Sewage treatment involves three main phases - pre-treatment, primary treatment, and secondary treatment. Pre-treatment screens out large solids. Primary treatment removes most solids but not dissolved organics, using sedimentation. Secondary treatment uses microorganisms to break down organics into simple compounds and uses sunlight to destroy pathogens, producing effluent safe for discharge. The sludge byproduct is further treated and can be used as fertilizer or disposed of through other methods.

1. PHYSICAL AND BIOLOGICAL
TREATMENT OF SEWAGE
CHAKAMBA
J

2. INTRODUCTION
• Sewage is a major carrier of disease (from
human wastes) and toxins (from industrial
wastes).
• The safe treatment of sewage is thus crucial to
the health of any community.
• This article focuses on the complex physical
and biological treatments used to render
sewage both biologically and chemically
harmless.

3. • The wastewater discussed in this section is
predominantly of domestic origin.
• Varying amounts of industrial and laboratory
wastewaters can be collected and treated with
the sanitary sewage.
• The primary purpose of the treatment of sewage
is to prevent the pollution of the receiving
waters.
• Many techniques have been devised to
accomplish this aim for both small and large
quantities of sewage.

4. SEWAGE TREATMENT
• Sewage is a mixture of domestic and industrial
wastes. It is more than 99% water, but the
remainder contains some ions, suspended
solids and harmful bacteria that must be
removed before the water is released into the
sea.
• The treatment of wastewater is divided into
three phases: pre-treatment, primary
treatment and secondary treatment.

5. Pre-treatment
• Large solids (i.e. those with a diameter of
more than 2cm) and grit (heavy solids) are
removed by screening.
• These are disposed of in landfills.

6. Primary treatment
• Primary treatment removes most of the solids
from the effluent, but doesn't remove or
degrade the dissolved organic matter
• The water is left to stand so that solids can
sink to the bottom and oil and grease can rise
to the surface.
• The solids are scraped off the bottom and the
scum is washed off with water jets. These two
substances are combined to form sludge.

7. Secondary treatment
• Secondary treatment uses micro-organisms to
convert these organics to simple compounds,
and uses the energy of the sun to destroy
pathogens.
• The effluent is then safe to be discharged into
the water body. The entire process is shown
• diagrammatically in Figure 1.

8. THE SEWAGE COLLECTION
SYSTEM

9. COLLECTION SYSTEM
• The purpose of a sewage collection system is
to remove wastewater from points of origin to
a treatment facility or place of disposal.
• The collection system consists of the sewers
(pipes and conduits) and plumbing necessary
to convey sewage from the point(s) of origin
to the treatment system or place of disposal.

10. • It is necessary that the collection system be
designed so that the sewage will reach the
treatment system as soon as possible after
entering the sewer.
• If the length of time in the sewers is too long,
the sewage will be anaerobic when it reaches
the treatment facilities.

11. • Sanitary sewage collection systems should be
designed to remove domestic sewage only.
• Surface drainage is excluded to avoid
constructing large sewers and treating large
volumes of sewage diluted by rainwater during
storms.
• Sewers which exclude surface drainage are called
sanitary sewers, and those which collect surface
drainage in combination with sanitary sewage are
called combined sewers.

12. • Sewers are laid to permit gravity flow of their
contents.
• Unlike water in a water distribution system,
the contents of a sewer do not flow under
pressure.
• The gradient must not be too steep or to
gentle

13. • This is a self-cleansing velocity and prevents
solids from settling in the sewer pipes. To the
maximum extent practical, sewers are laid in
straight lines.
• Corners and sharp bends slow the flow rate,
permit clogging, and make line cleaning
difficult.

14. • Pumping is necessary where the slope of the
sewer does not produce the required minimum
velocity or where sewage must be lifted to a
higher elevation.
• Sewage can be pumped from pumping stations
through pressure lines (force mains) regardless of
their slope, or it can be raised to a higher
elevation at pumping stations (lift stations), so
that gravity flow will again produce the required
velocity.

15. • For gravity flow lines, sewer pipes of vitrified
clay tile, concrete, cement-asbestos, or
bituminous-impregnated fibre may be used.
• For force mains and stream crossings, cast
iron or cement-asbestos pipes are used.

16. • Removing grease from sewage is essential to the
proper functioning of sewage systems.
• At fixed installations, grease is collected by
ceramic or cast iron grease interceptors installed
at kitchens and other facilities that generate
grease and by concrete or brick grease traps
outside the building.
• Approximately 90 per cent of the grease will be
removed from greasy wastes by properly
maintained grease interceptors and traps.

17. • On arrival at the treatment works sewage is soup
like, dirty liquid containing food, detergents,
human waste, oils, sand and sometimes harmful
chemicals such as acids.
• All these pollutants have to be removed and the
water cleaned before it can be returned to the
natural water cycle.
• The sewage treatment process can be complex.
However, the key elements of the process are:

18. Pre-treatment
• Pre-treatment removes the large solids (such as
rags and sticks) that are carried in with the
wastewater.
• These are removed by screens consisting of
metal bars spaced at 19 mm intervals which are
placed across the influent channels.
• Tines (metal combs) rake the collected matter off
these, and heavy objects such as rocks (which
would otherwise damage the equipment) are
then collected for land filling off site

19. 2. Grit and Sand Channels
• The sewage then flows along wide, deep
channels where grit and sand sink to the
bottom.
3. Sedimentation Tank
• Here, solid particles in the sewage settle at
the bottom of the tank to form a thick sludge.

20. 4. Filtration
A. Biological Filter
• The ‘settled sewage’ is then sprinkled onto large circular
beds, about two metres deep, filled with stones or clinker.
Biological filtration is based on the principle that where
enough air is present cultures of bacteria will form.
• Millions of bacteria and other tiny creatures live on the
stones and feed on the organic material in the sewage,
converting it into carbon dioxide, water and nitrogen
compounds.
• They literally ‘eat’ the sewage, removing harmful waste.
This biological activity produces a humus sludge which
settles out in special humus tanks.

23. • Activated Sludge
• In this alternative to biological filtration,
activated sludge containing bacteria is mixed
with the settled sewage in an aeration tank.
• The air the bacteria need is provided by water
in the tank.

24. 5. Final Settling Tank
• Small and fine particles settle out leaving the
cleaned water, or effluent, to flow back into
rivers and streams.

25. 6. Sludge
• Sludge can be defined as the solid waste
which has settled out of the sewage in the
sedimentation, humus and final settling tanks.
• tonnes of sludge are produced every year and
several methods of disposal are used:
a) Sludge contains nitrogen, phosphorus and
organic matter which makes it ideal for use as
an agricultural fertiliser. 68 per cent is used in
this way.

26. b) Sludge from sewage works in heavy industrial
areas may have a high metallic content and
therefore cannot be used on the land.
• In these cases the sludge can be dried into a
cake and used for landfill purposes and
tipping, or it can be incinerated.

28. The role of the laboratory
• A wide variety of analytical tests are used to
determine the purity of the wastewater at
various stages of treatment so that the
possibility of harm to either people or the
environment is minimised.

29. Sewage disposal
A detailed account

30. Pre-treatment
• Large solids (i.e. those with a diameter of
more than 2cm) and grit (heavy solids) are
removed by screening.
• These are disposed of in landfills.

31. Primary treatment
• Primary treatment removes most of the solids
from the effluent, but doesn't remove or
degrade the dissolved organic matter
• The water is left to stand so that solids can
sink to the bottom and oil and grease can rise
to the surface.
• The solids are scraped off the bottom and the
scum is washed off with water jets. These two
substances are combined to form sludge.

32. Secondary treatment
• Secondary treatment uses micro-organisms to
convert these organics to simple compounds,
and uses the energy of the sun to destroy
pathogens.
• The effluent is then safe to be discharged into
the water body. The entire process is shown
• diagrammatically in Figure 1.

34. • The sludge is further treated in 'sludge
digesters': large heated tanks in which its
chemical decomposition is catalysed by
microorganisms.
• The sludge is largely converted to 'biogas', a
mixture of CH4 and CO2, which is used to
generate electricity for the plant.

35. • The liquid is treated by bacteria which break
down the organic matter remaining in
solution.
• It is then sent to oxidation ponds where
heterotrophic bacteria continue the
breakdown of the organics and solar UV light
destroys the harmful bacteria.

36. Oxidation
• Conditions in the ponds promote the growth
of unicellular algae: minute plants which, like
any other plants, absorb carbon dioxide in
daylight and give off oxygen by
photosynthesis.

37. • This oxygen oxidises the organics, thus
purifying the sewage by reducing its oxygen
demand.
• The ponds absorb an amount of solar energy
equivalent to a 745 kW engine running
continuously.
• Such engines are in fact required by other
modern sewage treatment processes where
the works must be restricted to a smaller area.

38. The role of the laboratory
• A wide variety of analytical tests are used to
determine the purity of the wastewater at
various stages of treatment so that the
possibility of harm to either people or the
environment is minimised.

40. • Step 1 - Pre-treatment
• Pre-treatment removes the large solids (such as
rags and sticks) that are carried in with the
wastewater.
• These are removed by screens consisting of
metal bars spaced at 19 mm intervals which are
placed across the influent channels.
• Tines (metal combs) rake the collected matter off
these, and heavy objects such as rocks (which
would otherwise damage the equipment) are
then collected for land filling off site

41. • Step 2 - Primary Treatment
• Here grit (fine, hard solids), suspended solids and scum are removed in two stages.
• Preaeration
• Firstly the wastewater is aerated by air pumped through perforated pipes near the
floor of the
• tanks. This aeration makes the water less dense, causing the grit to settle out. As
the air jets are
• positioned such that the water is swirling as it moves down the tanks the
suspended solids are
• prevented from settling out. The air also provides dissolved oxygen for the bacteria
to use later
• in the process, but the wastewater is not in these tanks long enough for bacterial
action to occur
• here. The grit is collected in hoppers and washed, after which it is used for on site
land
• reclamation and landscaping.

42. • Sedimentation
• The water then flows slowly and smoothly through the
sedimentation tanks, where the suspended
• solids fall to the bottom and scum rises to the surface,
while clarified effluent passes on. The
• solids are removed from the bottom of the tanks by
scrapers, and scum is washed off with water
• jets. The scum and solids are brought to a common
collection point where they are combined to
• form 'sludge' and sent off for secondary treatment.

43. • Step 3 - Secondary Treatment
• After secondary treatment all effluent, both solid and liquid, is sufficiently safe to be released
• into the environment. The treatment of solids and liquids are covered separately below.
• Solids
• Sludge from the sedimentation tanks is digested anaerobically in large tanks, and then further
• digested in lagoons before being dried in dewatering beds. In the sludge digesters the sludge is
• kept at 37oC and mechanically mixed to ensure optimum operation. During this time the organic
• compounds within the sludge are converted to carboxylic acids and then finally to methane and
• carbon dioxide. This gaseous mix is known as "biogas", and is a valuable source of fuel. At
• Mangere it is used to generate electricity, which is primarily used to drive the plant machinery,
• with any excess electricity being sold to Mercury Energy. The exhaust created by burning the
• biogas is used to heat the sludge in the digesters.

44. • When the sludge leaves the digesters it has undergone a 50% volume
reduction. It is then sent to
• lagoons for about a year, and finally to dewatering beds. During this time
all pathogens are
• killed by the sunlight.
• A small proportion of the sludge is currently mechanically dewatered
instead of being treated in
• the lagoons and dewatering beds. Polyelectrolytes are added to the
sludge, and the attraction of
• opposite charges causes a floc (loose aggregation of particles) to form.
Most of the water is then
• removed by rollers squeezing the mixture. After this the sludge has
become concentrated from 4
• to 30% solids, and could potentially be used as a soil conditioner, although
currently it is simply
• landfilled.

45. • Liquids
• The liquids are either sent directly to open-air oxidation ponds, or sent to 'fixed growth reactors'
• to reduce their BOD before pond oxidation. The fixed growth reactors (FGR's) are tall, circular
• tanks covered with fibreglass. Each one is filled with approximately 36 million 10 cm diameter
• PVC 'wheels'. Microorganisms live on the wheels, and these reduce the BOD of the sewage by
• a further 75%. The wastewater is sprayed on the top of the wheels and percolates down, with the
• organics being reduced to CO2, CH4 and a small amount of foul-smelling H2S. From the bottom
• of the FGR the effluent is piped to one of the secondary sedimentation tanks where sludge
• (consisting mainly of dead microorganisms from the FGR's) is removed. This sludge is piped
• back to join the incoming wastewater and complete the cycle again.

46. • The wastewater then joins the liquid that was
sent straight to the ponds for oxidation, where an
• influent mix of primary and secondary treated
wastewater and some recycled pond water is
• received. The mix is precisely calculated to ensure
that the amount of organics entering the pond
• is the optimum amount for the bacteria to
process given the amount of oxygen available at
that time.

47. algae
O2
bacteria
CO2
N2
H2S
CH4
NH3

48. • A diagram of the processes taking place in the ponds is
given in Figure 2. The effluent entering
• the ponds is a mixture of primary and secondary
treated effluent, with the relative proportions
• varying during the year. During summer the pond algae
produce large amounts of O2 by
• photosynthesis, so the proportion of effluent that has
only been primary treated (i.e. the
• proportion that requires large amounts of O2) is
greater, whereas in winter more secondary
• treated effluent is used.

49. • THE ROLE OF THE LABORATORY
• The laboratory carries out a significant range of tests at various stages of the treatment process to
• ensure effluent purity and to protect water and equipment at other stages of the process. Many of
• these tests can only be used to monitor a single parameter, but a small number have much wider
• application. These are:
• • Atomic absorption spectroscopy (AA), which is used to detect the presence of many
• metals.
• • Inductively coupled plasma (ICP), also used to detect metals. Together these techniques
• can be used to monitor all the metals of interest in wastewater.
• • Gas Chromatography (GC), which is used to detect certain organic species.
• Specific tests are used to monitor a wide variety of substances, conglomerations of substances
• and more general properties of the wastewater.

50. • • Elements (phosphorous and nitrogen)
• • Molecules (ammonia and dissolved oxygen)
• • Ions (nitrite, nitrate and sulphide)
• In addition, some conglomerations of
substances (which include many individual
chemical
• species) are monitored:
• • Solids content (both suspended and total)

51. • And finally, some parameters of the solution as a whole are tested for:
• • pH
• • Alkalinity
• • BOD5 (the amount of oxygen consumed by biological degredation processes
over a five
• day period)
• • COD (the amount of oxygen required to completely oxidise the chemical species
present)
• • Turbidity
• There are different test frequencies for different sample sites. Tests such as BOD
and suspended
• solids are usually carried out frequently, e.g. daily or weekly, while other tests such
as those for
• metals are done only occasionally, e.g. monthly or quarterly. The tests are
discussed individually
• below.

52. • ENVIRONMENTAL IMPLICATIONS
• Odour control
• In the absence of adequate oxygen bacteria in the wastewater
break down essentially odourfree
• compounds to odorous compounds: fats and carbohydrates go to
alcohols, esters,
• aldehydes and carboxylic acids while proteins go to ammonia,
amides, mercaptans and
• hydrogen sulphide. All of these compounds can give off strong
smells, but those formed
• from protein degredation can emit very intense smells at
concentrations in the parts per
• billion range.

53. • The foul air containing these compounds is mostly formed in the pretreatment and
primary
• treatment phases. For this reason, the equipment used in these phases, as well as
the fixed
• growth reactors and the sludge dewatering plant, are roofed over and the gases
produced in
• these areas are removed using extractor fans. They are piped to one of six 'earth
filters':
• mounds of soil and finely ground sand and scoria with a total surface area of 9800
m2 and an
• average total depth of 1m. The air is released from perforated pipes underneath
the soil and
• percolates upwards. Organisms convert sulfurous compounds to sulfur,
nitrogenous ones to
• nitrogen and organics to CO2 and CH4.
• • Oil and grease (which includes anything that can dissolve in the solvent used

54. Figure 2–5: Primary settling tank
schematic

55. • Sludge digestors2.15
• 2.15The sludge which settles in the sedimentation
basin is pumped to the sludge digestors (see figure 2–
7) where a temperature of 30–35ºC is maintained. This
is the optimum temperature for the anaerobic bacteria
(bacteria that live in an environment that does not
contain oxygen). The usual length of digestion is 20–30
days but can be much longer during winter months.
Continual adding of raw sludge is necessary and only
well-digested sludge should be withdrawn, leaving
some ripe sludge in the digestor to acclimatise the
incoming raw sludge.

56. Figure 2–7: Sludge digestors

57. • Drying beds2.16
• 2.16Digested sludge is placed on drying beds
of sand (see figure 2–8) where the liquid may
evaporate or drain into the soil. The dried
sludge is a porous humus-like cake which can
be used as a fertiliser base.

59. • Trickling filters2.17
• 2.17The liquid effluent from the primary settling tank is passed to the secondary
part of the system where aerobic decomposition completes the stabilisation. For
this purpose, a trickling filter (see figures 2–9 and 2–10) is used.
• 2.18A trickling filter is a fixed bed, biological filter that operates under (mostly)
aerobic conditions. Pre-settled wastewater is ‘trickled’ or sprayed over the filter. As
the water migrates through the pores of the filter, organics are degraded by the
biomass covering the filter material.
• 2.19The Trickling Filter is filled with a high specific surface-area material such as
rocks, gravel, shredded PVC bottles, or special pre-formed filter-material. A
material with a specific surface area between 30 and 900m2/m3 is desirable. The
filter is usually 1–3 m deep but filters packed with lighter plastic filling can be up to
12 m deep. Pre-treatment is essential to prevent clogging and to ensure efficient
treatment. The pre-treated wastewater is ‘trickled’ over the surface of the filter.
Organisms that grow in a thin bio-film over the surface of the media oxidize the
organic load in the wastewater to carbon dioxide and water while generating new
biomass.

61. • Figure 2–9: Trickling filter
• The incoming wastewater is sprayed over the
filter with the use of a rotating sprinkler. In
this way, the filter media goes through cycles
of being dosed and exposed to air. However,
oxygen is depleted within the biomass and
the inner layers may be anoxic or anaerobic.

63. • circulate. Whenever it is available, crushed rock or gravel is the cheapest
option. The particles should be uniform such that 95 per cent of the
particles have a diameter between seven and 10 cm. Both ends of the
filter are ventilated to allow oxygen to travel the length of the filter. A
perforated slab that allows the effluent and excess sludge to be collected
supports the bottom of the filter.
• The bed consists of crushed rock or slag (1–2 m deep) through which the
sewage is allowed to percolate. The stones become coated with a
zoogloea film (a jelly-like growth of bacteria, fungi, algae, and protozoa),
and air circulates by convection currents through the bed. Most of the
biological action takes place in the upper 0.5 m of the bed. Depending on
the rate of flow and other factors, the slime will slough off the rocks at
periodic intervals or continuously, whenever it becomes too thick to be
retained on the stones. A secondary settling basin is necessary to clarify
the effluent from the trickling filter. The overall reduction of BOD for a
complete trickling filter system averages around 80–90 per cent.

64. • Secondary settling tank2.23
• 2.23With the majority of the suspended material removed
from the sewage, the liquid portion flows over a weir at the
surface of the secondary settling tank (see figure 2–11).
Chlorination of the effluent from the secondary settling
tank takes place in accordance with state and local laws.
Depending on the location most laws require that a free
available chlorine (FAC) residual (usually 0.2 mg/L) be
maintained after a 30-minute contact period. This contact
period is obtained through the use of chlorine contact
chambers which are designed to provide a 30-minute
detention time. From the chlorine contact chamber the
treated sewage is normally discharged into a receiving body
of water.

66. • Activated sludge system2.24
• 2.24Activated Sludge is a multi-chamber reactor unit that makes use of (mostly)
aerobic microorganisms to degrade organics in wastewater and to produce a
high-quality effluent. To maintain aerobic conditions and to the keep the active
biomass suspended, a constant and well-timed supply of oxygen is required.
Activated sludge systems (refer figures 2–12 and 2–13) normally make use of bar
screens and/or comminutors, grit chambers, primary settling tanks, secondary
settling tanks, and digesters, which are operated in the same manner as those of
trickling filter systems. They differ from the trickling filter systems in that they
make use of an aeration tank instead of a trickling filter.
• 2.25Different configurations of the Activated Sludge process can be employed to
ensure that the wastewater is mixed and aerated (with either air or pure oxygen)
in an aeration tank. The microorganisms oxidize the organic carbon in the
wastewater to produce new cells, carbon dioxide and water. Although aerobic
bacteria are the most common organisms, aerobic, anaerobic, and/or nitrifying
bacteria along with higher organisms can be present. The exact composition
depends on the reactor

67. • design, environment, and wastewater
characteristics. During aeration and mixing,
the bacteria form small clusters, or flocs.
When the aeration stops, the mixture is
transferred to a secondary clarifier where the
flocs are allowed to settle out and the
effluent moves on for further treatment or
discharge. The sludge is then recycled back to
the aeration tank, where the process is
repeated.

69. • 2.26Compressed air is continually diffused into the
sewage as it flows through the aeration tank. This
provides both a source of oxygen for the aerobic bacterial
floc that forms in the tank and the turbulence necessary
to bring the waste and the bacteria into contact. Aerobic
bacteria attack the dissolved and finely divided suspended
solids not removed by primary sedimentation. Some of
the floc is removed with the sewage that flows out of the
aeration tank and carried into the secondary settling tank.
Here the floc settles to the bottom of the tank, and is later
pumped back into the aeration tank. The liquid portion
then flows over a weir at the surface of the settling tank
to be chlorinated and released to a receiving stream.

71. • 2.27To achieve specific effluent goals for BOD,
nitrogen and phosphorus, different
adaptations and modifications have been
made to the basic Activated Sludge design.
Aerobic conditions, nutrient-specific
organisms (especially for phosphorus), recycle
design and carbon dosing, among others, have
successfully allowed Activated Sludge
processes to achieve high treatment
efficiencies.

72. • Rotating biological contactor system2.28
• 2.28Rotating biological contactor systems (refer figures 2–13, 2–14 and 2–15)
normally make use of bar screens and/or comminutors, grit chambers, primary
settling tanks, secondary tanks, and digesters, which are operated in the same
manner as those of trickling filter systems. The rotating biological contactor (RBC)
is a simple, effective method of providing secondary wastewater treatment. The
system consists of biomass media, usually plastic, that is partially immersed in the
wastewater. As it slowly rotates, it lifts a film of wastewater into the air. The
wastewater trickles down across the media and absorbs oxygen from the air. A
living biomass of bacteria, protozoa, and other simple organisms attaches and
grows on the biomass media. The organisms then remove both dissolved oxygen
and organic material from the trickling film of wastewater. Any excess biomass is
sloughed-off as the media is rotated through the wastewater. This prevents
clogging of the media surface and maintains a constant microorganism population.
The sloughed-off material is removed from the clear water by conventional
clarification. The RBC rotates at a speed of one to two rpm and provides a high
degree of organic removal.