1. Trickling Filters
TRICKLİNG FILTERS
Zozan GÜL

2. Trickling Filters
Trickling filters have been used to provide biological wastewater treatment of municipal and industrial wastewater for nearly 100 years.
The trickling filter is a nonsubmerged fixed-film biological reaktor using rock or plastic packing over which wastewater is distributed continuously.Treatment occurs as the liquid flows over the attached biofilm. The depth of the rock packing ranges from 0.9 to 2.5m and averages 1.8m .

3. Trickling Filters
Rock filter beds are usually circular and the liquid wastewater is distributed over the top of the bed by a rotary distributor.Many conventional trickling filters using rock as the packing material have been converted to plastic packing to increase treatent capacity.
Virtually all new trickling filters are now constructed with plastic packing.

5. Working Mechanism of Trickling Filter
The wastewater in trickling filter is distributed over the top area of a vessel containing non-submerged packing material.
Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm.
During operation, the organic material present in the wastewater is metabolised by the biomass attached to the medium. The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material.

6. The thickness of the aerobic layer is limited by the depth of penetration of oxygen into the microbial layer.
The micro-organisms near the medium face enter the endogenous phase as the substrate is metabolised before it can reach the micro-organisms near the medium face as a result of increased thickness of the slime layer and loose their ability to cling to the media surface. The liquid then washes the slime off the medium and a new slime layer starts to grow. This phenomenon of losing the slime layer is called sloughing.
The sloughed off film and treated wastewater are collected by an underdrainage which also allows circulation of air through filter. The collected liquid is passed to a settling tank used for solid- liquid separation.

8. TRICKLING FILTER CLASSIFICATION and APPLICATION
Trickling Filters designs are classified by hydraulic or organic loading rates. Rock filter designs have been classified as
low- or Standard-rate ,
intermediate rate and high –rate
Plastic packing is used typically for high –rate designs; however , plastic packing has also been used at lower organic loadings , near the high end of those used for intermediate –rate rock filters.Much higher organic loadings have been used for rock or plastic packing designs in ‘roughing’ applications where only partical BOD removal occurs.

10. Low –Rate Filters; do not include recycling Low hydraulic loading
Relatively simple
produces an effluent of consistent quality with an influent of varying strength
only top m of the filter packing will have appreciable biological slime
the lower portions of the filter may be populated by autotrophic nitrifying bacteria
can provide good BOD removal and a highly nitrified effluent

11. Intermediate- and High Rate Filters
recirculation of the filter effluent or final effluent permit higher organic loadings either rock or plastic packing
recirculation = 1-3 times inflow
may be designed as single or two stage processes
two filters in series operating at the same hydraulic application rate (m3/m2.hr) will typically perform as if they were one unit with the same total depth

12. Roughing Filters Very high organic and hydraulic loading
Plastic packing
Using to treat wastewater prior to secondary treatment
Low energy requirement

13. Two-Stage Filters
A two- stage filter system , with an intermediate clarifier to remove solids generated by the first filter, is the most ofter used with high-strength wastewater . two- stage systems are also used where nitrification is required. The first- stage filter and intermediate clarifier reduce carbonaceous BOD , and nitrification takes place in the second stage.

14. Nitrification
Both BOD removal and nitrification can be accomplished in rock or plastic packing trickling filters operated at low organic loadings . Heterotrophic bacteria , with higher yield coefficients and faster growth rates ,are more competitive than nitrifying bacteria for space on the fixed –film packing. Thus , significant nitrification and less than 15 mg/L for complete nitrification .Harremöes considered the soluble BOD , and conclued that a concentration less than 20 mg/L is neded to initiate nitrification . Nitrification can also be accomplished in seperate trickling filters following secondary treatment.

15. DESIGN OF PHYSICAL FACILITIES
Factors that must be considered in the design of trickling filters include
(1) type and physical characteristics of filter packing to be used
(2) dosing rate
(3) type and dosing characteristics of the distribution system
(4) configuration of the underdrain system
(5) provision for adequate airflow (i.e:, ventilation) , either natural of forced air
(6)settling tank design

16. Filter Packing High durability Low in cost High enough porosity
High surface area per unit volume

17. Rock Plastic Packing low cost
Low void volume limits the space available for airflow and increases the potential for plugging and flow short-circuiting
High hydraulic capacity
High void ratio Resistance to plugging
Require less area due to ability to use higher organic loadings and taller trickling filters

18. Dosing Rate
The dosing rate on a trickling filter is the depth of liquid discharged on top of the packing for each pass of the distributor. For higher distributor rotational speeds, the dosing rate is lower.
The rotational speed for a rotary distributor can be determined using the following relationships.
where n=rotational speed,rev/min
q=influent applied hydroulic loading rate,
R=recycle ratio
A=number of arms in rotary distributor assembly

19. The dosing rate,DR, has also referred to as the SK value, which stands for Spülkraft, a term used in German regulations to define dosing in the early 1980s.To achive the suggested dosing rates , the speed of the rotary distributor can be controlled by (1) reversing the location of some of the existing orifices to the front of distributor arm , (2)adding reversed deflectors to the existing orifice discharges or(3) converting the rotary distributor to a variable- speed electric drive.

20. Distrubution Systems
The application of wastewater onto the medium is accomplished by a rotating distribution system
Distributor consists of two or more arms that are mounted on a pivot in the center of the filter and revolve in a horizontal plane.
The arms are hollow and contain nozzles through which the wastewater is discharged over the filter bed

21. Nozzles are spaced unevenly so that greater flow per unit of length is achieved near the periphery of the filter than at the center
For uniform distribution over the area of the filter, the flowrate per unit length should be proportional to the radius from the center

22. Underdrains
The under drain system in Trickling Filters  serves two purposes:
1)To collect and convey the waste water passing through the filter and the sloughed solids from the filter to the SST, and
2)To provide for ventilation of the filter to maintain aerobic conditions.

23. Airflow
An adequate air flow is of fundemental importanc to successful opertaion of a trickling filter to provide efficient treatment and to prevent odor
Natural draft
Forced ventilation using low-pressure fans

24. Airflow
In case of natural ir draft driving force for airflow: temperature difference between the ambient air and air inside the pores
If wastewater is colder than the ambient air → the pore air will be cold direction of airflow will be downward
If the ambient air is colder than wastewater→ direction of air flow will be upward

25. PROCESS DESIGN CONSIDERATIONS
The trickling filter process appears simple , consisting of a bed of packing material through which wastewater flows and an external clarifier.
In reality , a trickling filter is a very comples system in terms of the characteristics of the attached growth and internal hyrodynamics .
In view of these complexities, trickling filter designs are based mainly on empirical relationships derived from pilot-plant experience.

26. Effuent Characteristics
Historically , trickling filters have been considered to have major advantages of using less energy than activated-sludge treatment and being easier to operate , but have disadvantages of more potential for odors and lower-quality effluent.
Some of these shortcomings , however , have been due more to inadequate ventilation , poor clarifier design, inadequate protection from cold temperatures and the dosing operation.
With proper design , trickling filters have been used succesfully in a number of applications.
Typically applications, process loadings, and effluent quality.

27. Effluent Recirculation
To dilute strong influent wastewater and supplement weak wastewater
To bring the filter effluent back in contact with the biological population for further treatment
Almost always included in high-rate trickling filter systems
improves distribution over the surface of filters
Reduces the clogging tendency
If sufficiently high, aids in the control of filter flies
the increase in applied total flow increases wetting efficiency, thus reducing short circuiting

28. Advantages of Trickling Filter
Rate of Filter loading is high as required less land areas and smaller quantities of filter media for their installations.
Effluent obtained from the trickling filter is sufficient stabilized.
Working of Trickling filter is simple and does not require any skilled supervision.
They are flexible in operation.
They are self cleaning
Mechanical wear and tear is small as they contain less mechanical equipment.

29. Disadvantages of Trickling Filter
The beds loss through these filters is high.
Construction cost is high
These filters cannot treat raw sewage and primary sedimentation is must.
Fly nuisance and odor nuisance may prevail.