PREPARATORY AND PRIMARY SEWAGE TREATMENT:A LITERATURE REVIEW Dipangkar kundu1 1 Agriculture 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. CONTENTS CHAPTER ONE 1.1 1.1.1 1.1.2 1.2 1.2.1 CHAPTER TWO 2.1 2.2 2.2.1 2.2.1.1 2.2.1.1.1 2.2.1.1.2 2.2.1.1.3 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.3 2.2.3.1 2.2. 3.2 2.2.3.3 2.2.3.4 CHAPTER THREE 3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7 3.3 3.3.1 3.3.2 3.4 3.4.1 3.4.2 3.4.3 4.0 INTRODUCTION SEWAGE Need for Sewage Treatment Purposes of Sewage Treatment SEWAGE TREATMENT Methods of Sewage Treatment PREPARATORY TREATMENT INTRODUCTION Treatment Process Screening Screens Control system and servicing Comminutors or cutting screens Disposal of screenings Grit chamber Placement Design consideration Cleaning and disposal Skimming Tanks Necessity Process Design parameter and efficiency Disposal of skimming PRIMARY TREATMENT INTRODUCTION Sedimentation Tanks Objective of sewage sedimentation Principle of sedimentation Classification Design parameters Design consideration Rectangular sedimentation tank Circular sedimentation tank Septic Tank Design consideration Advantage and disadva ntage of septic tank Imhoff Tank Construction feature Design consideration Advantages and disadvantages of imhoff tank CONCLUSION REFERENCES 2 PAGE NO 4 4 4 5 5 9 9 9 9 11 11 12 12 13 13 14 14 14 14 15 15 16 16 16 16 16 17 17 18 19 20 20 21 21 22 22 23 24 25 chemical plants etc) and some surface water from roofs and hard-standing areas. coastal waters and tidal estuaries. showers. various forms of dysentery and cholera. 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. slaughter houses. soil. paper and pulp mills. 1.1. textile processing mills. 3 . fever. canals.1. baths.1 SEWAGE Sewage is a complex substance which pollutes the various phases of human environment such as air. those bacteria may reach the milk during the process of milking • Sewage may contain bacteria that cause typhoid. water.1 Need for Sewage Treatment Sewage before being disposed of either in river.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. industrial wastes or trade wastes (liquids from manufacturing process such as tanneries.CHAPTER ONE INTRODUCTION 1. It may be defined as the liquid waste of a community which includes domestic wastes (human exc reta. 1. distilleries. lavatories etc from private or public buildings). land or other places has generally to be treated to make it safe both for environment and living beings. discharges from kitchen. 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. food and hence endangers human life as well as other living beings. Treatment and disposal of sludge v. pulp and paper mill and. Actually the distinction between primary.2. Preparatory treatment / Preliminary treatment ii. The segregated biological solids undergo additional treatment and neutralization prior to proper disposal or re-use. or collected and conveyed via a network of pipes and pump stations to a municipal treatment plant.). The choice of treatment. Table 1. 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.1. the treated water may undergo additional disinfection via chemical or physical processes.1 Methods of Sewage Treatment The process of sewage treatment can be typically classified into following five categories: i.2 SEWAGE TREATMENT Sewage treatment incorporates physical. methods depends upon several factors. septic tanks or onsite package plants). chemical and biological contaminants from water following human use. Typically. greenways. water-borne bacteria. It can be treated onsite at the point of which it is generated (e.g. etc. including the disposal facilities available. Treatment and disposal of liquids Combination of preparatory and primary and secondary treatment constitutes the complete treatment. Once the biological mass is separated or removed. Each type of treatment method uses various treatment units.1 shows the units used in different treatment methods and their performance.. chemical and biological process in the same operation. Primary treatment iii. Secondary treatment (Biological treatment) iv. 1. 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. chemical and biological processes which treat and remove physical. 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.. secondary and tertiary is rather arbitrary. chemical manufacturer wastewaters). golf courses. 4 . since many modern treatment methods incorporate physical.g. 20-30 40-60 25-75 30-40 60-65 25-75 The effluents are either treated for secondary treatment or directly disposed by land treatment. These units combine sludge digestion along with sedimentation. Removal of large a)Plain suspended organic sedimentation tanks solids Or 30-35 60-65 25-75 Sludge containing organic material has to be stabilized first in digestion tanks and the digested material then used as manure. Type of Purification Treatment Affected 2 Preparatory Treatment Primary treatment Units employed b) Septic tanks Or c) Imhoff tanks 5 Disposal of residuals .Table: 1.1 No 1 1 2 BOD removal (%) Suspended solids removal (%) Bacterial removal (%) 3 4 a) Removal of Coarse and fine floating materials screens of different designs b) Removal of Grit chambers or settleable inorganic detritus tanks solids c) Removal of fats Skimming tanks and grease 5 5-10 6 2-20 7 10-20 8 Screenings can be disposed easily either by burials or burnings 10-20 20-40 10-20 20-30 20-40 10-20 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. 1 3 4 2 Secondary or Biological Treatment Final Treatment 3 Removal of fine suspended and dissolved organic matter 4 5 a)Chemical flocculation 50-85 or sedimentation or 6 70-90 7 40-80 8 Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder 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 90-95 85-95 95-98 Sludge containing organic material has to be stabilized in diges tion tanks and the residue then used as manure or soil builder 65-95 65-92 80-95 Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder 90-95 70-92 90-95 Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder 75-95 85-90 90-98 Sludge containing organic material has to be stabilized in digestion tanks and the residue then used as manure or soil builder 85-90 85-90 90-98 Effluents are generally disposed of using them for irrigation Discharged to receiving environment. 100 100 100 Removal of Chlorinator pathogens and remaining very fine organic matter 6 . 1 Typical Flow Diagram of a Sewage Treatment Plant Fig 1. 7 .1 shows the positions of different sewage treatment methods in the sewage treatment facilitie s.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. Preliminary and Primary treatment will be discussed in the following chapter. 1 INTRODUCTION Preparatory treatment consists solely in separating the floating materials (like dead animals. 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. 2.1 Screens Depending on the size of the opening screens may be classified as following types: • Coarse Screens: Also known as Rack or Bar Screen.2.2 Treatment Process Preparatory treatment is carried out through the following process: ⇒ Screening o Removes floating paper. The spacing between the bars is about 50 mm or more. 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. These types of screen do help in removing large floating objects 8 . rags. 2. 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.CHAPTER TWO PREPARATORY TREATMENT 2. cloths etc ⇒ Grit chambers or Detritus tank o Removes grit and sand ⇒ Skimming tanks o Removes oil and grease 2.2. paper. tree branche s.1. 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.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 scree ns. They will collect about 6 liters of solids per million liter of sewage. pieces of rags. so as to trap and remove the floating materials. 1: Bar type Screen • Medium screens: In these types of screens the spacing between the bars is about 6 to 40 mm. • Fine screens: These types of screens have perforations of 1. The screening usually contains some quantity of organic material. The fine screens may be classified into disc or drum type and are operated continuously by electric motors. in order to increase opening area and reduce flow velocity.2: Disc type of Fine screen (Section) 9 . They are very effective and remove as much as 20% of suspended solids from sewage.1 Fig. Hence are only used for treating the industrial wastes or for treating those municipal wastes that are associated with heavy amount of industrial wastes. Bars are made of steel fixed parallel to one another at desired spacing on a rectangular steel frame.5 mm to 3mm in size.2. Nowa-days these screens are generally kept inclined at about 30 to 60° to the direction of flow. Fig 2. clogged very often and need frequent cleaning. These screens however .from sewage. It may be placed at either end of the chamber but generally at the inlet end. It is often used in conjunction with Grit chamber.2 shows the section of the fine screen. They will cordially collect 30 to 90 liters of material per million liter of sewage.2. Fig. A most commonly used bar type screen is shown in the figure 2. Manual cleaning is practiced at small plants with hand operated racks. Following are some provisions that should be ensured.2.8 m. high water elevation and a device to stop the cleaning operation after a predetermined length of time. fine screens or mechanically cleaned bar screens.1. They consist of revolving slotted drum (Fig. ⇒ The drive mechanism of a mechanically cleaned screen shall be enclosed.1. Cutters mounted Fig. ⇒ Assurance should be provided that the head loss through the screening unit at peak hourly flows will not surcharge the incoming sewer. when the sewage is screened through them. 2.2 Comminutor / Cutting screen 10 . In large plants mechanically operated racks are used to move over the screens either continuously or intermittently. ⇒ The maximum allowable head loss through clogged racks and bar screens is generally limited to 0. ⇒ Due consideration shall be given to the selection of materials because of corrosive conditions present.2. This kind of device breaks the larger sewage solids to about 6mm in size. assuming frequent attention by operating personnel. 2.2 Comminutors or Cutting screens Comminutors are usually provided in plants that do not have primary clarifiers.2. the absolute minimum head loss allowance through a manually cleaned bar screen is 15 cm.1.2. ⇒ 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. Generally.1 Control systems and servicing The material collected on the screens can be removed either by manually or by mechanically.1.2) through which the sewage is screened. • 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%. 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. design to separate the intended heavier inorganic materials by the process of sedimentation due to gravitational force. and the content can be used as manure. They remove the inorganic particles having specific gravity of about 2. Following methods of disposal are usually practiced. The duping on land for raising low lying areas is also adopted when screenings are from coarse screens. as such not containing much organic load. 2. • Digestion: In this process screenings along with the sewage sludge is digested in a sludge digestion tank. Dumping in the sea is only suitable only where strong forward current do exist to take the dump material away from the sore line.2mm larger to prevent damage to pumps or to prevent their accumulation in sludge digesters. • Dumping: In this process the screenings are dumped in low lying areas or in large bodies of water (Sea).65. They are nothing but kind of sedimentation tanks. eggshells.3 Disposal of Screenings The material separated by screens is called screening. 2. bones etc of size 0. It may also contain some organic load which may putrefy causing bad smells and nuisance.1. In this method the screenings are buried in 1m to 1. These are usually arranged in pairs to facilitated repairs and maintenance. 11 . It is not so successful method.2 .3 to 0.45m of porous earth.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.5m trenches and then covered with 0.2 GRIT CHAMBERS A grit chamber is an enlarged channel or a long basin. • Burial: The process is technically called Composting. and not from medium or fine screens.1. gravel grit.2. The incineration is carried out at temperatures of about 760 to 815°C. Such as sand. It contains 85 to 90% moisture and other floating material. In due course of time oxidation reduction of screenings will take place . Comminutors should always be preceded by grit chambers to prevent excessive wear. 2mm in dia and of specific gravity 2. maintenance and handling of materials. This intern also helps in manually cleaning of the chambers as one unit can work. However.2. 12 . Length of the tank = velocity × Detention period. Velocity of flow between 0.3 m/sec with a depth of about 0.3 Modern Grit chambers pass the low flow and the other to pass (along with the first of course) the high flow. In fact. 2. The grit chambers are generally designed for a flow velocity of 0. 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.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. but ahead of Comminutors.15 to 0.2. while other unit is shut down for cleaning. Two to three separate chambers in parallel should be provided (Fig 2. Where grit chambers are mechanized.65.15 to 0. pumps and other treatment units.2 m and a detention period of about 1 min.9 to 1.1 Placement Grit chambers should be located after the coarse bar racks or coarse bar screens.2.2 .3). nor should it would be too high as not to cause the settlement of the entire silt and grit-present in sewage. it should be readily accessible for inspection.225 m/sec is just sufficient to settle down all the inorganic matter larger than 0.2.2. theoretically a velocity of 0.3m/sec is practically considered sufficient to meet this purpose. One chamber to Fig. whether the screening and/or comminuting devices are located ahead of or after the grit chamber. 2. soap and candle factories. Precaution should be maintained that it should be used for any construction as it contains sufficient organic matter. which includes fats. waxes. by hand. 2. 13 .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. etc. from garages. 2. These materials may enter from sewage .3. ⇒ In mechanical cleaning. fatty acids etc. On the contrary heavy solids settle down at the bottom and slide back into the central aerated compartment.3).3. grit is removed with the help of machines and in hydraulic cleaning.2 Process In skimming tank air is blown by an aerating device through the bottom to create spiral circulation (Fig: 2. oil refineries.2. The silt.2. Consideration should be given in large plants for loading facilities to discharge grit directly by mechanical means to a transport vehicle.2. ⇒ Grit conveying and transporting equipment shall be provided with protection again st freezing and loss of material. ⇒ In manual cleaning grit is removed with shovels etc.2. 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.3 SKIMMING TANKS Skimming tanks are employed for removing oil and grease from sewage.5 million liters per day) ⇒ Mechanically or hydraulic cleaning is done for larger plants. Manually cleaning is done for small plants (Capacity of about 4. 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. 2. soaps.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. where it is moved to the outlet by the injected air. 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. from kitchens of restaurants and houses. 25 m/min in most cases 2. They can be disposed of by either burning or land burial. sometime it may be converted into soap lubricants. Diffusers placed in the bottom of the tank blows the compressed air to raise the grease and oil to the surface. It is done by means of vertical baffle walls. However.3 Skimming Tank Oil and grease that are collected are drawn off into a separating tank from where excess water is removed.3. They help in pushing the coagulated greasy material into side compartments.2.00622 q Vr 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.Fig: 2. 14 .3. They are so polluted that they can hardly be put into any economic use.2. candles and non-edible products.4 Disposal of skimming Skimming materials are mainly oil and grease. The surface area required for the tank can be found by the following formula: A = 0.3 Design Para-meter and Efficiency A typical skimming tank consists of long through-shaped structure divided into three lateral compartme nts. having slots in them. 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. 2. Basically three forces control the settling tendencies of the particle. size and shape of the particle can be altered by the addition of certain chemicals. 3. As regards. 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. 3. 3. The primary treatment effluents often contain a large amount of suspended organic material and have a high BOD.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.2 Principle of Sedimentation The process of sedimentation is based on the laws of settling velocities of granular particles. The primary treatment devices reduce the velocity and disperse the flow of waste water. 3.3 Classification On the basis of function sedimentation can be classified into following types: a. This situation can be brought under by specially designed tanks called sedimentation tanks. 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.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.CHAPTER THREE PRIMARY TREATMENT 3. Principal primary treatment devices are referred to as sedimentation tanks. Intermittent type 15 . septic tanks and imhoff tanks.1 INTRODUCTION Primary treatment is designed to remove solids which are not removed during preparatory treatment by the physical process of sedimentation and (flotation).2. 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.2. In most of the facilities primary treatment is used as preliminary step ahead of biological treatment. 3.1 and 1 million gallons per day. for 67 percent of the plant design flow ⇒ At treatment plants with less than 0.2.000 gallons per day per lineal foot. as a maximum. Continuous types are designed to operate continuously and extensively used now days. • Weir rate o The over floating rate on weirs should not exceed 5.5 Design Consideration Following consideration should be performed while designing • Detention period o Detention time is commonly specified as 2.2. o For plants designed between 0. Continuous type Now-a-days the intermittent type is not used as it is time consuming. They are mainly following two types: • Rectangular sedimentation tank • Circular sedimentation tank 3.1 million gallons per day. 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.000 gallons per day per lineal foot for plants designed for less than 0. ⇒ All tank piping. inlets.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. one unit is acceptable when an equalization tank or holding basin is constructed with adequate volume to dampen out peak inflow rates. o Plants when preceding activated sludge system detention time is specified as 1.000 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. the over flow rate should not exceed 10.5 hours o Selection of optimum detention time will depend on tank depth and the over flow rate. channel. 16 .b. labor is wasted and more units are required. They have become completely outdated on these days.1 million gallons per day treatment capacity. o In case of more than 1 million gallons per day the design value should not exceed 12.5 hours for primary tanks serving all types of plants. 3. • Suited to large capacity plants. • Easy adaptation to high-rate settlers and tolerant to shock loads. 3. • Commonly used in municipal and industrial applications.2. Fig. In existing installations. A typical rectangular sedimentation tank is shown below 17 .15m except final tanks in activated sludge plants.1 Typical Rectangular Tank which will be designed with a 3m minimum depth.6 Rectangular Sedimentation Tank Purpose of using a rectangular sedimentation tank: • Easy to operate and low maintenance costs. The minimum length of flow from inlet to outlet of a rectangular tank will be 3m. Tanks will be designed with a minimum depth of 2. tank length-to-width ratio varies between 3:1 and 5:1. 2 A Typical Circular Tank provided for sludge collection at the center of the tank. 3. S ide -water depths are 2m as a minimum.2. • Used for small to medium sized applications.5m to 2.3. • Best suited for plants with constant flow rates and quality.5m per minute will be Fig. Circular tank diame ters range from 8 to 45 m. 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 1.7 Circular Sedimentation Tank Purpose of using a circular sedimentation tank: • Easy sludge removal. Scum baffles extending down to 15cm below the water surface will be provided ahead of the overflow weir. A typical circular tank is shown below: 18 . and tank floor are deeper at the center. • High clarification efficiency. 1 Design Consideration • Capacity of Septic Tanks: A septic tank should be capable of storing the sewage flow during the detention period. 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. A septic tank is thus a horizontal continuous flow type of sedimentation tank.The tasks are as follows: • Clarification ⇒ Clarification is a function of the detention time and the water extraction method.3. • Treatment ⇒ Treatment consists of biological treatment by anaerobic digestion. which float to the surface along with soapsuds and “scum”. number of chambers. against a period of ordinary sedimentation tank) and with extra provisions for digestion of the settled sludge.3 SEPTIC TANK A septic tank may be defined as a primary sedimentation tank with a longer detention period (12 to 36 hours. Solids settle out of the water based on size and specific gravity. 3. The rate of accumulation of sludge has been recommended as 30 liters/person/year. Fig. shape. oils and greases. The variables of a septic tank are: size. Smaller lighter particles take longer to settle than heavier particles. Clarification also includes the removal of fats. • Free-board: A free-board of about 0.3 A typical Septic tank 3. Septic tank is designed to accomplish two criteria. The minimum capacity for about 8 to 10 persons may be kept 2250liters and 1400liters.3.3 m may be provided above the top sewage line in the tank 19 . directly admitting raw sewage and removing about 60% to70% of the dissolved matter from it. number and style of baffles and gas venting provisions. • Inlet and out let baffles: The baffles or tees should extend of about 20 cm above the top sewage line.5 cm below the inlet invert level • Detention period: The detention period should generally varies between 12 to 36 hours. Dr. Karl Imhoff of Germany.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. the effluent will be very foul.2 to 1. Imhoff. so they are sometimes also known as Two-storey Digestion tank.4 IMHOFF TANK The Imhoff tank obtained its name from its inventor. 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. The width should not be les than 90 cm. It is an improved version of septic tank designed to overcome two main defects of septic tank: 1. It provides an effluent amenable to further treatment 20 . and for isolated hospitals. but commonly 24 hours • Length to width ratio: They are usually rectangular with their length about 2 to3 times the width. Finally the outlet invert level should be kept 5 to 7. 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. An Imhoff tank is infact a two storied tank. The technology was developed in the Emscher District of Germany and patented in 1906 by Dr. buildings etc • Disadvantages o In case of not properly functioning.3. 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. The depth of tank generally varies between 1. Fig.4. The lower section or digestion compartment and 3. solids accumulate and slowly digest. 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. The upper section or sedimentation compartment 2. In the digestion chamber. 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.3.2 Design Considerations • Sedimentation chamber o Detention period: 2 to 4 hours (usually 2 hours) o Flowing velocity should not be more than 0.1 Construction Feature The Imhoff tank may be either rectangular or circular and is devided into three compartments: 1.000 liters/m2 of plant area per day o In case of effluent coming from activated sludge plant the surface loading can be adopted to 45.3m/minute o Surface loading should not exceed 30. Solids settle out in the upper sedimentation chamber and gradually flow into the lower digestion chamber.4. The two chambers are separated by a sloping partition that contains (Fig: 3.000 l/m2 /day o Length of tank should preferably not exceed 30m or so. By design. Length to the width ratio varies between 3 to 5 21 .4 Schematic Diagram of Imhoff Tank 3.4 Simplified Imhoff Tank) narrow slots through which the solids passes into the lower chamber. this makes them unsuitable for use in large treatment plants where separate sludge digestion is preferred in addition to sedimentation tank. collectors o It requires little operator time as sludge removal is periodic o At the time. 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.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. Practically a total depth of 9 to 11m has been found to be satisfactory. 22 . it offered a better solution to solid handling and digestion • Disadvantages o Depth of the tank is more.4. o The width of vent should be 60 cm or more 3. The free boar d 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.o Depth of the chamber should be kept shallow as far as possible. 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. kill plants. reduce O 2 in the water.0 CONCLUSION Sewage treatment is a complex process but it is most important to the community. Although the preparatory treatment is some time combined with the primary treatment and addressed as only primary treatment. where most of the sewage is left untreated. 23 . Here in Bangladesh sewage treatment is utmost necessary. Untreated sewage will spread diseases. as better selection and treatment in the initial step will lead to more effective treatment in the subsequent stages. Choice of treatment units depends on the accessibility and the quality of the sewage to be treated.4. 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. Though selection of sewage treatment process depends on several factors. attention should be made on cheapest and suitable treatment. create nuisance and give rise to many more side effects that can’t be expressed in a single sentence. RFEERENCES Aziz. J.wikipedia. International Textbook Company Imhoff. 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