selections because of the large non-clogging passages, maximum
re-distribution points, and good ventilation.
Nitrification
Bio-towers intended for ammonia oxidation following BOD roughing can use
higher surface area media with smaller passages, such as the CF-1900 (48
ft2/ft3) alone or in combination with VF-3800 (40 ft2/ft3) media. Thinner
bio-films in the nitrification process are less likely to cause plugging
of the narrow passages.
Other Applications for Structured Media
While not normally considered trickling filters, two other types of biological
waste treatment processes commonly use structured-sheet media. Commonly
referred to as “submerged fixed film,” anaerobic decomposition
of wastewater is achieved in covered tanks filled with media to support
the attached growth anaerobic organisms. Nitrogen removal via biological
de-nitrification is accomplished by mixing nitrified wastewater with a
carbon source, such as raw wastewater, and passing that liquid through
media-filled tanks containing denitrifying organisms.
| ACCUPAC PRODUCT |
SURFACE AREA ft2/ft3 (m2/m3) |
MIXING POINTS no./ft3 (no./m3) |
TYPICAL APPLICATIONS |
| CROSS FLOW MEDIA | |||
| CFS-3000 | 31 (102) | 180 (6,356) | Wastewater treatment, including BOD roughing and polishing |
| CF-1900 | 48 (157) | 720 (25,424) | Wastewater Treatment, especially shallow-depth BOD roughing and polishing, nitrification, and denitrification |
| VERTICAL FLOW MEDIA | |||
| VF-5000 | 30 (98) | — | BOD reduction of high strength wastes |
| VF-3800 | 40 (131) | — | Nitrification in mixed media applications |
| MIXED MEDIA | |||
| CFS-3000 & VF-5000 | All BOD oxidation applications greater than 16 ft. depth |
||
| CF-1900 & VF-3800 | All nitrification applications greater than 16 ft. depth |
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MOP 8 provides a large array of measured values for the coefficients in Germain’s formula for a variety of wastewater types. The effect of temperature is generally given as:
Process
Design Assistace from Brentwood
Brentwood can provide design assistance using a proprietary model based
on bio-filter performance data compiled from a number of sources, including
both published and non-published data.
The overall application rate of wastewater to the trickling filter, including
recirculation, expressed as gpm/ft2 of the filter area, is known as the
“Wetting Rate.” The desired wetting rate ranges from 0.05 gpm/ft2
to a maximum of 3 gpm/ft2, but is more typically in the range of 0.25
to 1 for BOD removal systems and 0.75 to 2 gpm/ft2 for
nitrification trickling filters.
If the average wetting rate is too low, the water may not penetrate the depth of the filter bed uniformly. It may channel away from some areas and leave damp unwetted areas that can act as incubators for pests like filter flies and snails (in nitrification towers). Also, biological populations not continuously wetted and fed by wastewater become ineffective. Those areas of the filter tower will not be available to provide effective treatment of wastewater during periods of higher flow. Semi-dry biomass can also putrefy and create odor problems.
Recycle of treated wastewater is an effective method of keeping all areas and depths of the trickling filter biologically active when the influent flow is too low for proper wetting.
Instantaneous Wetting Rate
From the work of the German wastewater treatment industry, a term has
been developed that identifies the Instantaneous Application Rate. This
term is the SpülKraft Rate, or SK Rate, that has the units of mm
of water per pass of the distributor arms.
Hydraulically-driven rotary distributors in the normal operating mode usually rotate at a rate of 1 revolution per 3/4 to 1-1/2 minutes and have two or four arms. The SK Rate may be in the range of 0.3 to 0.5 mm per pass in rock filters and from 5 to 30 mm per pass in more modern filters.
If recycle capacity is minimal and the operator has the ability to slow the rotation speed of the distributor, it is possible to compensate somewhat for low wetting rates by using higher SK values. Higher SK values will provide more complete penetration of the filter media depth and keep the bulk of the filter wetted.
Short cycle times of dryness between flushing will not be as detrimental to the biomass as a general starvation for water in pockets of media that are by-passed at low wetting rates.
Recirculating treated effluent to the trickling filter dilutes the influent wastewater entering the trickling filter. Since the BOD removal process is first order (i.e., the rate of removal of BOD is affected by the initial concentration of BOD), recirculation helps distribute the loading evenly through the depth of the filter. It also helps to manage the diurnal variation in loading while maintaining a minimum wetting rate throughout the day. In general, higher recirculation ratios (recirculation flow rate : influent flow rate) the better the effluent quality, at least to the point where the hydraulic retention time in the filter bed becomes too short. Typical recirculation rates are 1-3 times the daily average influent wastewater flow.
Media
Support SystemsIn a typical arrangement, the bottom layer of media modules are placed on 8 or 10-inch wide support beams spaced across the tank on 2 ft. centers. In the case of 10 inch support beams, a 2 inch wide center channel provides proper drainage. At the tank wall and around the center distributor column, a ledge 4 incheswide is used to support small pieces of cut media
AccuPier Support System
An alternative to the conventional concrete beam & pier system is the Brentwood AccuPier® System. This pre-engineered support system, consisting of field-adjustable plastic stanchions and fiberglass grating, is more economical and offers better air flow than concrete beam & pier supports. The open structure of the AccuPier system provides excellent ventilation and drainage. The glass-reinforced ABS piers have field-adjustable bases to accommodate sloping floors. The PVC pier stanchions are cut to length for the specific installation. Fiberglass grating in nominal 12 inch widths x 20 ft. lengths, pre-cut for the tank dimensions, spans the piers to give a flat, level surface to support the media. The piers are arranged in rows 2 or 3 ft. apart, and the spacing between piers within the row varies from 2 to 4 ft, depending on tower height and grating strength.
Dedicated
Bond JointsBecause the modules are constructed of vertical, corrugated sheets of PVC, the structural strength of the modules is dependent on the bonds between adjacent panels. Solvent welding at dedicated bond points, formed in the sheets to provide adequate bonding surface, ensures the structural integrity of AccuPac media
Compressive Strength of Modules
The structural integrity of the media is paramount to the longevity of the filter. Typically, each layer of the media is constructed to support the static weight of the media above, including the attached biomass and the transient loading of the applied wastewater. Industry practice is to use a factor of 40 lb/ft2 per foot of tower height. The bottom layer is constructed to a minimum standard of 1000 lb/ft2 to support the full height of the tower on the support beams. The top layer is also designed to support 1000 lb/ft2 to accommodate possible foot traffic during maintenance. This can be reduced to 700 lbs/ft2 when protective surface grating is used.
In addition to the physical and mechanical properties of the PVC sheet stock used in forming the media sheets, the structural strength of the modules is also determined by material sheet thickness and module configuration. For example, because the CF-1900 has more sheets per 24 inch wide module than the CFS-3000, it has greater surface area, and is, therefore, inherently stronger than a CFS-3000 module with the same sheet thickness. Crossflow modules are inherently stronger than vertical flow modules because of the crossed alignment of the bond joints. Therefore, empirical testing is necessary to ensure structural soundness of the wide variety of media types and sheet thicknesses of the PVC material. An industry-standard test procedure uses four modules in two layers placed at right angles to each other in a hydraulic test apparatus. The module deflection is measured as a function of the load applied. The deflection should not exceed 1.5% at the design load.
The hydraulic impact of the wastewater and braking jets of the hydraulic
distributor can, over time, damage the surface of the media. Also, it
is often necessary to walk on the surface of a trickling filter. Good
tower hygiene requires removal of debris that accumulates on the top of
the filters. The distributor arms and bearings in the distribution tower
also need to be serviced regularly to maintain proper operation and equipment
longevity.
Brentwood
AccuGrid
Brentwood AccuGrid™ polypropylene grating provides that additional
protection for the surface of the filter surface. This grid, when placed
over the top surface of the media in the trickling filter system, will
provide a non-skid walking surface that is strong and durable against
foot traffic and will help to reduce the hydraulic impact on the media.
While direct economic comparison to other treatment processes can only be made on a case-by-case basis, some general comparisons can be made.
• The containment vessel for bio-towers does not need to be constructed to hold the weight of the wastewater, as do activated sludge tanks. Vessels are often built of low-cost, pre-cast concrete panels or bolted steel plates.
• Power consumption for bio-towers is limited to pumping wastewater and re-circulated wastewater. No aeration power is needed (with the exception, in certain cases, of ventilation fans.)
• Maintenance for bio-towers is limited to the distributor arm and pumps. Blowers, air diffusers, return sludge pumps, and associated electrical equipment and controls are not needed.
• Less operator labor is needed to monitor, sample, and make adjustments to the process for the simpler trickling filter.
• Odor containment, if desired, is accomplished with the simple addition of a dome cover to the bio-tower tank.
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