Characteristics of sludge for disposal – Aerobic digestion – Gravity thickening – Flotation thickening –Rotatary drum screen – Gravity belt thickener – Centrifugation – Vacuum filtration – Pressure filtration – Belt filter press – Sand bed drying-factors affecting dewatering performance – Land disposal of sludges– Incineration.

1. UNIT - V
SLUDGE
TREATMENT
AND
DISPOSAL

3. INTRODUCTION
• Sludge is an odious, semisolid residual that resembles
thick soft mud produced from the solid–liquid
separation processes in wastewater treatment.
• It is usually very inconsistent in its composition and
most often unmanageable.
• The final destination of treated sewage sludge usually
is the land.
• Dewatered sludge can be buried underground in a
sanitary landfill.
• It also may be spread on agricultural land in order to
make use of its value as a soil conditioner and fertilizer.

4. SLUDGE IS CATEGORIZED INTO THE
FOLLOWING GROUPS
a. Primary sludge
• Primary sludge is generated by the separation
of settleable solids from the raw wastewater
during the primary sedimentation treatment
process.
• The total solids concentration in raw primary
sludge ranges between 5% and 9%, and is
typically 6%.

5. b. Secondary sludge
• Secondary sludge is the activated waste biomass
resulting from biological treatments.
• Some sewage plants also receive septic tank
solids from household on-site wastewater
treatment systems.
c. Sludge produced in advanced treatment
process
• It may contain viruses, heavy metals,
phosphorous, or nitrogen.

6. THE OBJECTIVES OF THE SLUDGE TREATMENT
• To decrease moisture content in the
sludge (Volume reduction)
• To remove organic matters
• To destroy microorganisms
• To eliminate toxic materials.

7. SLUDGE DISPOSAL METHOD
• Sludge from conventional wastewater
treatment plants (WWTP) is derived from
primary, secondary and tertiary treatment
processes.
• Most often, the sludge produced has a
concentration of a few grams per liter, and is
highly biodegradable.
• Each process has a different impact on the
water pollution load.

8. SLUDGE DISPOSAL METHOD
• Pre-treatment
•

9. Pre-treatment
• Pre-treatment consists of various physical and
mechanical operations, such as screening,
sieving, blast cleaning, oil separation and fat
extraction.
• Pre-treatment allows the removal of
voluminous items sands and grease.
• The residues from pretreatments are not
considered to be sludge.
• They are disposed of in landfills.

10. Primary sludge
Primary sludge is produced following primary
treatment.
This step consists of physical or chemical
treatments to remove matter in suspension
(e.g. solids, grease and scum).
The most common physical treatment is
sedimentation.
Sedimentation is the removal of suspended
solids from liquids by gravitational settling.

11. • Chemical treatments are coagulation and
flocculation.
• Coagulation and flocculation are used to
separate suspended solids when their normal
sedimentation rates are too slow to provide
effective clarification.

12. Secondary
sludge
• Secondary sludge is generated from the use of
specially provided decomposers to break down
remaining organic materials in wastewater after
primary treatment.
• The active agents in these systems are micro-
organisms, mostly bacteria, which need the
available organic matter to grow.
• There are various techniques such as lagooning,
bacterial beds, activated sludge as well as
filtration or biofiltration processes.

13. Tertiary sludge
Tertiary sludge is generated when carrying out
tertiary treatment.
It is an additional process to secondary treatment
and is designed to remove remaining unwanted
nutrients (mainly nitrogen and phosphorus)
through high performance bacterial or chemical
processes.
These treatments are necessary when a high level
of depollution is required, for example in
sensitive areas identified in the Member States.

14. SLUDGE
CHARACTERISTICS
• Wastewater sludge type is solid, semi-
solid, or muddy liquid where each of
those consists of the various organic or
non-organic materials, heavy metals,
pesticides, polycyclic aromatic, phenols,
and many other materials.

15. Screening
• Screening grinders are beneficial for medium-size
plants. Reduced-size solids are returned to raw
sewage or mixed with sewage sludge depending
on grinder location related to the treatment units
It contains both organic and inorganic matter.
Grit
• Final grit disposal is by burial. It may be in either
a sanitary landfill or other accepted landfill
operation. The grit must be having a minimum of
6 inches (15 cm) of soil covering. This is to
prevent vector attraction and odors It involves
organic and inorganic matter, especially fats and
grease.

16. Characteristics of Primary and
Secondary Sludge

17. ANAEROBIC AND AEROBIC
DIGESTION
 Anaerobic digestion is a common method of readying
sludge solids for final disposal.
 All solids settled out in primary, secondary or other
basins are pumped to an enclosed air tight digester,
where they decompose in an anaerobic environment.
 The rate of their decomposition depends primarily on
proper seeding, ph, character of the solids, temperature
etc. digestion serves the dual purpose of rendering the
sludge solids readily drainable and converting a portion
of the organic matter to gaseous end products.
 It may reduce the volume of sludge by as much as 50%
organic matter reduction. After digestion, the sludge is
dried and /or burned or used for fertilizer or landfill.

18. The following factors are measures of the
effectiveness of digestive action
• Gas production,
• Solids balance,
• B.O.D,
• Acidity and ph,
• Sludge characteristics and odors.

19. Aerobic sludge treatment
process in waste water

20. Aerobic Digestion
Works
Aerobic digestion is the degradation of the
organic sludge solids in the presence of oxygen.
The oxygen is introduced as fine bubbles of air
into the reactor.
The micro-organisms in the sludge convert the
organic material to carbon dioxide and water, and
the ammonia and amino species to nitrate.
Aerobic digestion is generally lower in capital cost
than anaerobic digestion (AD) for plants below
20,000 m3/d wastewater capacity.

21. In addition, aerobic digestion
• It is simple in construction, requiring no tank
covering
• More easily operated and controlled than AD
• It does not generate nuisance odors
• It can process sludge with a high nutrient
content
• Generates supernatant streams having low in
BOD and nutrient concentrations
• Removes ammoniacal compounds, and
• Achieves a similar VSS removal (40−50%) as
that for AD.

22. THICKENING
• Thickening is a first step to reduce
sludge water content.
• Sludge reaches 10 to 30 % dryness,
and can still be pumped.

23. Gravity thickening

24. Gravity thickening

25. Gravity thickening
Gravity thickening is the process by which bio-
solids are condensed to produce a concentrated
solids product and a relatively solids-free
supernatant.
Water is collected at the top.
The process is capable of thickening the sludge by
2 to 8 times, bringing it from a few grams/litre to
a few tens of grams/litre.
Performing costs are relatively low, as only an
electricity supply is needed to operate the harrow
and the pumps.

26. • Gravity thickeners generally retain 80−90% of
the solids in the thickened product stream,
depending on the sludge characteristics and
the efficiency of the preceding conditioning
stage.
• Gravity thickeners are widely used for primary
and mixed sludge, but are less effective for
waste activated sludge (WAS).

27. Advantages
• Gravity thickening equipment is simple to
operate and maintain.
• Gravity thickening has lower operating costs
than other thickening methods such as DAF,
gravity belt or centrifuge thickening.
Truck traffic at the plant and the farm site can be
reduced;
• Trucking costs can be reduced;
• Existing storage facilities can hold more days of
biosolids production

28. Disadvantages
• Scum build-up can cause odors. This buildup, which can
occur because of long retention times, can also increase
the torque required in the thickener. Finally, scum build-
up is unsightly.
• Grease may build up in the lines and cause a blockage.
This can be prevented by quick disposal or a back flush.
• Septic conditions will generate sulfur-based odors. This
can be mitigated by minimizing detention times in the
collection system and at the plant, or by using oxidizing
agents.
• Supernatant does not have solids concentrations as low
as those produced by a DAF or centrifuge thickener. Belt
thickeners may produce supernatant with lower solids
concentrations depending on the equipment and solids
characteristics.

29. Dissolved Air Floatation
Thickener

30. Dissolved Air Floatation
Thickener

31. Dissolved Air Floatation
Thickener
• The technique of air flotation can be used when
the solid particles have a low rate of settlement,
and in sewage sludge treatment the process is
used to thicken surplus activated sludge.
• Dissolved air flotation is a water treatment
process that clarifies wastewater by removal of
suspended solids, oils, greases, BOD, COD, and
metals.
• This is achieved by dissolving air in the
wastewater under pressure and then releasing
the air at atmospheric pressure in a flotation
tank.

32. Rotary drum
filtering

33. Rotary drum
filtering

34. Rotary drum filtering
 Rotary drum thickening increases the sludge solids
concentration (i.e. thickens the sludge) by agitating
the solids in a slowly-rotating vessel with porous
walls though which the water (or filtrate) drains the
sample of rotary drum filtering for sludge disposal.
 A rotary drum thickener (RDT) is based on the same
principle as a gravity belt thickener (GBT), in that
water drains from the sludge through a retaining
porous medium.
 For an RDT the porous medium is the cylindrical wall
of a 0.5−1.5 m diameter drum which rotates at
speeds between 5 and 20 RPM while the sludge
continually passes through it.

35. Applications
• The rotary filter is most suitable for continuous
operation on large quantities of slurry.
• If the slurry contains considerable amount of
solids, that is, in the range of 15-30%.
• Examples of pharmaceutical applications include
the collection of calcium carbonate, magnesium
carbonate and starch.
• The separation of the mycelia from the
fermentation liquor in the manufacture of
antibiotics.
• Block and instant yeast production.

36. Gravity Belt
Thickening
Gravity belt thickening (GBT) increases the sludge
solids concentration (i.e. thickens the sludge) by
allowing the water (or filtrate) to drain from the sludge
under gravity through a permeable medium (a moving
belt) on which the sludge sits.
Gravity thickening is the process by which biosolids are
condensed to produce a concentrated solids product and
a relatively solids-free supernatant.
The biosolids technology fact sheet for gravity
thickening provides a description of gravity thickening
applicability to wastewater treatment plants.

37. Gravity Belt Thickening
• GBT is a low-pressure process which operates
by allowing the water to drain from the sludge
under gravity.
• A belt filter press, on the other hand, operates
by squeezing water from the sludge under
pressure, and achieves a higher dry solids
concentration as a result.

38. Gravity Belt Thickening

39. Gravity Belt Thickening

40. Centrifugation
Centrifugal thickening increases the sludge
concentration (i.e. thickens the sludge) by
encouraging the particles to migrate to the
walls of a rapidly rotating cylindrical vessel
under the influence of a centrifugal force.
There are a number of configurations of
centrifuge used for reducing the water
content of sludge.
They can be used for both thickening and
dewatering.

41. Centrifuges are available in a number of
configurations, including
• Solid bowl
• Basket, and
• Disc-nozzle.

42. VACUUM FILTER

43. Operation of a Vacuum Filter Cycle
The filter cycle, which is one full revolution of
the drum, consists of three parts; the form time,
the drying time, and the discharge time the first
part of the cycle, when the drum is submerged, is
the cake formation or form time.
It is during this part of the cycle that sludge solids
are being drawn to the media by the effect of the
vacuum and are receiving the initial compression
necessary to form a cohesive cake.
Initially, the water and fine particles are drawn
through the media, leaving only the coarser
particles on the face.

44.  However, as the drum continues to rotate, and the
thickness of the sludge cake is increased, the finer
particles are trapped as well as the coarse solids.
 There is an indication that the rate of cake formation is
proportional to the square root of the time elapsed since
the start of cake formation, but this is modified by an
upper limit of cake thickness beyond which the cake
formation falls rapidly.
 This upper limit occurs as the flow resistance of the
cake approaches the available pressure differential
supplied by the vacuum.
 This consequently places an upper limit, which is the
minimum time necessary to form a cake of sufficient
thickness to be successfully discharged.
 Within these limits, the form time can be varied by
changing the total cycle time, or by changing the
submergence

45. The second portion of the cycle is the drying time.
During this part of the cycle, moisture is removed
from the cake and ascertains amount of
compression takes place.
The amount of moisture removed is dependent
upon two controlling factors.
First, the cake may be compressed to a level
beyond which resistance to air flow prevents
additional dewatering at the pressure differential
available.
Secondly, drying may be carried to a point where
the cake begins to crack and the pressure
differential across the cake drops due to leakage
of air through the cracks.

46. The moisture content of the cake may be
altered by making adjustments in the total
cycle time, or by changing the submergence.
The third and final portion of the total cycle
time is the discharge time.
In the case of-a belt type filter, the media with
the cake is separated from the drum, the cake
is discharged, and the media is washed and
returned to the drum.

47. PRESSURE FILTERS
Pressure filter press dewatering is a batch process
in which dewatering is achieved by forcing the
water from the sludge under high pressure.
It produces a cake that is drier than that
produced by any other dewatering alternative.
Another advantage is that the high solids capture
results in good filtrate quality.
Disadvantages include high capital cost, relatively
high operation and maintenance costs, high
chemical costs, and a large area requirement for
the equipment in small wastewater treatment
plants.

48. Among the various types of pressure filter
presses that have been used for dewatering
sludge, the two most commonly used are the
fixed-volume recessed plate filter press and
the variable-volume recessed plate filter
press.
Fixed-Volume Recessed Plate Filter Press
The fixed-volume press consists of a series of
recessed plates held in a frame and pressed
together either hydraulically or
electromechanically between a fixed end and
a moving end

49. Solids capture solids in feed - solids in
filtrate solids in feed x 100
Cake under Compression
Shape of filter chamber during filtration
Shape of filter chamber during cake
compression by diaphragm

50. Pressure Filters

53. BELT PRESSURE FILTER
The belt filter (sometimes called a belt press filter, or
belt filter press) is an industrial machine, used for
solid/liquid separation processes, particularly the
dewatering of sludge in the chemical industry, mining
and water treatment.
A belt filter press (BFP) provides sludge dewatering
by pressing the sludge to force the water through a
permeable medium.
The process produces a cake (the dewatered product)
having a dry solids (DS) content of 30% or more in the
case of primary sludge.
This is to be distinguished from the lower-energy
gravity belt thickener process which achieves a
maximum DS content of ~10%.

54. A BFP combines drainage and mechanical
pressure sequentially to remove water.
The equipment normally comprises 2−3
recirculating belts, with two belts combining
at some point to compress the sludge and
squeeze water from it.
The conditioned sludge feed is first conveyed
along a gravity drainage section of one of the
porous belts, as with a gravity belt thickener.
It is then subject to pressure as it is passed
between two recirculation belts, forming a
wedge zone at the inlet, which travel over a
roller.

55. In common with the gravity belt thickener, the
belts are subject to cleaning with water spray
to limit the plugging of the filter belt pores.
 The waste wash water is combined with the
filtrate.
The overall performance of the BFP is
dependent on the feed sludge source and
characteristics and the dosing conditions.
As with all other thickening and dewatering
technologies, primary sludge – both raw and
anaerobically digested – are more readily
filtered than waste activated sludge (WAS).

56. SLUDGE SAND DRYING BED FILTER

57. SLUDGE SAND DRYING BED FILTER
Sludge drying beds provide sludge dewatering by
allowing the liquid to both drain under gravity through
a permeable medium on which the sludge sits, and to
evaporate under ambient conditions.
Lagoons provide dewatering through evaporation alone.
Drying beds are uncomplicated in design and operation,
since they involve simply spreading the sludge as a thin
(up to 300 mm) layer over a porous bed of sand and
gravel.
The water is then allowed to drain through the bed and
into a network of embedded drainage pipes to allow it
to be collected.
Further water removal is provided by evaporation from
the sludge surface.

58. • The degree of dewatering attainable depends on
the drying duration and, if the bed is uncovered,
the weather conditions.
• Once sufficiently dewatered, which takes 10−15
days and is evident from the appearance of cracks
and a dark brown surface, the solids (30−40% DS
or dry solids) are mechanically removed.
• Removal is normally using a front end loader or
specialist mechanical sludge removal equipment

59. FACTOR AFFECTING DEWATERING
PERFORMANCE
Under land disposal the following methods may
be included
• Burial
• Fill
• Application as fertilizer or soil conditioner

60. Burial Sludge Disposal

61. Burial Sludge Disposal
This method is used principally for raw sludge,
where, unless covered by earth, serious odor
nuisances are created.
The sludge is run into trenches two to three
feet wide and about two feet deep.
The raw sludge in the trenches should be
covered by at least 12 inches of earth

62. Fill
• Use of sludge for fill is confined almost entirely
to digested sludge which can be exposed to the
atmosphere without creating serious or
widespread odor nuisances.
• The sludge should be well digested without any
appreciable amount of raw or undigested mixed
with it.
• Either wet or partially dewatered sludge, such as
obtained from drying beds or vacuum filters can
be used to fill low areas

63. Fill
Lagoons used for disposal are usually fairly
deep. Sludge is added in successive layers
until the lagoon is completely filled.
Final disposal of digested sludge by lagoons is
economical as it eliminates all dewatering
treatments.
It is applicable, however, only where low
waste areas are available on the plant site or
within reasonable piping distance.
They are frequently used to supplement
inadequate drying bed facilities.

64. Soil Conditioning or Fertilizer
• Sewage sludge contains many elements
essential to plant life, such as nitrogen,
phosphorous, potassium, and in addition, at
least traces of minor nutrients which are
considered more or less indispensable for
plant growth, such as boron, calcium, copper,
iron, magnesium, manganese, sulfur, and
zinc.

65. Soil Conditioning or Fertilizer
In fact, sometimes these trace elements are
found in concentrations, perhaps from
industrial wastes, which may be detrimental.
 The sludge humus, besides furnishing plant
food, benefits the soil by increasing the water
holding capacity and improving the tilts, thus
making possible the working of heavy soils
into satisfactory seed beds.
It also reduces soil erosion.

66.  Nitrogen is required by all plants, particularly where
leaf development is required.
 Thus, it is of great value in fertilizing grass, radishes,
lettuce, spinach, and celery.
 It stimulates growth of leaf and stem.
 Phosphorous is essential in many phases of plant
growth.
 It hastens ripening, encourages root growth and
increases resistance to disease.
 Potassium is an important factor in vigorous growth.
 It develops the woody parts of stems and pulps of
fruits.
 It increases resistance to disease, but delays ripening
and is needed in the formation of chlorophyll.

67. INCINERATION
Incineration − or combustion of sewage sludge
is the most widely-accepted alternative end
disposal method after land spreading.
It is an oxidative method, converting the
organic carbon, nitrogen, sulphur, nitrogen
and phosphorus into gaseous and
predominantly mineral (i.e. inorganic) solid
products.

68. The incineration of sludge waste, derived from
wastewater, uses thermal treatment to oxidize organic
matter contained in the sludge.
Incineration is carried out in a furnace using oxygen
and high temperatures (over 850 degrees Celsius)
resulting in total, or near-total, oxidization of the
organic material.
Incineration is the most established and widely
implemented of the sludge thermo chemical treatment
processes, with the original sewage sludge incinerators
installed in the mid-1930s.
There are currently hundreds of large sewage sludge
incinerators in operation across the world, most of
which generate power as well as converting the sludge
to ash.

69. Sludge sand drying bed filter

70. Incineration of Sewage Sludge in Coal-
Fired Power Plants

71. Incineration of Sewage Sludge in Coal-
Fired Power Plants
To avoid the high costs of a stand-alone
incineration plant for sludge and to improve the
energy recovery efficiency, possibilities have been
investigated to incinerate dried sludge in a coal-
fired power plant.
In this case, beneficial use can be made from
existing coal combustion installations and existing
exhaust gas treatment systems.
Because the amount of incinerated sludge is
small compared to the amount of coal, the effect
of the incineration of the sludge on the air and
ash qualities can be neglected.

72. THANK
YOU