Recommended Design Criteria for Trickling Filters...
General Considerations...
Trickling filters may be used where the treatment of wastewater is amenable to aerobic biological treatment processes. Trickling filters shall be preceded by primary clarifiers equipped with scum and grease collecting devices, or other
suitable pretreatment facilities. If fine screening is provided the screen size shall have from 0.03 to 0.06 inch
openings. Bar screens and/or communitors are not suitable as the sole means of primary treatment. Filters shall be
designed to provide for reduction in carbonaceous and/or nitrogenous oxygen demand in accordance with the Federal Secondary Treatment requirements and/or the South Dakota Water Quality Standards, or to effectively
reduce organic loading on the downstream treatment processes. Information as well as all functional design
calculations used in sizing trickling filter facilities shall be included in an engineer's report with the
facilities plan, plans and specifications and/or be submitted separately for review and approval. This report
shall include the following :
1. Influent wastewater characteristics;
2. Temperature range of applied wastewater;
3. Pretreatment processes;
4. Type of filter, i.e., single stage or multi- stage;
5. Hydraulic and organic loadings applied to the filter;
6. Hydraulic factors involving proper distribution computations;
7. Recirculation rate and piping system;
8. Filter beds-volume, area and depth;
9. Media-type, specific weight, void space;
10. Underdrainage and ventilation systems;
11. Equations utilized for treatment efficiency computations;
12. Degree of treatment anticipated.
Trickling filters are affected by diurnal load conditions. The volume of media shall be determined from either pilot
plant studies or use of acceptable design equations and shall be based upon the design peak hourly organic loading rate rather than the design average rate. An alternative would be to provide flow equalization.
Design Basis...
1. Trickling filters shall be designed as standard-rate, high-rate, or super-rate based on hydraulic and biological loadings. ( a ) The hydraulic loading on standard-rate trickling filters shall be between 1 and 4 million gallons per acre per day with an organic loading between 5 and 20 pounds BOD5 per 1,000 cubic feet of media per day. ( b ) The hydraulic loading, including recirculation, on high-rate or super-rate filters shall be between 10 and 40 million gallons per acre
per day with an organic loading between 30 and 100 pounds BOD5 per 1,000 cubic feet of media per day.
2. High-rate trickling filters may be used as roughing filters prior to further biological treatment. High-rate filters can
accommodate highly variable hydraulic overloading conditions without significant deterioration of the biological growth.
3. A minimum of two trickling filters shall be provided and designed, so that with one unit out of operation, the design average flow can be handled by the remaining unit.
Hydraulic Considerations...
1. Distribution :
a. Rotary distributors or fixed-nozzle systems shall be designed to provide uniform flow distribution over the surface of the entire filter media. Flow distribution devices shall be designed not to deviate by more than 10 percent from the normal rate of liquid application at any point on the media surface at design average flow conditions. Flow distribution between multiple units of stationary media systems shall be by weirs, valves, meters and other positive flow splitting devices.
b. For rotary/reaction type distributors, a minimum head of 24 inches between the low water level in the siphon chamber and center of the arms shall be required. Similar allowance in design shall be provided for added pumping head requirements where pumping to the reaction type distributor is used. Surge relief, to prevent damage to distributor parts, shall be provided where wastewater is pumped directly to the distributors. The revolving speed varies with the flow rate for the reaction-driven unit, but it should be in the range of one revolution every 10 minutes or less for a two-arm distributor. At peak hourly flow, the peripheral speed of the arm should not exceed 4 feet per second.
c. A minimum clearance of 12 inches shall be provided between the filter's surface and any moving parts of the distributor arms. A minimum of 18 inches clearance should be provided when severe icing problems are anticipated.
d. Design features that should be considered in selecting a distributor are the durability of construction, corrosion resistance, ease of cleaning and its ability to handle large flow variations while maintaining adequate rotational speed. The distributor arms may be designed with a constant or tapered cross section to provide minimum transport velocities at low flows. The distributor arms should be designed with adequate adjustments to maintain proper alignment and balance. End caps or gates that are quick-opening shall be provided on the arms for cleaning and draining. Orifice nozzles shall be adjustable for regulating the flow rate and spray dispersion. Mercury seals are not permitted. Distributor units should allow removal of the bearings without complete disassembly. The rotary distributor should be designed to prevent icing of the center column and filter walls.
2. Dosing and Controls :
a. When suitable flow characteristics have been developed, wastewater may be applied to the filters by dosing siphons, pumps or by gravity flow from preceding treatment units. Where recirculation is not provided to standard-rate filters,
dosing intervals should not be more than 5 minutes. Application of wastewater to high-rate filters should be practically continuous. Provisions for continuous application of wastewater to the filter media by the use of recirculation is strongly recommended.
b. The piping system, including dosing equipment and distributor, shall be designed to provide capacity for the peak hourly flow rate and recirculation to achieve the design efficiency. The recirculation rate shall be variable and subject
to plant operator control.
c. Devices shall be provided to permit measurement of the recirculation rate. Time lapse meters and pump head recording devices are acceptable for facilities treating less than 1 MGD. Automatic pump controls shall have manual override capability.
Media...
1. The media may be crushed rock, slag, redwood, or specially manufactured artificial material. The media should be durable, resistant to spalling or flaking and be insoluble in the wastewater being treated. The material should be free of fine particles, grease and oil and also should be properly screened and/or washed to remove dust and dirt before placement. Slag media shall be free from iron.
2. If rock media is used, the top 18 inches shall have a loss by the 20-cycle, sodium sulfate soundness test of not
more than 10 percent. The balance of the media shall pass a 10-cycle test using the same criteria. All tests shall be conducted as prescribed by ASCE Manual of Engineering Practice, No. 13. Manufactured media shall be resistant to ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalies, organic compounds, and fungus and biological attack. Such media shall be capable of supporting a person's weight or a suitable access walkway shall be provided to allow for distributor maintenance.
3. Rock and/or slag filter media for standard- rate filters shall be at least 5 feet but not more than 10 feet in depth above the underdrains. The filter media for high-rate filters shall be at least 3 feet but not more than 7 feet in depth above the underdrains. Manufactured filter media should be at least 10 feet deep to provide adequate contact time with the wastewater. Manufactured filter media depths shall not exceed 30 feet except where special construction is justified by extensive pilot studies or additional proven engineering data. The suitability of the manufactured media may be evaluated on the basis of full scale experience with other plants treating similar wastes and loadings or through actual use of a pilot plant on site. Super-rate trickling filters must have manufactured media.
4. Rock, slag and similar media shall not contain more than 5 percent by weight of pieces whose longest dimension is three times the least dimension. They shall be free from thin, elongated and flat pieces, dust, clay, sand or fine material and shall conform to the following size and grading when mechanically graded over vibrating screen with square openings.
Passing 4-1/2 inch screen - 100% by weight
Retained on 3 inch screen - 95-100% by weight
Passing 2 inch screen - 0-2% by weight
Passing 1 inch screen - 0-1% by weight
If hand picked field stone are used, the dimensions shall be from 2 1/2 to 5 inches in diameter.
5. Material delivered to the filter site shall be stored on wood-planked or other approved clean, hard- surfaced areas. All material shall be rehandled at the filter site and no material shall be dumped directly into the filter. Crushed rock, slag and similar media shall be washed and rescreened or forked at the filter site to remove all fines. Such material shall be placed by hand to a depth of 12 inches above the tile underdrains. The remainder of material may be placed by means of belt conveyors or equally effective methods approved by the engineer. All material shall be carefully placed so as not to damage the underdrains. Manufactured media shall be handled and placed as approved by the engineer. Trucks, tractors, and other heavy equipment shall not be driven over the filter during or after construction.
6. The required volumes of rock or slag media filters shall be based upon pilot testing with the particular wastewater or any of the various empirical design equations that have been verified through actual full scale experience. Such calculations must be submitted if pilot testing is not utilized. Pilot testing is recommended to verify performance predictions based upon the various design equations, particularly when significant amounts of industrial wastes are present.
Underdrainage System...
1. Subfloor : The floor of the filter shall be able to support the underdrainage system, the filter media, and the
water load. A minimum gradient of 1 percent shall be provided for the subfloor which slopes to a collection channel.
2. Filter Block : Precast filter blocks, made of vitrified clay or concrete should be used. The underdrainage system shall cover the entire floor of the filter. Inlet openings of the filter blocks into the underdrains shall have an unsubmerged gross combined area equal to at least 20 percent of the surface area of the filter. Underdrains shall have a minimum slope of 1 percent to a collection channel. Redwood timbers resting on concrete sleepers may be used in the place
of filter blocks, when stacked redwood media is used.
3. Collection Channel : The collection channels (central or peripheral) shall provide capacity to carry the flow from the underdrains and permit free passage of air to the underdrains for ventilation. Channels should be sloped to provide a minimum flow velocity of 2 feet per second. The water level in the collection channel should be below the bottom of the filter blocks during peak flow conditions. Drains, channels and pipe shall be designed to have 50 percent of their cross-sectional area unsubmerged at peak hourly flows.
4. Ventilation : All trickling filters shall be provided with ventilation openings to the underdrains. Ventilation openings will be provided with dampers or other adjustable devices to permit adjustment of the ventilation rates. Natural draft ventilation openings shall have a minimum gross area of 4 square feet per 1,000 square feet of filter area.
Forced ventilation providing a mimimum air flow of 1 cubic foot per minute per square foot (1 cfm/ft2) shall be provided
for covered filters and deep manufactured media filters.
5. Flushing : Provision shall be made for flushing the underdrains such as, adjustable ventilation ports. In small filters, use of a peripheral head channel with vertical vents is acceptable for flushing purposes. Inspection facilities shall be provided.
Special Design Considerations...
1. Flooding : Appropriate valves, sluice gates, walls and other structures shall be provided to permit flooding of filters comprised of rock or slag media.
2. Climatic Protection : ( a ) The surrounding wall shall extend at least 4 feet above the media of all uncovered filters to maximize the containment of windblown spray and reduce heat losses. ( b ) Other protection such as covers, wind screens or windbreaks shall be provided to maintain operation and treatment efficiencies when climatic conditions are expected to result in icing problems.
3. Maintenance : ( a ) All distribution devices, underdrains, channels and pipes shall be installed so that they may be properly maintained, flushed or drained. Access shall be provided around the periphery of the underdrain system to allow flushing of the underdrains. ( b ) Suitable and safe access shall be provided to the top of the structure for inspection
and maintenance. ( c ) Treatment operators should be supplied with a complete set of operational instructions, including maintenance schedules, tools and spare parts as may be necessary.
4. Structure : ( a ) The walls, floor and underdrain system should be constructed of concrete, concrete blocks,
vitrified clay or other approved material for containment of wastewater. ( b ) The wall structure shall be designed such
that a cover, dome or other means may be easily installed over the trickling filter at a later date. ( c ) Enclosed structures shall be well lighted and ventilated. ( d ) Electrical equipment and controls shall meet the requirements of
the National Electrical Code.
Deviations from Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.
