The priliminary treatment of sewage involves the use of only physical unit operations such as screening, comminution, grit removal, skimming, floatation, etc. The physical unit operations involved in preliminary treatment of seawge are accomplished by employing various units or appurtenances which include screens, comminutors, grit chambers or detritus tanks, skimming tanks, floatation units, etc.
1. Screening—Screens:
Screening is the first operation carried out at sewage treatment plant. It consists of passing sewage through a screen so as to trap and remove floating materials present in sewage, which would otherwise clog and damage pumps and other equipment, interfere with the satisfactory operation of treatment units or equipment or cause objectionable shoreline conditions where disposal into sea is practised.
A screen is a device with openings generally of uniform size. The screening element may consist of parallel bars or flats, rods or wires, grating, wire mesh or perforated plate, and the openings may be of any shape but generally they are circular or rectangular.
A screen composed of parallel bars or fiats or, rods is called a rack or bar rack or bar screen. The bars or flats or rods used for bar screens are of rectangular or trapezoidal section placed vertically or inclined at a slope varying from 30° to 80° with horizontal or curved and spaced at close and equal intervals across a chamber or a channel through which sewage flows. The function performed by a screen is called screening and the materials removed by it are known as screenings or rakings.
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Quantity of Screenings and Their Disposal:
The quantity of screenings varies with the size of screen used and on the nature of sewage. Generally it has been found that the screenings from domestic or sanitary sewage vary from 0.0015 m3/Ml with screen size of 100 mm to 0.015 m3/Ml in case of 25 mm size.
Screenings should not be left in the open or transported in uncovered conveyors as it would create nuisance due to flies and insects. If conveyors are used, they should be kept as short as possible for sanitary reasons.
Screening are disposed of by the following methods:
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(i) Burial,
(ii) Incineration,
(iii) Composting, and
(iv) Digestion.
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In small installations screenings can be disposed of by burial in trenches usually 75 mm to 100 mm deep. At large installations where sufficient land for burial is not available within a reasonable distance from the plant screenings are incinerated. However, screenings usually contain about 80 per cent moisture by weight and will not burn without pre-drying.
As such before incineration screenings need to be dried. In most incinerators, the fresh screenings are dried by the heat of the fire before they enter the firebox. Screenings may also be dried by spreading them over the ground and exposing them to the sun.
The dried screenings are incinerated either by utilizing the sludge gas obtained from the digestion tank or by using fuel gas or oil as fuel for the incinerator. Where possible the screenings are transported and mixed with town refuse for production of compost. Screenings can also be placed in sludge digestion tanks for digestion.
2. Comminution—Comminutors:
Comminution may be defined as the process of cutting the large size solids present in sewage into smaller pieces of more or less uniform size of about 6 mm. This is carried out to improve the downstream operations and processes and to eliminate problems caused by the varied sizes of solids that are present in sewage. Devices that are used to comminute or cut up the solids in sewage are known as comminutors.
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Various types of comminutors are available out of which two types of comminutors are described below:
A type of comminutor consists of a vertical revolving cast iron drum screen with 6 mm slots in small machines and 10 mm slots in large machines. The drum screen is rotated by means of a motor located on top. A stationary comb-shaped steel cutting plate is provided with its comb-teeth opposite the drum slots and just clearing the slot surface.
A set of steel projections called shear bars are mounted on the surface of the drum and these projections pass between the comb-teeth when the drum rotates. The larger solids are cut by the cutting teeth and the shear bars as the solids are carried past the stationary comb. The small sheared particles pass through the drum slots along with the sewage. The sewage then comes out through the open bottom of the drum and flows through an inverted siphon into the downstream channel which carries it to the treatment plant.
In another type of comminutor sewage enters a slotted cylinder within which another similar cylinder with sharp-edged slots rotates rapidly by means of a motor located on top. The larger solids caught between the two cylinders are cut into smaller pieces due to the shearing action. As the solids are reduced in size, they pass through the slots of the cylinders and move on with the liquid to the treatment plant.
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Where coarse screens or bar racks are not used, comminutors can be used to cut up the solids without removing them from the sewage. Often, comminutors are used to cut up the material retained on the screens so that it may be returned to the flow stream for removal in the subsequent downstream treatment operations and processes.
Thus comminutors eliminate the problem of disposal of screenings by reducing the solids to a size that can be processed elsewhere in the treatment plant. Comminutors are also frequently installed in the wet well of pumping stations to protect the pumps against clogging by rags and large objects.
Provisions must be made to bypass comminutors in case flows exceed the capacity of the comminutor or in case there is a power or mechanical failure. Comminutors are, however, not yet in common use in India.
3. Grit Removal-Grit Chambers:
Sewage contains considerable amount of grit which consists of sand, gravel, silt, ash, cinders, clinkers, egg shells, bone chips and several other inert inorganic materials.
Both quality and quantity of grit varies depending upon:
(a) Types of street surfaces encountered,
(b) Relative areas served,
(c) Climatic conditions,
(d) Types of inlets and catch basins,
(e) Amount of storm water (or rain water) diverted from combined sewers at overflow points,
(f) Sewer grades,
(g) Construction and condition of sewer system,
(h) Ground and groundwater characteristics,
(i) Industrial wastes,
(j) Relative use of dumping chutes or pail depots where night soil and other solid wastes are admitted to sewers, and
(k) Social habits.
The specific gravity of grit is usually in the range of 2.4 to 2.65. Grit is non-putrescible and possesses a higher hydraulic subsidence value (HSV) or settling velocity than organic solids. Hence it is possible to separate the gritty material from organic solids by differential sedimentation.
Grit removal is necessary to protect the moving mechanical equipment and pump elements from abrasion and accompanying abnormal wear and tear, to reduce formation of heavy deposits in pipes, channels and conduits, and to reduce the frequency of cleaning of sludge digesters.
For removing grit from sewage, grit chambers (also called grit channels or grit basins) are provided. A grit chamber is an enlarged channel or long basin in which the cross-section is increased to reduce the velocity of flowing sewage to such an extent that the heavier grit settles down at the bottom of the grit chamber while the lighter organic matter remains in suspension and thus carried along with the effluent of the grit chamber for further treatment.
Thus in the design of grit chambers the most important consideration is that the velocity of flow of sewage should neither be so low as to cause the settling of the lighter organic matter, nor be so high as to not to cause the settlement of the entire grit present in the sewage.
Design of Grit Chambers:
The basic data essential for a rational approach to the design of grit chambers are hourly variations of sewage flow and typical values for minimum, average and peak flows. Since grit chamber is designed for peak flows and the velocity of flow through grit chamber is maintained constant within the range of flow, the accurate estimation of the flows is necessary for successful design and operation of grit chamber.
The quantity and quality of grit varies from sewage to sewage. Data relating to these two factors is very useful in proper design of grit collecting, elevating and washing mechanisms. However, in the absence of specific data, grit content may be taken as 0.05 to 0.15 m3 per million litre for domestic sewage and 0.06 to 0.12 m3 per million litre for combined sewage. The quantity of grit may increase three to four fold during peak flow hours which may last for 1 to 2 hours.
Detritus Tanks:
Detritus tanks are the grit chambers designed with a smaller flow through velocity (about 0.1 to 0.2 m/s) and a longer detention period (about 3 to 4 minutes) so as to separate out not only the larger size grit but also very fine sand particles. Further due to smaller flow through velocity and longer detention period a large amount of organic matter will also settle down along with grit.
A detritus tank is a continuous flow settling tank of rectangular or square shape. The sides of the tank are vertical and these are tapered at the bottom so as to form trough for the collection of the settled material or detritus, which is a mixture of heavier grit and lighter organic solids.
The lighter organic matter is then separated from the detritus by the following methods:
(i) Resuspending the lighter organic matter by passing compressed air through the deposited detritus;
(ii) Removing the detritus from the tank and washing it with water which is then sent with the effluent of the detritus tank;
(iii) Placing the detritus on a conveyor and passing the conveyer through water such that the fine organic matter is flushed back into the sewage.
The grit is removed continuously by means of scraper mechanism. All other details of detritus tanks remain the same as those of grit chambers.
Disposal of Grit:
Clean grit is characterized by the lack of odours. Washed grit may resemble particles of sand and gravel, interspersed with particles of egg shell and other similar relatively inert materials from the households. Grit washing mechanism has to be included whenever the detention time is more and flow through velocity is less.
Unless washed, it may contain considerable amount of organic matter. This becomes an attraction to rodents and insects and is also unsightly and odorous. The grit may be disposed of by dumping or burying or by sanitary land fill.
The ultimate method used however depends upon the quantity and characteristics of the grit, availability of land for dumping, filling or burial. In general, unless grit is washed, provision for burial should be made. However, unwashed grit, when mixed with soil, is valuable as soil conditioner and will give good yield of garden crops such as cucumbers, squashes, tomatoes, etc.
Grease Removal:
Plenty of greasy materials such as fats, oils, grease, waxes, soaps, fatty acids, etc., may be present in sewage obtained from kitchens of restaurants and houses, motor garages, oil refineries, etc. In ordinary domestic or sanitary sewage the amount of greasy materials is usually too small but in industrial sewage these materials may be present in large amounts.
The removal of greasy materials from sewage is essential to avoid the formation of unsightly and odorous scum on the surface of receiving waters in case the untreated sewage is to be disposed of by dilution.
In case sewage is treated before its disposal then also the removal of greasy materials from sewage is essential to avoid formation of unsightly and odorous scum on the surface of settling tanks, or to avoid interference with the process of aeration in the activated sludge process of sewage treatment, or to avoid inhibition of biological growth on trickling filters.
For removal of greasy materials from sewage skimming tanks are used in which the other lighter floating materials such as pieces of corks and wood, vegetable debris and fruit skins, etc., are also removed. Skimming tanks are placed before sedimentation tanks.
4. Skimming Tanks:
Skimming tank is a rectangular or circular tank in which as sewage flows air is blown by an aerating device through the bottom of the tank. The rising air bubbles tend to coagulate and congeal (solidify) the greasy material and cause it to rise to the surface and push it to a side compartment from where it is removed.
A long trough-shaped skimming tank, divided into two or three lateral compartments by vertical baffles having slots for a short distance below the sewage surface. The baffle walls help in pushing the rising coagulated greasy material into the side compartments called stilling compartments.
The rise of greasy material is brought about by blowing compressed air into the sewage from diffusers placed at the bottom of the tank. The collected greasy materials are removed (i.e., skimmed off) either manually or with the help of some mechanical equipment.
Sewage enters the skimming tank at one end, flows longitudinally and leaves the tank through a narrow inclined duct at the other end. The duct is so narrow that the suspended heavier particles are carried up its slope and out of the tank.
The detention period for skimming tanks in the range of 1 to 15 minutes and the value commonly adopted in the design is 3 to 5 minutes. The amount of compressed air required is about 200 m3 per million litres of sewage. The surface area required for a skimming tank can be found from the following formula-
In which,
A = surface area of tank (in m2);
q = rate of flow of sewage (in m3/day); and
Vr = minimum rising velocity of greasy material to be removed (in m/minute)
= 0.25 m/minute in most cases
The efficiency of a skimming tank can be increased 3 to 4 times by passing chlorine gas (about 1.5 to 2 mg per litre of sewage) along with compressed air. Chlorine may also be added as a solution with the sewage discharge, just ahead of the air diffuser plates. The action of chlorine is to destroy the protective colloidal effect of protein, which holds the grease in emulsified form.
The aeration of sewage in skimming tanks has following advantages:
(i) It skims greasy matter out of sewage and raises it to the surface.
(ii) It freshens sewage by supplying some dissolved oxygen.
(iii) Objectionable gases such as H2S, etc., are driven out.
(iv) It causes flocculation of colloidal matter.
Greasy materials can also be removed from sewage by subjecting the aerated sewage to a vacuum pressure of about 250 mm of mercury for 10 to 15 minutes in a vacuator. This process is known as vacuum floatation. The vacuum pressure causes the air bubbles, present in the aerated sewage, to expand and move upward through the sewage to the surface. The rising air bubbles lift greasy and other lighter materials to the surface, where they are removed through skimming troughs.
The vacuum floatation unit consists of a covered cylindrical tank in which a partial vacuum is maintained. The tank is provided with scum-and sludge-removal mechanisms. The floating material is continuously swept to the tank periphery, automatically discharged into a scum trough, and removed from the unit to a pump also under partial vacuum.
However, prior to the application of partial vacuum sewage is saturated with air. As such in vacuum floatation method an aeration tank for saturating the sewage with air is required. The other auxiliary equipment required in this method includes a short-period detention tank for removal of large air bubbles, vacuum pumps, and sludge and scum pumps.
The greasy materials removed as skimmings from skimming tanks or vacuators can be disposed of either by burning or burial. It is generally too polluted to be of any economic use. However, it may sometimes by converted in soap lubicants, candles and other non-edible products. It may sometimes be digested along with sludge in sludge digesters, which would, however, prove beneficial only if mineral oils are in less amount, and vegetable and organic matter predominate, because the later digest easily, and produce gases of high fuel value.
Necessity and Use of Skimming Tanks in India:
In hot countries like India, it is very difficult to skim out greasy materials, because they do not coagulate and congeal (solidify due to cooling) easily. Moreover, in normal municipal sewage greasy materials are generally found in very meagre quantities. As such skimming tanks are generally not provided in India at the treatment plants.
Instead of these attempts are made to remove greasy materials before they enter the sewer by providing grease traps at industries, garages, hotels, and other sources which produce sewage containing large amount of greasy materials. However, skimming tanks may be provided at the treatment plants of industrial towns. Similarly these may also be provided at treatment plants of colder hill stations where greasy materials may congeal easily.
There may be considerable variation in flowrate and strength of sewage received at almost all sewage treatment plants. On account of such variations there may be deterioration in performance from the optimum value that can be achieved. In order to overcome the operational problems caused by these variations and to improve the performance of the downstream processes flow equalization may be used.
Flow equalization is simply the damping of flowrate variations so that a constant or nearly constant flowrate is achieved. In sewage treatment flow equalization (or flowrate equalization) may be achieved either by inline arrangement or off-line arrangement. In the in-line arrangement the entire flow passes through the equalization basin.
In the off-line arrangement only the flow above the average daily flowrate is diverted into the equalization basin. In the in-line arrangement it is possible to achieve considerable damping of constituent-concentration and flowrate. On the other hand in the off-line arrangement the amount of constituent-concentration damping is considerably reduced, though pumping requirements are minimized in this arrangement.
Location of Equalization Basins:
The best location for equalization basins must be determined for each system of sewage treatment because the optimum location will vary with the type of treatment and the characteristics of the collection system and the sewage. Thus detailed studies should be performed for several locations throughout the system.
Probably the most common location will be at the existing or the proposed treatment plant sites. Further there is also a need to consider location of equalization basin in the treatment-process flowsheet. In most cases, equalization after primary treatment and before biological treatment may be appropriate. Equalization after primary treatment causes fewer problems with sludge and scum.
However, if flow- equalization systems are to be located ahead of primary settling and biological systems, the design must provide for sufficient mixing to prevent solids deposition and concentration variations, and aeration to prevent odour problem.
Volume Requirements for Equalization Basin:
The volume required for flowrate equalization basin is determined by using an inflow mass diagram in which the cumulative inflow volume is plotted against the time of day. The average daily flowrate, also plotted on the same diagram, is the straight line drawn from the origin to the end point of the diagram.
To determine the required volume, a line parallel to the line representing the average daily flowrate is drawn tangent to the mass inflow curve. The required volume is then equal to the vertical distance from the point of tangency to the straight line representing the average flowrate.
If the inflow mass curve goes above the line representing the average flowrate, the inflow mass diagram must be bounded with two lines that are parallel to the average flowrate line and tangent to the extremities of the mass inflow diagram. The required volume is then equal to the vertical distance between the two lines.
In practice the volume of the equalization basin is made larger than that theoretically determined to account for the following factors:
(i) Continuous operation of aeration and mixing equipment will not allow complete drawdown, although special structures can be built.
(ii) Volume must be provided to accommodate the concentrated plant recycle streams that are expected, if such flows are returned to the equalization basin (a practice that is not recommended).
(iii) Some contingency should be provided for unforeseen changes in diurnal flow. The additional volume to be provided for the equalization basin varies from 10 to 20 per cent of the theoretical value.
Benefits of Flow Equalization:
The principal benefits derived from the application of flow equalization are as follows:
(i) Sewage treatability is reportedly enhanced after equalization (this remains to be demonstrated conclusively);
(ii) Biological treatment is enhanced, because shock loadings are eliminated or can be minimized, inhibiting substances can be diluted, and pH can be stabilized;
(iii) The effluent quality and thickening performance of secondary sedimentation tanks following biological treatment is improved through constant solids loading;
(iv) Effluent-filtration surface-area requirements are reduced, filter performance is improved, and more uniform filter-backwash cycles are possible; and
(v) In chemical treatment, damping of mass loadings improves chemical feed and process reliability. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants because of the relatively low costs involved.