secondary treatment units

1. Biological treatment

2. Objectives
 To oxidize dissolved and particulate biodegradable
constituents into non-polluting end products.
 To remove or transform nutrients such as nitrogen
and phosphorous.
 To capture non-settleable and suspended solids into
a biofilm.
 To remove specific trace organic compounds.

3. Types of biological treatment
 Aerobic and anerobic
 aerobic processes-Aerobic means in the presence of air
(oxygen)
 anaerobic processes - Anaerobic means in the absence
of air (oxygen).
 Attached and suspended
 Attached growth process-Biofilim attached to a media
 Suspended growth process-Biofilm in suspension

5. Aerobic Anerobic
 50% carbon is converted
into carbon di-oxide (CO ).
 40%-50% of carbon is
converted into biomass
 60% of energy is stored in
biomass. Rest removed
as process heat
 High energy input for
aeration
 Nutrient addition
requirement is substantial
 Process requires large
area
 94% of carbon is
converted into biogas
(CH) . 5% of carbon is
converted into biomass
 90% of energy is
retained as (CH ). 3%-
5% is wasted as heat
and rest is converted to
biomass.
 No external energy input
 Low nutrient requirement
 Low area requirement

6. Attached Growth process
 Microorganisms that are used for the conversion of
nutrients or organic material are attached to the inert
packing material.
 The organic material is removed from the
wastewater flowing past the biofilm or the attached
growth.
 Sand, gravel, rock and a wide variety of plastic and
other synthetic material is used as the packing
material.
 Eg. Trickling Filter, RBC

7. Suspended Growth Process
 Microorganisms responsible for treatment are
maintained in liquid suspension by mixing methods.
 Mostly aerobic method
 Eg. ASP, Oxidation pond

8. Secondary treatment
 The effluent from primary treatment is treated
further for removal of dissolved and colloidal
organic matter in secondary treatment.
 This is generally accomplished through
biochemical decomposition of organic matter,
which can be carried out either under aerobic or
anaerobic conditions.
 In these biological units, bacteria’s decompose
the fine organic matter, to produce clearer
effluent.
 The end products of aerobic decomposition are
mainly carbon dioxide and bacterial cells, and
that for anaerobic process are CH4, CO2 and

9. Secondary treatment
Activated Sludge
Process (ASP)
Trickling Filter
Oxidation Pond
Aerated Lagoon
Membrane
Bioreactor
Moving Bed
Bioreactor
Rotating Biological
Contactors
Upflow Anaerobic
Sludge Blanket (UASB)

10. Rotating Biological Contactor (RBC)

12. RBC
 Attached growth treatment process
 It consists of a series of closely spaced circular
plastic disks that are attached to a rotating hydraulic
shaft.
 The disc are of 3m diameter10mm thick placed at
30-40mm spacing on the shaft.
 40% of the bottom of each plate is dipped in the
wastewater and the film which grows on the disk
moves in and out of the wastewater.
 The shaft rotates 1 to 2 revolutions per minute.
 Provides large surface area for the biological growth.
 When rotated out of water air enters the voids and
aerobic condition maintained

13. RBC
 Biofilm is formed attached to the surface of disc
 Film absorbs organic pollutant when submerged
period of rotation
 Oxygen transfer occur during exposed to
atmosphere other half rotation
 Sloughing occurs when the thickness of biofilm
increases and attachment decreases
 It is removed in the clarifier stage
 Hydraulic loading rate 0.04-0.06m3/day/m2
 BOD loading rate are 0.05-0.06 kg BOD /m2 /day
 Stable operation during variable HRT and SRT

14. RBC- Biofilm Layer

17. Contact Bed Filter
 A contact bed consists of a watertight tank filled with
filtering media.
 The tank is usually constructed below the ground
surface by excavating the earth and it is provided
with a lining of cement concrete or watertight cement
plaster on masonry, on sides as well as on bottom.
 The filtering media in this case is gravel, ballast or
broken stones. The size of the particles of filtering
media varies from 20 to 40 mm.
 The depth of the filter bed is kept between 1 and 1.8
m, the common value being 1.2 m.

18. Contact Bed Filter
 Usually underground with water tight concrete lining
 Sewage is received from primary sedimentation tank
 A dosing tank with a siphon is provided to serve all
the beds
 Supplied to different contact beds at regular interval
by siphonic action.
 The area of the filter bed is usually less than 2000
m2. Usually 3 or 4 beds are provided adjacent to
each other so that they can work in rotation.

20. Operation of contact bed filters
 Filling
 The outlet valve of the under drain is closed and the tank is slowly
filled with sewage effluent from the primary settling tank through
the dosing tank.
 The depth of sewage applied may be 50 to 100 mm over the top
of the bed.
 This filling may take about 1 to 2 hours.
 Contact
 The dosing tank outlet is then closed, and the sewage admitted
over the contact bed is allowed to stand for about 2 hours.
 During this period the fine suspended, colloidal and dissolved
organic matter present in the sewage gets transferred to the filter
media,
 Sewage comes in contact with the bacterial film covering the filter
media.

21. Operation of contact bed filters
 Emptying
 The outlet valve of the under drain is then opened and the
sewage present in the contact bed is withdrawn slowly without
disturbing the organic film of the bed.
 This operation may take about 1 to 2 hours.
 Oxidation
 The contact bed is then allowed to stand empty for about 4 to 6
hours.
 During this period of rest, atmospheric air enters the void
spaces of the contact media,
 Supply oxygen to the aerobic bacteria, resulting in the oxidation
of the organic matter present in the film.

22. Performance
 Effluent is slightly turbid and odor less
 Removes 80% of Suspended solids
 Removes 60-75% of BOD
 Performance reduces after long term operation
 After 4-5 years the filter have to change completely
 Rate of loading should not exceed 110L/day/m2 for
filter beds of 1.2m to 1 m depth

23. Contact Bed Filters
Advantages Disadvantages
 Contact beds can be
operated without exposing
the sewage effluent to view.
 Contact beds can work
under small heads.
 There is no nuisance of filter
flies.
 The problem of undesirable
odour is less as compared to
that in the case of trickling
filters.
 Rate of loading is much less
in comparison to trickling
filters.
 Large areas of land is
required for their installation.
 Operation of contact beds
requires skilled supervision.
 Cost of contact beds is more
as compared to that of
trickling filters.
 Contact beds require long
rest periods.
 There is relatively high
incidence of clogging.

24. Intermittent Sand Filter

25. Intermittent Sand Filter
 The intermittent sand filter is the earliest type of
sewage filter used in the biological treatment of
sewage.
 It consists of a rectangular tank with a specially
prepared bed of sand.
 Rarely used now because of large area requirement
 Contact media is finer than contact bed filters and no
concrete lining around the filter media.

26. Intermittent Sand Filter
 The tank is about 1 to 1.25 m deep
 1000 to 4000 m2 in area with ratio of length to width
about 3 to 4.
 The filtering media consists of sand with an effective
size of 0.2 to 0.5 mm and uniformity coefficient of 2
to 5.
 The sand is filled in the tank and a sand bed of depth
about 0.75 to 1.05 m is prepared.
 Below the sand bed, a bed of gravel of thickness
about 0.15 to 0.30 m is provided in order to facilitate
the drainage of the effluent.
 Hydraulic loading applied 80-110L/m2/day

27. Intermittent Sand Filter
 To carry off the effluent, open jointed drain pipes are
laid in the gravel bed.
 Usually 3 to 4 beds are provided adjacent to each
other so that they can work in rotation.
 A dosing tank with a siphon is provided to serve all
the beds.

28. Operation of Intermittent Sand Filter
 Sewage effluent from primary sedimentation tank is
applied intermittently, through dosing tank with
siphon
 Flooding is done from 5-10cm depth after an interval
of 24 hrs
 The sewage effluent percolates through the sand
bed
 Suspended organic matter gets trapped in voids of
the top portion of sand through straining action.
 During rest period the trapped organic matter is
acted upon aerobic bacteria in the filter media.

30. Performance
 The effluent from the intermittent sand filter is quite
clear, well nitrified and stable.
 The effluent contains suspended solids less than 10
ppm and also the BOD of the effluent is less than 5
ppm.
 However, in order to maintain the efficiency of the
filter, the topmost layer of depth about 25 mm should
be raked at regular intervals to break up the
materials caught in the top part of the filter.
 The sand of the filter should also be renewed from
time to time.

31. Advantages
 Quality of effluent obtained from the intermittent sand
filters is very clean and stable
 Does not require any further treatment before disposal
except chlorination in some cases.
 Oxidation of organic matter of sewage is brought about
by aerobic bacteria as the sewage is applied
intermittently, there are less chances for anaerobic
conditions to develop, and hence there is no trouble of
odour, flies, etc.
 The operation is very simple, requiring no mechanical
equipment except for dosing and hence does not require
constant skilled supervision.
 Small head is required for applying the sewage on the
surface of the filter.
 There is no secondary sludge which is to be disposed of,
except for the occasional sand scraping.

32. Disadvantages
 The rate of filtration is low, and hence that of loading
is very small, per unit surface area of the filter.
Hence they cannot be employed for medium and
large size sewage treatment plants.
 Since a sand filter of about one hectare area is
required per 2500 population, they require large
areas of land and larger quantity of sand on account
of which their construction is costly.

33. Secondary Sedimentation tank
 The secondary sedimentation facility is provided
after the biological reactor to facilitate the
sedimentation of the cells produced during biological
oxidation of organic matter.
 If these cells produced are not removed, complete
treatment will not be achieved as these cells will
represent about 40 to 60% of the organic matter
present in untreated wastewater in aerobic
treatment.
 Depending on the type of reactor used fraction of
these settled cells is returned back to the reactor and
remaining cells are wasted as excess sludge for
further treatment.