This document provides information on aerobic attached growth systems, specifically trickling filters. Key points include: - Trickling filters are fixed film bioreactors that use media like rock or plastic to develop biofilms, treating wastewater as it trickles through the media. - Wastewater flows over the biofilms, exposing them alternately to wastewater and air to facilitate treatment. - Design considerations include media type, wastewater distribution, ventilation, and secondary clarification after treatment. - Empirical equations are provided to help design trickling filters based on parameters like organic loading, temperature, media characteristics, and wastewater flow.

1. Aerobic Attached Growth
Systems
Dr. Akepati S. Reddy
Professor, School of Energy & Environment
Thapar University, Patiala (Punjab) 147001
INDIA

2. Aerobic Attached Growth Systems
• Treatment units are made compact through disconnecting SRT
from HRT and increasing the SRT
• In suspended growth systems it is achieved through providing
secondary sedimentation tanks or membrane filtration units
and recycling the sludge
• Aerobic attached growth systems it is achieved by developing
biofilms on support media within the reactor
– loss of microbial biomass is minimized
– biomass is retained within the reactor in the form of biofilms
– sludge recycling is not needed
• Aerobic attached growth systems are two types:
– Submerged attached growth systems
• Mechanical aeration is needed
• SAFF, MBBR, FAB
– Exposed to atmosphere attached growth systems
• Usually mechanical aeration is not practiced.
• Trickling filters, Rotating biological contact reactors

3. Trickling Filters

4. Trickling filters
• Most conventional technology and relatively less used
• Circular or rectangular non-submerged fixed film bioreactors
• Includes
– Rock or plastic packing as a medium for bio-film development
– Wastewater dosing or application system
– Under drain system
– Structure containing packing
– Secondary settling unit
• Clarified or fine screened primary effluent is applied on the
packing and allowed to trickle through

5. • The bio-film is alternatively exposed to wastewater and air
• Wastewater made to flows as a thin film over the bio-film
• Treated effluent from TF is passed through a secondary clarifier
and let out as treated secondary effluent
• Portion of the clarified effluent may be recycled to TF for
– Diluting the strength of incoming wastewater
– Maintaining enough wetting of the bio-film during minimal or no
flow conditions
Trickling filters

6. Wastewater
Trickling filter
Primary
clarifier
Trickling
filter
Secondary
clarifier
Treated
effluent
To sludge
Handling & disposal
Recirculation
sludge

9. Trickling Filter
In some wire mesh screen is placed over the top
Requires less energy and easy to operate
Have more potential for odors and quality of treated effluent is low –causes are
Inadequate ventilation
Poor clarifier design
Inadequate protection from cold temperatures
Dosing operations
Biofilm development may not be uniform in the trickling filter
Liquid may not uniformly flow over the entire packing surface (wetting efficiency)

10. Packing
Low cost material with high specific surface area, high enough
porosity and high durability is ideal
• Rock (rounded river rock, crushed stone or high quality
granite), blast furnace slag, or plastic can be used
Corrugated plastic sheet packing
– High hydraulic capacity, high porosity, and resistance to plugging
(for better air circulation and bio-film sloughing)
– Most used and two types: cross flow and vertical flow
– Packing depth is 4 to 12 m (6 to 6.5 m is typical), hence require
less land area
– Superior to stone packing for higher organic loading rates
Rock packing
– High density limits depth – 0.9 to 2.5 m (1.8 m is typical)
– Size is 75 to 100 mm (95% of the material)

11. Wastewater dosing/application system
Two types of wastewater dosing/distribution systems:
Rotary distributors & Fixed Nozzle Distribution Systems
Rotary distributors
• Has 2 or more hollow arms mounted on a central pivot
– Arms have nozzles and extend across the TF diameter
– Spacing between nozzles decreases center to perimeter (ensures
uniform distribution of wastewater)
• Dynamic reaction of the discharged wastewater from nozzles
revolves the distribution arm
– Rotational speed varies with flow
– Alternatively electrical motor can also be used

12. • Clearance between distributor arm and filter packing top is 150
to 225 mm
• Typical head loss in the distributor is 0.6 to 1.5 m
• Rotational speeds of distributors
– In the past these were 0.5 to 2 rpm – but now reduced for better
filter performance
– Change from conventional 1 to 5 min/rev. for rock filters to dosing
every 30-55 min. once improved performance (reduced bio-film
thickness & odour observed)
– Can be decided by
N = (1+R)q / (A.DR)
R, recycle ratio q, influent flow rate
A, no. of distributor arms DR, dosing rate
Wastewater dosing/application system

13. • Dosing volumes
– Higher dosing volumes are reported to improve wetting efficiency,
agitation, flushing and wash away of fly eggs, and resulted in
thinner bio-film
– Daily intermittent high flushing doses are shown to control bio-
film thickness and solids inventory
Fixed nozzle distribution system
– In case of square or rectangular filters fixed flat spray nozzles are
used
– Used for shallow rock filters
– Flat spray pattern nozzles are used (at periphery half spray nozzles
are used)
– Water pressure is varied systematically in order to spray
wastewater first at maximum distance and then at decreasing
distance
Wastewater dosing/application system

14. Under-drain system
Meant to catch filtered wastewater and solids and convey to
secondary clarifier
• Should facilitate ventilation, easy inspection and flushing if
needed
– Drains are open at both the ends and open into a circumferential
channel
Floor and drains of the filter should be strong enough to support
packing, slime growth and wastewater
– Rock filters use pre-cast blocks of vitrified clay or fiber glass
grating laid on reinforced concrete sub-floor
– Plastic packing filters use beams over columns
Free draining out of filtered wastewater should be ensured
– 1 in 5 bottom slope is provided towards the outlet
– under-drains are designed for flow velocity >0.6 m/sec. at average
flow
– Beams are provided with channels on their top

15. Ventilation and air flow
Natural draft is the primary means for providing the needed air flow
• Often inadequate – forced ventilation by low pressure fans can
be solution – FD or ID fans can be used
• Driving force is temperature difference between ambient air
and wastewater
• During periods of no temperature difference natural draft may
not be there
• Cooler wastewater forces downward air flow through the filter
and warmer wastewater forces upwards air flow
• Upward air flow not desirable - supply of oxygen is lower where
demand is higher and odours can be a problem

16. For better air flow
• Drains should not flow beyond half full
• Open area over the top of the under-drains should be >15%
• Each 23m2 filter area should have 1 m2 area of ventilation
manholes or vent stacks
Ventilation and air flow

17. Secondary clarifier
Settling is meant for the removal of suspended flocs and
discharging the clarified sewage –
– Sludge is not recycled – but can be returned to primary
clarifier
– Part of the clarified effluent may be recycled for reducing
strength or for keeping wet of the bio-film
• Sludge is relatively easily settlable
Settling tanks with side wall depth of 3.6 to 3.7 and overflow
rate 1m/hr for average flow and 2 m/hr for peak flow are
used.
With proper design and operation of a secondary clarifier
clarified effluent has <20 mg/l of TSS

18. Trickling filters
Classified as
• Low or standard rate filters
– Produce effluent of consistent quality though influent is of varying
strength
– Top 0.6 to 1.2 m of the filter has appreciable bio-film and lower
portion may have nitrifying bacteria
– Odours and filter flies may be the problems
• Intermediate and high rate filters
– Flow is usually continuous
– Recirculation of effluent allows higher organic loadings
– May be a single or two stage process
– High rates filters usually use plastic packing

19. • Roughing filters
– Much higher organic loadings (>1.6 kg/m2.day) are used
– Hydraulic loading can be upto 190 m3/m2.day
– Used to treat wastewater prior to secondary treatment (require
low energy than ASP – 2 to 4 kg BOD per kWh for TF against 1.2 to
2.4 kg BOD per kWh for ASP)
2-stage filters
• High strength wastewaters prefer 2-stage filter with
intermediate clarifier
• Preferred when nitrification is required
– Nitrification demands low organic loading (<30 mg/l) – complete
nitrification demands <15 mg/l of organic load
• Separate TF, after secondary treatment, can also be used for
nitrification
Trickling filters

21. Biological community of trickling filter
Biological community of trickling filter includes
– Aerobic, anaerobic and facultative bacteria
– Fungi, algae and protozoans
– Worms, insect larva and snails
Bio-film on the medium surface is mainly composed of aerobic,
facultative and anaerobic bacteria
– Lower reaches of the filter can have nitrifying bacteria
– At lower effluent pH, fungi play important role
– Algae may grow in the upper reaches of the filter
Problems caused by biological community
– Protozoa, worms, insect larva and snails feed on biological films
and increase effluent turbidity
– Snails are troublesome in the TFs used for nitrification
– Algae can cause filter clogging and odors problem
– Fungal growth can also clog the filter

22. Sloughing of bio-films
Bio-film can be up to 10 mm thick – in the top 0.1 to 0.2 mm aerobic degradation
occurs
• Organic loading increases thickness of the film
• Aerobic environment and substrate can not penetrate inner
depths of the bio-film
• In case of thick films, due to anaerobic environment and
substrate limiting conditions and consequent endogenous
respiration, bio-film looses its clinging ability
• Under hydraulic loading sloughing off of bio-film occurs
• Mechanisms of sloughing for rock packing is apparently
different from that of plastic packing
– In plastic packing sloughing off is a small scale process and occurs
from hydrodynamic shear
– In rock packing large scale spring sloughing occurs mainly due to
insect larva activity – affects total BOD of the effluent (can be
higher than that of the influent)

23. Trickling Filter: design
Design is based on empirical relationships derived from
pilot plant studies and full-scale plant experiences
Following parameters are related with treatment efficiency
and used as design and operating parameters
– Volumetric organic loading
– Unit area loading
– Hydraulic application rates
Treatment efficiency: >90% at <0.5 kg/m3.day BOD loading;
and <60% at >3.5 kg/m3.day BOD loading
Nitrification:
– For effective nitrification BOD loading must be very little
– Nitrification rates can be 0.5 to >3 g/m3.day
– Efficiencies depend on the packing surface area and on the
specific NH4-N loading rate
– Higher temperature and better wetting efficiencies can enhance
the nitrification efficiency

24. Empirical formula for BOD removal for 1st stage TF is
E1 is BOD removal efficiency at 20C
W1 is BOD loading kg/day
V is volume of filter packing (in m3)
F is recirculation factor; Defined as
Here R is recycle ratio and its value can be 0 to 2.0
VF
W
E
1
1
4432.01
1


 2
10
1
1
R
R
F



Empirical formulae for BOD removal for
rock filter media

25. Empirical formulae for BOD removal
for rock filter media
BOD removal efficiency at 20°C in the 2nd stage TF
W2 is BOD loading to 2nd stage TF in kg/m3
BOD removal efficiency is temperature dependent and
temperature correction can be made by
VF
W
E
E
2
1
2
1
4432.0
1
1



20
20 )035.1( 
 T
T EE

26. Empirical formulae for plastic packing
Contact time of wastewater with bio-film depends on the
filter depth and the hydraulic application rate
 Wastewater contact time (in min.)
q, hydraulic loading rate in L/m2.min.
Q, wastewater flow rate (L/min.)
A, cross sectional area of the filter in m2
c, constant for the packing used
D, depth of the packing in m (typical is 6.1 -6.7 m)
n, hydraulic constant for packing
Depends on temperature and specific surface area
Taken as 0.67 at 20C and as 0.5 for a packing of 90 m2/m3
specific surface area
n
q
Dc

A
Q
q 

27. Change in BOD concentration with time is
k is taken as 0.69/day at 20C
It incorporates the constant ‘C’ (constant for the packing)
Its value is temperature dependent
kS
dt
dS

n
q
kD
kte
S
S 

 expexp
0
20
20 035.1 
 T
T kk
Empirical formulae for plastic packing

28. Modified Velz equation for the effluent BOD
for the plastic packing
R is recycle ratio
 is temperature correction coefficient (1.035)
So is influent BOD and Se is effluent BOD, mg/L
As is specific surface area (m2/m3)
D is depth of the filter (m)
‘q is hydraulic application rate (L/m2.sec.)
recommended value is >0.5L/m2.sec.
‘n’ is constant characteristic of the packing used (depends on
temperature and specific surface area (taken as 0.67)
K20 is rate constant at 20°C
 
  
R
Rq
DAK
R
S
S
n
S
Te









 
1
exp1
20
20
0


29. Modified Velz equation
K value
Its value is 0.059 to 0.351
for domestic wastewater it is 0.21
it requires adjustment for both packing depth and influent BOD
K1 is K value for D1 = 6.1 m and S1 = 150 mg/l of BOD
For influent BOD of 400 to 500 mg/l oxygen transfer can
become limiting in trickling filters
Loading of soluble bCOD >3.3 kg/m3.day can also limit oxygen
5.0
2
1
5.0
2
1
12 












S
S
D
D
KK

30. Natural air draft through filter can be determined by
Dair, draft in mm of water column
Z, height of the filter in meters
Tc, cold temperature in K
Th, hot temperature in K
Air flow velocity through the filter is proportional to the natural air
draft
Pressure drop across filter is related to flow velocity by
total head loss in kPa (kPa=100 mm water column) - 101.325 kPa
is equal to 1 atmos or 10132 mm water column;)
V is superficial air flow velocity (m/sec.)
Np is filter tower resistance factor in terms of velocity heads
Ventilation and air flow
Z
TT
D
hc
air 




  11353





g
VNP pT 2
2
P

31. Np is estimated as
Np is number of velocity heads
D is depth of packing (m)
L/A is liquid mass loading rate (kg/m2.hr)
A is filter cross-section area (m2)
Total head loss through the filter (Npt) is usually taken as 1.3
to 1.5 times to Np
2.0 times for rock packing of 45 m2/m3 specific surface area
1.6 times for plastic random packing with 100 m2/m3 specific surface
area
For plastic packing with cross flow conditions the multiplying factor is
1.3 when specific surface area is 100 m2/m3 and 1.6 for the
specific surface area of 140 m2/m3 packing
 A
L
p DN
)1036.1( 5
exp33.10



Ventilation and air flow

32. Air flow requirements
Oxygen required per kg of BOD loaded to the TF is
Ro, kg of oxygen needed per kg of BOD loaded
Lb, BOD loading rate kg/m3.day (typical: 0.3-1.0 kg/m3.day)
If nitrification is also considered then
Nox/BOD, ratio of influent nitrogen oxidized to influent BOD
72 m3 of air at 20°C, at assumed O2 transfer efficiency of 5%, can supply one kg of
O2 to the TF – transfer efficiency is taken as 2.5% in case of nitrification
– Oxygen content of air 0.279 kg/m3 at 20°C
bb -0.17L-9L
o exp1.2exp0.8R 
BOD
NO
4.6exp1.2exp0.8R x0.17L-9L-
o
bb


33. Air flow requirements
Air flow (in m3/day) required (AR) for a TF is
For temperature and pressure different form 20C and one
atmos, air flow rate is
If temperature is >20C air requirement will increase -
solubility of oxygen decreases with increasing
temperature
Recommended air flow is 0.3 m3/m2.min.
a
A
20T
P
760
293
T273
ARAR







100
20-T
1ARAR A
T
'
T
00QSR72AR 
Q, flow rate of influent in m3/day
S0 is influent BOD in kg/m3

34. Nitrification
Used either as a combined system or as a tertiary treatment
Influent BOD and DO within the TF impact nitrification rates
– Maximum rates occur when sBOD is <5mg/l, rates are
inhibited at >10 mg/l and rates are insignificant at >30 mg/l
– For rock media filters for BOD loading of <0.08 kg/m3.day,
nitrification efficiencies are >90% and for the loading of 0.22
kg/m3.day it is 50%
– For >90% nitrification surface loading of NH3-N of <2.4
g/m2.day is needed (biofilm surface)
Linear relationship exists between specific nitrification rate
and BOD/TKN ratio and can be shown by
– Here RN is specific nitrification rate (in g/m2.day)
44.0
82.0








TKN
BOD
RN

35. Nitrification
Packing design, temperature, oxygen availability and ammonia
loading rate all affect nitrification rates
• DO concentration has very great effect
• Follows zero order reaction (against NH4-N) for most portion of
the packing (for NH4-N in the > 5-7 mg/L range)
• Rates decline from top with depth may be from the decrease of
bio-film growth, from predation (grazing by snails!) and from
changed wetting efficiency
• Rates can be 1 to 3 g/m2.day (bio-film) and may decline by 20-
50% with decrease in temperature from 20 to 10°C
For higher nitrification rates
– NH4-N should be >5 mg/L
– hydraulic loading rates should also higher
– Specific surface area of 100 m2/m3 may be appropriate
Periodic floods can minimize the predation problem

36. For determining the volume of packing and the hydraulic
application rate, the following empirical equation is used
– Here RN is surface nitrification rate for Z depth of TF and T
operating temperature
– N is NH4-N concentration in the bulk liquid
– r is an empirical constant
– KN is N conc. at RN=RN.Max./2 – its expected value is 1.5 mg/l
– RN.Max. is in g.N/m2.day – its value is 1.1 to 2.9 g.N/m2.day -
temperature correction is considered not needed for 10-25C
range – beyond this correction is done by
rZ
N
MaxNTZN
NK
N
RR 







 exp..,),(
10
)10.(.).(. 045.1 
 T
MaxNTMaxN RR
Nitrification

37. Rotating Biological Contactors

38. Rotating biological contactor (RBC)
• Series of closely spaced polystyrene/polyvinyl chloride
circular disks on horizontal shaft constitute RBC unit
• Standard RBC unit includes
– 3.5 m dia disks with total disk area of 9300 m2/unit to support
microbial film
– A shaft of 8.23 m length (of this 7.23 m is occupied by disks)
• RBC unit is placed in a 45 m3 capacity tank
– Shaft orientation is either perpendicular to or parallel to the
wastewater flow
• RBC unit is usually provided with an enclosure
– Prevents algal growth
– Discs are protected from sunlight (UV light)
– Prevents heat loss and exposure to cold weather

40. RBC
Treatment process (sec. or advanced level)
• Treatment is for BOD removal, or nitrification, or both, or
for pretreatment of higher strength industrial effluent
• Wastewater clarified in primary clarifier or fine screened is
fed to the reactor
• 40% of disc surface is submerged in wastewater
maintained in a 45 m3 tank
• Disk surface is alternatively brought in contact with
wastewater and atmosphere by rotating at 1 to 1.6 rpm
rate either mechanical or pneumatically
• Treatment occurs through bio-sorption of organic matter of
wastewater into bio-film and aerobic biooxidation of the
sorbed matter when bio-film is exposed to atmospheric air
• Bio-film as it thickens and looses its ability to cling to disc
surface due to hydrodynamic shears sloughs off
• Treated effluent needs secondary clarification

41. • A number of RBC units may be operated in series to form a process
train
– Exploit the benefits of staged biological reactor design –
facilitates maintaining different conditions in different stages
– For reliability two or more parallel flow trains are employed
– 2-4 units in series are used for BOD removal
–  6 units are used for combined BOD removal & nitrification
– To avoid overloading on initial stages, stepped feed or
tapered systems are opted
– RBC units decrease as one moves to higher stages
• RBC units are of low, medium and high density types
– Low density or standard type units are used for initial stages
– Medium and high density units (11000 and 16,700 m2 area)
are used in the mid and final stages
RBC

42. Simple to operate and involves low energy costs
Performance is related to specific surface loading of BOD and/or NH4-N
– For first stage it is 12-20 g/m2.day of soluble BOD (as total
BOD it is 24-30 g/m2.day)
– For nitrification maximum loading rate is 1.5 g/m2.day
Associated with odor and bio-film sloughing problems
– Occurs when oxygen demand exceeds supply
– Sulfur oxidizing bacteria form tenacious whitish film and
prevent sloughing off
Structural failure of shafts, disks and disk support systems can occur
– Excessive bio-film growth and sloughing problems cause it
RBC

43. RBC design considerations
Principal elements of RBC
• Disc material and configuration
– HDPE of different configurations or corrugation patterns
– Corrugation increases available surface area and enhances
structural stability
• Shaft
– Shape is square, round or octagonal
– Steel shafts coated for protection against corrosion of 13-30
mm thickness are used
• Tankage
– Requirement is 0.0049 m3/m2 film area
– Typical side wall depth is 1.5 m
– At 0.08 m3/m2 day hydraulic loading rate HRT is 1.44 hrs

44. Drive system
– Mechanical or pneumatic drives are used for shaft rotation
– Mechanical drive capacity is 3.7 or 5.6 kW per unit
– Deep plastic cups are attached to the perimeter of the disks
and compressed air is released into the cups for rotation
– Air requirement is 5.3 m3/min for standard density shaft and
7.6 m3/min for high density shaft
Enclosures
– Segmented fiberglass reinforced plastic enclosures are used
RBC design considerations

45. RBC design
First stage RBC disk area is determined by using 12-20 g/m2.day sBOD loading
Disk area of subsequent stages is found by second order model by Opatken
Sn is soluble BOD concentration in mg/l
As is disk surface area for stage-n (in m2)
Q is flow rate in m3/day
Here sBOD/BOD is taken as
0.5 for secondary clarified effluent
0.5 to 0.75 for primary clarified effluent
)/(00974.02
)/(00974.0411 1
QA
SQA
S
s
ns
n


 

46. For nitrification stages area required is found by using maximum
nitrification rate (rn.max) as 1.5 g/m2.day
– Applicable if sBOD of wastewater is <10 to 15 mg/l -
otherwise rn.max should be corrected by:
Here Frx is fraction of nitrification rate possible
sBOD is soluble BOD loading in g/m2.day
sBODFrx  1.00.1
RBC design