This document discusses the treatment of municipal waste and industrial effluents through various biological processes. It describes the four main treatment processes: 1) preliminary treatment to remove solids, 2) primary treatment using sedimentation to remove settleable organic materials, 3) secondary or biological treatment using microorganisms to break down dissolved and suspended organic matter, and 4) tertiary or advanced treatment for additional removal of nutrients or contaminants. Key biological treatment methods discussed include activated sludge processes, trickling filters, and rotating biological contactors.

1. TREATMENT OF MUNICIPAL WASTE AND INDUSTRIAL EFFLUENTS
Dr. Naveen Gaurav
Associate Professor and Head
Department of Biotechnology
Shri Guru Ram Rai University
Dehradun

2. TREATMENT OF MUNICIPAL WASTE AND INDUSTRIAL EFFLUENTS
Biological treatment is an important and integral part of any wastewater treatment plant that
treats wastewater from either municipality or industry having soluble organic impurities or a
mix of the two types of wastewater sources.
The four processes are: (1) Preliminary Treatment (2) Primary Treatment (3) Secondary or
Biological Treatment and (4) Tertiary or Advanced Treatment
1. Preliminary Treatment:
As already stated, preliminary treatment involves the removal of floating materials (leaves,
papers, rags) and settleable inorganic solids (sand, grit), besides oily substances (fats, oils,
greases). The three major types of equipment-screeners, grit chambers, and skimming tanks,
employed in preliminary screening are briefly described.
A screener is a device with openings (usually uniform in size) to remove the floating
materials and suspended particles. The process of screening can be carried out by passing
sewage through different types of screeners (with different pore sizes).
The screeners are classified as coarse, medium or fine, depending on the size of the
openings. The coarse screen has larger openings (75-150 mm). The openings for medium and
fine screens respectively are 20-50 mm and less than 20 mm. Different types of screens-fixed
bar screen (coarse or medium) disc type fine screen, drum type fine screen are in use.
A diagrammatic representation of a fixed bar screen is shown in Fig. 57.1A.
A shredder or comminatory is a special screen that can cut and retain the floating and
suspended materials (Fig. 57.1B).

4. Grit Chambers:The heavy inorganic
materials (specific gravity 2.4-2.7) like sand,
ash and others can be removed by using
grit chambers. This technique is based on
the process of sedimentation due to
gravitational forces. Grit chambers may be
kept either before or after the screens. A
diagrammatic representation of a typical
grit chamber is depicted in Fig. 57.2A.
Skimming Tanks:
Several greasy and oily materials (fats, oils,
waxes, soaps etc.) from the domestic or
industrial outlets find their entry into the
sewage. They can be removed by using a
skimming tank which is fitted with baffle
walls that divide the tank (Fig. 57.2B). The
skimming tank is divided into three
compartments that are interconnected.
As the compressed air is pushed from the
floor of the tank, the raising air bubbles
coagulate and solidify the oily and greasy
materials present in the sewage. This
material is pushed to the side compartment
referred to as stilling compartment from
where it can be removed manually or
mechanically.

5. 2. Primary Treatment:
Primary treatment is aimed at the removal of fine suspended organic solids that cannot be
removed in the preliminary treatment. Primary treatment basically involves the process of
sedimentation or settling. In the normal process of sewage treatment, sedimentation is usually
carried out twice-once before the secondary treatment, referred to as primary sedimentation, and
then after the secondary treatment is complete, a process known as secondary sedimentation. It is
sometimes necessary to use chemical coagulants to facilitate or aid sedimentation, and this
process is referred to as chemical precipitation or coagulation-aided sedimentation.
Principle of Sedimentation:
The solid particle of the sewage tend to settle down due to gravity. However, most of the solid
particles of organic compounds remain in a suspended state in a flowing sewage. If the flow of the
sewage is stopped and if it is stored in a tank referred to as sedimentation tank, the solid particles
can settle down at the bottom. The process of sedimentation is influenced by several factors.
These include the size, shape and specific gravity of particles, besides viscosity and flow velocity of
sewage.
Types of Settling: There are four major types of settling—discrete settling, flocculent settling,
hindered or zone settling and compression. This categorization is mainly based on the tendency of
the particles to interact and form solids.
Discrete settling:The particles which do not change their size, shape and weight are referred to as
discrete particles or granular particles. The use of grit in sewage may be considered as an example
of discrete settling.
Flocculent settling: The flocculent particles can change their size, shape and weight, and thus lose
their identity. These particles actually coalesce during settling. Settling of bioflocs, and chemical .
floes in secondary sedimentation tanks are good examples of flocculent settling.

6. Hindered or zone settling: The particles as such, tend to remain in a fixed position with
respect to each other. When flocculated, the whole mass of particles settle as a unit or a
zone. In the hindered settling the concentration of particles increases from top to the bottom
and this results in the thickening of the sludge. Zone settling is employed in conjunction with
biological treatment facilities.
Compression:
Settlement of particles in the lower layers can occur by compression of the weight of the
particles on the upper layers. This process facilitates sludge thickening at the bottom.
Types of Sedimentation Tanks (Clarifiers):
There are different ways of classifying sedimentation tanks.
Based on the shape:
Rectangular, circular and square.
Based on the flow of sewage:
Longitudinal, vertical, radial and spiral.
Based on the purpose and position:
Primary, secondary, coagulation-cum-sedimentation tanks, grit chambers, septic and Imhoff
tanks.
Based on the operation:
Batch type and continuous flow type.
For the sake of illustration, some sedimentation tanks are depicted in Fig. 57.3.

8. Chemical-aided Sedimentation:
It is not always possible to remove the colloidal wastes in sewage by plain sedimentation.
However, addition of certain chemicals aids sedimentation, a process referred to as
chemical precipitation or chemical-aided sedimentation. By this technique, about 60-80%
of the suspended particles can be removed. Chemical precipitation involves three stages—
coagulation, flocculation and sedimentation.
Coagulation is mainly a chemical process wherein the charged particles are destabilized (by
the addition of chemical agents). On the other hand, flocculation involves the physical
phenomena of aggregating the destabilized particles to finally form settleable solids (i.e.
sedimentation). The chemicals used in chemical-aided sedimentation are of two types-
coagulants and coagulant aids.
Coagulants:
These are the chemicals (normally positively charged) which form insoluble and gelatinous
precipitates with colloidal particles (negatively charged ones present in sewage). The most
commonly used coagulants in sewage treatment are alum (alluminium sulfate) iron salts
(ferric sulfate, ferrous sulfate, ferric chloride), lime and soda ash (sodium carbonate),
sodium silicate and sodium aluminate.
Coagulant-aids:
These chemicals aid or facilitate the process of coagulation. This is brought out by en-
chancing the action of coagulants and reducing the amount of sludge formed. The common
coagulant aids are activated silica, weighting agents (e.g. powdered lime stone or silica) and
polyelectrolytes.

9. 3. Secondary or Biological Treatment:
Biological treatment of sewage is
required for the removal of dissolved
and fine colloidal organic matter. This
process involves the use of
microorganisms (bacteria, algae, fungi,
protozoa, rotifers, nematodes) that
decompose the unstable organic
matter to stable inorganic forms.
The biological treatment processes of
sewage are broadly classified as
aerobic, anaerobic and pond processes.
Depending on the nature of the use of
the microorganisms, the biological
processes are categorized as suspended
growth systems and attached growth
systems. A list of the major secondary
(biological) treatment processes is
given in Table 57.1, and the most
important ones are briefly described.

10. Aerobic Suspended-Growth Treatment Processes:
The most important suspended-growth biological treatment systems used for the removal
of organic matter are listed:
i. Activated sludge process
ii. Aerated lagoons
iii. Sequencing batch reactor
iv. Aerobic digestion.
Among these, activated sludge process is the most widely used for the secondary treatment
of sewage.
i. Activated Sludge Process:
The activated sludge process, first developed in England in 1914, continues to be the most
commonly used modern process for the biological treatment of sewage. In this method, the
sewage containing organic matter with the microorganisms is aerated (by a mechanical
aerator) in an aeration tank. The reactor contents are referred to as mixed liquor. Under
aerobic conditions, the microorganisms metabolize the soluble and suspended organic
matter. The generalized metabolic reaction is as follows.

11. A part of the organic matter is utilized for the synthesis of new bacterial cells while the
remaining gets oxidized to CO2 and H2O. The newly formed microorganisms are
agglomerated to form floes, technically referred to as sludge.
The separated sludge which is not in contact with organic matter becomes activated. It is
separated from the settling tank, and returned to the aeration tank, and recycled. The
activated sludge recycled in aeration tank serves as a seed or inoculum. The excess and
waste sludge can be removed.
For efficient operation of activated sludge process, it is necessary to maintain a constant
supply of O2 which can be done by mechanical aeration or through the use of rotating
paddles. Growth of protozoa in a sludge is an indication of its healthy condition. The disposal
of a waste sludge is a problem. It may be used as a fertilizer in crop lands or as landfills, after
drying.
Factors affecting performance:There are several factors that influence the efficiency of
activated sludge process, the most important being the type of the reactor, aeration, food
microorganism (F/M) ratio, nutrients, sludge recirculation rate, besides pH and temperature.
Advantages:The activated sludge process is a very compact, low-cost and an efficient
biological treatment system for sewage treatment. It is worked out that under ideal
conditions, up to 95% of BODs, 98% of bacteria (particularly coliform) and 95% of suspended
solids can removed by activated sludge process. The excess and waste sludge has a higher
fertilizer value compared to other treatment processes.
Disadvantages:There is production of large volumes of sludge which sometimes becomes
difficult to handle. Power consumption is relatively high for operation. Supervision by skilled
personnel is necessary.

12. Conventional activated sludge process:
In the normal treatment of sewage, the activated sludge is proceeded by primary
sedimentation tank. The conventional activated sludge system consists of a separation tank,
settling or sedimentation tank and sludge removal line (Fig. 57.4). The sewage after the
primary treatment is introduced at the head of the tank. It is desirable to supply
O2 uniformly throughout the tank.
Modified activated sludge processes:
For increasing the performance of the activated sludge system, several modifications have
been done in the recent years. Most of them are directed to bring out efficient aeration.
Aeration can be done by step aeration, tapered aeration, and high rate aeration by
complete mixing and extended aeration.

13. ii. Aerated Lagoons:
Aerated lagoons, also called as aerated ponds, are the facultative stabilization ponds
wherein surface aerators are installed to overcome the bad adours (due to overload of
organic materials). The microbiological treatment of aerated ponds is comparable to the
activated sludge process. The major difference is the large surface area in aerated ponds
and this is more susceptible for temperature effects. It is possible to carry out continuous
nitrification in aerated lagoons. This however, depends on the design and operating
conditions of the pond (particularly the temperature).
iii. Sequencing Batch Reactor:
Sequencing batch reactor (SBR) is a modification of activated sludge treatment system. The
processes namely aeration and sedimentation are carried out in both the systems. The
major difference is that while in the conventional activated sludge system, aeration and
sedimentation occur simultaneously in separate tanks, these two processes are carried out
sequentially in the same tank in SBR. Thus, the sequencing batch reactor may be regarded
as fill- and-draw activated sludge process.
The operating sequence of a typical SBR is depicted in Fig. 57.5. The process is carried out in
a sequence of five steps — filling, aeration (reacting) sedimentation (settling), decanting and
idle. Several modifications and improvements have been made in the SBR for more efficient
operation.

15. iv. Aerobic Digestion:
The organic sludge’s produced from various treatment processes (activated sludge
treatment, trickling filter-sludge) are subjected to aerobic digestion in special reactors
referred to as aerobic digesters.
Aerobic Attached — Growth Treatment Processes:
Aerobic attached-growth treatment processes are commonly used to remove the organic
matter found in the sewage. These processes are also useful for the nitrification
(conversion of ammonia to nitrate).
The commonly used attached-growth processes are listed:
a. Trickling filters
b. Roughing filters
c. Rotating biological contractors
d. Packed bed reactors.
e. Facultative pond
Among these, trickling filter is most widely used.
a. Trickling Filters:
Trickling filters, also known as percolating or sprinkling filters, are commonly used for the
biological treatment of domestic sewage and industrial waste water. In a strict sense,
trickling filters are not filters, but they are oxidation units.
A diagrammatic representation of trickling filter is depicted in Fig. 57.6. It has a bed of
course, hard and porous material over which sewage is sprayed. In about two weeks time,
the biomass attached to the media surface grows and forms a layer, referred to as
biological film or microbial slime.

17. This film has a thickness of 0.1 to 2.0 mm and is rich in microorganisms. As the liquid
(sewage) trickles through the biofilm, the organic matter gets oxidized to CO2 and NO2 by
the microbial metabolism. This oxidation is carried out by the aerobic organisms
(particularly bacteria) that are present on the upper portion of the biological film.
The biological film is rich in the bacteria- Pseudomonas, Flavobacterium, Alcaligenes, and
algae-Chlorella, Utothrix, and Stigeoclonium, besides some fungi and yeasts. Biofilms with a
thickness in the range of 70-100µm are efficient for the treatment process.
As the biofilm ages, its thickness increases and it automatically settles to the bottom of the
tank. The waste waters obtained in milk processing, paper mills and pharmaceutical
industries are treated by trickling filters.
Types of trickling filters: The trickling filters are classified as low rate (the conventional one),
high rate and super rate. This categorization is mainly based on the hydraulic and organic
loading rates of the sewage. The low rate filters are suitable for the treatment of domestic
sewage, while high rate filters and super rate filters are useful for industrial sewage.
Factors affecting performance of trickling filters: The type of the media and its depth,
organic and hydraulic loading, filter staging, recirculation rate and flow distribution are the
important factors that influence the performance of the trickling filters.
Advantages: Trickling filters are simple, occupy less space and the operating costs are low.
They operate efficiently in hot climate and thus are suitable for most developing countries
(like India).
Disadvantages:Removal of BOD is moderate (around 70%), and disposal of excess sludge is
necessary. Primary sedimentation is required, since trickling filters cannot handle raw
sewage.

18. b. Roughing Filters:
Roughing filters are a special type of trickling filters that are designed to operate at high
hydraulic loading rates. These filters are mostly used to “reduce the organic matter in
downstream processing, besides nitrification applications. Due to high hydraulic loading,
there is a continuous sloughing of the biological film. In such a case, the unsettled filter
effluent can be recycled, and this increases the efficiency of the treatment process. The
retention time on the biofilm being less, organic materials that are not readily degradable
remain unaffected.
c. Rotating Biological Contactors (RBC):
Rotating biological contactor (RBC) is a recent device for the biological treatment of
sewage. It operates on the principle of aerobic attached- growth system operated on the
moving media. RBC are suitable for the treatment of domestic and industrial sewage in
small and medium towns.
A rotating biological contactor is composed of a series of closely spaced and light weight
circular discs (Fig. 57.7). They are made up of inert materials such s polystyrene or polyvinyl
chloride (PVC) or polyethylene. These discs are mounted on a horizontal shaft in a tank
through which waste water flows. The shaft is rotated slowly (less than 10 revolutions per
minute) by a low speed motor.
The discs of the shaft, referred to as bio discs, are partially (40-60%) submerged in sewage.
As the bio discs are rotated, the biomass attached to them is alternately submerged in
sewage. This enables the discs to pick up a thin layer of sewage, and then to oxidize the
absorbed substrates. The un-oxidized substrates fall back into the sewage. And this process
in repeated again and again by the rotating bio discs.

20. Rotating biological contractor is basically a film flow bioreactor. The microbial biofilm is
built upon the partly submerged support medium containing bio discs. The RBC is very
efficient for the removal of organic matter in the sewage (about 90% of BOD). As and when
there is an excess growth of biomass, it has to be removed. This process can be carried in a
similar fashion, as it is done for the trickling filter.
RBC are commonly used for the treatment of municipal waste water. In addition, they are
widely used for the biological processing of industrial wastes, coming from several
industries such as vegetables, pulp, meat and textiles.
Factors affecting performance of RBC:
The treatment process in RBC is influenced by rotation speed of shaft, waste water
retention time, temperature, disc submergence, organic loading, media density, and
number of stages.
Advantages:
RBC is compact and requires moderate energy input. It has high BOD removal efficiency.
Disadvantages:
Disposal of sludge formed is a major problem with RBC. The treatment process is
frequently associated with odour formation. RBC operation requires skilled personnel.
d. Packed-Bed Reactors:
Packed-bed reactors or fluidized bed reactors are used for the removal of BOD and
nitrification. A reactor is packed with a medium to which the microorganisms get attached
and form biofilms. The sewage along with air (or pure oxygen) is introduced from the
bottom of the reactor (Fig. 57.8). The main advantage with packed bed reactors is that they
have high surface area of biofilms for unit of reactor.

22. e. Facultative Ponds:
In facultative ponds, the treatment of sewage is carried out by a combination of both aerobic and
anaerobic processes. Three types of microorganisms aerobic, anaerobic and facultative (both aerobic and
anaerobic) are employed in facultative ponds.
The term oxidation pond or stabilization pond is frequently used for facultative ponds.
A diagrammatic view of a facultative pond is depicted in Fig. 57.12. It consists of three zones.
1. The surface aerobic zone: It has the aerobic bacteria and algae, existing in a symbiotic relation
2. The bottom anaerobic zone: It contains anaerobic bacteria and solids that undergo decomposition.
3. Intermediate facultative zone: This zone is partly aerobic and partly anaerobic and contains both
types (aerobic and anaerobic) of bacteria.
Processes that occur in facultative ponds:
The sewage organic matter is stabilized by both aerobic and anaerobic processes. The algae present in
the aerobic zone carry out photosynthesis and release O2. This oxygen is utilized by the aerobic and
facultative bacteria to oxidize soluble and colloidal organic matter.
The organic solids present in the anaerobic zone (bottom sludge) are degraded to dissolved organic
compounds (organic acids) and gases such as CO2, CH4 and H2S. The organic acids can be oxidized by the
aerobic bacteria while the gases produced (CO2, CH4, H2S, NH3) may be vented to the atmosphere. In
fact, most of CO2 is utilized by the algae for photosynthesis, while H2S combines with O2 to form sulfuric
acid.
H2S + 2O2 → H2SO4
In the absence of adequate O2 in the upper layers of the pond, gases with unpleasant and foul odours
(H2S) are vented to the atmosphere. These foul smells often cause nuisance to the surroundings.
It is generally possible to maintain good O2 supply by the algae and facultative bacteria. Sometimes,
surface aerators are used to enhance the efficiency of facultative ponds (particularly when the sewage
contains high organic content).

23. Advantages: For the facultative ponds, the initial and operating costs are low. There is no
need for skilled personnel.
Disadvantages: Unpleasant odours and mosquito breeding are frequently seen in
facultative ponds. The requirement of land area for construction of these ponds is more.

24. Thank you
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