Different type of treatment units for wastewater treatment

1. TREATMENT UNITS

3. Main sewer line
Screening
Grit chamber
Skimming tanks
Primary sedimentation tanks
Trickling filters/ASP/aerated lagoon
Secondary sedimentation tanks
Disinfection
Discharge
Pre-
treatment
Primary
treatment
Secondary
treatment
Advanced
treatment

4. SCREENS

5. SCREENS
 The primary treatment incorporates unit operations for removal of
floating and suspended solids from the wastewater.
 Screen is the first unit operation in wastewater treatment plant.
 Types of Screens
 Coarse Screen
 Bar Screens- This screen can be manually cleaned or mechanically cleaned
 Clear spacing between the bars in these screens may be in the range of 50mm
or more
 Disposed by incineration, burial or dumping
 6 litres of solids / million litre of sewage
 Medium screen
 6-40mm opening size
 30-90 litres of material /million litre of sewage
 30-60 degree direction of flow
 Fine Screen
 Fine screens are mechanically cleaned screens using perforated plates, woven
wire cloths
 Commonly these are available in the opening size ranging from 0.035 to 6 mm
 Remove 20% of suspended solids removed from sewage

6. BAR SCREEN

7. FINE SCREENS

8. DISPOSAL OF SCREENINGS
 Screening can be discharged to grinders or
disintegrator pumps, where they are ground and
returned to the wastewater.
 Screenings can be disposed along with municipal
solid waste on sanitary landfill.
 In large sewage treatment plant, screenings can be
incinerated
 For small wastewater treatment plant, screenings
may be disposed off by burial on the plant site
 Composting also can be done

9. GRIT CHAMBER

10. GRIT CHAMBER
 Grit chamber is the second unit operation used in
primary treatment of wastewater and it is intended
to remove suspended inorganic particles such as
sandy and gritty matter from the wastewater.
 This is usually limited to municipal wastewater and
generally not required for industrial effluent
 Specific gravity >2.65 so that the organic particles
are not getting settled
 Diameter 0.15-0.2 mm or more
 Types
 Horizontal flow grit channels
 Aerated grit channels
 Detrius tank

11. HORIZONTAL FLOW GRIT CHAMBER
Grit is removed by maintaining a constant upstream velocity of 0.3 m/s
Velocity is controlled by proportional weirs or rectangular control sections,
such as Parshall flumes.
Grit is removed by a conveyor with scrapers, buckets, or plows.

12. AERATED GRIT CHAMBER
 Grit is removed by causing the wastewater to flow in a spiral pattern
Air is introduced in the grit chamber along one side, causing a
perpendicular spiral velocity pattern to flow through the tank.
Heavier particles are accelerated and diverge from the streamlines,
dropping to the bottom of the tank
 Lighter organic particles are suspended and eventually carried out of the
tank

13. VORTEX TYPE GRIT CHAMBER
The vortex-type grit chamber consists of a
cylindrical tank in which the flow enters
tangentially, creating a vortex flow pattern.
Grit settles by gravity into the bottom of the tank (in
a grit hopper) while effluent exits at the top of the
tank.
The grit that settles into the grit hopper may be
removed by a grit pump or an air lift pump.
There are no submerged bearings or parts that
require maintenance
Headloss through a vortex system is minimal

14. DETRITUS TANK
 A detritus tank (or square tank degritter) is a
constant-level, short-detention settling tank. These
tanks require a grit-washing step to remove organic
material.
 Detritus tanks do not require flow control because
all bearings and moving mechanical parts are
above the water line.
 There is minimal headloss in this type of unit

15. DISPOSAL OF GRIT
 In large treatment plant, grit is incinerated with
sludge
 In the past grits along with screening was dumped
into sea.
 Generally, grit should be washed before disposal to
remove organic matter.
 Land disposal after washing is most common.

16. SKIMMING TANK

17. SKIMMING TANK
 The floating solid materials such as soap, vegetables,
debris, fruit skins, pieces of corks, etc. and oil and
grease are removed from the wastewater in skimming
tanks.
 A skimming tank is a chamber designed so that floating
matter rises and remains on surface of the wastewater
until removed, while the liquid flows continuously through
outlet or partition below the waterlines.
 The detention time in skimming tank is 5 minutes.
 To prevent heavy solids from settling at the bed,
compressed air is blown through the diffusers placed in
the floor of the tank.

18. SKIMMING TANK
 Due to compress air supply, the oily matters rise
upward and are collected in the side trough, from
where they are removed.
 In conventional sewage treatment plant separate
skimming tank is not used and these materials are
removed by providing baffle ahead of the effluent
end of the primary sedimentation tank.
 The chamber is a long trough shaped structure
divided up into 2 or 3 lateral compartments by
vertical baffle walls

19. DISPOSAL OF SKIMMINGS
 Burning or burial
 Converted into soaps candles etc
 Digested in digesters

20. PRIMARY SEDIMENTATION TANK

21. PRIMARY SEDIMENTATION TANK
 The objective of treatment by sedimentation is to remove
readily settleable solids and floating material and thus to
reduce the suspended solids content
 Preliminary step to biological treatment,
 function is to reduce the load on the biological treatment
units
 The primary sedimentation tanks are usually designed
for a flow through velocity of 1 cm/sec at average rate of
flow

22. PRIMARY SEDIMENTATION TANK
 The detention period in the range of 90 to 150 minutes
may be used for design.
 These tanks may be square, circular, or rectangular in
plan with depth varying from 2.3 to 5 m.
 The diameter of circular tanks may be up to 40 m. The
width of rectangular tank may be 10 to 25 m and the
length may be up to 100 m.
 But to avoid water currents due to wind, length is limited
up to 40 m.

23. 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 bacterial cells.

24. 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)
Membrane
Bioreactor

25. OXIDATION POND
 Oxidation ponds are the stabilization ponds, which
received partially treated sewage.
 Comparatively long detention time – stabilization
done by natural forces
 Classified into 3
 Aerobic ponds
 Anaerobic ponds
 Facultative ponds

26.  Aerobic ponds
 In aerobic pond the microbial population similar to ASP
exists along with algae.
 The aerobic population release CO2, which is taken up by
the algae for their growth, also releases NH3, Phosphates
 Algae in turn release O2, which helps in maintaining the
aerobic condition in the pond.
 Both algae and microorganism help in maintaining the
oxygen demand in the pond called algal symbiosis
 Very shallow depth of aerobic pond (0.15 to 0.45 m) is used
for the treatment of wastewater for removal of nitrogen by
algae growth. For general wastewater treatment depth of
0.5 to 1.2 m may be used.
 The solar radiation should penetrate to the entire depth of
the pond to support photosynthesis to keep entire pond
content aerobic

27.  Anaerobic pond:
 In anaerobic pond, the entire depth is under anaerobic
condition except an extremely shallow top layer.
 Normally these ponds are used in series followed by
facultative or aerobic pond for complete treatment.
 The depth of these ponds is in the range of 2.5 to 6 m.
 They are generally used for the treatment of high strength
industrial wastewaters and sometimes for municipal
wastewater and sludges.
 Depending upon the strength of the wastewater, longer
retention time up to 50 days is maintained in the anaerobic
ponds.
 Anaerobic lagoons are covered these days by polyethylene
sheet for biogas recovery and eliminating smell problem and
green house gas emission in atmosphere.

28.  Facultative pond
 Most of the ponds exist in facultative nature.
 Three zones exist in this type of ponds.
 The top zone is an aerobic zone in which the algal photosynthesis and
aerobic biodegradation takes place.
 In the bottom zone, the organic matter present in wastewater and cells
generated in aerobic zone settle down and undergo anaerobic
decomposition.
 The intermediate zone is partly aerobic and partly anaerobic. The
decomposition of organic waste in this zone is carried out by facultative
bacteria.
 The nuisance associate with the anaerobic reaction is
eliminated due to the presence of top aerobic zone.
 Maintenance of an aerobic condition at top layer is important
for proper functioning of facultative stabilization pond, and it
depends on solar radiation, wastewater characteristics, BOD
loading and temperature.
 Performance of these ponds is comparable with conventional
wastewater treatment.

29. Facultative pond

30. OXIDATION POND
 The detention time in the pond is usually 2 to 6 weeks depending upon
sunlight and temperature.
 Better efficiency of treatment is obtained if several ponds are placed in
series so that the sewage flows progressively from one to another unit
until it is finally discharged.
 Organic loading which may range from 150 –300 kg/ha/day in hot tropical
countries like India.
 Each unit may have an area ranging between 0.5 to 1.0 hectare.
 The length of the tank may be kept about twice the width.
 A free board of about 1 m may also be provided above a capacity
corresponding to 20-30 days of detention period.
 Properly operated ponds may be as effective as trickling filter in reducing
the BOD of sewage.
 The BOD removal efficiency of pond is up to 90% and coliform removal
efficiency of pond is up to 99%.

32. ADVANTAGES AND DISADVANTAGES
Advantages
 Low operational and maintenance cost
 Lagoons provide effective treatment with minimal threat to the
environment.
 Work well in clay soils where conventional subsurface on-site absorption
fields will not work.
 Skilled supervision is not required
 Quite flexible - not affected by fluctuations in organic loading
 Useful for hot dry climate like Indian
Disadvantages
 Lagoons must be constructed in clay soil or be lined to prevent leakage.
 May overflow occasionally during extended periods of heavy rainfall.
 Nuisance due to offensive odors and mosquito breeding
 Lagoons are not aesthetically acceptable to some people

33. REMOVAL OF SETTLED SLUDGE
 Average sludge accumulation is between 2-5 cm
depth /year
 Rate of sludge removal is required
 once in 6 years for a depth of 1.2 m deep ponds
 once in 12 years for a depth of 1.5 m deep ponds

34. 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.