Manjeet

A SEMINAR REPORT

ON

STUDY OF WASTE WATER TREATMENT

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGY (CIVIL ENGINEERING)

SUBMITTED BY SUBMITTED TO

Sunil Sharma Er. Pawan Kashyap (7109762) Civil Engineering Department

January - June2013

R.P.I.I.T. TECHNICAL CAMPUS, BASTARA, KARNAL

Affiliated to

KURUKSHETRA UNIVERSITY KURUKSHETRA

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ACKNOWLEDGEMENT

Words are inadequate and out of place at times particularly in the context of expressing sincere feeling in

the contribution of this work, is no more than a mere rituals. It is our privilege to acknowledge with

respect & gratitude, the keen valuable and ever-available guidance rendered to us by Er. Pawan

Kashyap (Lecturer) , Civil Engineering Department, R.P.I.I.T. Technical Campus (Bastara)

Karnal, Haryana, India without the wise counsel and able guidance, it would have been impossible to

complete the work in this manner.

We would like to place on record my deep sense of gratitude to Dr. Sorabh Gupta (Director), Dr. G.S.

Shrama (Executive Director), Dr. Harish Abhichandani (Additional Director), Dr. S.L. Verma,

Head of Civil Engineering Department, R.P.I.I.T. Technical Campus (Bastara) Karnal, Haryana,

India, for providing us infrastructural facilities to work in, without which this work would not have been

possible.

We express gratitude to Teaching & Non-Teaching Staff of Civil Engineering Department

R.P.I.I.T. Technical Campus, Bastara (Karnal) Haryana, India, for their intellectual support

throughout the course of this work.

At last but not least we are also thankful to our parents & family member to support us

financially & morally.

Above all we are thankful to the almighty god for giving strength to carry out the present work.

Sunil Sharma(7109762)

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CERTIFICATE

We hereby certify that the work which is being presented in the Seminar Report entitled

“Study of Waste Water Treatment”, in partial fulfillment of the requirements for the award of

the degree of Bachelor of Technology in Civil Engineering and submitted to the Department

of Civil Engineering of R.P.I.I.T. Technical Campus (Bastara) Karnal, Haryana, India is

an authentic record of our own work carried out during a period from January 2013 to June

2013 (8th semester) under the supervision of Er. Pawan Kashyap (Lecturer), CIVIL

ENGINEERING DEPARTMENT, R.P.I.I.T. TECHNICAL CAMPUS (BASTARA) KARNAL,

HARYANA, INDIA.

The matter presented in this Project Report has not been submitted by us for the award of

any other degree elsewhere.

Sunil Sharma(7109762)

This is to certify that the above statement made by the students is correct to the best of our

knowledge.

Er. PAWAN KASHYAP

Civil Engineering Department.

Seminar viva of Mr. Sunil Sharma Roll No. (7109762) is held today 08/05/2013 for partial fulfillment of the requirements the award of the degree of bachelor of technology and accepted.

Dr. S.L. Verma

Head.Civil Engineering Department

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TABLE OF CONTENTS

Title PageNo.

Acknowledgement 2

Certificate 3

List of figures 6

1: INTRODUCTION 7-8

1.1 General

1.2 System of sanitation

1.3 Need of waste water treatment

1.4 Types of sewage system

1.4.1 Combined sewage system

1.4.2 Seprate sewage system

2: TREATMENT OF SEWAGE 9

2.1 Classification of waste water treatment

3: PRELIMINARY TREATMENT OF SEWAGE 10-17

3.1 Screening

3.2 Types of Screen

3.2.1 Coarse screen

3.2.2 Medium screen

3.2.3 Fine screen

3.3 Grit Chamber

4

3.4 Skimming tank

3.5 Pre-aeration tank

4: PRIMARY TREATMENT OF SEWAGE 18-23

4.1 Septic tank

4.2 Imhoff tank

5: SECONDARY TREATMENT OF SEWAGE 24-31

5.1 Trickling filter

5.2 UASB reactor

6: TERTIARY TREATMENT OF SEWAGE 32

6.1 Tertiary Treatment

7: DISPOSING OF SEWAGE EFFLUENT 33-34

7.1 Disposing of sewage effluent

7.1.1 Disposal by dilution

7.2 Effluent Irrigation

7.2.1 Sewage farming

5

LIST OF FIGURES

Figure Title Page

1.4.1 Combined sewage system 8

1.4.2 Seprate sewage system 8

2.1 Schematic of a typical wastewater treatment plant 9

3.2.1 Coarse screen 11

3.2.2 Manually racked screen 12

3.3 Two chamber aerated grit chamber 15

3.4 Skimming Tank 16

4.1 Septic Tank 19

4.1.1 Wastewater comes into the septic tank from the sewer pipes in the house 19

4.2 Imhoff Tank 22

5.1 Trickling Filter 25

5.1.1 High Rate Trickling Filter 29

5.2 UASB REACTOR 31

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1. Sewerage System

Introduction

1.1 GENERAL

Sewage before being disposed of either in river streams or on land, has generally to be treated, so

as to make it safe. The degree of treatment required,however,depends upon the characterstics of

the souce of disposal.

1.2 System of sanitation

The waste products of a society including the human excreta had been collected, carried and

disposed of manually to a safe point of disposal, by the sweepers, since time immemorial. This

Primitive method of collecting and disposing of the society’s wastes, has now been modernized

and replace by a system, in which these wastes are mixed with sufficient quanity of water and

carried through closed conduits under the condition of gravity flow. This mixture of water and

waste products, popularly called Sewage.

1.3 Need of Wastewater Treatment

In addition to water that we want to recycle, wastewater contains pathogens (disease organisms),

nutrients such as nitrogen and phosphorus, solids, chemicals from cleaners and disinfectants and

even hazardous substances. Given all of the components of wastewater, it seems fairly obvious

that we need to treat wastewater not only to recycle the water and nutrients but also to protect

human and environmental health. Many people, however, are not very concerned about

wastewater treatment until it hits home. They can ignore it until bacteria or nitrates show up in

their drinking water, the lake gets green in the summer and the beach is closed, or the area begins

to smell like sewage on warm days. Sometimes residents discover they can’t get a building

permit or sell their home without a septic inspection or upgrade, or they find out there is no room

on their property for a new or replacement septic system. Often when one homeowner has a

sewage treatment problem, others in the neighborhood have the same problem. People don’t

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always talk to their neighbors about sewage problems for a variety of reasons, including risk of

enforcement actions.

1.4 Types of sewage system

Combined sewage system

Seprate sewage system

1.4.1 Combined sewage system

Fig.1.4.1 Combined sewage system

1.4.2 Seprate sewage system

Fig.1.4.2 Seprate sewage system

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2. Treatment of sewage

Treatment of sewage

Sewage before being disposed of either in river streams or on land, has generally to be treated, so

as to make it safe. The degree of treatment required,however,depends upon the characterstics of

the souce of disposal.

2.1 Classification of waste water treatment

Wastewater treatment options may be classified into groups of processes according to the

function they perform and their complexity:

Preliminary Treatment

Primary Treatment

Secondary Treatment

Tertiary Treatment

Fig. 2.1 Schematic of a typical wastewater treatment plant

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3. Preliminaty Treatment of sewage

Preliminary Treatment

Preliminary Treatment includes simple processes that deal with debris and solid material.The

purpose of preliminary treatment is to remove those easily separable components. This is usually

performed by screening (usually by bar screens) and grit removal. Their removal is important in

order to increase the effectiveness of the later treatment processes and prevent damages to the

pipes, pumps and fittings.

3.1 Screening

Screening is the very first process carried out at a sewage treatment,plant and consist of passing

the sewage through different type of screen,so as to trap and remove the floating materials, such

as piece of cloth, paper , wood, cork ,hair, fibre, kitchen refuse etc. These floating materials, if

not removed, will choke the pipes, or adversely affect the working of the sewage pumps. The

idea of providing screens is to protect the pumps and other equipments from the possible

damages due to the floating matter of the sewage.Screen should preferably be placed before the

Grit chambers.

3.2 Types of Screens

Depending upon the size of the opening, screens may be classified as:-

Coarse screens

Medium screen

Fine screen

3.2.1 Coarse screens

Coarse screen are defined as screens with openings between 6 to 150 mm. They are typically

used in wastewater treatment applications in order to remove large solids from the influent in

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order to protect pumps, piping, instruments, valves and other downstream equipment. Coarse

screening as a physical unit operation in wastewater treatment applications is effectively used in

order to remove rags, clothes, logs, debris or any other large objects that could clog, damage or

alter the process pumps and piping downstream. Depending upon the size of the wastewater

treatment plant and the requirements the coarse screens can be manually or mechanically

cleaned. An installation with two or more screen channels is usually preferred as one unit can be

taken out of service for maintenance without interrupting the screening operation. If only one

screen is used, a by-pass channel with a manually cleaned bar screen is essential. For small

wastewater treatment applications, a manually cleaned bar screen in a box is sometimes preferred

due to its simplicity. Napier-Reid designs and fabricates manual screens with a large box in order

to reduce the cleaning frequency by the operator. This box can also be designed as a flow splitter

box in order to evenly distribute the flow in two aeration tanks continuously if a conventional

activated sludge process is used or in an interrupted alternating on-off sequence if a Sequence

Batch Reactor (SBR) is selected. In SBR applications, control valves usually follow the outlets

of the splitter box in order to regulate the flow into the tanks.

Fig.3.2.1 Coarse screen

3.2.2 Medium Screen

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The spacing between the bar is about 6 to 40 mm. These screen will ordinary collect 30 to 90

liters of material per million litre of sewage. The rectangular shaped coarse and medium screens

are now a days widely used at sewage treatment plant. They made of steel bar, fixed parallel to

one another at desired spacing on rectangular steel frame, and are called bar screens. The

screens are set in a masonary or R.C.C chamber, called the screen chamber.

Fig.3.2.2 Manually racked screen

3.2.3Fine Screen

Have perforations of 1.5 to 3mm in size. The installation of these screen prove very effective,

and they remove as much as 20% of the suspended solid from sewage. These screen, however

get clogged very often and need frequent cleaning.

3.3 Grit Chambers

Wastewater usually contains a relatively large amount of inorganic solids such as sand, cinders

and gravel which are collectively called grit.  The amount present in a particular wastewater

depends primarily on whether the collecting sewer system is of the sanitary or combined type. 

Grit will damage pumps by abrasion and cause serious operation difficulties in sedimentation

tanks and sludge digesters by accumulation around and plugging of outlets and pump suctions. 

Consequently, it is common practice to remove this material by grit chambers.  Grit chambers

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are usually located ahead of pumps or comminuting devices, and if mechanically cleaned, should

be preceded by coarse bar rack screens.  Grit chambers are generally designed as long channels. 

In these channels the velocity is reduced sufficiently to deposit heavy inorganic solids but to

retain organic material in suspension.  Channel type chambers should be designed to provide

controlled velocities as close as possible to 1.0 foot per second.  Velocities substantially greater

than 1.0 foot per second cause excessive organic materials to settle out with the grit.   The

detention period is usually between 20 seconds to 1.0 minute.  This is attained by providing

several chambers to accommodate variation in flow or by proportional weirs at the end of the

chamber or other flow control devices which permit regulation of flow velocity.  There are also

patented devices to remove grit.  One development is the injection of air several feet above the

floor of a tank type unit.  The rolling action of the air keeps the lighter organic matter in

suspension and allows the grit relatively free from organic matter to be deposited in the quiescent

zone beneath the zone of air diffusion.  Excessive quantities of air can cause the roll velocity to

be too high resulting in poor grit removal.  Insufficient quantities of air result in low roll

velocities and excessive organic matter will settle with the grit.  These grit chambers are usually

called aerated grit chambers. 

Cleaning. 

Grit chambers are designed to be cleaned manually or by mechanically operated devices.   If

cleaned manually, storage space for the deposited grit is usually provided.  Grit chambers for

plants treating wastes from combined sewers should have at least two hand-cleaned units or a

mechanically cleaned unit with by-pass.  Mechanically cleaned grit chambers are recommended. 

Single, hand-cleaned chambers with by-pass, are acceptable for small wastewater treatment

plants serving sanitary sewer systems.  Chambers other than channel type are acceptable, if

provided with adequate and flexible controls for agitation and/or air supply devices and with grit

removal equipment. 

There are a number of mechanical cleaning units available which remove grit be scrapers or

buckets while the grit chamber is in normal operation.  These require much less grit storage

space than manually operated units. 

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Washing Grit

Grit always contains some organic matter which decomposes and creates odors.  To facilitate

economical disposal of grit without causing nuisance, the organic matter is sometimes washed

from the grit and returned to the wastewater.  Special equipment is available to wash grit. 

Mechanical cleaning equipment generally provides for washing grit with wastewater as it is

removed from the chamber. 

 Quantity of Grit

This depends on the type of sewer system, the condition of the sewer lines and other factors. 

Strictly domestic wastewater collected in well constructed sewers will contain little grit, while

combined wastewater will carry large volumes of grit, reaching a peak at times of severe storms. 

In general, 1.0 to 4.0 cu.ft. of grit per million gallons of wastewater flow can be expected. 

 Operation

Manually cleaned grit chambers for combined wastewater should be cleaned after every large

storm.  Under ordinary conditions these grit chambers should be cleaned when the deposited grit

has filled 50 to 60 percent of the grit storage space.  This should be checked at least every ten

days during dry weather. 

When mechanically cleaned grit chambers are used, they must be cleaned at regular intervals to

prevent undue load on the cleaning mechanism.  Recommendations of the manufacturer should

be rigidly observed.   This plus experience, will determine the cleaning schedule. 

A grit in which marked odors develop indicates that excessive organic matter is being removed

in the grit chamber.  Alternately, if sludge from a settling tank is excessively high in grit, or if

there is excessive wear in pumps, comminutors, sludge collectors or other mechanical

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equipment,  the reason is likely to be inefficient functioning of the grit removing process.  In

either case, a study of this unit should be made. 

Fig.3.3 Two chamber aerated grit chamber

Disposal of Screenings and Grit 

Screenings decompose rapidly with foul odors.  They should be kept covered in cans at the

screens and removed at least daily for disposal by burial or incineration.  The walls and

platforms of the screen chamber and screen itself should be hosed down and kept clean.  Grit

containing much organic matter may have to be buried to prevent odor nuisances. 

3.4 Skimming Tank

It is a chamber so arranged that floating matter rises and remains on the surface of wastewater

until removed, while liquid flows out continuously through deep outlets or under partition or

deep scum board. This may be accomplished in separate tank or combined with primary

sedimentation. In conventional sewage treatment plants separate skimming tanks are not used,

15

unless specifically required, and this is achieved by providing baffle ahead of effluent weir in

primary sedimentation tank. Skimming tanks are used to remove lighter, floating substances,

including oil, grease, soap, pieces of cork and wood, vegetable debris, and fruit skins. Tank can

be rectangular or circular, designed for detention period of 1 to 15 minutes. Typical detention

time of about 5 min is adopted in design (Metcalf and Eddy, 2003). The submerged outlet is

opposite the inlet and at lower elevation to assist in flotation and remove any solids that may

settle.

Fig.3.4 Skimming Tank

3.5 Pre-Aeration Tanks

Pre-aeration of wastewater, that is aeration before primary treatment is sometimes provided for

the following purposes: 

To obtain a greater removal of suspended solids in sedimentation tanks.

To assist in the removal of grease and oil carried in the wastewater.

To freshen up septic wastewater prior to further treatment.

BOD reduction.

Pre-aeration is accomplished by introducing air into the wastewater for a period of 20 to 30

minutes at the design flow.  This may be accomplished by forcing compressed air into the

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wastewater at a rate of about 0.10 cu.ft. per gallon of wastewater when 30 minutes of aeration is

provided or by mechanical agitation whereby the wastewater is stirred or agitated so that new

surfaces are continually brought into contact with the atmosphere for absorption of air.  To insure

proper agitation when compressed air is forced into the wastewater, air is usually supplied at the

rate of 1.0 to 4.0 cubic feet per minute per linear foot of tank or channel.   When air for

mechanical agitation (either with or without the use of chemicals) is used for the additional

purpose of obtaining increased reduction in BOD, the detention period should be at least 45

minutes at design flow.  The agitation of wastewater in the presence of air tends to collect or

flocculate lighter suspended solids into heavier masses which settle more readily in the

sedimentation tanks.  Pre-aeration also helps to separate grease and oil from the wastewater and

wastewater solids and to carry them to the surface.  By the addition of air, aerobic conditions are

also restored in septic wastewater to improve subsequent treatment. 

The devices and equipment for introducing the air into the wastewater are the same or similar to

those used in the activated sludge process.

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4. Primary Treatment of sewage

Primary Treatment

Primary treatment is mainly the removal of solids by settlement. Simple settlement of the solid

material in sewage can reduce the polluting load by significant amounts. It can reduce BOD by

up to 40%. Some examples of primary treatment is septic tanks, septic tanks with upflow filters,

Imhoff tanks.

4.1 Septic Tank

In rural areas where houses are spaced so far apart that a sewer system would be too expensive

to install, people install their own, private sewage treatment plants. These are called septic

tanks.

A septic tank is simply a big concrete or steel tank that is buried in the yard. The tank might hold

1,000 gallons (4,000 liters) of water. Wastewater flows into the tank at one end and leaves the

tank at the other. The tank looks something like this in cross-section:

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Fig.4.1 Septic Tank

In this picture, you can see three layers. Anything that floats rises to the top and forms a layer

known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the

middle is a fairly clear water layer. This body of water contains bacteria and chemicals like

nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.

Wastewater comes into the septic tank from the sewer pipes in the house, as shown here:

Fig.4.1.1 Wastewater comes into the septic tank from the sewer pipes in the house

19

A septic tank naturally produces gases (caused by bacteria breaking down the organic material in

the wastewater), and these gases don't smell good. Sinks therefore have loops of pipe called P-

traps that hold water in the lower loop and block the gases from flowing back into the house.

The gases flow up a vent pipe instead -- if you look at the roof of any house, you will see one or

more vent pipes poking through.

As new water enters the tank, it displaces the water that's already there. This water flows out of

the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches

filled with gravel. The following diagram shows an overhead view of a house, septic tank,

distribution box and drain field:

A typical drain field pipe is 4 inches (10 centimeters) in diameter and is buried in a trench that is

4 to 6 feet (about 1.5 m) deep and 2 feet (0.6 m) wide. The gravel fills the bottom 2 to 3 feet of

the trench and dirt covers the gravel, like this:

The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain

field is determined by how well the ground absorbs water. In places where the ground is hard

clay that absorbs water very slowly, the drain field has to be much bigger.

A septic system is normally powered by nothing but gravity. Water flows down from the house

to the tank, and down from the tank to the drain field. It is a completely passive system.

You may have heard the expression, "The grass is always greener over the septic tank." Actually,

it's the drain field, and the grass really is greener -- it takes advantage of the moisture and

nutrients in the drain field.

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4.2 Imhoff Tank

The Imhoff tank was developed to correct the two main defects of the septic tank. 

It prevents the solids once removed from the sewage from again being mixed with it, but

still provides for the decomposition of these solids in the same unit.

It provides an effluent amenable to further treatment.

Contact between the waste stream and the anaerobic digesting sludge is practically eliminated

and the holding period in primary settling compartment at the tank is reduced. The Imhoff tank

may be either circular or rectangular and is divided into three compartments: 

The upper section or sedimentation compartment

The lower section known as the digestion compartment and

The gas vent and scum section

It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids

at one end of the sedimentation compartment.  Periodically reversing the flow will result in an

even accumulation of sludge across the bottom of the tank. In operation, all of the wastewater

flows through the upper compartment.  Solids settle to the bottom of this sloped compartment,

slide down and pass through an opening or slot to the digestion compartment.  One of the bottom

slopes extends at least six inches beyond the slot.  This forms a trap to prevent gas or digesting

sludge particles in the lower section from entering the waste stream in the upper section.  The gas

and any rising sludge particles are diverted to the gas vent and scum section. 

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Fig.4.2 Imhoff Tank

Imhoff Tank Operation

There are no mechanical parts in an Imhoff tank.  Attention should, however, be given to the

following: 

Daily removal of grease, scum and floating solids from the sedimentation compartment.

Weekly scraping of the sides and sloping bottoms of the sedimentation compartment by a

rubber squeegee to remove adhering solids which may decompose.

Weekly cleaning the slot at the bottom of the sedimentation compartment.  This can be

done by use of a chain drag.

Periodic reversal of flow where provided for in the design of the tank.

Control of the scum in the scum chamber, by breaking it up, hosing with water under

pressure, keeping it wet with supernatant from the digestion compartment and removal if

the depth approaches two to three feet.

Removal of sludge should be done before the sludge depth approaches within 18 inches

of the slot in the sedimentation compartment.  It is better to remove small amounts

frequently than large amounts at long intervals.  Sludge should be removed at a slow

regular rate to avoid coning (i.e. the formation of a channel through the sludge) which

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would permit partially digested sludge and liquid held in storage above the digested

sludge to be withdrawn from the tank.  Before winter temperatures are expected, most of

the digested sludge except that necessary for seeding (about 20 percent) should be

removed to provide space for winter accumulations when digestion is very slow.  The

height of the sludge in the sludge compartment should be determined at inlet and outlet

end of the tank at least once a month.

After each time that sludge is removed, the sludge pipes should be flushed and drained to

prevent sludge from hardening in and clogging the pipes.

Prevention of "Foaming".  Every effort should be made to prevent "foaming" because

correction after the condition arises is sometimes difficult.  "Foaming" is usually

associated with an acid condition of the sludge and in such cases may be prevented or

corrected by treatment with lime or sodium bicarbonate to counteract the acidity of the

sludge.  There are a few simple measures which may, under certain circumstances,

remedy or improve the condition.

The Imhoff tank has no mechanical parts and is relatively easy and economical to operate.  It

provides sedimentation and sludge digestion in one unit and should produce a satisfactory

primary effluent with a suspended solids removal of 40 to 60 percent and a BOD reduction of 15

to 35 percent.  The two-story design requires a deep over-all tank.  Primary tanks with separate

digesters have largely replaced the Imhoff tank for large municipal installations.  The Imhoff

tanks is best suited to small municipalities and large institutions where the tributary population is

5,000 or less, and a greater degree of treatment is not needed. 

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5. Secondary Treatment of sewage

Secondary Treatment

In secondary treatment the organic material that remains in thewastewater is reduced

biologically. Secondary treatment actually involves harnessing and accelerating the natural

process of waste disposal whereby bacteria convert organic matter to stable forms. Both aerobic

and anaerobic processes are employed in secondary treatment. Some examples of secondary

treatment are UASB, reed bed systems, trickling filters and stabilisation ponds.

5.1 Trickling Filters

A trickling filter consists of a bed of highly permeable media on whose surface a mixed

population of microorganisms is developed as a slime layer.  The word "filter" in this case is not

correctly used for there is no straining or filtering action involved.  Passage of wastewater

through the filter causes the development of a gelatinous coating of bacteria, protozoa and other

organisms on the media.  With time, the thickness of the slime layer increases preventing oxygen

from penetrating the full depth of the slime layer.  In the absence of oxygen, anaerobic

decomposition becomes active near the surface of the media.  The continual increase in the

thickness of the slime layer, the production of anaerobic end products next to the media surface,

and the maintenance of a hydraulic load to the filter, eventually causes sloughing of the slime

layer to start to form.  This cycle is continuously repeated throughout the operation of a trickling

filter.  For economy and to prevent clogging of the distribution nozzles, trickling filters should be

preceded by primary sedimentation tanks equipped with scum collecting devices. 

Primary treatment ahead of trickling filters makes available the full capacity of the trickling filter

for use in the conversion of non-settleable, colloidal and dissolved solids to living microscopic

organisms and stable organic matter temporarily attached to the filter medium and to inorganic

matter temporarily attached to the filter medium and to inorganic matter carried off with the

effluent.  The attached material intermittently sloughs off and is carried away in the filter

effluent.  For this reason, trickling filters should be followed by secondary sedimentation tanks to

24

remove these sloughed solids and to produce a relatively clear effluent. 

Construction and Design

The primary factors that must be considered in the design of trickling filters include: 

Type of filter media to be used.

Type and dosing characteristics of the distribution system.

The configuration of the under-drain system. 

Fig.5.1 Trickling Filter

Filter Media

The ideal filter medium is a material that has a high surface area per unit volume, is low in cost,

has a high durability, and does not readily clog.  The choice of filter media is more often

governed by the material locally available which may include field stone, gravel, broken stone,

blast furnace slag and antracite stones.  Stones less than one inch in diameter do not provide

25

sufficient pore space and may therefore result in plugging of the media and ponding.  The

tendency is to use larger sizes with 2 1/2 inches in diameter now considered the minimum size. 

Large diameter stones tend to avoid ponding situations but also limit the surface area per unit

volume available for the slime layer to grow.  An upper size limit of about 4 inches is therefore

recommended.

Distribution System:  The rotary distributor has become standard for the trickling filter

process because of its reliability and ease of maintenance.  The rotary distributor consists of a

hollow vertical center column carrying two or more radial pipes or arms, each of which contains

a number of nozzles or orifices for discharging the wastewater onto the bed.  All of these nozzles

point in the same direction at right angles to the arms and the reaction of the discharge through

them causes the arms to revolve.  The necessary reaction is furnished by a head of 18" to 24". 

The speed of revolution will vary with the flow rate, but it should be in the range of one

revolution in 10 minutes or less for a two-arm distributor.  A dosing tanks and siphon should be

provided for standard rate trickling filters to shut off the flow when the head falls below that

necessary to revolve the arms at the required speed.  In some cases positive drive mechanisms

are being used.

A clearance of 6 to 9 inches should be allowed between the bottom of the distributor arm and top

of the bed.  This will permit the waste streams from the nozzles to spread out and cover the bed

uniformly, and it will also prevent ice accumulation from interfering with the distributor motion

during freezing weather.

Fixed spray nozzles were used when trickling filters were first developed.  The nozzles were

attached to pipes laid in the filter medium and were fed intermittently from a siphon controlled

dosing tank.  By this method, wastewater is applied to the filter for short periods of time.  

Between applications the filter has rest periods while the dosing tank is filling.  Many types and

shapes of nozzles were developed and the siphon dosing tank was designed to attain the best

possible even distribution of wastewater over the entire surface of the filter.  At best, the

distribution was not even and there were areas of the filter on which very little wastewater was

sprayed. 

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In addition, due to the greater number of nozzles used for the distribution of the wastes, clogging

and increased operational and maintenance problems were encountered. 

Under-drain System

The under-drain system in trickling filters serves two purposes:  (a)  to carry the wastewater

passing through the filter and the sloughed solids from the filter to the final clarification process,

and (b)  to provide for ventilation of the filter to maintain aerobic conditions.  The underdrains

are specially designed vitrified clay blocks with slotted tops that admit the wastewater and yet

support the media.  The blocks are laid directly on the filter floor, which is sloped toward the

collection channel at a 1 to 2 percent gradient.  Since the underdrains also provide ventilation for

the filter it is desirable that the ventilation openings total at least 20% of the total floor area.  

Normal ventilation occurs through convection currents caused by a temperature differential

between the wastewater and the ambient air temperature.  In deep filters or heavily loaded filters,

there may be some advantage in force ventilation. 

Filter Classification

 Trickling filters are classified by hydraulic or organic loading, as high-rate or low-rate. 

The organic load on a filter is the BOD content in pounds applied to the filter.  This is usually

expressed as pounds of BOD per day per 1000 cubic feet of filter medium or pounds of BOD per

day per acre foot.  The hydraulic load, including recirculation flow if used, is the gallons of flow

per acre of filter surface per day. 

Low-rate filters are relatively simple treatment units that normally produce a consistent effluent

quality even with varying influent strength.  Depending upon the dosing system, wastewater is

applied intermittently with rest periods which generally do not exceed five minutes at the

designed rate of waste flow.  With proper loadings the low-rate trickling filter, including primary

and secondary sedimentation units, should remove from 80 to 85 percent of the applied BOD. 

While there is some unloading or sloughing of solids at all times, the major unloadings usually

occur several times a year for comparatively short periods of time. 

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High-rate filters are usually characterized by higher hydraulic and organic loadings than low-rate

filters.  The higher BOD loading is accomplished by applying a larger volume of waste per acre

of surface area of the filter. 

One method of increasing the efficiency of a trickling filter is to incorporate recirculation. 

Recirculation is a process by which the filter effluent is returned to and reapplied onto the filter. 

This recycling of the effluent increases the contact time of the waste with the microorganisms

and also helps to "seed" the lower portion of the filter with active organisms. 

When recirculation is used, the hydraulic loading per unit area of filter media is increased.  As a

result, higher flow velocities will usually occur causing a more continuous and uniform

sloughing of excess growths.  Recirculation also helps to minimize problems with ponding and

restriction of ventilation. 

Recirculation can be continuous or intermittent.  Return pumping rates can either be constant or

variable.  Sometimes recycling can be practiced during periods of low flow to keep the

distributors in motion, to prevent the drying of the filter growths, and to prevent freezing during

colder temperatures.  Also, recirculation in proportion to flow may be utilized to reduce the

organic strength of the incoming wastes, and to smooth out diurnal flow variations. 

Aero Filter:  The aero-filter is still another process which distributes the wastewater by

maintaining a continuous rain-like application of the wastewater over the filter bed.  For small

beds, distribution is accomplished by a disc distributor revolving at a high speed of 260 to 369

rpm set 20" above the surface of the filter to give a continuous rain-like distribution over the

entire bed.  For large beds a large number of revolving distributor arms, 10 or more, tend to give

more uniform distribution.  These filters are always operated at a rate in excess of 10 million

gallons per acre of surface area per day. 

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Fig. 5.1.1 High Rate Trickling Filter

High-rate trickling filters, including primary and secondary sedimentation, should, under normal

operation, remove from 65 to 85 percent of the BOD of the wastewater.  Recirculation should be

adequate to provide continuous dosage at a rate equal to or in excess of 10 million gallons per

acre per day.  As a result of continuous dosing at such high rates, some of the solids accumulated

on the filter medium are washed off and carried away with the effluent continuously. 

High-rate trickling filters have been used advantageously for pretreatment of industrial wastes

and unusually strong wastewaters.  When so used they are called "roughing filters".  With these

filters the BOD loading is usually in excess of 110 pounds of BOD per 1000 cubic feet of filter

medium. 

Generally, most organic wastes can be successfully treated by trickling filtration.  Normally food

processing, textile, fermentation and some pharmaceutical process wastes are amenable to

trickling filtration. 

Some industrial wastewaters which cannot be treated by trickling filtration are those which

contain excessive concentration of toxic materials, such as pesticide residues, heavy metals, and

high acidic and alkaline wastes. 

Since the organisms growing on the media are temperature dependent, climatic changes will

affect the filter's performance.  The organisms metabolic rate increases with increasing

temperature and warmer weather.  Therefore, higher loadings and greater efficiencies are

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possible in warmer temperatures and climates, if aerobic conditions can be maintained in the

filter. 

5.2 Up-flow anaerobic sludge blanket (UASB)

UASB technology, normally referred to as UASB reactor, is a form of anaerobic digester that is

used in the treatment of wastewater.The UASB reactor is a methanogenic (methane-producing)

digester that evolved from the anaerobic clarigester. A similar but variant technology to UASB is

the expanded granular sludge bed (EGSB) digester. A diagramatic comparison of different

anaerobic digesters can be found here.

UASB uses an anaerobic process whilst forming a blanket of granular sludge which suspends in

the tank. Wastewater flows upwards through the blanket and is processed (degraded) by the

anaerobic microorganisms. The upward flow combined with the settling action of gravity

suspends the blanket with the aid of flocculants. The blanket begins to reach maturity at around 3

months. Small sludge granules begin to form whose surface area is covered in aggregations of

bacteria. In the absence of any support matrix, the flow conditions creates a selective

environment in which only those microorganisms, capable of attaching to each other, survive and

proliferate. Eventually the aggregates form into dense compact biofilms referred to as "granules".

A picture of anaerobic sludge granules can be found here.

Biogas with a high concentration of methane is produced as a by-product, and this may be

captured and used as an energy source, to generate electricity for export and to cover its own

running power. The technology needs constant monitoring when put into use to ensure that the

sludge blanket is maintained, and not washed out (thereby losing the effect). The heat produced

as a by-product of electricity generation can be reused to heat the digestion tanks.

The blanketing of the sludge enables a dual solid and hydraulic (liquid) retention time in the

digesters. Solids requiring a high degree of digestion can remain in the reactors for periods up to

90 days. Sugars dissolved in the liquid waste stream can be converted into gas quickly in the

liquid phase which can exit the system in less than a day.

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UASB reactors are typically suited to dilute waste water streams (3% TSS with particle size

>0.75mm).

Fig. 5.2 UASB REACTOR

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6. Tertiary Treatment of sewage

6.1 Tertiary treatment

Tertirary treatment is the polishing process whereby treated effluent is further purified to

acceptable levels for discharge. It is usually for the removal of specific pollutants e.g. nitrogen or

phosphorus or specific industrial pollutants. Tertiary treatment processes are generally

specialised processes. Some examples of tertiary treatment are bank’s clarifiers, grass plots, etc.

Tertiary treatment is part of the treatment process which wastewater must go through before it

can be discharged into the environment. The process includes four or five stages. These are

preliminary, primary, secondary, and tertiary wastewater treatment; sometimes, this is followed

by an additional step. There are several different types of tertiary treatment, all of which involve

improving the quality of the waste to reduce its impact on the environment into which it is

released.

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7. Disposing of sewage Effluent

7.1 Disposing of Sewage Effluents

The various treatments that may be given yo the raw sewage before disposing it of, it shall be

worthwhile to first discuss the various method and sources of disposal of sewage. The study of

the source of disposal is important, because the amount of treatment required to be given to

sewage depends very much upon the source of disposal, its quality and capacity to tolerate the

impurities present in the sewage effluent, without itself getting potentially polluted or becoming

less useful.

There are two methods of disposing sewage effluents:

Disposal by Dilution

Effluent irrigation or Broad irrigation or Sewage Farming, i.e disposal on land.

7.1.1 Disposal by Dilution

Treated sewage or the effluent from the treatment plant is discharged into a river or lake or sea.

The discharge sewage in due course of time is purified by self purification of natural water.

Conditions favouring disposal by dilution:

When sewage is fresh, 4 to 5 hour old, free from floating and suitable solids.

When diluting water has high dissolved oxygen content

When diluting water is not used for the purpose of navigation water supply.

When flow currents of the deposition water are favourable causing no deposition,

nuisance or destruction of aquatic life.

When the outfall sewer of the city of the treatment plant is situated near some natural

waters having large volume.

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7.2 Effluent Irrigation

In Effluent irrigation (broad irrigation), the raw sewage is discharge on a vacant land, which is

provided underneath with a system of properly laid under-drains. These under drain, usually,

consist of 15 to 20 cm dia porous tile pipes, laid open joint at a space of 12 to 30 m. The effluent

collected in these drain after getting filtered through the soil pores.

7.2.1 Sewage farming

In the case of sewage farming , however, the stress is laid upon the use of sewage effluent for

irrigation crops and increasing the fertility of the soil. The pre-treatment of sewage, in removing

the ingredients which may prove harmful and toxic to the plants is, therefore, necessary in this

case.

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REFERENCES

A Text book of “ Sewage Disposal and Air Polution Engineering” By “ S.K. Garg”.

www.google.com

www.wikipedia.org

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