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PART I INTRODUCTION
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PART II INSTITUTIONAL AND REGULATORY FRAMEWORK
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PART III TECHNICAL BASIS
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3 Overview of Rural Sanitation and Wastewater Management
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4 Rural Wastewater Treatment Technology
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5 Wastewater Treatment Process Design
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PART IV PROJECT PLANNING AND DESIGN
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6 Project Planning and Design
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6.1 Diagnosis for Project Villages – Initial Community Assessment
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6.2 Establishment of Stakeholder Group
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6.3 Assessment on Existing Conditions and Community’s Capacity
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6.4 Baseline Engineering Survey and Assessment
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6.5 Project Feasibility Study and Environmental Impact Assessment
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6.6 Selection of Operation Model
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6.7 Project Cost Estimate
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7 Community Participation
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PART V PROJECT FINANCING
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PART VI PROJECT IMPLEMENTATION AND MANAGEMENT
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9 Procurement and Implementation
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10 System Adminstration, Operation, Maintenance and Monitoring
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Appendix: Case Studies – Rural Wastewater Management in Zhejiang, Shanxi, and Jiangsu Province
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REFERENCES
4.3.1.1 Trickling Filter
- Categories: 4.3.1 Attached Growth Process
- Time of issue: 2022-04-28 18:24:24
- Views: 0
Trickling Filter
A trickling filter, a non-submerged fixed-film biological reactor, is typically a cylindrical tank filled with a high specific surface area material (e.g., rocks, gravels, or special preformed plastic filter media) and equipped with a water dosing system and an underdrain as a porous structure through which air can circulate.
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Figure 4.5 Schematic diagram of a trickling filter
Treatment Process:
The trickling filter operates mostly under aerobic condition without forced ventilation or aeration. Rocks and gravels were usually used as the packing material in the past and structured plastic fill media is now applied to increase treatment capacity. Wastewater is trickled or sprayed continuously and evenly over the top of the filter by a rotary distributor. While wastewater is travelling down to the bottom of the trickling filter, organisms in the attached biofilm degrade the organic matters and nutrients in the wastewater. A portion of effluent is recycled to the trickling filter feed flow. It is important to maintain enough microorganisms for BOD and ammonia removal but prevent excessive microorganism growth and clogging of fill media channels.
Treatment Efficiency:
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Pollutant/Parameter |
Removal efficiency |
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BOD |
60%-85% |
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TN |
0-35% |
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P |
10%-15% |
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Suspended solid |
Low |
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Total Coliforms |
60%-90% |
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Note: The treatment efficiency varies depending on wastewater characteristics, hydraulic and organic loading, medium type, operation conditions, maintenance of optimal dissolved oxygen levels, temperature, and recirculation rates. |
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Source: SASSE & BORDA 1998; U.S.EPA 2000; UNEP 2004; WSP 2008.
Design Criteria:
General
- When designing trickling filters, key factors that shall be taken into consideration include: (i) wastewater characteristics of the influent; (ii) type and physical characteristics of filter packing to be used; (iii) hydraulic loading rate and organic loading rate; (iv) type and dosing characteristics of the distribution system; (v) configuration of the underdrain system; and (vi) ventilation systems, either natural or forced air.
- The support structure of trickling filters shall withstand high vertical loading as well as the weight of attached sludge and water. Enough space is required for sludge removal.
- The trickling filter tank should be well ventilated with at least two vents above the filter bed. Tees should be installed at the top of the vents to increase air flow and the vent openings should be covered by screens to prevent flies from entering or leaving the filter bed.
- Recirculation by recycling a portion of the trickling filter effluent back through the filter must be designed to remain the filter media wet. The recycled effluent can also dilute higher strength influent flow to minimize loading variations and improve the treatment efficiency. Typical recirculation ratios range from 1:1 to 4:1.
Filter media
- The filter media should be supported 30 cm above the bottom of the tank.
- The media layer shall have enough surface area for attached growth of microorganisms and have enough void to allow good water and air mixing. Typically, the specific surface area of filter media should be at least 97.5 m2/m3 and the void ratio should be greater than 90%. Synthetic materials, typically plastics, provide approximately 95% void space between the media, while the rock media only has 35% void space.
- Where locally available, rock has the advantage of low cost. The most suitable material is rounded rock or crushed stone, graded to a uniform size so that 95% is within the range of 75-100 mm.
- The size of rocks at lower layer shall be larger than that at upper layer to avoid blockage caused by accumulation of deciduous biological film from upper layer.
- Plastic filters require additional provisions, including ultraviolet protective additives on the top layer of the filter media, and increasing plastic wall thickness for media packs that are installed in the lower section of the filter where loads increase.
- The minimum depth of filter media should be 2 m.
- The media layer distribution shall be professionally designed to maximize the lifetime.
- The organic loading rate should be no greater than 1.5 kg BOD/m3•d. Due to the limited void space in rock media, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/ m3•d.
- A high rate trickling filter shall be sized applying a hydraulic loading rate between 8 to 12 m3/m2•d. Recirculation flows should always be taken into consideration when calculating hydraulic loading.
- Typically configured with two filters in parallel.
Distribution system
- The wastewater should be well and uniformly distributed. On the rotary distributor, the number of nozzles should increase as the distance between the nozzle and the center shaft increases to realize even spray pattern. Static clog resistance spray nozzles are recommended.
- A space of 150 to 225 mm should be allowed between the bottom of the distributor arm and the top of the filter bed. The space permits the wastewater streams from the nozzles to spread out and cover the bed uniformly and prevents ice accumulations from interfering with the distributor motion during freezing weather.
Underdrain system
- The floor and underdrains must have sufficient strength to support the packing, slime growth, and the wastewater.
- The floor and underdrain block slope to a central or peripheral drainage channel at a 1 to 5 percent grade.
- The effluent channels shall be sized to produce a minimum velocity of 0.6 m/s at the average daily flowrate.
- The underdrains may be open at both ends, so that they may be inspected easily and flushed out if they become plugged.
- The underdrains should be open to a circumferential channel for ventilation at the wall as well as to the central collection channel.
Operation and Maintenance:
General
- A well operated trickling filter shall have:
- Healthy, thin layer of biomass with entire surface wetted.
- All orifices are open and flowing.
- Level distributor arms that rotate continuously and smoothly.
- No ponding.
- No leaking from center column seal.
- Require a moderately skilled and technical operator to manage the system.
- It will require several weeks of wastewater application to a new trickling filter before any appreciable bio-growth be attached to the media. High rate of recirculation will help to establish growth.
- Trickling filters should be inspected regularly for signs of: (i) ponding; (ii) uneven distribution of flow; (iii) clogging; (iv) roughness or vibration; (v) leakage; (vi) filter flies; and (vii) unusual odors.
- All equipment must be tested and calibrated as recommended by the equipment manufacturer. A routine O&M schedule should be developed and followed for trickling filter system.
- Ensure continuous supply of power and constant wastewater flow.
Control of distributor arm speed
To adjust the rotation speed of distributor arms, the operator can:
- Adjust the recirculation rate: increased circulation results in increased flow and increased arm speed, and vice versa.
- Change the number of nozzles on the distributor arms: less openings for a given flow result in a higher speed, while more openings result in a slower speed.
- Install speed-retarder nozzles: install nozzles on the opposite side of the distributor arms from the normal flow will act as speed retarders to slow the rotation.
Odor control
Excessive organic loading and inadequate ventilation may lead to unpleasant odors. To eliminate odor, the operator can:
- Reduce the organic loading in the influent.
- Increase hydraulic loading to wash out excess biological growth.
- Remove debris from filter effluent channels, underdrains, and the top of the filter media.
Clogging and ponding control
Increasing of hydraulic loading to slough off excessive biomass and removal of debris in the filter are required to avoid clogging and anaerobic digestion. If ponding or clogging occurs, the operator can:
- Increase the recirculation rate to the trickling filter to sheer the accumulated biomass and open plugged void spaces in the media.
- Remove debris on the top of the filter media.
- Slow the rate of revolution of the rotary distribution arm (if applied) to increase the instantaneous hydraulic load to each section of the media to flush biomass.
- Apply low doses of chlorine to kill excess biomass. The operator should consult the O&M manual or professionals to determine the chemical dosages.
- Shut down the filter to dry out media.
- Replace media when necessary.
Flies control
To reduce or eliminate the fly population on the filter, the operator can:
- Increase the recirculation rates, that is increase hydraulic loading, to flush the filter.
- Unplug the trickling filter’s distributor arm orifices or spray nozzles.
- Periodically flooding the filter and flush the walls.
- Use natural techniques such as the installation of bird houses.
- Enhance housekeeping.
- Application of chemicals (such as insecticide) may be necessary if fly population keeps increasing.
Sloughing
- If excessive sloughing is caused by low organic loading, the operator can: (i) unplug the trickling filter’s distributor arm orifices or spray nozzles; (ii) increase recirculation flow to the trickling filter.
- If excessive sloughing is caused by high hydraulic loading, the operator can: (i) divert a portion of the flow to an equalization tank, if possible; (ii) decrease the recirculation flow to the trickling filter.
Icing
In areas with very low temperature, such as winter in northeast China (-30 ℃), to protect the trickling filter from icing, the operator can:
- Use high-pressure stream of water to remove ice from orifices, nozzles, and distributor arms.
- Decrease the recirculation rate, as the water temperature is lowered when being recirculated.
- Construct covers or wind breaks for the filter.
Capital Cost and O&M Cost:
- Capital costs are moderate to high depending on type of filter materials and feeder pumps used.
- Operation and maintenance costs are moderate to high depending on electricity consumption of feeder pumps and the sprinkler system. Compared to activated sludge process, trickling filter requires less energy consumption as aeration blowers is not required.
- Expert design and skilled labor is required for construction and maintenance.
Applicability
- Suitable in areas where large parcels of land are not available.
- Appropriate for small to medium sized communities.
