PREPARATORY AND PRIMARY SEWAGE TREATMENT :A LITERATURE REVIEW

Dipangkar kundu1

1Agriculture Engineer, Agriculture Engineering Technology Extension Project (AETEP)Upazilla Agriculture Office,Sador,Joypurhat-5900

ABSTRACT

This paper describes the preparatory and primary treatment process which is performed in

any sewage treatment plant as initial steps to the treatment. It focuses on the treatment

procedure and the various units involved in the operation. Descriptive analysis is done

through the books and some internet based journals that establish the specific objectives,

design parameters, design consideration and advantages and disadvantages of the units

used.

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CONTENTS

CHAPTER ONE INTRODUCTION PAGE NO 1.1 SEWAGE 4 1.1.1 Need for Sewage Treatment 4 1.1.2 Purposes of Sewage Treatment 4 1.2 SEWAGE TREATMENT 5 1.2.1 Methods of Sewage Treatment 5

CHAPTER TWO PREPARATORY TREATMENT 2.1 INTRODUCTION 9 2.2 Treatment Process 9 2.2.1 Screening 9 2.2.1.1 Screens 9 2.2.1.1.1 Control system and servicing 11 2.2.1.1.2 Comminutors or cutting screens 11 2.2.1.1.3 Disposal of screenings 12 2.2.2 Grit chamber 12 2.2.2.1 Placement 13 2.2.2.2 Design consideration 13 2.2.2.3 Cleaning and disposal 14 2.2.3 Skimming Tanks 14 2.2.3.1 Necessity 14 2.2.3.2 Process 14 2.2.3.3 Design parameter and efficiency 15 2.2.3.4 Disposal of skimming 15

CHAPTER THREE PRIMARY TREATMENT 3.1 INTRODUCTION 16 3.2 Sedimentation Tanks 16 3.2.1 Objective of sewage sedimentation 16 3.2.2 Principle of sedimentation 16 3.2.3 Classification 16 3.2.4 Design parameters 17 3.2.5 Design consideration 17 3.2.6 Rectangular sedimentation tank 18 3.2.7 Circular sedimentation tank 19 3.3 Septic Tank 20 3.3.1 Design consideration 20 3.3.2 Advantage and disadva ntage of septic

tank 21

3.4 Imhoff Tank 21 3.4.1 Construction feature 22 3.4.2 Design consideration 22 3.4.3 Advantages and disadvantages of imhoff

tank 23

4.0 CONCLUSION 24 REFERENCES 25

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CHAPTER ONE

INTRODUCTION

1.1 SEWAGE

Sewage is a complex substance which pollutes the various phases of human environment such as air, water, soil, food and hence endangers human life as well as other living beings. It may be defined as the liquid waste of a community which includes domestic wastes (human excreta, discharges from kitchen, baths, showers, lavatories etc from

private or public buildings), industrial wastes or trade wastes (liquids from manufacturing process such as tanneries, paper and pulp mills, slaughter houses,

distilleries, textile processing mills, chemical plants etc) and some surface water from roofs and hard-standing areas.

1.1.1 Need for Sewage Treatment

Sewage before being disposed of either in river, land or other places has generally to be

treated to make it safe both for environment and living beings. Following points can direct us to the need for sewage treatment:

• Sewage is both dangerous to health and a potential nuisance

• Raw or untreated sewage discharged into the stream forms objectionable deposits on the bed

• Grease and soap contained in the sewage will rise to the surface of the stream and from a scum

• Fish may be killed and stream may be spoiled from any recreational use

• Disease bearing bacteria may be carried in the bodies of the cows that wade in contaminated stream, those bacteria may reach the milk during the process of milking

• Sewage may contain bacteria that cause typhoid, fever, various forms of dysentery and cholera.

1.1.2 Purposes of Sewage Treatment

Sewage treatment is done in order to remove or alter its objectionable properties so as to

render it less dangerous or offensive. Following reasons may be taken into account :

• To stabilize the sewage without causing odor and nuisance and without endangering public health

• To prevent damage done to receiving waters, whether they are rivers, canals, coastal waters and tidal estuaries.

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1.2 SEWAGE TREATMENT

Sewage treatment incorporates physical, chemical and biological processes which treat

and remove physical, chemical and biological contaminants from water following human use. It can be treated onsite at the point of which it is generated (e.g., septic tanks or onsite

package plants), or collected and conveyed via a network of pipes and pump stations to a municipal treatment plant. Efforts to collect treat and discharge domestic wastewater are typically subject to local regulations and standards . It should be noted that specific industrial sources of wastewater often require specialized treatment processes (e.g., pulp

and paper mill and, chemical manufacturer wastewaters).

Typically, sewage treatment is achieved by the initial physical separation of solids from the raw wastewater stream followed by the progressive conversion of dissolved biological

matter into a solid biological mass using indigenous, water-borne bacteria. Once the biological mass is separated or removed, the treated water may undergo additional

disinfection via chemical or physical processes. This 'final effluent' can then be discharged or re-introduced back into a natural surface water body (stream, river or bay) or other

environment (wetlands, golf courses, greenways, etc.). The segregated biological solids undergo additional treatment and neutralization prior to proper disposal or re-use.

1.2.1 Methods of Sewage Treatment

The process of sewage treatment can be typically classified into following five categories: i. Preparatory treatment / Preliminary treatment

ii. Primary treatment iii. Secondary treatment (Biological treatment)

iv. Treatment and disposal of sludge v. Treatment and disposal of liquids

Combination of preparatory and primary and secondary treatment constitutes the complete

treatment. The choice of treatment, methods depends upon several factors, including the disposal facilities available. Actually the distinction between primary, secondary and tertiary is

rather arbitrary, since many modern treatment methods incorporate physical, chemical and biological process in the same operation. Each type of treatment method uses various

treatment units. Table 1.1 shows the units used in different treatment methods and their performance.

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Table: 1.1

No Type of Treatment

Purification Affected

Units employed BOD removal (%)

Suspended solids removal (%)

Bacterial removal (%)

Disposal of residuals

1 2 3 4 5 6 7 8 1 Preparatory

Treatment a) Removal of floating materials b) Removal of settleable inorganic solids c) Removal of fats and grease

Coarse and fine screens of different designs Grit chambers or detritus tanks Skimming tanks

5-10 10-20 20-30

2-20 20-40 20-40

10-20 10-20 10-20

Screenings can be disposed easily either by burials or burnings The grit can be easily disposed of either by burials or burnings or for raising low lying areas Contents should be disposed first by stabilizing them in digestion tank by anaerobic process.

2 Primary treatment

Removal of large suspended organic solids

a)Plain sedimentation tanks Or b) Septic tanks Or c) Imhoff tanks

30-35 20-30 30-40

60-65 40-60 60-65

25-75 25-75 25-75

Sludge containing organic material has to be stabilized first in digestion tanks and the digested material then used as manure.

The effluents are either treated for secondary treatment or directly disposed by land treatment. These units combine sludge digestion along with sedimentation.

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1 2 3 4 5 6 7 8 3 Secondary or

Biological Treatment

Removal of fine suspended and dissolved organic matter

a)Chemical flocculation or sedimentation or b) Intermittent sand filters followed and preceded by plain sedimentation or c)High rate trickling filters followed and preceded by plain sedimentation or d) Low rate trickling filters followed and preceded by plain sedimentation or e) Activated sludge treatment process and secondary settling tanks or f) Oxidation ponds

50-85 90-95 65-95 90-95 75-95 85-90

70-90 85-95 65-92 70-92 85-90 85-90

40-80 95-98 80-95 90-95 90-98 90-98

Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder Sludge containing organic material has to be stabilized in diges tion tanks and the residue then used as manure or soil builder Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder Effluents are generally disposed of using them for irrigation

4 Final Treatment

Removal of pathogens and remaining very fine organic matter

Chlorinator

100 100 100

Discharged to receiving environment.

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The individual operations of these units may be combined in different ways depending upon the topography and other local needs so as to create different types of treatment

plants.

Fig.1.1 Typical Flow Diagram of a Sewage Treatment Plant

Fig 1.1 shows the positions of different sewage treatment methods in the sewage treatment

facilities. Preliminary and Primary treatment will be discussed in the following chapter.

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CHAPTER TWO

PREPARATORY TREATMENT

2.1 INTRODUCTION

Preparatory treatment consists solely in separating the floating materials (like dead animals, tree branches, paper, pieces of rags, wood etc) and also the heavy settleable inorganic solids along with grease and oil which impede efficient sewage treatment and are undesirable at the end-product biosolids.

The treatment reduces the Biochemical Oxygen Demand (BOD) of the waste by about 15 to 30% which is a measure of the strength or pollution potential of the waste matter.

2.2 Treatment Process

Preparatory treatment is carried out through the following process:

⇒ Screening o Removes floating paper, rags, cloths etc

⇒ Grit chambers or Detritus tank o Removes grit and sand

⇒ Skimming tanks o Removes oil and grease

2.2.1 Screening

Screening is the very first operation carried out in a sewage treatment plant and consists in

passing the sewage through the different types of screens, so as to trap and remove the floating materials. The specific objectives are:

• To prevent formation of unsightly scums in settling and aeration tanks

• To prevent clogging sprinklers nozzles or the surface of trickling filters

• To prevent pumping machinery and siphons, etc from damage and

• To prevent formation of sludge banks or unsightly floating matter in the receiving bodies of water

Screening operation is performed with Screens and in some cases with Comminutors or also known as cutting screens.

2.2.1.1 Screens

Depending on the size of the opening screens may be classified as following types:

• Coarse Screens: Also known as Rack or Bar Screen. The spacing between the bars is about 50 mm or more. They will collect about 6 liters of solids per million

liter of sewage. These types of screen do help in removing large floating objects

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from sewage. It is often used in conjunction with Grit chamber. It may be placed at either end of the chamber but generally at the inlet end. Bars are made of steel

fixed parallel to one another at desired spacing on a rectangular steel frame. Now-a-days these screens are generally kept inclined at about 30 to 60° to the direction

of flow, in order to increase opening area and reduce flow velocity. A most commonly used bar type screen is shown in the figure 2.1

Fig.2.1: Bar type Screen

• Medium screens: In these types of screens the spacing between the bars is about 6 to 40 mm. They will cordially collect 30 to 90 liters of material per million liter of sewage. The screening usually contains some quantity of organic material.

• Fine screens: These types of screens have perforations of 1.5 mm to 3mm in size. They are very effective and remove as much as 20% of suspended solids from

sewage. These screens however , clogged very often and need frequent cleaning. Hence are only used for treating the industrial wastes or for treating those

municipal wastes that are associated with heavy amount of industrial wastes. The fine screens may be classified into disc or drum type and are operated

continuously by electric motors. Fig 2.2 shows the section of the fine screen.

Fig.2.2: Disc type of Fine screen (Section)

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2.2.1.1.1 Control systems and servicing

The material collected on the screens can be removed either by manually or by mechanically. Manual cleaning is practiced at small plants with hand operated racks. In

large plants mechanically operated racks are used to move over the screens either continuously or intermittently. Following are some provisions that should be ensured;

⇒ Manually cleaned screening facilities shall include an accessible platform from which the operator may rake screenings easily and safely

⇒ Mechanically cleaned bar screens shall be properly controlled by differential water level and timing devices or continuous operation

⇒ Mechanical units which are operated by timing devices shall be provided with auxiliary controls which will start the cleaning operation at a preset, high water elevation and a device to stop the cleaning operation after a predetermined length of time.

⇒ The drive mechanism of a mechanically cleaned screen shall be enclosed.

⇒ The maximum allowable head loss through clogged racks and bar screens is generally limited to 0.8 m. Generally, the absolute minimum head loss allowance

through a manually cleaned bar screen is 15 cm, assuming frequent attention by operating personnel.

⇒ Assurance should be provided that the head loss through the screening unit at peak hourly flows will not surcharge the incoming sewer.

⇒ Due consideration shall be given to the selection of materials because of corrosive conditions present.

2.2.1.1.2 Comminutors or Cutting screens Comminutors are usually provided in plants that do not have primary clarifiers, fine

screens or mechanically cleaned bar screens. This kind of device breaks the larger sewage solids to about 6mm in size, when the sewage is screened through them. They consist of

revolving slotted drum (Fig. 2.2) through which the sewage is screened. Cutters mounted

Fig.2.2 Comminutor / Cutting screen

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on the drum shear the collected screen against a comb until they are small enough to pass through 5mm to10mm wide slots of the drum. These are usually arranged in pairs to

facilitated repairs and maintenance. Comminutors should always be preceded by grit chambers to prevent excessive wear.

2.2.1.1.3 Disposal of Screenings

The material separated by screens is called screening. It contains 85 to 90% moisture and

other floating material. It may also contain some organic load which may putrefy causing bad smells and nuisance. Following methods of disposal are usually practiced.

• Burning or incineration: In this method the screenings are first dried with the 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.

• Burial: The process is technically called Composting. In this method the screenings are buried in 1m to 1.5m 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 content can be used as manure.

• Dumping: In this process the screenings are dumped in low lying areas or in large bodies of water (Sea). Dumping in the sea is only suitable only where strong

forward current do exist to take the dump material away from the sore line. The duping on land for raising low lying areas is also adopted when screenings are

from coarse screens, and not from medium or fine screens, as such not containing much organic load.

• Digestion: In this process screenings along with the sewage sludge is digested in a sludge digestion tank. It is not so successful method.

2.2 .2 GRIT CHAMBERS

A grit chamber is an enlarged channel or a long basin, in which the cross section is

increased so as to reduce the flow velocity of sewage to such an extent that the heavy inorganic material settle down by the force of gravity and lighter inorganic materials

remain in suspension. They are nothing but kind of sedimentation tanks, design to separate the intended heavier inorganic materials by the process of sedimentation due to

gravitational force. They remove the inorganic particles having specific gravity of about 2.65. Such as sand,

gravel grit, eggshells, bones etc of size 0.2mm larger to prevent damage to pumps or to prevent their accumulation in sludge digesters.

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2.2.2.1 Placement

Grit chambers should be located after the coarse bar racks or coarse bar screens, but ahead of Comminutors, pumps and other treatment units. Where grit chambers are mechanized, it

may be desirable to locate the bar screens and/or comminuting devices ahead of grit chambers to reduce the effect of rags and other gross particles. However, whether the

screening and/or comminuting devices are located ahead of or after the grit chamber, it should be readily accessible for inspection, maintenance and handling of materials.

2.2.2.2 Design Consideration

The most important in the design of the grit chambers is that the flow velocity should

neither be too low to cause the settling of lighter organic matter, nor should it would be too high as not to cause the settlement of the entire silt and grit-present in sewage.

Velocity of flow between 0.15 to 0.3m/sec is practically considered sufficient to meet this purpose. In fact, theoretically a velocity of 0.225 m/sec is just sufficient to settle down all

the inorganic matter larger than 0.2mm in dia and of specific gravity 2.65. The grit chambers are generally designed for a flow velocity of 0.15 to 0.3 m/sec with a

depth of about 0.9 to 1.2 m and a detention period of about 1 min. Length of the tank = velocity × Detention period. Two to three separate chambers in parallel should be provided (Fig 2.3). One chamber to

Fig.2.3 Modern Grit chambers

pass the low flow and the other to pass (along with the first of course) the high flow. This intern also helps in manually cleaning of the chambers as one unit can work, while other

unit is shut down for cleaning.

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2.2.2.3 Cleaning and Disposal

The grit chambers can be cleaned in the following two ways:

• Manually • Mechanically or Hydraulically

⇒ Cleaning should be done periodically at about 2 weeks interval. Manually cleaning is done for small plants (Capacity of about 4.5 million liters per day)

⇒ Mechanically or hydraulic cleaning is done for larger plants.

⇒ In manual cleaning grit is removed with shovels etc, by hand.

⇒ In mechanical cleaning, grit is removed with the help of machines and in hydraulic cleaning, grit is removed by the force of water-jet directed from a central point and

removed through the pipes in the side walls or bottom of the chamber.

⇒ Grit conveying and transporting equipment shall be provided with protection against freezing and loss of material. Consideration should be given in large plants for loading facilities to discharge grit directly by mechanical means to a transport

vehicle. The silt, grit etc removed by the grit chambers can easily disposed of either by burial or burning (incineration) or for raising low lying areas by dumping. Precaution should be maintained that it should be used for any construction as it contains sufficient organic

matter.

2.2.3 SKIMMING TANKS

Skimming tanks are employed for removing oil and grease from sewage, which includes

fats, waxes, soaps, fatty acids etc. These materials may enter from sewage , from kitchens of restaurants and houses, from garages, oil refineries, soap and candle factories, etc.

2.2.3.1 Necessity

Skimming tanks are required for the following reasons:

⇒ To avoid forming of unsightly and odorous scums on the surface of settling of

settling tanks

⇒ To avoid interfere with the activated sludge treatment process

⇒ Inhibit biological growth on trickling filters.

2.2.3.2 Process

In skimming tank air is blown by an aerating device through the bottom to create spiral circulation (Fig: 2.3). The rising air tends to coagulate and congeal (solidify) the grease

and cause it to rise to the surface (being pushed in separate compartments) and are collected from the surface. On the contrary heavy solids settle down at the bottom and

slide back into the central aerated compartment, where it is moved to the outlet by the injected air.

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Fig: 2.3 Skimming Tank

Oil and grease that are collected are drawn off into a separating tank from where excess

water is removed.

2.2.3.3 Design Para-meter and Efficiency

A typical skimming tank consists of long through-shaped structure divided into three lateral compartme nts. It is done by means of vertical baffle walls, having slots in them.

They help in pushing the coagulated greasy material into side compartments. Diffusers placed in the bottom of the tank blows the compressed air to raise the grease and oil to the

surface. Detention time can be set up to 3 to 5 minutes and the amount of compressed air required is about 300 to 6000 cubic meters per million liters of sewage. The surface area required for the tank can be found by the following formula:

A = 0.00622Vr

q

Where q = Rate of flow of sewage in m3/day

Vr = Minimum raising velocity of the greasy material to be removed in m/min = 0.25 m/min in most cases

2.2.3.4 Disposal of skimming Skimming materials are mainly oil and grease. They can be disposed of by either burning or land burial. They are so polluted that they can hardly be put into any economic use.

However, sometime it may be converted into soap lubricants, candles and non-edible products.

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CHAPTER THREE

PRIMARY TREATMENT

3.1 INTRODUCTION Primary treatment is designed to remove solids which are not removed during preparatory treatment by the physical process of sedimentation and (flotation). The primary treatment

devices reduce the velocity and disperse the flow of waste water. The primary treatment effluents often contain a large amount of suspended organic material and have a high

BOD. In most of the facilities primary treatment is used as preliminary step ahead of biological treatment. Principal primary treatment devices are referred to as sedimentation

tanks, septic tanks and imhoff tanks.

3.2 SEFDIMENTATION TANKS Sedimentation is a process used for various stages within a waste treatment plant to reduce the concentration of suspended and settleable material.

The clarification of the sewage by the process of sedimentation can be preceded by providing condition under which the suspended material in sewage settles out. This

situation can be brought under by specially designed tanks called sedimentation tanks.

3.2.1 Objective of Se wage Sedimentation Primary objective of the sewage treatment is to remove the materials that are not removed during the preparatory treatment. The specific objectives are as follows:

• To bring down the load on secondary treatment units

• To remove the settlea ble solids that are capable of forming sludge banks in the receiving water

• To reduce the excessive organic loading where sewage irrigation is practiced.

3.2.2 Principle of Sedimentation The process of sedimentation is based on the laws of settling velocities of granular

particles. Basically three forces control the settling tendencies of the particle. These are:

• Velocity of the flow

• Size and shape of the particles

• Viscosity of the sewage or the temperature of the sewage Out of the three forces only the first two are controlled to perform the sedimentation in the sedimentation tank and the last one is left uncontrolled as this is not practically possible to

control. The velocity of flow can be reduced by increasing the length of the travel and by detaining the particle longer time in the sedimentation basin. As regards, size and shape of

the particle can be altered by the addition of certain chemicals.

3.2.3 Classification On the basis of function sedimentation can be classified into following types:

a. Intermittent type

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b. Continuous type Now-a-days the intermittent type is not used as it is time consuming, labor is wasted and

more units are required. They have become completely outdated on these days. Continuous types are designed to operate continuously and extensively used now days.

They are mainly following two types:

• Rectangular sedimentation tank

• Circular sedimentation tank 3.2.4 Design Parameters

⇒ The tank should be designed for the daily average flow or the daily flow equivalent to peak hourly flow that require the largest surface area.

⇒ All tank piping, channel, inlets, outlets and weirs will be designed to accommodate the peak flows

⇒ Average hourly should be used 3 times if specific peak flows are not documented

⇒ Each tank will be sized, as a maximum, for 67 percent of the plant design flow

⇒ At treatment plants with less than 0.1 million gallons per day treatment capacity, one unit is acceptable when an equalization tank or holding basin is constructed with adequate volume to dampen out peak inflow rates.

3.2.5 Design Consideration Following consideration should be performed while designing

• Detention period o Detention time is commonly specified as 2.5 hours for primary tanks

serving all types of plants. o Plants when preceding activated sludge system detention time is specified

as 1.5 hours o Selection of optimum detention time will depend on tank depth and the

over flow rate.

• Weir rate o The over floating rate on weirs should not exceed 5,000 gallons per day per

lineal foot for plants designed for less than 0.1 million gallons per day.

o For plants designed between 0.1 and 1 million gallons per day, the over flow rate should not exceed 10,000 gallons per day per lineal foot.

o In case of more than 1 million gallons per day the design value should not exceed 12,000 gallons per day per lineal foot

o Inlets to a settling tank will be designed to dissipate inlet velocity, distribute the flow uniformly and prevent short circuiting

o The inlet outlet channels will be designed for a minimum velocity of 2 feet per second at the average flow rate.

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3.2.6 Rectangular Sedimentation Tank Purpose of using a rectangular sedimentation tank:

• Easy to operate and low maintenance costs.

• Easy adaptation to high-rate settlers and tolerant to shock loads.

• Commonly used in municipal and industrial applications.

• Suited to large capacity plants. The minimum length of flow from inlet to outlet of a rectangular tank will be 3m. In

existing installations, tank length-to-width ratio varies between 3:1 and 5:1. Tanks will be designed with a minimum depth of 2.15m except final tanks in activated sludge plants,

which will be designed with a 3m minimum depth. A typical rectangular sedimentation tank is shown below

Fig;

3.1

Typ

ical

Rec

tang

ular

Tan

k

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3.2.7 Circular Sedimentation Tank Purpose of using a circular sedimentation tank:

• Easy sludge removal.

• High clarification efficiency.

• Used for small to medium sized applications.

• Best suited for plants with constant flow rates and quality. Circular tank diame ters range from 8 to 45 m. S ide -water depths are 2m as a minimum,

and tank floor are deeper at the center. Scum baffles extending down to 15cm below the water surface will be provided ahead of the overflow weir; and the distance between scum

collection troughs will not exceed 23m along the periphery of the clarifier. A circular sludge-removal mec hanism with peripheral speeds of 1.5m to 2.5m per minute will be

provided for sludge collection at the center of the tank. A typical circular tank is shown below:

Fig;

3.2

A T

ypic

al C

ircu

lar T

ank

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3.3 SEPTIC TANK A septic tank may be defined as a primary sedimentation tank with a longer detention

period (12 to 36 hours, against a period of ordinary sedimentation tank) and with extra provisions for digestion of the settled sludge.

A septic tank is thus a horizontal continuous flow type of sedimentation tank, directly admitting raw sewage and removing about 60% to70% of the dissolved matter from it.

Septic tank is designed to accomplish two criteria.The tasks are as follows:

• Clarification

⇒ Clarification is a function of the detention time and the water extraction method. Solids settle out of the water based on size and specific gravity. Smaller lighter particles take longer to settle than heavier particles. Clarification also includes the removal of fats, oils and greases, which

float to the surface along with soapsuds and “scum”. The variables of a septic tank are: size, shape, number of chambers, number and style of

baffles and gas venting provisions.

• Treatment

⇒ Treatment consists of biological treatment by anaerobic digestion. Anaerobic treatment partially decomposes the organic matter into simpler

compounds that can be treated further in the septic tank or discharged into the soil for aerobic treatment.

Fig; 3.3 A typical Septic tank

3.3.1 Design Consideration

• Capacity of Septic Tanks: A septic tank should be capable of storing the sewage flow during the detention period. The rate of accumulation of sludge has been recommended as 30 liters/person/year. The minimum

capacity for about 8 to 10 persons may be kept 2250liters and 1400liters.

• Free-board: A free-board of about 0.3 m may be provided above the top sewage line in the tank

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• Inlet and out let baffles: The baffles or tees should extend of about 20 cm above the top sewage line. Inlet should penetrate by about 30 cm below

the top sewage line and outlet should penetrate of about 40% of the depth of the sewage. Finally the outlet invert level should be kept 5 to 7.5 cm

below the inlet invert level

• Detention period: The detention period should generally varies between 12 to 36 hours; but commonly 24 hours

• Length to width ratio: They are usually rectangular with their length about 2 to3 times the width. The width should not be les than 90 cm. The depth of tank generally varies between 1.2 to 1.8m

3.3.2 Advantages and Disadvantages of septic Tanks:

• Advantages o Can be easily constructed and d not require any skilled super vision

o Cost is reasonable compared to the advantages and sanitation they offer o A proper functioning septic tank can considerably reduce solids and BOD

from sewage

o The effluent from septic tank can be disposed of on land in a soak pit or a cess-pool without much trouble

o They are best suited for isolated rural areas, and for isolated hospitals, buildings etc

• Disadvantages o In case of not properly functioning, the effluent will be very foul, dark and

even worser than the influent

o Require larger sizes for serving too many people o Leakage of gases from the septic tank may cause bad smells and

environmental pollution o Working of septic tank is unpredictable and non uniform.

3.4 IMHOFF TANK The Imhoff tank obtained its name from its inventor, Dr. Karl Imhoff of Germany. The

technology was developed in the Emscher District of Germany and patented in 1906 by Dr. Imhoff. An Imhoff tank is infact a two storied tank, so they are sometimes also known

as Two-storey Digestion tank. It is an improved version of septic tank designed to overcome two main defects of septic tank:

1. It prevents the solids once removed from the sewage from again being mixed with it still provides the decomposition of these solids in the same unit

2. It provides an effluent amenable to further treatment

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3.4.1 Construction Feature The Imhoff tank may be either rectangular or circular and is devided into three

compartments: 1. The upper section or sedimentation compartment

2. The lower section or digestion compartment and 3. The gas vent or scum section

The settling of solids occurs in an upper chamber and digestion of the solids in the lower chamber. The two chambers are separated by a sloping partition that contains (Fig: 3.4

Simplified Imhoff Tank) narrow slots through which the solids passes into the lower chamber. Solids settle out in the upper sedimentation chamber and gradually flow into the

lower digestion chamber. In the digestion chamber, solids accumulate and slowly digest. By design, gas and scum are prevented from entering the sedimentation chamber due to

the narrow slots that disallow gas and sludge particles from entering the sedimentation chamber.

Fig; 3.4 Schematic Diagram of Imhoff Tank

3.4.2 Design Considerations

• Sedimentation chamber

o Detention period: 2 to 4 hours (usually 2 hours)

o Flowing velocity should not be more than 0.3m/minute

o Surface loading should not exceed 30,000 liters/m2 of plant area per day

o In case of effluent coming from activated sludge plant the surface loading

can be adopted to 45,000 l/m2/day

o Length of tank should preferably not exceed 30m or so. Length to the width

ratio varies between 3 to 5

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o Depth of the chamber should be kept shallow as far as possible. Practically

a total depth of 9 to 11m has been found to be satisfactory. The free board

should be provided may be about 45 cm

• Digestion chamber

o The chamber is generally designed for a minimum capacity of 57 liters per

capita

o In warmer climates the capacity may be reduced to about 35 to 40 liters per

capita, where shorter periods between the sludge withdrawals are possible

• Gas vent or Scum chamber

o The surface area of the scum chamber should be about 25 to 30% of the

area of horizontal projection of the digestion chamber.

o The width of vent should be 60 cm or more

3.4.3 Advantages and disadvan tages of imhoff tank

• Advantages

o It requires little space and has a small “foot print”

o It is simple as it has no moving parts as mixers, collectors

o It requires little operator time as sludge removal is periodic

o At the time, it offered a better solution to solid handling and digestion

• Disadvantages

o Depth of the tank is more, which make the construction costlier

o If improperly operated the tank may give out offensive odors

o They are unsuitable and do not function properly where sewage is highly

acidic in charac ter

o There is no adequate control over their operation; this makes them

unsuitable for use in large treatment plants where separate sludge digestion

is preferred in addition to sedimentation tank.

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4.0 CONCLUSION Sewage treatment is a complex process but it is most important to the community.

Untreated sewage will spread diseases, kill plants, reduce O2 in the water, create nuisance

and give rise to many more side effects that can’t be expressed in a single sentence. Here

in Bangladesh sewage treatment is utmost necessary, where most of the sewage is left

untreated. Though selection of sewage treatment process depends on several factors,

attention should be made on cheapest and suitable treatment. From the descriptive analysis

of this paper on preparatory and primary treatment it appears that they are the most initial

process in any sewage treatment plant and comprises of variety of units. Although the

preparatory treatment is some time combined with the primary treatment and addressed as

only primary treatment. Choice of treatment units depends on the accessibility and the

quality of the sewage to be treated, as better selection and treatment in the initial step will

lead to more effective treatment in the subsequent stages.

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