Chapter six
Sewage Treatment:
6.1 Introduction
Wastewater treatment is a process to treat sewage or wastewater to remove suspended solid
contaminants and convert them into an effluent that can be discharged back to the environment
with acceptable impact. The plants where the wastewater treatment process takes place are
popularly known as Wastewater treatment plants, Water resource recovery facilities, or Sewage
Treatment Plants. Pollutants present in wastewater can negatively impact the environment and
human health. So, these must be removed, broken down, or converted during the treatment
process.
6.2 Type and quantity of sewage
A number of activities help in the formation of wastewater.
Domestic wastewater is generated because of activities like bathing, washing, using the toilet, etc
in residences, restaurants, and businesses.
Surface rainwater runoff is generated due to the mixing of debris, grit, nutrients, and various
chemicals. Industrial wastewater results because of chemical and manufacturing industry
discharges. So, wastewater is essentially the used water that has been affected by domestic,
commercial, or industrial use.
Domestic wastewater is relatively easy to treat as compared to industrial wastewater due
to its high-strength nature.
The wastewater originating from various sources can be broadly divided into two categories:
1. Biodegradable wastewater
The wastes in general have a predominance of biodegradable organic matter, and are generally
treated in a similar manner. The stabilization of organic matter is accomplished biologically
using a variety of microorganisms. The microorganisms are used to convert the colloidal and
dissolved carbonaceous organic matter into various gases and into cell tissue.
The general term that describes all of the chemical activities performed by a bacterial cell is
metabolism which is divided into catabolism and anabolism. Catabolism includes all the
biochemical processes by which a substrate (food) is degraded to end products with the release
of energy. Anabolism includes all the biochemical processes by which the bacterium synthesizes
new cells.
2. Non-biodegradable wastewater
The non-biological wastes in general and the wastewater in particular are rich in non-
biodegradable matter consisting of solids and liquids in suspended or dissolved form, including
various inorganic and organic, many of which may be highly toxic. Examples are domestic or
industrial wastewater containing excessive dissolved solids (minerals), inorganic or organic
compounds or naturally occurring organics such as humic and fulvic acids.
Objectives of Wastewater Treatment
1. To introduce fundamentals of the wastewater treatment plants and their unit operations and
processes
2. To provide basic design skills and knowledge on the wastewater treatment plants and their unit
operations and processes
3. To experience a design project on a hypothetical wastewater treatment plant
a. Identify kinds and sources of wastewater
b. Describe hazards in wastewater
c. Describe ways of treating wastewater
d. Describe the products of wastewater treatment, including the production and use of biosolids
Wastewater Treatment Standards
Effluents from different establishments should be treated before being discharged to receiving
bodies so that it should be:
1. Free from materials and heat in quantities, concentrations or combinations which are toxic or
harmful to human, animal, aquatic life.
2. Free from anything that will settle in receiving waters forming putrescence or otherwise
objectionable sludge deposits, or that will adversely affect aquatic life.
3. Free from floating debris, oil, scum and other materials in amounts sufficient to be noticeable
in receiving waters;
4. Free from materials and heat that alone, or in combination with other materials will produce
color, turbidity, taste or odour in sufficient concentration to create a nuisance or adversely affect
aquatic life in receiving waters;
5. Free from nutrients in concentrations that create nuisance growths of aquatic weeds or algae in
the receiving waters.
Characteristics of wastewater – Assignment
Contaminants in wastewaters are usually a complex mixture of organic and inorganic
compounds. It is usually impractical, if not nearly impossible to obtain complete chemical
analysis of most wastewaters. However, since it is comparatively easy to measure the amount of
oxygen used by the bacteria as they oxidize the wastewater, the concentration of organic matter
in the wastewater can easily be expressed in terms of the amount of oxygen required for its
oxidation. The most important standard methods for analysis of organic contaminants are:
a. Theoretical Oxygen Demand (ThOD)
This is the theoretical amount of oxygen required to oxidize the organic fraction of the
wastewater completely to carbon dioxide and water.
b. Chemical Oxygen Demand (COD)
The chemical oxygen demand (COD) of a raw water or a wastewater is determined by
performing a laboratory test on the given wastewater with a strong oxidant like dichromate
solution; and the theoretical computations of COD are only performed on water solutions
prepared with the known amounts of specific organic compounds in laboratory situations to
compare the theoretical and test results, and to establish the limitations of the test procedures.
The advantage of COD measurements is that they are obtained very quickly (within 3 hours), but
they have the disadvantages that they do not give any information on the proportion of the
wastewater that can be oxidized by bacteria, nor on the rate at which bio-oxidation occurs.
c. Biochemical Oxygen Demand (BOD)
Oxygen demand of wastewaters is exerted by three classes of materials:
(1) Carbonaceous organic materials usable as a source of food by aerobic organisms
(2) oxidizable nitrogen derived from nitrite, ammonia, and organic nitrogen compounds which
serve as food for specific bacteria (e.g., Nitrosomonas and Nitrobacter).
(3) Chemical reducing compounds, e.g., ferrous ion (Fe2+), sulfites (SO32-), and sulfide (S2-)
which are oxidized by dissolved oxygen.
For domestic sewage, nearly all oxygen demand is due to carbonaceous organic materials and is
determined by BOD dilution test. For effluents subjected to biological treatment, a considerable
part of the oxygen demand may be due to nitrification.
6.3 Sewage Treatment
Preliminary and primary wastewater treatment methods
6.3.1 Preliminary Treatment
Preliminary treatment consists solely in separating the floating materials (like dead animals, tree
branches, papers, pieces of rags, wood, etc.), and also the heavy settleable inorganic solids. It
also helps in removing the oils and greases, etc. from the sewage. This treatment reduces the
BOD of the wastewater, by about 15 to 30%. The processes used are: - Screening for removing
floating papers, rags, clothes, etc - Grit chambers or Detritus tanks for removing grit and sand;
and - Skimming tanks for removing oils and greases
Screening
Screening is the very first operation carried out at a sewage treatment plant, and consists of
passing the sewage through different types of screens, so as to trap and remove the floating
matter, such as pieces of cloth, paper, wood, cork, hair, fiber, kitchen refuse, fecal solids, etc.
present in sewage. These floating materials, if not removed, will choke the pipes, or adversely
affect the working of the sewage pumps. Thus, the main idea of providing screens is to protect
the pumps and other equipments from the possible damages due to the floating matter of the
sewage. Screens should preferably be placed before the grit chambers (described in the next
article). However, if the quality of 'grit' is not of much importance, as in the case of land fillings,
etc., screens may even be placed after the grit chambers.
Comminutors
Comminutors or Shredders are the patented devices, which break the larger sewage solids to
about 6mm in size, when the sewage is screened through them. Comminutors are of recent
origin, and eliminate the problem of disposal of screenings, by reducing the solids to a size
which can be processed elsewhere in the plant. Such devises are used only in developed
countries like, and generally not adopted in our country
Disposal of Screenings
The material separated by screens is called the screenings. It contains 85 to 90% of moisture and
other floating matter. It may also contain some organic load which may putrefy, causing bad
smells and nuisance.
a. Incinerators; burning of the screenings is done in the incinerators, similar to those used
for burning garbage. The process of burning is called Incineration. The screenings are
first dried with sun's heat by spreading on ground or by compressing through hydraulic or
other presses, so as to reduce the moisture content to about 60%. The incineration is
carried out at temperatures of about 760 to 815°c. This will avoid bad smells.
b. Composting the screenings are buried in 1 to 1.5m deep trenches, and then covered with
0.3 to 0.45m of porous earth. In due course of time, oxidation-reduction of screenings
will take place, and the contents can be used as manure.
c. Dumping them in low lying areas (away from the residential areas) or in large bodies of
water, such as sea. Dumping in sea will be suitable only where strong forward currents do
exist to take the dumped material away from the shore line. The dumping on land for
raising low lying areas is also adopted only when screenings are from the course screens
and not from the medium or fine screens, and as such not containing much organic load.
Grit Removal Basins
Grit removal basins, such as Grit chambers or Grit channels or Detritus tanks are the
sedimentation basins placed in front of the wastewater treatment plant. The grit chamber remove
the inorganic particles (specific gravity about 2.65 and nominal diameter of 0.15 to 0.20mm or
larger) such as sand, gravel, grit, egg shells, bones, and other non-putresible materials that may
clog channels or damage pumps due to abrasion, and to prevent their accumulation in sludge
digesters.
Tanks for Removing Oils and Grease
1. Skimming Tanks
Skimming tanks are sometimes employed for removing oils and grease from the sewage, and
placed before the sedimentation-tanks. They are, therefore, used where sewage contains too
much of grease or oils which include fats, waxes, soaps, fatty acids, etc. These materials may
enter into the sewage from the kitchens of restaurants and, houses, from motor garages, oil
refineries, soap and candle factories, etc. They are, thus, normally present in large amounts in the
industrial wastewaters.
These oil and greasy materials may be removed in a skimming tank, in which air is blown by an
aerating device through the bottom
Disposal of Skimming’s
The oil and greasy material removed as skimmings from the skimming tanks can be disposed of
either by burning or burial.
6.3.2 Primary Wastewater Treatment
Sedimentation
The sedimentation tanks are thus designed to remove a part of the organic matter from the
sewage effluent coming out from the grit chambers.
In a complete sewage treatment, the sedimentation is, in fact, carried out twice; once before the
biological treatment (i.e. primary sedimentation) and once after the biological treatment (i.e.
secondary sedimentation). When chemical coagulants are also used for flocculating the organic
matter during the process of sedimentation, the process is called chemical precipitation or
sedimentation aided with coagulation. This is generally not used in modern days.
Sedimentation basins are thus designed for effecting settlement of particles by reducing the flow
velocity or by detaining the sewage in them. They are generally made of reinforced concrete and
may be rectangular or circular in plan.
Sedimentation Aided with Coagulation (Type II Sedimentation)
1. Chemical Precipitation and Coagulation
Very fine suspend particles, present in wastewaters, which cannot be removed in plain
sedimentation, may sometimes, be settled by increasing their size be changing them into
flocculated particles. For this purpose, certain chemical compounds (like ferric chloride, ferric
sulphate, alum, chlorinated copperas, etc.) called coagulants are added to the wastewaters, which
on thorough mixing form a gelatinous precipitate called floc.
Merits and Demerits of Coagulation Process in Sewage Treatment
As pointed out earlier, the coagulation process is generally not adopted in modern sewage
treatment plants, mainly because of the following reasons:
1. More advanced methods of sewage treatment based on biological actions are available these
days, and they are preferred to coagulation.
2. The coagulation process has various disadvantages, such as discussed below:
(i) The biological secondary treatments used these days for treating sewage is complete in
themselves, and do not require coagulation. Moreover, coagulation rather makes some of these
processes more difficult.
(ii) The chemicals used in coagulation react with sewage, and during these reactions, they
destroy certain micro-organisms, which are helpful in digestion of the sludge, thus creating
difficulties in sludge digestion.
(iii) Cost of chemicals is added to the cost of sedimentation, without much use, and thereby
making the treatment costlier.
(iv) The process of coagulation and subsequent sedimentation produces larger quantities of
sludge than that produced in plain sedimentation, and thus adding to the problems of sludge
disposal.
(v) The process of coagulation requires skilled supervision and handling of chemicals. In view
of all these disadvantages, the coagulation of sewage has become obsolete these days. It may
still, however, be adopted in certain special cases, such as:
(a) For treating sewage from industries, using some specific chemicals in their processes.
(b) It is particularly advantageous, where there is large seasonal variation in sewage flow or as an
emergency measure to increase the capacity of an overloaded plain sedimentation tank.
6.3.3 Secondary/biological and tertiary wastewater treatment
Micro-organisms, such as bacteria, play an important role in the natural cycling of materials and
particularly in the decomposition of organic wastes. The role of micro-organisms is elaborated
further here because they are also important in the treatment of wastewater.
Biological Wastewater Treatment
Types of Biological Process for Wastewater Treatment
The common methods of biological wastewater treatment are:
a) Aerobic processes such as trickling filters, rotating biological contactors, activated sludge
process, oxidation ponds and lagoons, oxidation ditches,
b) Anaerobic processes such as anaerobic digestion and anaerobic lagoons
c) Anoxic processes such as denitrification
Trickling Filters
The conventional trickling filters and their improved forms, known as high rate trickling filters
are now almost universally adopted for giving secondary treatment to sewage. These filters, also
called as percolating filters or sprinkling filters, consist of tanks of coarser filtering media, over
which the sewage is allowed to sprinkle or trickle down, by means of spray nozzles or rotary
distributors. The percolating sewage is collected at the bottom of the tank through a well
designed under-drainage system. The purification of the sewage is brought about mainly by the
aerobic bacteria, which form a bacterial film around the particles of the filtering media. The
action due to the mechanical straining of the filter bed is much less. In order to ensure the large
scale growth of the aerobic bacteria, sufficient quantity of oxygen is supplied by providing
suitable ventilation facilities in the body of the filter; and also to some extent by the intermittent
functioning of the filter. The effluent obtained from the filter must be taken to the secondary
sedimentation tank for settling out the organic matter oxidized while passing down the filter. The
sewage influent entering the filter must be given pre-treatments including screening and primary
sedimentation.
Merits and Demerits of trickling filters
The various advantages of the trickling filters are:
(i) Rate of filter loading is high, as such requiring lesser land areas and smaller quantities
of filter .media for their installations.
(ii) Effluent obtained from the trickling filters is sufficiently nitrified and stabilized. They
can remove about 75% of BOD and about 80% of suspended solids. The effluent can,
therefore be easily disposed of in smaller quantity of dilution water.
(iii) Working of tricking filters is simple, and does not require any skilled supervision.
(iv) They are flexible in operation, and they can, therefore, withstand the application of
variety of sewages having different concentrations and compositions. Even if they are
overloaded, they can recoupe after rest.
(v) They are self-cleaning.
(vi) Mechanical wear and tear is small, as they contain less-mechanical equipment.
(vii) Moisture content of sludge, obtained from trickling filters, is as high ~ 99% or so.
(viii) Trickling filters have been found to operate more efficiently in warm weather, and
produce an effluent appreciably lower in BOD. Hence, they are of immense-use in
hot countries.
The disadvantages of the trickling filters are:
(i) The head loss through these filters is high making automatic dosing of the filters
necessary (through siphonic dosing tanks). The total head loss through such a filter
will be equal to the head loss through the circular distributor [about 45 to 58 cm] plus
the loss in the feed piping and in the siphon as well as the entrance loss. Generally,
these extra losses will total to at least 15 cm.
(ii) Cost of construction of trickling filters is high.
(iii) These filters cannot treat raw sewage, and primary sedimentation is a must.
(iv) These filters pose a number of operational troubles such as given below:
Fly nuisance, Odour nuisance, Ponding trouble
Activated Sludge Process
The activated sludge process provides an excellent method of treating either raw sewage or more
generally the settled sewage. The sewage effluent from primary sedimentation tank, which is,
thus normally utilized in this process is mixed with 20 to 30 percent of own volume of activated
sludge which contains a large concentration of highly active aerobic micro-organisms. The
microorganisms utilize the absorbed organic matter as a carbon and energy source for cell
growth and convert it to cell tissue, water, and oxidized products (mainly carbon dioxide, CO2).
Some bacteria attack the original complex substance to produce simple compounds as their waste
products. Other bacteria then use these waste products to produce simpler compounds until the
food is used up.
The effluent obtained from a properly operated activated sludge plant is of high quality usually
having a lower BOD than that of a trickling filter plant. BOD removal is up to 80 - 95 percent,
and bacteria removal is up to 90 - 95 percent.
There is the problem of obtaining activated sludge at the start of a new plant. Hence, when a new
plant is put into operation a period of about 4 weeks may be required to form a suitable return
sludge and during this period almost all the sludge from the secondary sedimentation tank will be
returned through the aeration tank. A new plant may also sometimes be seeded with the activated
sludge from another plant so as to quickly start the process in the new plant.
Methods of Aeration: There are two basic methods of introducing air into the aeration tanks,
(1) Diffused air aeration or Air diffusion
(2) Mechanical aeration
Activated Sludge Process Vs Trickling Filter Process
As discussed earlier, the conventional biological or secondary treatment of sewage is usually
carried out, either by using trickling filters or by using an activated sludge process. The basic
difference between an activated sludge process and the action involved in a trickling filter is that;
whereas in a trickling filter, the bacterial film coating the grains of the filter media is stationary
and likely to become clogged after sometime; in the activated sludge process, on the other hand,
the finer suspended organic particles of sewage (settled as activated sludge) are themselves
coated with the bacterial film, which is kept moving by the constant agitation. In the activated
sludge process, therefore, the sludge floc are coated with bacteria, and they act like free moving
organisms, which are being continuously swept through the sewage, and which in their search for
food and work, oxidize the organic matter present in sewage in a much more efficient way than
that carried out in a filter by the bacteria coated around the particles of filter media. As such, it
can be stated that an activated sludge process is more efficient than a trickling filter. The quality
of the effluent obtained in a conventional activated sludge plant is also better than that of a
trickling filter plant. But since a conventional activated sludge plant requires a lot of skilled
attendance and supervision during its operation, the modified activated sludge processes are
generally used these days.
Waste Stabilization Pond
WSP are considered as the most effective and appropriate method of wastewater treatment in
warm climates where sufficient land is available and where the temperature is most favorable for
their operation (Mara, 1976). WSP are employed for treatment of a range of wastewaters, from
domestic wastewater to complex industrial wastes. The design of WSP depends on the treatment
objectives. It may be designed to receive untreated domestic or industrial wastes, to treat primary
or secondary treatment plant effluents, excess activated sludge. Anaerobic, facultative and
maturation ponds are the three major types of pond in a WSP system. These ponds are normally
arranged in series to achieve effective treatment of raw wastewater (Marais, 1974). Anaerobic
and facultative ponds are employed for BOD removal, while maturation ponds remove excreted
pathogens.
Advantages and Disadvantages of Waste Stabilization Ponds
a. Simplicity
WSP are simple to construct: earthmoving is the principal activity; other civil works are minimal
– preliminary treatment, Inlets and outlets, pond embankment protection and, if necessary, lining
pond. They are also simple to operate and maintain: routine tasks comprise cutting the
embankment grass, removing scum and any floating vegetation from the pond surface, keeping
the inlets and outlets clear, and repairing any damage to the embankments, only unskilled, but
carefully supervised, labor needed for pond O&M.
b. Low cost
Because of their simplicity, WSP are much cheaper than other is wastewater treatment processes.
There is no need for expensive, electromechanical equipment (which requires regular skilled
maintenance), nor for a high annual consumption of electrical energy.
c. High Efficiency
BOD removals > 90 percent readily obtained in a series of well-designed ponds. The removal of
suspended solids is less, due To the presence of algae in the final effluent. Total nitrogen
removal is 70 - 90 percent, and total phosphorus removals 30 - 45 percent. WSP are particularly
efficient in removing excreted pathogens, whereas in contrast all other treatment processes are
very inefficient in this, and require a tertiary treatment process such as chlorination (with all its
inherent operational and environmental problems) to achieve the destruction of fecal bacteria.
Disadvantages of Waste Stabilization Ponds
The major disadvantage of WSPs is the large area that is required (2 - 5 m2/capita), the
potentially high algal content of the effluent, evaporation losses, the potential odour and
mosquito nuisance and the sensitivity of algae to toxic matter present in raw municipal sewage.
Tertiary Treatment Processes
Nutrient removal
Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release
to the environment can lead to a buildup of nutrients, called eutrophication, which can in turn
encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause
an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable
and eventually most of them die.
Nitrogen removal
The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia
to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas.
Nitrogen gas is released to the atmosphere and thus removed from the water.
Phosphorus removal
Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water
systems. (For a description of the negative effects of algae, see Nutrient removal). It is also
particularly important for water reuse systems where high phosphorus concentrations may lead to
fouling of downstream equipment such as reverse osmosis. Phosphorus can be removed
biologically in a process called enhanced biological phosphorus removal. In this process, specific
bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and
accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass).
When the biomass enriched in these bacteria is separated from the treated water, these biosolids
have a high fertilizer value.
Disinfection
The purpose of disinfection in the treatment of waste water is to substantially reduce the number
of microorganisms in the water to be discharged back into the environment. The effectiveness of
disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type
of disinfection being used, the disinfectant dosage (concentration and time), and other
environmental variables.
6.4 Sewage effluent disposal techniques
Disposal by Dilution and Oxygen Sag Curve
Disposal by dilution is the process whereby the treated sewage or the effluent from the sewage
treatment plant is discharged into a river stream, or a large body of water, such as a lake or sea.
The discharged sewage, in due course of time, is purified by what is known as self purification
process of natural waters. The degree and amount of treatment given to raw sewage before
disposing it off into the river-stream in question, will definitely depend not only upon the quality
of raw sewage but also upon the self purification capacity of the river stream and the intended
use of its water.
Land Disposal and Treatment
In this method, the sewage effluent (treated or diluted) is generally disposed of by applying it on
land. The percolating water may either join the water-table, or is collected below by a system of
under drains. This method can then be used for irrigating crops. This method, in addition to
disposing of the sewage, may help in increasing crop yields (by 33% or so) as the sewage
generally contains a lot of fertilizing minerals and other elements. However, the sewage effluent
before being used as irrigation water must be made safe
Effluent standards for Irrigation under Environment (Protection) Rules, 1986
6.5 Agricultural run-off and solid waste removal
Water quality caused by agricultural activities
A. Sedimentation; the most prevalent source of agricultural water pollution is soil that is
washed off fields. Rain water carries soil particles (sediment) and dumps them into
nearby lakes or streams. Too much sediment can cloud the water, reducing the amount of
sunlight that reaches aquatic plants. It can also clog the gills of fish or smother fish larvae.
Farmers and ranchers can reduce erosion and sedimentation by 20 to 90 percent by
applying management practices that control the volume and flow rate of runoff water,
keep the soil in place, and reduce soil transport.
B. Nutrients; Farmers apply nutrients such as phosphorus, nitrogen, and potassium in the
form of chemical fertilizers, manure, and sludge. They may also grow legumes and leave
crop residues to enhance production. When these sources exceed plant needs, or are
applied just before it rains, nutrients can wash into aquatic ecosystems. There they can
cause algae blooms, which can ruin swimming and boating opportunities, create foul taste
and odor in drinking water, and kill fish by removing oxygen from the water.
To combat nutrient losses, farmers can implement nutrient management plans that help
maintain high yields and save money on fertilizers.
C. Livestock Grazing; Overgrazing exposes soils, increases erosion, encourages invasion by
undesirable plants, destroys fish habitat, and may destroy stream banks and floodplain
vegetation necessary for habitat and water quality filtration. To reduce the impacts of
grazing on water quality, farmers and ranchers can adjust grazing intensity, keep
livestock out of sensitive areas, provide alternative sources of water and shade, and
promote vegetation of ranges, pastures, and riparian zones.
D. Irrigation; water is applied to supplement natural precipitation or to protect crops against
freezing or wilting. Inefficient irrigation can cause water quality problems. Farmers can
reduce NPS pollution from irrigation by improving water use efficiency. They can
measure actual crop needs and apply only the amount of water required. Farmers may
also choose to convert irrigation systems to higher efficiency equipment.
E. Pesticides; Insecticides, herbicides, and fungicides are used to kill agricultural pests.
These chemicals can enter and contaminate water through direct application, runoff, and
atmospheric deposition. They can poison fish and wildlife, contaminate food sources, and
destroy the habitat that animals use for protective cover. To reduce contamination from
pesticides, farmers should use Integrated Pest Management (IPM) techniques based on
the specific soils, climate, pest history, and crop conditions for a particular field. IPM
encourages natural barriers and limits pesticide use and manages necessary applications
to minimize pesticide movement from the field.
