To counter the great variety and quantity of material flowing to the plant, raw wastewater is considered to contain materials in four classifications: 1) floating large and/or stringy objects; 2) fine, high specific gravity solids; 3) flocculent and colloidal suspended solids; and 4) dissolved organic matter. The task of treatment would be simple if the materials in wastewater would faithfully follow the above lines of distinction, with no variability of flow or composition. Under optimum conditions, following such a treatment scheme would seem to result in a high level of efficiency. The difference between theory and reality though is substantial and treatment processes are always somewhat compromised. The first step in the treatment train is that large floating and stringy objects can be removed by screening or can be ground up and returned to the flow. The materials in classifications 2 and 3 can then be removed by differential settling in flow-controlled conditions in grit chambers and large detention basins. This would leave only oxidizable dissolved organic matter to be dealt with.
Following the four-step classification for arranging process equipment and systems, design engineers generally install screens and/or comminutors, followed by grit chambers and sedimentation basins. These together are conventionally classified as primary treatment. The oxidation of dissolved organics is, then, secondary treatment.
Stormwater. Point sources of water pollution from wastewater treatment facilities are, if not completely, at least well under some sort of control. The same cannot be said for stormwater and sources of nonpoint pollution. Rain water runoff from streets, parking areas, construction sites, industrial sites, agricultural lands, and the such, flowing directly overland into water bodies or being discharged by storm sewers, is now the biggest factor affecting water quality. As a start toward control, a National Pollutant Discharge Elimination System program has been established for permit applications. Numerous business concerns and municipalities will find themselves facing a whole new set of rules in a somewhat different ball game. There are requirements for"actual storm" monitoring of runoff and various compliance deadlines. There will undoubtedly be more changes and refinements as this program evolves, new regulations are developed, and a better determination is made how to cope with some of the program's more difficult issues. For more information on this program contact your state regulatory agency, your regional EPA office, or the Water Environment Federation.
Screens are the first line of defense in the treatment process. Screens for domestic wastewater treatment are generally of the coarse variety, consisting of racks of bars placed in the influent channel or ahead of the raw wastewater pump. These are anchored at an angle of 30 [degrees] to 45 [degrees] with the horizontal, with the acute angle on the downstream side. They may be hand-raked, but most modern plants are provided with mechanical raking facilities.
Screening techniques have been adapted to the removal of very fine solids, down to seven microns, by high mesh metal or cloth media, so that the techniques overlap the sedimentation processes in. capability. In wastewater treatment, however, blinding of the media occurs with application to raw domestic wastewater. Consequently, most of the fine screening devices with this type of flow sheet must provide safeguards. Fine screening is applicable to a wide range of industrial wastes and to domestic wastewaters that have been stabilized by biological or chemical procedures. In the latter application, they can act as "advanced treatment devices."
Screenings removed from domestic sewage by coarse screens range in quantity from 0.5 to 3 cu ft per million gallons; by fine screens, up to 30 cu ft per million gallons. Straight racks are used for hand-raked screens and some that are mechanically-raked. Curved screens are mechanically cleaned. The clear openings are ordinarily 1 in. to 2 in.; the minimum net area of submerged openings is about 2 sq ft per mgd for domestic sewage. In terms of velocity, the area should allow a minimum flow of 1 fps to avoid forcing objects through the bars. If the screens are mechanically cleaned, an overload protector is needed.
The screen channel or chamber needs to be of such dimensions and configuration that sedimentation and undesirable accumulations do not occur. A head loss must be allowed for, which may be as much as 2 or 3 ft. Frequency of cleaning, if done by hand, will influence this figure. It is good practice to provide two or more small units rather than depend on one excessively wide screen. A platform should be included for temporary storage and draining of screenings prior to disposal.
Commercially available screens are made in widths from 2 to 16 ft.
Mechanically Operated Rakes. There are three general types of mechanical rake, two for use with the common flat screen and one requiring curved bars.
One type for fiat screens consists of a rake or series of rakes, the ends attached to a pair of endless chains which continuously move the rakes slowly upward over the back face, or effluent side of the screen, carrying the screenings to the top of the screen. There they are dropped into a conveyor or bucket, into a grinder, or onto a draining platform.
In the second type, the rakes pass upward on the face or influent side of the screen, dump their burden, disengage the screen, and then move downward.
Automatic Operation. During much of the time, the amount of screenable matter in the sewage is much less than the maximum for which the rake was designed. Several of the makers provide clock-actuating mechanisms to provide automatic intermittent operation at adjustable intervals.
Most screen manufacturers furnish an automatic float control. This energizes the motor circuit to operate the rake when sufficient screenings accumulate to hold back the sewage in front of the screen and produce a predetermined differential head between the upstream and downstream sides (generally about 2 in.). Removing the screened material permits the lost head to return to normal, the electric current cuts off and the rake stops.
Water-level control electrodes also can be used for starting the screen cleaning mechanism.
By the use of multiple screens, the cleaning of one at a time permits use of a smaller motor or other power unit. Because screens are usually placed at the outlet of the sewer system, where gas discharge is greatest, motors should be explosion-proof. Limit switches can be installed to prevent over-travel of the rake and to effect automatic opening of the teeth upon striking an obstruction.
Fine screens are effectively utilized in industrial waste pretreatment for removal of solids not easily handled by sedimentation basins. Wastes of a fibrous character, or particulate matter easily separated from water are widely subjected to treatment with fine screens.
They arc employed in England and at some plants in the United States as tertiary treatment devices, for polishing effluents where effluent quality requirements are unusually high.
Disc Screens. In these, stainless or alloy wire cloth up to around 60 mesh is mounted on a rigid circular frame that rotates on a shaft in a channel perpendicular to the direction of flow. The lower half is submerged so that solids impinge on the surface of the screen and are lifted in the rotation cycle above the level of flow, where they can be continuously removed by water spray.
Drum Screens. In this type, the screening or straining fabric is mounted on a cylinder that rotates in a flow channel. Construction varies, principally with regard to direction of flow through the fabric - either into one end of the cylinder, with solids collection on the interior of the fabric covering; or from outside the drum through the fabric to the interior, with solids collection on the exterior.
Another major variation is the fabric mesh, which can be as fine as 200. The term "microstrainer" is applied to such devices and their primary use is for potable water treatment or exceptionally high degree of wastewater effluent polishing. For preliminary or intermediary stages of domestic wastewater treatment, 10 or 20 mesh is common. For wastes from canneries, leather tanneries, etc., up to 40 mesh may be employed. The fabric should be of corrosion resistant, sturdy material such as stainless steel.
Provision is made for continuous removal of collected solids, by means of washing conveyors, for example, supplemented by water sprays to keep the fabric clean. In wastewater applications, periodic treatment of the fabric surface is desirable to avoid microorganism buildup. High pressure steam has been used for cleaning and microorganism control.
Vibrating Screens. If the straining surface is mounted such that it can be placed in vertical or horizontal motion or a combination of both, as the flow stream is passed through it, the strainer is referred to as a vibrating screen. Their primary use is in industrial waste applications but have been studied with reference to processing combined sewer system discharges. Also, they could be used for intermediary treatment of domestic wastes containing specific industrial wastes where rather uniform discrete particles are encountered.
Unless mechanical or hydraulic conveyance is provided, the screenings are deposited on a platform or in containers, to be removed at intervals by hand. Belt or other conveyors are used in some large plants, transporting the screenings to a desirable disposal point or to hoppers for intermittent removal by trucks. Pneumatic equipment is available for conveying screenings. Methods of disposal include incineration, burial, digestion with sludge, or shredding for return to the sewage flow.
Incineration temperatures of screenings are around 1,300 [degrees] to 1,400 [degrees] E The heating value is about 5,000 to 8,000 Btu per pound of dry material, and dewatering can be carried to the point where screenings will burn without auxiliary fuel.
If screenings are to be disposed of by shredding, two methods are in common use. One involves continuous shredding in the influent channel and the second involves the collection of solids on a screen, with removal from the screen by scrapers and final grinding. These methods transfer the solids removal problem to the settling tanks where the screenings are removed with other sewage solids and pumped to sludge digesters with the sludge collected in the settling tanks.
Comminution. This technique involves the reduction of particles to small size, and in sewage treatment, the term applies to a process of cutting, or shredding by apparatus inserted in the channel of flow. Trituration and disintegration are terms having similar application. Comminutors are made in several sizes, from 30,000 gpd to many million gallons per day.
Comminution can take place as part of influent pumping or in a pipeline system when impellers or cutting blades mounted on the impeller shaft revolve against a stationary surface.
Screenings grinders are often of the hammer-mill type with knife-edge hammers operating against stationary, wedge shaped surfaces. These are usually separate from the raked screen and receive the screenings as they are removed. After disintegration, the screenings drop into the plant flow stream.
The more dense suspended materials - sand, cinders, metal filings - are classified under the term "grit," and may be removed by differential settling; i.e., by retarding the flow to a velocity of about one foot per second. This permits the higher specific gravity solids to settle out and others to remain in suspension. If not removed, grit can damage pumps and will accumulate in the digesters. Removal of grit from digesters is not easily accomplished and is certain to interfere seriously with the sludge digestion program.
Channel type grit chambers are elongated and rectangular in plan, with the cross section or outlet arrangements such that a fairly constant velocity is maintained within normal flow limits. Other designs provide for separation of grit and organic solids by washing or by air agitation. Mechanical grit collection systems are advisable. Retention times for grit chambers in current design practice range from 10 to 100 seconds. The quantity of grit removed ranges from one to 27 cu ft per million gallons.
The flow of sewage varies, being larger during the day than at night. It is therefore difficult to provide a grit chamber that will have the same desirable velocity for all volumes of flow.
Other velocity control devices for the rectangular channel are the Sutro weir and the Parshall flume. Either will provide good velocity control, but the latter causes less head loss.
All grit channels should be designed with a sufficient detention period to allow the smallest grit particle to settle to the bottom before the flow leaves the channel: 65-mesh silica sand is generally considered the minimum size. Overflow rates govern the basis of design whether the channel is rectangular, round or square. Since the velocity through these channels is relatively high, disturbances from wind and from convection currents can be ignored, but careful consideration should be given to turbulence at the inlet and outlet because only in the portion of the channel, where true streamline flow exists, is effective settling accomplished. If the design cannot overcome turbulence at these points, an additional length of channel equal to twice the water depth at maximum flow must be added to the length theoretically required.
The 65-mesh silica and particles (0.22 mm or 0.009 in. in size) settle at a rate of about four feet per minute, which means that grit channels must have an overflow rate of less than 43,000 gpd per sq ft. Grit channels can be designed to remove smaller or larger particles by selecting the proper settling rate.
It should be noted that low gravity solids of three times the size of the high gravity solids settle at the same rate. It is therefore obvious that only the relatively small organic solid particles will pass out of the channel. Controls to maintain a constant velocity of one foot per second regardless of depth will not re-suspend the 65-mesh silica sand particles, but will re-suspend organic particles less than three times their size.
Since a considerable portion of the organic solids such as grains of corn, peas, beans, etc., are much larger than three times the size of the 65-mesh grit particles, they are bound to settle out with the grit. These can only be separated from the true grit in separate washers if really clean grit is required.
Conveyor Washing. As solids are collected in the grit chamber, wherever the size of the installation warrants, mechanized removal is desirable. This usually takes the form of a screw or flight conveyor or bucket elevator. With the conveyors operating countercurrent to wastewater flow some washing of the grit occurs in the grit chamber. Supplemental washing can be added as the conveyor exits from the chamber by water or effluent sprays. If grit is disposed of for reuse, it should be relatively clean. Otherwise, a fairly thorough job of separation of putrescible organics is all that is necessary.
Agitation Washing. Grit can be washed by a diaphragm jig, placed in the flow stream or otherwise or by means of a separate unit in which agitation is accomplished by diffused air.
Instead of a conveyor, a reciprocating rake for agitation or a spiral classifier operating in connection with the grit collection mechanisms offer alternatives.
Cyclone grit separators provide a cylindrical feed chamber with tangential inlet, tapered to the grit discharge point. The centrifugal force imparted to the tangential flow separates the grit easily and a high degree of removal - down to 150 mesh is possible.
Odors from grit collection may develop and are subject to control by aeration of the recirculation fluid line. Hydrogen peroxide is also an aid.
In the non-mechanized types of grit chambers, cleaning is necessary after every storm, and also when the channels are half full or more. In dry weather, measurements should be made once a week.
Removal is accomplished by shutting off the flow in the compartment or channel to be cleaned, pumping or bailing out the sewage, and removing the residue with shovels, pails, or in large plants, with grab buckets. Such grit usually contains so much organic matter that it should be buried or burned; although recycling of this material should be investigated.
Mechanized final removal of the material discharged by any of these methods can be provided by a monorail and bucket carrier, or a belt, bucket, scraper, screw, or other conveyor.
To further the liquid-solids separation step of primary treatment beyond screening and grit removal, the usual method is gravity separation, though others have been attempted. This consists of reducing the rate of flow to the point where the solids drop out.
Domestic wastewater solids are somewhat flocculent and are easily settled, but the process can be hastened and made more complete by applying chemicals. In some cases the wastes are of such a character that fine screening may be employed. Filtration is difficult to apply as a primary step, but is used for polishing an effluent. Centrifuging is usually applied to dewatering of sludge.
If the waste being treated contains appreciable concentrations of oil, grease, or fibrous materials, the principle of flotation may be applied to achieve clarification. This is accomplished by dispersing air in the form of fine bubbles, causing the suspended materials and oil globules to form a dense scum on the liquid surface, which can be removed, burned in an incinerator, or transferred to a digester.
Some criteria of design for settling basins are the surface overflow rate (gallons of flow per day per square foot of surface area, with a range of 500 to 2,000 for domestic sewage); the weir overflow rate (gallons of flow per day per lineal foot of weir length, with a range of 5,000 to 20,000 for sewage); and surface-area-weir-length ratio (-square feet of surface per foot of weir length, with range of 5 to 150).
Effluent weirs are high maintenance items. This is reduced by employing thermoplastic construction materials such as fiberglass-reinforced plastics.
Centrifugal separation of solids from the liquid waste stream has been employed for dewatering sludge.
A number of manufacturers offer factory-built plants that include in addition to clarification complete secondary and advanced treatment facilities.
The principal maintenance problem of settling tanks is removal of sludge. The sludge collectors generally move the sludge to a hopper in which it is stored until removed by pumping or by gravity flow. In some designs sludge is drawn by gravity through a telescoping valve to a sump, from which it is pumped to the digester. The latter arrangement may offer more freedom in controlling the solids concentration in sludge being pumped to the digester.
For most economical use of digester capacity, the solids concentration should be held in the neighborhood of six percent. At this concentration the sludge is sufficiently fluid to offer few pumping problems and an excess amount of water is not being transferred to the digester to become supernatant liquor.
At small plants the problem may be handled by operating the pumps a few times a day and coordinating the pumping with collector operation, sampling frequently to observe sludge density. At large plants and for most efficient operation all around, sludge collection and pumping should be as frequent as possible and theoretically match sludge production. Where the sludge is pumped from a separate sump, the control valve allowing gravity withdrawal from the basin hoppers can be left open continually at a predetermined setting and the pump turned on only when the sump is full.
Some floating material collects on the top of the tanks. Mechanical apparatus should be provided to remove such material. Most plants are so designed that skimmings may be swept into a trough, which discharges into the digester or into a pump sump so that further handling is not necessary.
The side walls of the settling tank sometimes collect fine solids, which in warm weather decompose; also at the water line, some grease is deposited. Such collections should be removed daily with a stiff brush or with a squeegee, and the walls washed down with a hose, care being taken not to disturb the tank contents more than is necessary.
The inlet channel should be washed out with a hose every day and scrubbed at least once a week, using a stiff broom; the same treatment should be given to the outlet channel.
Manufacturers furnish with their equipment a lubrication chart that shows where to oil and grease and tells what kind of lubricant to use. These instructions should be followed carefully.
Velocity control and dispersion in settling tanks are usually handled by configuration and other construction details. The methods for sludge and scum removal that are least expensive in labor costs and produce satisfactory results involve the use of power-driven equipment operating on a predetermined cycle. Most sludge removers push the sludge to a central well in circular tanks or to a hopper at one end of rectangular basins. One type of circular collector draws the sludge directly from the bottom by hydrostatic pressure.
Tube and Plate Settlers. A development based on an old principle is the tube or inclined plate settler. This consists of parallel tubes or plates constructed of extruded PVC or metal installed at a 30 [degrees] to 60 [degrees] inclination. As wastewater flows upward through the tubes or plates, solids settle along the inclined walls and drop to the bottom of the basin. The tubes or plates are assembled in modules, with the size of the individual tubes varying, depending on the application. For most wastewater treatment applications, the tubes measure 2 in. by 6 in. in cross section and 4 ft long. They may be installed in existing rectangular or circular basins to increase capacity.
Four general designs of sludge collectors are in common use. In one, used in circular tanks, an arm carrying scraper plows or blades, revolves around a central shaft pushing the sludge into a central well. In a second type, used in rectangular tanks, a series of blades or flights attached to endless chains push the sludge into a hopper at one end. A third type, also used in rectangular tanks, consists of a traveling bridge spanning the tank, from which is suspended a sludge-collecting blade. In another, for circular tanks, revolving blades are equipped with intake nozzles for picking up sludge, actuated by static head differentials.
Revolving Collectors. The revolving mechanism of this collector has a number of horizontal arms which carry "plows," "blades," or "squeegees" set at an angle so as to push the sludge continuously but slowly toward a central well as the arms revolve; or in another pattern the arms carry, instead of short blades, two spiral scraper blades that extend from the outside circular wall to the central well.
The power is almost always an electric motor. In some designs this motor, together with the revolving equipment, is supported either on a central pier or by a truss that spans the tank, and revolves a central shaft to which the arms are attached. In other designs the motor and tractor are at the outer end of the rotating bridge, which is carried by wheels that travel on a track surrounding the tank and the other end of the bridge being centrally supported.
Chain Collectors. These consist of two endless chains which carry, at intervals of several feet, scrapers or flights. The scrapers extend entirely across the tank and are drawn by the chains along the bottom of the entire length of the tank. The chains then pass around sprockets at the end, back to the other end and around other sprockets to the starting point. In primary tanks the return trip of the flights is made at the tank surface and they thus carry scum and floating materials to the effluent end. These collectors draw the sludge into a hopper usually at the inlet end of the tank. Some of these collectors make use of pivoted flights that are constructed to remain in contact with the floor and have overload release and alarm devices.
Cross-collectors often are provided at the hopper end of large tanks, or two or more parallel tanks discharging into a common cross-channel.
Chains, Sprockets, and Other Components. In the chain and sprocket arrangement for flight collectors, the chain is of limited life and periodically has to be replaced. Sprockets are also subject to wear as are the flights. In the past a metal wearing surface was usually provided. Sprockets are designed for minimizing chain wear, using special configurations. Metallic chains are furnished by the manufacturers of sludge collectors. In wastewater treatment devices, the long-wearing characteristics are deserving of consideration because of the grit content of wastewater.
Nonmetallic chains are constructed of non-metallic links and pins along with fiberglass flights and polyurethane wearing shoes. Besides long wearing characteristics these non-metallic components are lightweight, thus aiding maintenance operations.
Bridge-type Collectors. This type of collector has a bridge spanning the tank and running back and forth on rails on the side walls. From this bridge, one or more scraper blades are suspended and are lowered for sludge collection and raised for scum collection. The bridge is propelled by electric power, as is the motion of the blades.
Keeping the surface free from scum is particularly a problem with primary clarifiers and is seldom used on secondary units. Any of the sludge collectors described can carry scum-collecting arms at the surface of the tank for discharge to a trough for ultimate disposal. Where there is no scum-removing mechanism, horizontal spraying with water under pressure has been used to collect the scum.
Special devices are furnished for disposal of the accumulated scum. Among them are a manually revolved trough or one that is mechanically tipped in coordination with a skimmer, a power driven helical, screw or flight conveyor or more sophisticated arrangements whereby wiper blades operate in an apron or trough. Flushers are also provided to assist removal. Hydraulically operated scum removal apparatus can also be obtained.
The factor that primarily governs the rate of operation of a plant is the variation in influent flow. Measuring devices should always be provided preferably with remote transmission of flow data.
The balancing of flow between the pumps and different plant units can be accomplished by recirculation of effluent, if desired or necessary to proper operation.
Pump operation may be programmed between several pumps of different capacities or by using variable speed pumps. Their operation should be indicated on the control panel through an appropriate transmission system.
Levels throughout the plant should also be indicated on the panel. This is made possible by using liquid level sensors at strategic points with transmission to the central point to operate gauges, recorders and alarms.
Drives for sludge collectors can be programmed for intermittent operation as desired. Those for flocculation equipment may be equipped with variable speed motors to obtain optimum rotation speed for the type of floc.
All variable factors are subject to integration in a supervisory control system, which provides command and report back features. Provisions for manually over-riding are necessary to assure continued plant operation in the event of function failure or unusual conditions.
Lubricants. Keeping equipment lubricated is an important part of proper operation and preventive maintenance. Plant personnel should be aware of any special requirements of the engines, compressors, pumps, and other machinery at their facilities.
Protecting pump stations and treatment facilities from the destructive effects of sewage is of paramount concern. These facilities often have high initial costs and may be difficult to repair. If high levels of maintenance are not followed operational inefficiencies and premature failure may occur. Taking these facilities out of service even for short periods of time can be costly and disruptive to the community. Keeping corrosion to a minimum is an important part of good design and ease of maintenance.
Cathodic Protection. Since corrosion of metal surfaces in clarifiers is a continuing maintenance problem, cathodic protection is used by many utility agencies.
Protective Coatings. Rubber, synthetic resins, sprayed metal, epoxies, urethanes, and reinforced plastic coatings are a few examples of the protective systems available.
Corrosion Resistant Components. Pump stations and treatment works are usually constructed of steel, concrete, and fiberglass reinforced plastic. If steel housing is used, it should be corrosion-proofed with a durable coating system and cathodic protection. Other features to be protected may include the wet and dry wells, sump pumps, ventilators, access ladders, lights, covers, structural elements, dehumidifiers, control and security systems, doors and hatches, windows, beams, walkways, and other appropriate items. The designer must consider the nature and strength of the sewage, those parts of the system most likely to come in contact with it, access to components for maintenance, and other factors. Predicting the effect of all of these factors is certainly not easy, but it should be remembered that it is more desirable to protect a component from corrosion than it is to replace it.
One consideration in lift station design, whether on-site or factory-built, is septic conditions arising from wet well detention periods. One means of combating the problem is injecting air into the force main. A "wet well-mounted" pump station is offered by Smith and Loveless, Inc., in which all equipment except piping and sensors are contained in an above-ground enclosure mounted over a standard wet well that keeps the critical components from having direct contact with corrosive liquids and gases.
Information. NACE International is a membership organization that provides publications and training programs on corrosion control designed specially for water and wastewater systems. For information contact the group at tel(713) 492-0535, fax-(713) 579-6694.
Flotation Units. Clarification by utilizing compressed air to "float" suspended matter is advantageously applied in some plants. Air is injected in a pre-aeration tank in the form of rising bubbles and carries to the surface partially emulsified particles of oil and grease, which collect in the scum on the surface and may be removed with it.
One design employs diffuser plates in a modification of the ridge and furrow system, the total plate area being about 10 percent of that of the tank. Another uses a down draft tube aerator.
Most flotation units, especially those designed for treatment of difficult wastes or sludge employ the principle of dissolved air flotation. In this, the influent is supersaturated with air under pressure. As the flow enters an open tank and the pressure released, minute air bubbles are formed that carry the floatables to the tank surface. Various configurations of tank design are employed and the skimming mechanism is an essential part of the unit and must be reliable.
Grease Interceptors. Interception of grease before it reaches the sewer is greatly to be desired, as it prevents grease deposits in the sewers. It probably is practicable to require interceptors only at establishments where large amounts of grease are wasted.
It has been found that mechanical flocculation prior to sedimentation will promote the rapid growth of floc and enhance settling.
There is little, if any, difference between the flocculator mechanisms used in wastewater treatment and in potable water treatment. The difference is in method of application. Wastewater in the raw state contains flocculent solids and additives are not required. For advanced waste treatment, where a high degree of clarity of primary effluent is desired, flocculents, such as alum or polymers can be added to improve unit efficiency.
It is generally desirable to discharge to the digester a sludge with as high a solids content as possible. Sludge can be transferred to a separate thickening tank and allowed to settle. Usually, the thickener consists of a series of vertical arms attached to a raking mechanism to stir the sludge gently and release entrained water and gas while scraping the mixture to a central well. It is applicable to both primary and secondary sludge treatment.
In gravity thickeners, it is important that the prevalence of anaerobes be minimized. Any procedure to accomplish this will hasten concentration of sludge, such as use of hydrogen peroxide. Anaerobic bacteria tend to promote gas formation resulting in a lighter product.
Another type of equipment concentrates sludge by flotation without chemical treatment. Fine air bubbles bring sludge solids to the top of the tank, forming a concentrated sludge blanket. The floated sludge is removed by continuous skimmer conveyors. High specific gravity solids will settle to the tank bottom and be removed with separate conveying equipment. The thickeners can be used in steel tank construction or concrete. Gravity thickeners are furnished with generous floor slopes, high torque stirring mechanisms, and manual or automatic lift devices.
The dissolved air flotation principle is also applied in thickening. Many units are shop fabricated and shipped assembled.
A number of manufacturers market pre-designed, factory assembled sewage treatment units or components that can be used to assemble a complete plant at a desired location. These range in size from the capacity required for a single residence to that for a small town. Intermediary sizes are finding widespread application in shopping centers, highway rest areas, institutions, and housing developments.
The general design and equipment of these units are described in appropriate sections of this manual, depending on the type of treatment involved, the number of units to be serviced, and the nature of the waste to be treated. Because of their intended use in sensitive and unsewered areas, these units usually treat wastes to a high degree, including tertiary levels.