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9.PRELIMINARY TREATMENT


9 PRELIMINARY TREATMENT SYSTEMS
INTRODUCTION
Before the treatment process itself, raw water must be pretreated. This procedure consists of a certain number of operations which are e

xclusively physical and mechanical, aimed at removing as many elements as possible which, because of their nature and size, could hinder future treatment procedures. Pretreatment operations are listed below (a treatment plant may include one or more of these operations, according to its size and the quality of the raw water): - screening, - comminution, -grit removal, - presedimentation, - grease removal (frequently simultaneous with grit removal), - oil removal, - straining, - removal and treatment of by-products.

The terms screening and straining need to be defined, since they are both mechanical procedures. Screening uses bar screens. The width of the openings between the very long bars in the screen rack is usually greater than 5 mm. Cleaning is done mechanically (raking). Strainers are slender structures with round, virtually square or crossed-mesh openings which, in general, are under 3 mm in size. The strainers can be either fixed or rotating, and cleaning is done either mechanically or hydraulically. However, as a result of technological advances, straining operations may be carried out with very fine, custom made bar screens with slots less than 1 mm wide. These bar screens, like fine strainers, reduce the BOD5 of MWW.

Chap. 9: Preliminary treatment systems

1. SCREENING
1.1. OPERATING CONDITIONS
Screening is the first treatment station, both for surface and wastewater. Its purpose is to: - protect the structure downstream against large objects which could create obstructions in some of the facility's units, - easily separate and remove large matter carried along by the raw water which might negatively affect the efficiency of later treatment procedures or make their implementation more difficult. The efficiency of the screening operation depends on the spacing between screen bars: - fine screening, for a spacing of under 10 mm, - medium screening, for a spacing of 10 to 40 mm, - coarse screening, for a spacing of over 40 mm. Usually, fine screening is preceded by a preliminary screening operation for purposes of protection. Screening is carried out by a manually cleaned bar screen (large in size, in order to reduce the frequency of screenings collection operations) or, preferably, by an automatically cleaned bar screen (essential in cases of high flow rates or for water with a high solids content). The automatic bar screen is usually protected by a sturdy preliminary bar screen which should also be provided with an automatic cleaning system in large facilities, and in the case of raw water containing a high volume of coarse matter. To reduce manual operations as much as possible, screening procedures have become increasingly automated, even in small facilities. Automation is essential in situations where large amounts of plant matter (such as leaves during the autumn season) are carried by the water and arrive all at once at the bar screen, tending to mat the bars and completely clogging the screen in a few minutes. Fine screens must be automated. The collected refuse is stored in a container of a given capacity, calculated according to the acceptable frequency of refuse disposal operations. Usual spacings are: - for surface waters, between 20 and 40 mm (upstream of strainer), - for municipal wastewater: for raw water, from 15 to 30 mm (but upstream from a straining and/or lamellae settling process, fine screening is necessary); for sludge (if necessary), 10 mm or less, - for some industrial effluents, especially agrifood effluents, fine bar screening (or at times, medium screening followed by straining). 1.1.1. Hydraulic sizing - Clogging Under normal circumstances, the crossing velocity through the bar screen should be sufficient for matter to attach itself to the screen without producing an excessive

1. Screening

loss of head or a complete clogging of the bars, or allowing matter to be carried by the flow; normally acceptable crossing velocities between bars average between 0.6 and 1.0 m.s -1 and 1.2 to 1.4 m.s -1 at the maximum water flow. These velocities apply to the area of the clogged bar screen that is still clear. The degree of clogging (as a percentage of the clear wetted section) depends on the water quality and on the system used to recover waste from the bar screen. For automatic bar screens it can be a nywhere between 10% (surface water) and 30% (wastewater with a high solids content). For manually cleaned bar screens, the area of immersed bar screen must be larger, so as to avoid frequent cleanings. Approach velocities upstream of the bar screen are generally slow, especially in the case of fine bar screens with an open area of less than 50%; frequent sediment accumulation occurs upstream of the bar screen, which must either be prevented (by stirring) or disposed of, for instance, by daily self-cleaning.

1.1.2. Automatic control and the protection of bar screens In general, the bar screen cleaning system works on an intermittent basis. It can be controlled in three ways: a) by a cyclic system of controllable frequencies (1 min to 1 h) and lengths of time (1 to 15 min); b) by a differential head loss indicator; c) (even better) by a combination of both systems. When the bar screen is located downstream from a pumping station, the control mechanism can be linked to the start-up of the pumps, with a built-in timer to keep the screen in operation for 1 to 30 minutes. Automatic bar screens must be equipped with torque limiters to prevent equipment damage in case of overloading or blocking. Normally, reciprocating cleaning bar screens, both curved and straight, include a device to ensure that the rake automatically stops moving at a point outside of the screen area, so as to avoid jamming upon restarting.

1.2. DIFFERENT SCREENS

TYPES

OF

BAR

In wastewater lifting stations, liftable cage screens are used instead of bar screens, thus avoiding the problem of access; however, their handling and cleaning are difficult. 1.2.1.2. Upstream cleaning bar screens Automatically cleaned bar screens are usually cleaned from the upstream side, especially those described in this section. There are three main types of automatic bar screens, differing from one another only with regard to some technical

1.2.1. General construction principles 1.2.1.1. Manual bar screens To make raking easier, manually cleaned bar screens, made up of straight, round or rectangular bars, are, in general, at an angle of 60° to 80° to the horizontal. They can be travelling (on slides), or pivoting, for downstream cleaning of covered channels .

Chap. 9: Preliminary, treatment systems details. On the screen rack, the bars' crosssection is round or trapezoidal (thus lessening the risk of solid matter jamming), with sharp or rounded-off edges. Some of these bar screens allow for fine screening (and even straining) through the use of a custom made bar screen rack (of the Johnson type, where spacing ranges from a few millimetres to 0.5 mm) or a perforated steel sheet. Screenings disposal is carried out downstream of the screen. . Type I: curved bar screens This is the best bar screen for medium size facilities, where the water does not carry a high volume of matter and screenings are removed at a low elevation. Also, the effective cross-section of the opening is large and it is mechanically simple. Cleaning is done with one or two rakes located at the end of the rotating arm which revolves around the horizontal axis. . Type II: straight bar screens with reciprocating cleaning mechanism In general, the bar screen surface (at an angle of around 80° to the horizontal) stops slightly above the maximum level of liquid; it is extended with an apron. A reciprocating device (rake, doctor grab or swivelling ladle) brings up the screenings, first along the screen area and then along the apron; it then comes down away from the screen area. The screenings are removed from the elevated position by an extractor (which may be motor driven). When handling very l rge flows (surface a water intakes), greater for instance than 30,000 m3.h -1 , of water that does not have a large volume of matter, the operation can be carried out with a mobile screen which cleans only part of the bar screen, and moves laterally after each cycle. . Type III: continuous cleaning straight bar screens These are the best bar screens for fine screening when there is a risk of matting. Preliminary screening must be done first if there is a probability that the water is carrying coarse matter. Straight bar screens, at an angle of 80° to the horizontal, are cleaned by doctor blades, rakes or brushes driven by chains. In general, screenings are ejected by means of a motor-driven mechanism.

Figure 263. Back-cleaned screen installation.

Table 27. Upstream cleaning mechanical bar screens.
Depth of channel m 0.75 to 1.75 Width of channel m 0.5 to 1.6 Bar spacing mm 10 to 40 Bar thickness mm 10 Height of disposal m 0 Water depth m 0.50 to 1.5

Medium screening

Fine screening

Type of bar screen Curved bar screen DC type Hydraulic straight bar screen GDH type Cable straight bar screen GDC type Rack and pinion bar screen Grab bar screen Fine curved bar screen GFC type Fine straight bar screen GFD type Endless moving bar screen

Cleaning operation Contin.

Recipr.

0.75 to 2.80

0.6 to 1.2

10 to 40

10

0 to 1.2

0.50 to 1.5

Recipr.

2.00 to 10.0

0.1 to 2.6

10 to 40

10

0.65 and 1.2

1.5 to 9.5

Recipr. Recipr.

1.50 to 5.00 2.50 to 10.0

0.6 to 2.0 1.5 to 10

12 to 80 12 to 100

0.65 and 1.3

Contin.

0.75 to 1.75

0.5 to 1.6

1 to 10

0

0.50 to 1.5

Contin.

2.0 to 10.0

1.0 to 2.6

1 to 10

0.85 and 1.2

1.5 to 9.5

Contin.

0.6 to 15.0

0.3 to 4.0

1 to 15

0 to 1.2

0.4 to 14.5

Chap. 9: Preliminary treatment systems

1.2.1.3. Back-cleaned bar screens Some bar screens include a downstream endless chain cleaning system. With this set-up there is a risk that some of the screenings will fall back into the water downstream; however, for water carrying a high volume of matter, it may be advisable to have a back-cleaned preliminary screen with a high removal capacity (figure 263).

torque limiter with a reaction arm, and a horizontal automatic stopping device, - a noiseless extractor equipped with dampeners.

1.2.3. Fine curved bar screen (GFC The general design of this type I bar screen is the same as the DC bar screen, except that it includes a fine screen or a stainless steel perforated sheet for spacings 1.2.2. Curved bar screen, (DC type) of less than 10 mm. Instead of rakes, polyurethane scrapers or nylon bristle This type I bar screen (fig. 264) includes: brushes are used. - a curved bar screen rack in a quadrant, 1.2.4. Straight bar screen (GDH type) This type II bar screen uses hydraulic supported by a rigid frame, jacks to ensure a simple kinematic operation - a diametral rotating arm with two adjustable rakes, with a reduction gear, a (figure 265).

1. Screening It specifically includes: - a vertical bar screen rack supported by a rigid frame (1), - a frame (2) that tilts upstream by means of a jack (3), thus allowing the cleaning rake to descend away from the bar screen surface, - a cleaning device made up by a rake carriage (4) which slides in the moving frame, driven by a jack (5), - an ejector (6) driven by a jack. 1.2.5. Straight bar screen with cables (GDC type) The rake carriage of this type II bar screen is driven by two cables. With this device, facilities can reach great depths, since pneumatic jacks drive the rake away from the frame (and descent therefore occurs away from the bar screen surface); t is h ensures a greater operational safety. It includes (figure 266): - an inclined bar screen rake on a rigid frame (1), - a moving carriage (2), which slides on the guides of the rigid frame, - an ejector (3) driven by jacks. 1.2.6. Fine straight bar screen (GFD type) This type III bar screen comprises a monobloc lifting unit, placed on guides sealed to the channel walls. Cleaning of the bar screen rack is carried out by chaindriven brushes. It includes (figure 268): - an inclined stainless steel bar screen rack, mounted on a rigid monobloc frame, - nylon brushes mounted on two endless chains driven by a reduction gear, for scraping and lifting of screenings,

Chap. 9: Preliminary treatment systems

Figure 267. Straight bar screens (GDC type).

1. Screening

- an assembly for ejection and cleaning of lifting brushes, with a revolving brush roll driven by a chain.

the block, is driven by a third axial cable. Its construction and operation are simple, but its electrical drive (track limit switches, for instance) must be extremely reliable.

Figure 269. Rack and pinion bar screens.

1.2.7. Other bar screens .Inclined straight bar screen with rack and pinion system This type II bar screen is excellent for medium screening of water carrying a high volume of matter when the screenings do not require significant lifting. Because of its movable parts, its mechanical operation is simple and sturdy: all the movable parts are outside of the water, except for the reciprocating rake carried by two long hinged arms which, in turn, are held by a self-driven block which slides along the racks (figure 269). . Vertical grab bar screen This type II bar screen (figure 270) is excellent for heavy water flows carrying a high volume of matter, and when facilities Figure 270. Vertical grab bar screens. reach great depths. The raising and lowering of the carriage which holds the grab (or ladle) is driven by two cables; the pivoting movement of the grab, hinged on

Chap. 9: Preliminary treatment system

Continuous moving screen This machine is used for fine screening of wastewater, after preliminary screening and grit removal. Its bar screen surface is made up of a group of custom made hinged rakes (made of plastic) inscribed in a cylinder,

that mesh with one another, hinged on two lateral chains (which ensure rotation of the assembly). Through the relative movement of two successive rakes, matter picked up by the machine can be removed.

1.3. COMMINUTION
This treatment is applied mainly to MWW, in order to shred solid matter carried by the water so that it may later undergo further treatment. Its purpose is to eliminate the constraints and complications brought about by the disposal and removal of coarse screening waste and to benefit from the increase in volume of the digestion gas produced. However, in practical terms, comminution has some short-comings, particularly the risk of a large accumulation of comminuted textile fibres or plants mixed with grease (clogging pumps and pipes, and possibly producing a scum layer in anaerobic digesters), which would mean that relatively frequent cleaning would have to be carried out on rather delicate equipment. For the above mentioned reasons, comminution is no longer used in France, except on raw water at the head o the f plant. However, when treating sludge, it sometimes replaces fine screening in which case it is done through a pressure comminutor, so as to obtain the desired degree of comminution fineness. Both gravity flow and pressure comminutors are particularly well suited for treatment of MWW, due to the fact that they are capable of absorbing matter

normally carried by this kind of water (after preliminary coarse screening of 40 to 80 mm, depending on the size of the machine), and reducing it into elements of a few millimetres average diameter. 1.3.1. Gravity flow comminutors (raw water) These produce only a slight loss of head, and use little energy. The more traditional comminutors include a vertical revolving drum, with horizontal slots. In the Infilco Degrémont Griductor (figure 270 a), the drum is made up of round bars equipped with cutting teeth; there are fixed cutting rakes bolted on the frame. The machine is located within a flat-bottom, open, rectangular channel; the water goes through the drum horizontally, at which time the matter is shredded. Another model, installed in the same way, includes two revolving vertical drums, with meshing cutting teeth. 1.3.2. Pressure comminutors (raw water and sludge) They are mounted between flanges and have to be extremely robust. They are equipped with a large electrical motor, in order to counter the possibility of clog-

1. Screening

ging. In general, the comminutor system (which is in a closed casing) includes a rotating element with bars, cutting teeth and rakes, which does not make the water circulate; rather, water circulation is achieved through a pump placed in series in the conduit. Some of these comminutors operate differently: they include a sharp revolving propeller with pumping capabilities (working at low pressure), which drives flow circulation.

Table 73. General comminutors.
Type of comminutor Gravity flow comminutors In-line comminutors Flo m3.h-1

characteristics

of

Power of the electrical motor kW 0.25 to 4

5,000 to 8,000 50 to 300

7.5 to 20

Figure 270 a. Infilco Degrémont Griductor.

Chap. 9: Preliminary treatment systems

2. GRIT REMOVAL
2.1. OPERATING CONDITIONS
Grit removal operations remove gravel, sand and fine mineral particles from raw water, in order to prevent deposits in channels and pipes, to protect pumps and other machines against abrasion, and in order to avoid problems in later treatment stages. The normal size of particles treated

by grit removal operations is equal to or greater than 200 ?m; smaller particles are removed by presedimentation or settling treatments. The theoretical principles of both grit removal and settling of discrete particles are closely related (see page 158). In practice, the following data can be used (valid in unhindered settling for grit particles with a specific gravity of 2.65).

Table 74. Corrected settling velocity of grit particles.
d cm Vc cm.s-1 Vc' . cm.s-1 Vc'' cm.s-1 VI cm.s-1 0.005 0.010 0.020 0.030 0.040 0.050 0.2 0.7 2.3 4.0 5.6 7.2 0 0.5 1.7 3.0 4.0 5.0 0 0 1.6 3.0 4.5 6.0 15 20 27 32 38 42 0.10 15 11 13 60 0.20 27 21 25 83 0.30 35 26 33 100 0.50 47 33 45 130 1.00 74 65 190

Where: d : diameter of the grit particle, Vc : settling velocity for a fluid with zero horizontal velocity, Vc' : settling velocity for a fluid with horizontal velocity equal to VI, Vc: settling velocity for a fluid with horizontal velocity of 0.30 m.s -1 , VI: critical horizontal velocity to entrain the settled particle. When treating wastewater, the objective is to extract as much inorganic matter as possible, and as little organic matter as possible (as this causes problems both during removal and storage of the extracted grit). This separation procedure

uses energy, which in turn lessens the efficacy of the settling procedure. So a compromise has to be made between the removal capacity (limit particle size adopted) and the acceptable amount of organic matter in the grit. 2.1.1. Grit removal in surface water The water intake must be designed to avoid sand carry-over. If local conditions are inadequate, a grit remover must be included unless arrangements have been made to remove grit at another structure. If the facility includes a presedimentation tank, those particles larger than 300 ?m can be removed by a rough grit channel with hydraulic flushing.

2. Grit removal

If the facility includes a strainer (1 to 2 mm-mesh, for instance), grit removal should be done upstream, so as to avoid any problems at the strainer itself. In general the grit remover is a rectangular channel-type device. Its cross-section will depend on the desired horizontal flow velocity: this velocity should be greater than the critical velocity VI of the settled particles (table 74), if sand is to be disposed of hydraulically, and lower if disposal is to be done by a bottom scraper. The horizontal surface is calculated dividing the maximum flow to be conveyed, by the settling rate Vc of the smallest particles to be retained, corrected according to the horizontal flow velocity selected. The grit can also be separated by cycloning (a hydrocyclone at the discharge end of the lift pumps). (See page 611.) 2.1.2. Grit wastewater removal in municipal

pumps, air lifts) must be used, so as to reduce the chance of abrasion and clogging. In increasing order of size and effectiveness, the following are used: - single grit channel, where flow velocity varies according to the flow rate. This structure is rarely recommended, and only for small and simple facilities. The grit is extracted manually from a longitudinal gutter with a storage capacity of 4 to 5 days; - a grit channel improved by the inclusion of an outlet weir obeying linear equation (i.e., water depth proportional to flow rate). Flow velocity remains constant at 0.3 m.s 1. Retention time is around 1.5 to 2 min; - circular grit chamber, used for mechanical extraction of grit and hydraulic extraction of floating matter and scum (see page 608). Retention time is around 2 to 3 min; - aerated rectangular grit chamber, used for mechanical extraction of grit and hydraulic extraction of floating matter and scum (see page 608). Retention time is around 2 to 5 min. The last two above-mentioned devices are increasingly used in combination with grease removal (see page 615). 2.1.3. Grit removal in industrial wastewater Grit removal is less necessary in IWW. Aerated grit chambers used for MWW can sometimes be used for IWW, especially in agricultural and food industry effluents. When treating effluents of the metallurgical and mechanical industry, the purpose of grit removal is to separate the very

Since the nature of the medium is heterogeneous, separation of grit and other matter contained in the water cannot be fully carried out: the extracted grit will still contain some organic matter that settled with it; this amount can be minimised if a sweeping flow of about 0.3 m.s -1 is maintained at floor level. Separation can be improved by washing the grit extracted from the grit remover (see page 630), whereby the content of organic matter in the washed grit decreases to less than 30%. Because of the nature of the extracted product, very specific equipment (vortex

Chap. 9: Preliminary treatment systems

dense particles of iron oxide, of granulated oily scale with a bulk density 4. These abrasive particles that quickly, are present in initial con-

centrations ranging from 0.2 to several slag and of grammes per litre, and therefore should of 2.5 to be recovered by special equipment in the settle grit remover.

2.2. CIRCULAR GRIT REMOVERS
The diameter of these tapered cylindrical structures is 3 to 8 m, with a liquid depth of 3 to 5 m. Water enters tangentially either at the periphery of the structure or in a central cylindrical baffle; it is recovered through a submerged opening in the cylindrical wall. The grit is deposited on a slightly sloping floor, moves due to hydraulic forces, and falls into a c entral hopper for storage and recovery. The sweeping velocity of the floor is kept practically constant at a level greater than 0.3 m.s 1 in three ways (characteristic of this type of structure):

- rotation of the liquid mass through a vortex effect resulting from the tangential entrance of the water, - rotation of the liquid mass by a revolving vertical shaft, blade type mechanical mixer, whereby a specific power of 10 to 20 W.m-3 can be maintained regardless of the flow rate, thus allowing the level of liquid to remain practically constant throughout the operation, - gyration (in a vertical plane) of the liquid mass by blowing air into a submerged cylindrical baffle through special diffusers. With this machine, the operation can be carried out with a nearly constant level of liquid. The grit collected in the central hopper is extracted by a pump or an air lift, and dried by gravity or sent to a mechanical recovery system.

2.3. AERATED RECTANGULAR GRIT CHAMBERS
The width of these structures can range from 4 m (single structure) to 8 m (double structure); the liquid depth is around 4 m; the maximum length is around 30 m. They can handle large flow rates.

The shape of the floor depends on the system used to recover grit. Water is introduced at one end of the structure, and recovered at the other end through a submerged opening; often, it goes through a downstream weir designed to maintain a constant water level. All along this slow horizontal flow structure, there is an in-line air-injection system, equipped with special air diffusers such as Vibrairs, whereby a 15 to 30 W.m-3 specific aeration power is

2. Grit removal

achieved. The liquid can be maintained at a nearly constant level. The blown air produces cross-circulation velocity, promotes (through turbulence) the separation of the organic matter bound to the particles of grit, and partially removes floating matter. Grit extraction is done automatically, as follows:

- by a group of air lifts operating in a pulsed fashion (recovery in the lower hoppers), - by scraping (scraper bridge) towards a collection pit at one end (recovery by a fixed pump or air lift), - by suction pump or air lift set up on a movable bridge, discharging the diluted grit into a lateral disposal trough.

Figure 271. Aerated rectangular grit chamber. -10 m. These cylindroconical structures play a dual role: - they separate discrete particles through vertical settling, Depending on the level of incoming - they separate large amounts of incoming water, two different grit removing oil using a scum baffle. techniques are used: Recovery of these deposits is always . tangential separators (figure 272), carried out by a grab. frequently called "hydrocyclones°, which is These structures built by Degrémont incorrect since the centrifugal energy with diameters of 4 to 32 m, are located developed is low. In the case of rolling upstream of the settling tanks or filters. mills, hot strip mills and continuous casting Accordingly they must be capable of mills, separators are used when wastewater removing particles upwards of 100 ?m. inlets can go down to levels of :

2.4. "METALLURGY" GRIT REMOVERS

Chap. 9: Preliminary treatment systems

.

2. Grit removal

"Classifier" separators: their only role is to remove discrete particles larger than 200 or 250 gm. They are located upstream of the clarifier thickeners, and protect the sludge pumps and dewatering equipment. Built by Degrémont in the range 5-12 m diameter, these units are fed, in general, by overhead conduits, and they comprise a

circular zone for rapid settling with a low water depth. A centrally driven diametral scraper arm discharges sediments towards an outside pit, from where dry extraction is carried out through a screw or a reciprocating rake (figure 273).

2.5. HYDROCYCLONES
These machines separate particles through centrifugal hydraulic classification. They are made up of a cylindroconical compartment in which, because of the tangential feed, water rotates before leaving through an axial overflow pipe (figure 274). The concentrated sludge leaves through the cone bottom (underflow). Even in the smallest machines, the centrifugal acceleration can be greater than 600 g, and the feed pressure ranges from 0.5 to 2 bar. The separation factor d50 generally expressed in gym, and improperly called removal capacity, corresponds to the particle diameter for which there is a 50% separation. Its construction has the following characteristics: compartment diameter D, length L/diameter D ratio, diameter a of the inlet opening and diameter s of the outlet opening, and cone angle a. Different ratios have been considered in order to define a cyclone in terms of its size; according to Rietema, typically it is as follows (based exclusively on the geometric aspects)

Chap. 9: Preliminary treatment systems

Where: : water density ? : difference in densities between solid particles and water ?p : differential pressure in the machine ? : dynamic viscosity q : output of the machine. The experimental differential pressure variation ?p is set as a function of the throughflow rate. There are two basic types of machines: ? Monotubular hydrocyclones Their diameter ranges from 150 to 800 mm for the treatment of 20 to 250 m3 .h 1 , flows, with d 50 factors of 50 to 80 ?m.

They are protected from abrasion. They operate on concentrated sludge suspensions, or even on raw water that does not have a high volume of matter if the removal capacity can be increased. ? Multitubular hydrocyclones For grit removal of larger flows with a low volume of matter, very small diameter cyclones can be used, set in parallel in one compartment, where a smaller d 50 factor (10 ?m) can be reached. Their diameter is of several centimetres, and they are made in anti-abrasive plastic material. The head loss is between 1 and 2 bar. The feed water must first go through appropriate straining.

3. Presedimentation

3. PRESEDIMENTATION

3.1.FIELD OF APPLICATION
Presedimentation, which precedes clarification, is a solid/liquid separation stage of surface waters containing a very high amount of solid matter, carried out when the conventional one-stage settling procedure cannot be done. The purpose of this settling procedure is to remove most of the suspended solids in raw water, to dispose of them as concentrated sludge, and to provide the main settling stage with an acceptable water quality. The concentration threshold of suspended solids after which presedimentation becomes necessary is a function of the type of main settling tank to be used: - 1.5 to 2 g.l-1 for non-scraper type or sludge blanket settling tanks, - 5 g.l-1 for scraper settling tanks. If grit removal has been carried out, presedimentation is the next stage, and it

includes two phases of solid/liquid separation, i.e., hindered settling, and sludge thickening, the relative magnitude of which depends on the content and nature of suspended solids in the raw water, and the treatment being considered. In general, the presedimentation stage is designed to dip occasional peaks of suspended solids of up to about 30 g.l-1 . Other than during these potential peak periods, the presedimentation tank can be bypassed. Above 30 g.l-1 , this same structure can be used but at lower flows; thus, the sludge disposal flow represents a considerable proportion of the incoming flow. In general, it is inadvisable to consider presedimentation in water with an SS content > 50 g.l-1 ; in that case, it is usually better to isolate the unit so as not to damage the equipment. A raw water holding tank can be set up upstream of the unit.

3.2. APPLICATION
In general, coagulant and/or flocculant reagents must be used to improve the quality of settled water and the hydraulic performance of presedimentation tanks. Without a chemical reagent, the rate used in the unit must be lower than the natural settling velocity of the effluent's suspended solids which, in turn, depends on

the nature and concentration of these solids (about 0.5 to 1 m.h -1 ). With an inorganic coagulant only, a fraction of the colloidal phase is removed. The settling velocity obtained with iron chloride (about 1.5 to 3 m.h -1 ) is often greater than that obtained with aluminium sulphate (about 1 to 2 m.h -1 ); in addition, aluminium salts produce large amounts of sludge. The treatment rate to be applied is one third of the rate necessary for optimal coagulation/flocculation.

Chap. 9: Preliminary treatment systems

These inorganic coagulants should only be used on water with SS definitely lower than 30 g.l-1 . With an organic flocculant only, settling velocity increases considerably; the colloidal fraction can be decreased if the polymer has been correctly selected. Of the three options, this is the best, because the floc produced is very compact, and the sludge very concentrated (more than 100 g.l-1 ). For a treatment rate of 1 g.m-3 of active product, applicable velocities can reach the following levels: - 3 to 5 m.h -1 for raw water with 30 g.l-1 of SS; - 8 to 10 m.h -1 for raw water with) 10 g.l-1 of SS. In some cases, an inorganic coagulant and an organic flocculant can be used together. When only an organic coagulant is

used, performance is usually lower than when only an organic flocculant is used. The need for, and the sizing of a presedimentation facility are often difficult to determine. There are four basic factors that must be considered: - nature and concentration of particles (fine sand, silt, clay, colloids, etc.); - range and frequency of peaks, - need to maintain quality; - operational costs: reagents, labour. Presedimentation tanks are rectangular (suction bridges, chain scraper) or circular (diametral scraper). Their construction is similar to that of settling tanks (chapter 10, subchapter 3). They should be preceded by a flash mixer to add the reagents, and sometimes even by a flocculator. Sludge removal by pumps is strongly recommended because of the concentration and large quantities of sludge that must be removed.

Fig. 275. Circular scraper settling tanks for presedimentation.

4. Grease and oil removal

4. GREASE AND OIL REMOVAL
4.1. PRODUCTS TO BE SEPARATED
Grease and oil removal operations separate products with a slightly lower specific gravity than water; through a natural or aided (chapter 3, paragraph 4.1.1) flotation effect, in a compartment with a sufficient volume of liquid. Greases are solid products (as long as the temperature is sufficiently low), of animal (or vegetable) origin, present in MWW and in some IWW (from the agricultural and food industries) and, in low quantities, in storm water tanks, lagoons, ponds, etc. They are present either in the form of free particles or, more frequently, coalesced with different suspended solids (which must be dislodged so that flotation may occur). The separation technique used permits the recovery, not only of the grease itself, but also of floating products such as various vegetable or animal waste (slaughterhouses), soap, foam (detergents), elastomers and plastics, etc. Grease removal is a liquid/solid separation procedure whereby a compromise is reached between a maximum retention of grease and a minimum deposit of fermentable settled sludge. Its performance is difficult to estimate because of sampling and analysis difficulties. "Oils" is the name given to various liquid products such as vegetable oils, mineral oils and light hydrocarbons. If they are present only as traces (in surface water, heating condensates of petroleum product reservoirs), their separation is carried out by adsorption and filtration. The term oil removal (or oil separation) is usually used only for the removal of oil present in appreciable quantities in IWW, especially the petroleum industry (normally absent in MWW, since it is illegal to dispose of it in the sewage system). Oil removal is a liquid/liquid separation procedure. for many small-scale enterprises, restaurants, communities, etc. Standardised grease separators (or grease traps) are manufactured in series for maximum flows of 20 to 30 l.s -1 . These devices are designed for a retention time of 3 to 5 min and a rising velocity of about 15 m.h -1 . If operated correctly they can retain up to about 80% of solidified grease, and store 401 of lighter matter per ls -1 of inlet flow. Regular cleaning is essential. Water temperature must be less than 30°C at the outlet. These devices are designed so

4.2. GREASE SEPARATORS
4.2.1. Operating conditions 4.2.1.1. Grease removal in MWW before sewage disposal This "at source" preliminary treatment is recommended, and sometimes compulsory,

Chap. 9: Preliminary treatment systems

as to avoid, as far as possible, the deposit of heavy matter; but it might be advisable to include upstream a settling tank for coarser matter, easy to clean, and with a retention time of 1 to 3 min. 4.2.1.2. Grease removal as preliminary treatment for a wastewater purification plant A primary settling tank can separate grease that settles at the surface but, in general, it is unable to recover large amounts of grease. This situation could lead to operational difficulties. For household wastewater, grease separation is essential if there is no primary settling; its effectiveness is maximised if carried out together with grit removal. Structure size should therefore be estimated accordingly (a retention time of around 15 min), and provisions should be made to separate the organic matter settled with the grit. In wastewater from the food and agricultural industry containing high amounts of grease to be retained (particularly slaughterhouses and the meat industry), it may be advisable to have a separate grease separator designed for a hydraulic loading of 10 to 20 m3.h-1 per m of effective surface. It would protect the sewer system because it would be located before the discharge to sewer. These units are not designed to retain oils and hydrocarbons which, when necessary, are removed through primary settling. 4.2.2. Circular grit/grease separator The diameter of this cylindroconical unit is 3 to 8 m, and its liquid depth (at the centre) is 3 to 5 m. It is equipped with a submerged Turboflot mixer/aerator placed

along the axis. The Turboflot mixer/aerator (figure 276) includes a centrifugal pumping impeller, submerged under some 2 m of

4. Grease and oil removal

water, driven by a submerged electrical approximately 45°. Settled grit slides on motor which releases a specific power of this slope towards the recovery point at 15 to 30 W per m3 of liquid capacity. The impeller: - induces a revolving flow in the lower areas of the unit, - creates an area of concentrated turbulence which promotes the separation of grease and coalesced matter, - draws some atmospheric air through an open air pipe, and releases this air into the liquid in the form of very small dispersed bubbles. The air produces a slow revolving movement of the liquid mass through an airlift effect, which promotes the collection of grease and scum at the surface. Water is introduced tangentially into a central, submerged cylindrical baffle which surrounds the Turboflot; it is recovered

through a submerged opening in the circular wall. The lower tapered zone of the unit becomes a hopper with an angle of the bottom of the unit; this movement is aided by a sweeping velocity greater than 0.15 m.s -1 produced by the mixer. Once the grit that collects at the bottom of the hopper has been separated from the settled organic matter by direct mixing in the unit (air injection to the base of the air lift), it is drawn off by an air lift; the emulsion is dried by gravity or sent towards a mechanical recovery system. The grease floating on the surface is continuously recovered by a low-speed rotating scraper assembly; the scraped grease is pushed on an inclined surface to an abovewater weir and falls into a collection trough. It is generally disposed of by gravity flow into a storage skip.

Figure 277. Circular grit/grease separator.

Chap. 9: Preliminary treatment systems

4.2.3. Rectangular grit/grease separator Units with a width of 4 m (single unit) to 8 m (double unit) have a liquid depth of about 4 m and a maximum length of about 30 m. They are able to treat large flows (figures 278 and 279). The unit's cross section has a shape that works well with sweeping crossflows, with slopes that promote grit collection on the bottom of the unit. Water is introduced at the head of the unit, and recovered at the other end through a wide submerged opening in the wall, passing through a downstream weir to maintain the water level constant. The unit, with a slow horizontal flow, is generally equipped with two interrelated mixing and aeration systems which create transverse spiral flows independent of the flow of water. This permits significant variations in the velocity of horizontal flow

which can be slow without causing any problem: - a possible preliminary grit removal zone at the inlet (which can be up to 1/3 of the unit's length), includes an in-line air-injection system equipped with custom made air diffusers such as Vibrairs, which deliver a specific aeration power of 20 to 30 W.m-3 . The blown air maintains a transversal circulation velocity, promotes (through turbulence) separation of the organic matter coalesced to the grit, and prevents the massive accumulation of large grit particles at the head of the unit; - the rest of the unit is used for grease separation and fine grit remo val; it includes a series of in-line Turboflots, which produce a slower spiral flux and allow grease to float. Grit is automatically extracted by a reciprocating travelling bridge with a programmed sequence:

Figure 278. Rectangular grit/grease separators.

4. Grease and oil removal

- either by scraping towards an end collection pit, followed by discharge by means of a pump or a fixed air lift, - or by suction pump or air lift mounted on the travelling bridge, discharging the suspended grit into a side disposal trough. The grease floating on the surface is scraped towards the end of the unit by a travelling bridge, and is removed according to a programmed sequence: - either by pushing it onto an inclined surface and over a non-submerged weir: dry" option - immediate recovery in a pit or skip, - or by weir penstock (motor-driven and programmed): "wet" option - hydraulic conveying to an additional separation unit (see paragraph 6.3). 4.2.4. Rectangular grease separator with aerator/mixers at the head of the unit For water containing small amounts of large grit particles, it may be advisable to use a variant of the unit described above, with only one or two aerator/mixers (figure 280); these devices can be of the Vortimix D type, which includes a submerged propeller on a vertical shaft, under which a controlled pressurised air flow is injected through an appropriate diffuser. This kind of aerator/mixer, to be used in large units, plays the same role as the Turboflot, with the possibility of including separately the mixing and aeration functions.

Chap. 9: Preliminary treatment systems

4.3. OIL SEPARATORS
4.3.1. Operating conditions Two types of industrial effluents are involved: - regular amount of oil (petroleum production and refining, edible oil mills, cold rolling mills, airports), - small amount of oil, but with high accidental flow peaks (storm water from refineries, storage heating condensates, oilfired power plants, hot rolling mills). Oils and hydrocarbons are present: - either in a free state, - or as fine but unstable mechanical emulsions, more or less adsorbed on suspended solids - or (less frequently) as chemical emulsions (such as aqueous cutting fluids).

Oil separation by gravity is applicable only in the first two, and is a function of: - specific gravity of oil which, in general, ranges from 0.7 to 0.95, but can be greater than 1 in some heavy hydrocarbons, - temperature, the rise of which always promotes separation, - the dynamic viscosity of the oil, which can range from several hundredths of a Pa.s to more than 0.2, a limiting value for the use of certain processes, - the congealing point. Oil separation in these effluents includes one or two stages: - preliminary oil separation, or the removal of floating hydrocarbons, which can be combined with grit removal,

4. Grease and oil removal

- oil separation which, depending on the objective, almost completely

removes the dispersed hydrocarbons (table 74).

Table 74. Preliminary separators and separators. Feed Preliminary Medium separators Polishing separators separators (40-50 mg.l-1 HC) (5-20 mg.l-1 ) Under pressure Closed separators Cyclones Coalescer filters Granular media filters Gravity . API type Mechanical flotation Dissolved air flotation , parallel plates . circular units units 4.3.2. Gravity preliminary oil separators The performance of preliminary separation units, which are generally operated without the use of a reagent, cannot be quantified: - the hydrocarbon specific gravity and size distribution of droplets in water are usually unknown, - the nature of the emulsion is poorly defined, - it is practically impossible to do upstream sampling. These devices eliminate very large and irregular peaks of oil, as well as larger droplets. There are three types: ? longitudinal separators (API): operate according to American Petroleum Institute standards (separation of droplets greater than 150 ?m in diameter); their width ranges from 1.8 to 6 m, their water depth from 0.6 to 2.4 m. These separators are difficult to cover (for smell reduction purposes), and sludge removal from the bottom is impractical; ? lamellae separators: through the use of lamellae settling procedures with plastic plates spaced at about 4 cm (figure 281), retention time has decreased from several hours to less than 60 min and even 30 min. These devices require some maintenance; also, it is advisable to use them for relatively warm water (solidification should be avoided) with small amounts of suspended solids (bottom scraping is expensive). Since their construction is of a modular type, treatment of heavy flows requires a large number of modules (each one able to treat 15 to 30 m3 .h -1 , with distribution units that are both difficult to cover and to skim;

Chap. 9: Preliminary treatment systems

? circular separators: separation is carried out in two successive chambers (figure 282): the first is a covered one, which prevents outgassing of light, volatile

products; the second includes a bottom scraper as well as a surface skimmer.

4.4. RECOVERY OF OIL AND FLOATING MATTER IN BASINS OR LAGOONS
The collection of oil layers (and/or floating matter) found at the surface of still water can be done with devices called oil recovery units, which do not carry out any purifying action on t e underlying water. h There are four types: . Adjustable direction troughs and weirs Fixed (or floating for variable water levels), they require an additional device to approach the oil layer and carry away a large quantity of water.

Figure 283. Radeg.

4. Grease and oil removal

? Drum or belt oil collectors Their main advantage is that they can recover oil with very little water and, especially in the case of belt collectors, they can tolerate a large variation in water level. For large surfaces, they also need an oillayer skimmer. ? Fixed mechanical oil recovery units The oil layer is conveyed over a great distance towards the collection zone, by water currents created by a moving pump set.

? Movable oil recovery units With these floating devices (self-propelled or towed), large water surfaces (storm water tanks or lagoons) can be maintained. They take up a large flow of water, which is then cydoned or clarified. With Radeg (figure 283), there is a recovery of not only oil and grease but also, through its grinder, of papers, rags and other floating matter which are frequently found on water surfaces.

Chap. 9: Preliminary treatment systems

5. STRAINING
5.1. STRAINING OF WASTEWATER
In addition to mechanical bar screens with narrow slots spaced at 3 or 6 mm, necessary for some municipal wastewater treatment schemes, it may be useful to have strainers with smaller openings. That is the case for some AFI wastewater in which, together with straining, a significant portion of suspended polluting matter can be removed and, possibly, recycled. The strainers' operating equipment is made up of perforated sheet plates or, more often, Johnson-type bar screen elements. Spacing ranges from 0.5 mm (or even 0.25 mm) to 2 mm. There are: . concave vertical bar screens (figure 284) with assisted self-cleaning and a capacity limited to 100-200 m3 h -1 . Since the raw water is distributed in the form of a vertical water curtain, the retained particles are carried hydraulically towards a lower container, 9 . rotating drums with a maximum capacity of 1500 m3 .h -1 : - in some devices, most of the matter retained outside of the drum is removed by scraping, whereas the remaining mat

5. Straining

ter is carried by the strained water which crosses the strainer in the opposite direction (figure 285),

- in other devices (figure 286), matter retained inside the drum is removed because of its slope, and cleaning is carried out through water spraying (also in the opposite direction).

5.2. STRAINERS FOR SURFACE WATER
5.2.1. Macrostraining The filtering elements are made up of perforated sheets or, more often, of crossmesh stainless steel or synthetic fabric sheets with 0.15 to 2 mm openings. Strainers come in the form of 15 to 6 m diameter drums, or 1 to 3 m wide bands; their 3 to 15 m height is well suited to rivers with a variable water level (figure 287).

Depending on the direction in which the water circulates, the sprinkler washing system is located over or inside the filter. Ditty wash water is removed through a trough. In both cases, by using dihedral or semi-cylindrical shaped panels, the specific filtration surface increases (chapter 3, figures 59 and 60). The maximum capacity of these devices is several thousand m3 h -1 , 5.2.2. Microstraining The mesh size of the synthetic fabric filtering sheets ranges from 30 - 40 to

Chap. 9: Preliminary treatment systems

150 ?m. They are mounted on drums. The washing equipment must be capable of intense spraying of the fabric because of the increased clogging risk due to fine sand

and silt. Joint watertightness is essential. These devices have a limited capacity (figure 289).

Figure 288. Rotating microstrainers.

5. Straining

Chap. 9: Preliminary treatment systems

5.3. MECHANICAL FILTRATION OR PRESSURE STRAINING
These filters (see page 182) comprise a semi-continuous washing system which ensures sludge removal by applying atmospheric pressure, in countercurrent, on a sector of the filter. Washing pressure must be consistent with the mechanical strength of the fabric. So as to avoid irreversible clogging and excessive use of wash water, the fineness of the pressure straining system should

usually be limited, for all practical purposes, to: - 100-150 ?m, for water with a large volume of organic matter and vegetable debris, - 40-50 ?m, for water containing only hard inorganic matter. Fibres are especially troublesome, since they tend to stick to the fabric. There are three basic types of design (figure 290). Unit flow of these devices decreases with straining fineness. Above 250 ?m, it can reach a level of 3 5,000 m .h -1 ; and in microstraining, up to 500 m3 .h -1 . Wash water consumption, under the same conditions, ranges between 2 and 8%.

6. Disposal and treatment of byproducts

6. DISPOSAL AND TREATMENT OF BY-PRODUCTS
6.1. SCREENINGS
This type of matter (see page 76) is often landfilled or buried. It can also be incinerated in a screenings furnace or in a household refuse furnace. Comb ustion temperature should be greater than 800°C to avoid smells. Screenings inside the works are handled as follows: - either manually (in small facilities): a tray (perforated or non-perforated), rolling skip, movable skip, - or mechanically: conveyor belt, continuous or reciprocating conveyor with squeegees, - or hydraulically: trough fed by pumped water. Because of transportation and environmental constraints, drying or compacting treatments have been given increased consideration, i.e.: - drying (and transportation) by a movable, Sita-type, integrated compactor skip (figure 291): the water content is reduced by 75 to 80%, and the bulk density of the compacted matter in the skip is 0.75 to 0.8, - compacting through a mechanical or hydraulic custom made p ress (figure 292): the compacted product may have a water content of 55 to 65%, and a bulk density of 0.6 to 0.65.

Figure 291. Sita integrated compactor skip.

Chap. 9: Preliminary treatment systems

Figure 292. Screenings press.

6.2. GRIT
Grit (see page 76) extracted by a shovel from small grit channels cannot be reused, and must be buried or landfilled together with screenings. For medium-size facilities, grit hydraulically extracted from grit removers can be separated from its water by: - settling in a shallow tank: water is removed through filtering slabs or over a weir, -mechanical recovery (Archimedean screw, or reciprocating rake classifier) and storage in a fixed hopper or skip,

- hydrocycloning and storage in a hopper with an overflow weir, - hydrocycloning and recovery by Archimedean screw before storage in a fixed hopper or movable skip. Washing by make-up water on the Archimedean screw can also be considered. In large facilities, grit is sometimes washed before hopper storage, in a basin equipped with an efficient air mixing system which receives the water/grit mix pumped from the grit chambers. A washed grit that is quite clean can be reused on-site (setting up drying beds).

6. Disposal and treatment of byproducts

Figure 293. Grit washer.

6.3. GREASE AND SCUM
In general, grease and scum collected at the surface of grit removers, grease separators and primary settling tanks, cannot be reused. One possibility is to send this kind of waste to anaerobic digestion (after having been fine screened during its hydraulic transfer): this arrangement usually increases gas production, but at the risk of producing a scum layer. It is preferable to store it in a skip, which could be equipped with an overflow outlet scum baffle, and then remove it periodically for burial or landfill. It can also be incinerated with sludge or screened matter, if the furnace and handling conditions allow it.

Chap. 9: Preliminary treatment systems

In large facilities, grease and scum from different units are sometimes hydraulically transported towards a static flotation unit, which is equipped with a mechanical blade skimmer for dry recovery of grease, which is

then stored in a skip or pit. After supplementary screening and, possibly, reheating to liquefy them, the products are pumped at a constant rate into an incinerator or sludge treatment furnace.


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