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Activated sludge processes.

In the conventional activated sludge process, raw wastewater is settled in a primary tank with a retention period of two hours or less and then subjected to aeration for six to eight hours. After aeration, the sludge drops out in a final clarifier with a two-hour retention period. A portion of the sludge is returned to the inflow point, usually a maximum of 25 percent of the total flow volume. The contents of the aeration tank are referred to as "mixed liquor" and may have a suspended solids concentration of at least 1,000 mg/L.

Criteria for sizing aeration units are in terms of pounds of BOD in the tank influent per day per 1,000 cu ft of tank volume, ranging from 25 to 100; or pounds of BOD per 100 lb of volatile solids under aeration (or "mixed liquor suspended solids"), ranging from 10 to 100 or optimally 25 to 50 lb. Where diffused air is employed, the aeration rate is around 500 to 700 cu ft of air per pound of BOD removed.

The age of the sludge is an important factor and with sufficiently long aeration the sludge would contribute to its own eventual destruction. The development of these concepts in the last couple of decades resulted in a number of modifications. One of these is called "contact stabilization," in which aeration tanks for wastewater plus sludge could be made smaller while a separate tank was provided for aeration of the sludge.

Another modification provides aeration of raw wastewater for long periods of time to reduce the amount of sludge to be wasted or to be disposed. Termed "extended aeration," this modification is the basis for design of the small activated sludge plants factory built for subdivision, institution, and even residential use. Manufacturers of factory-built plants are listed in Section D-5.

Often, the choice of the types of aeration process applied in activated sludge treatment depends on local conditions, such as waste characteristics, land available, and local energy considerations. Pilot plant testing may be indicated in determining a final choice. Several firms offer pilot plant services or facilities.

Generally, lagoons or ponds that receive settled wastewater and consequently function only as secondary treatment devices are known as "oxidation ponds." Those that receive raw wastewater and function over the entire range of treatment are called "stabilization" ponds or lagoons. The activity in these ponds and lagoons can be increased by installing aeration systems, making possible equivalent BOD reduction with a reduced retention period. Various issues of PUBLIC WORKS contain articles on the design of polishing ponds to help remove excess suspended solids from plant effluent.

Aeration serves at least three functions: 1) Mixing the effluent from primary treatment with the returned activated sludge, 2) keeping the sludge in suspension, and 3) supplying oxygen required in the biological oxidation process. Compressed air may be used by discharging it through submerged diffusers made of various types of porous materials, or through nozzles on a pipe grid, placed in the contact tank in such a manner that a rotary motion is imparted to the sewage. Through constant renewal of the liquid surface, oxygen is also absorbed from the atmosphere.

Low-pressure air may also be introduced by multi-valved tubing laid transversely in rows across a lagoon or tank bottom. Water circulates thoroughly, but without turbulence to place oxygen through all levels of sewage.

A similar effect may be obtained by using paddles or an impeller to agitate the sewage, and when this latter method is used, it is referred to as mechanical aeration. By combining a rotor and a source of compressed air a high oxygenation efficiency is obtained.

The use of pure oxygen under controlled conditions with agitation is now being employed in a number of plants. See Section D-4.

The American Society of Civil Engineers has developed a standard for the "Measurement of Oxygen Transfer in Clean Water." It is useful for the preparation of specifications for compliance testing and by manufacturers in the development of performance information. The standard is available from the ASCE Publications Fulfillment Department.

Diffused Aeration

In pressure aeration the air is forced by means of compressors (also called blowers) through porous diffusers or other devices which create small bubble diffusion, usually installed near the bottom of a tank. The rate at which air is supplied for satisfactory treatment varies with the strength of the wastewater as received. It is desirable to maintain a small amount of dissolved oxygen at the head end of the aeration tank with a gradual increase up to 1 to 2 mg/L at the discharge end.

Diffusers. As the air is discharged into the liquid it should be finely dispersed because of the larger surface offered by the greater number of small bubbles. This may be done by passing the air through porous plates or tubes, by combining air and sewage under jet pressure or by impingement.

Porous plates and tubes for this purpose are made from ceramically bonded fused aluminum oxide by Aercor-Aeration Engineering Resources Corp.; Ferro Corp., Filtros Plant, Specialty Ceramics Div.; and Refractron Corporation.

When diffuser plates are built into a tank, they are generally set over an air channel in special holders, either of pre-cast concrete or fabricated corrosion-resistant metal. Each holder contains a row of several plates and can be removed separately for cleaning or replacement, or to clear out the air channel. If portland cement mortar is used in contact with the plates, the plates must be saturated with water to prevent absorption of water from the concrete. After being set, plates should be protected from dirt and oil.

Several plants have been equipped with a combination of air main and diffuser plate holders that hold two lines of vertical plates a foot above the bottom of the tank, each line of plates supplying one of the two adjacent tanks and the two in. space between them serving as an air channel. Advantages claimed are uniform air distribution at all times, simplicity of installing and removing, and freedom from clogging of plates by sediment.

Tubes are usually installed not as a continuous connected line, but in units, each consisting of two horizontal tubes with their outer ends closed, clamped in a frame and fed by an air pipe connected between the two and serving to suspend them in the tank. By disconnecting its air pipe from the air main (which is installed above the sewage, generally on top of the wall) a unit can be removed for cleaning or renewal. By furnishing these unit air pipes with valves, the air supply can be reduced or shut off from any part of the tank.

The standard size tube is 2 1/2 in. internal diameter, walls 5/8 in. thick, and a length of 24 in. However, tubes can be made in any dimensions desired.

The diffuser tube offered by Ferro Corp., Filtros Plant, Specialty Ceramics Div. is a ceramic tube provided with a standard 3/4 in. NPT pipe fitting in either galvanized, stainless steel or PVC, all with the standard orifice. A similar tube is offered by Aercor-Aeration Engineering Resources Corp.

Aercor-Aeration Engineering Resources Corp.'s aeration system employs a porous "Alundum" diffuser dome, with PVC piping and fixtures. A ceramic disc diffuser is also supplied by Ferro Corp., Filtros Plant, Specialty Ceramics Div.

"Swing diffusers" consist of units with air pipes so connected to the air header that they can be raised to the tank wall for cleaning, rearrangement, or replacement. The accessibility of swing diffusers allows adjustment of air supply to meet the oxygenation requirements of sewage without dewatering the tanks as would be required in the case of fixed diffusers.

Air diffusers of various types ranging from plastic fiber-wound perforated tubes to synthetic media bags and specialized orifices are supplied by Aer-O-Flo Environmental, Inc.; AEROMIX Systems, Inc.; Aercor; Air Diffusion Systems; Aqua-Aerobic Systems Inc.; Arlat Inc.; Davis/EMU; Eco Equipment Inc.; Envirex Inc.; EnviroQuip Int'l. Inc.; FMC Corp., Material Handling Systems Div.; G-H Systems, Inc.; Hydro-Aerobics, Inc.; Jones & Attwood; I. Kruger Inc.; Pollution Control, Inc.; Liquid Air Corp.; Roediger Pittsburgh; Sanitaire-Water Pollution Control Corp.; and Suburbia Systems Inc.

Other Aeration Devices. Air entrainment in a liquid stream and release through a nozzle or other hydraulic mixing and diffusing device is a principle incorporated in several proprietary systems. High efficiency with regard to energy consumption per quantity of air or oxygen is claimed.

Among these is the system of Chemineer Inc. In this, wastewater is pumped into a manifold simultaneously with air supplied by blowers and the air and liquid streams are mixed in vortex changers to provide fine bubble diffusion. Longitudinal or radial headers can be furnished for installation in existing tanks. The jet aerator of Amwell also utilizes this principle in a longitudinal header. Fine and coarse bubble aeration systems and diffusers are available from Napier-Reid Ltd. The Air Flo system from Barebo, Inc./Otterbine for deep water bodies (15 ft or more) uses a shore-mounted oil-less air compressor and weighed tubing connected to the air lift diffuser.

Static diffusers, consisting of tubes with helix-shaped vanes, baffles, or other means of imparting turbulence to an air-water mixture are provided by Chemineer Inc.; and Polcon Sales Ltd. These are installed upright over air headers along the bottom of a basin or oxidation pond.

Schreiber Corp. has rotating, submerged diffusers designed to increase the oxygen transfer rate by extending the contact time with the air bubbles. The rotating diffusers can be combined with stationary diffusers.

The Vari-Cant from Jet Tech Inc. is self-cleaning while discharging at a downward angle in order to increase transfer efficiency.

Seghers-Dinamel, Inc. has a carousel type arrangement that is set into a circular basin; the diffuser system is said to promote high efficiencies with low energy costs.

GS Metals Corp. has aeration decking with a specially designed louver pattern that provides optimum air flow for high rates of biological activity.

Solar Electric Systems of KC, Inc. has solar powered pond aeration systems especially useful for remote sites or as an energy-saving feature.

The design of some of the aeration devices is such that they can be used for process mixing, in particular, "jet mixing." Companies supplying equipment suitable for this include Chemineer Inc.; and Jet Tech Inc.

Air Filters. The air should be filtered to prevent the clogging of diffusers, and all the air piping and channels should be made of non-corrodible material - heavily galvanized iron or one of the non-corrodible iron alloys, copper, aluminum, etc. Filtration is necessary before the air reaches the diffusers to remove dust, oil, or other impurities. Air delivered to the diffusers should contain not over 0.5 mg to 1.00 mg of dust per 1,000 cu ft. Filtration is accomplished by passing the air through one of the commercial filters made for this purpose; usually the filter is located on the suction side of the blower for most efficient operation.

Air filters may be of either the dry, the viscous, or the electronic type. The first screens out the impurities, the second retains them on a viscous surface. In electronic precipitation, the dust particles are given an electrostatic charge of definite polarity by ionization and are then removed from the air stream by electrical attraction. This method is especially desirable for a smoky atmosphere. In some cases, two and even all three types are combined in one filter. Manufacturers of various types of filters include American Air Filter; and Hankison Int'l.

Blowers and Compressors. At one cubic foot of air per gallon of sewage, there is required an average capacity of 700 cu ft of air per minute per mgd; or, assuming that the maximum equals twice the average, a capacity of 1,400 cu ft per minute per average mgd is required of the equipment. Air for activated sludge tanks is furnished by rotary positive displacement and centrifugal compressors (also called blowers). To insure that no oil is taken up by the air in passing through the blower, there should be no internal lubrication. The pressure required seldom exceeds eight or ten psi. Current use is about equally divided between displacement and centrifugal types in the 300 to 15,000 cfm range; centrifugal compressors are preferred for units larger than 10,000 to 15,000 cfm. Provision should be made for regulating the air output to conform to the requirements of variable sewage flow, especially in large plants. Rotary compressors furnish constant volume at variable pressures. Centrifugal compressors vary the volume but maintain constant pressure. They may be throttled to reduce both horsepower and flow.

Compressors are usually driven by electric motors or by internal combustion engines. Use of the latter, operating on gas from sludge digestion tanks, is common in larger plants.

Manufacturers include Dresser Industries, Roots Division; DuroFlow Corp.; EnviroQuip Int'l. Inc.; FMC Corp., Material Handling Systems Div.; Gardner Denver Industrial Machinery-Cooper Ind.; Hoffman Air & Filtration Systems; Lamson Corp., Centrifugal Air Systems Div.; Pollution Control, Inc.; SEMBLEX, Inc.; SIHI Pumps Ltd.; Spencer Turbine Co.; Turblex, Inc.

Air meters. Meters of the orifice plate and venturi tube types may be used for air measurement with a low loss of head. Leeds & Northrup Unit of General Signal furnishes either type of meter. Bailey Controls Co.; Preso Meters Corp.; and others also furnish meters for this purpose. Where ratios of sewage flow to air flow are desired, Leeds & Northrup Unit of General Signal Corp. furnishes air-sewage ratio gauges.

Air Dryers. Water vapor entering the compressor with atmospheric air can condense from temperature changes and contaminate the entire compressed air system. Possibly causing corrosion, damage to the system, and higher maintenance costs. Controlling temperature in a refrigerated evaporator is one method used to produce dry air. Van Air Systems Inc. has an energy saving regenerative compressed air dryer that automatically adjusts to actual moisture load conditions.

Mechanical Aeration

Mechanical aerators are of several general types - horizontal paddle, vertical turbine, and vertical draft tube. In the former a paddle wheel (or brush) with a horizontal axis revolves partly submerged in the sewage. In the third, the sewage is drawn up or down through a central vertical tube by a revolving impeller, providing aeration and agitation.

The vertical turbine types may operate partially submerged or completely submerged. In the former mode, they are termed "surface aerators." Floating models are available as well as fixed.

Manufacturers of the horizontal paddle type, which is a modification of the Kessener brush system developed in the Netherlands, include Air-O-Lator; Eimco Process Equip. Co.; Envirex Inc.; Hydro Group Inc., Environmental Products Div.; I. Kruger Inc.; Lakeside Equipment Corp.; Purestream, Inc.; United Industries, Inc.; Zimpro Environmental, Inc.

Those firms supplying vertical turbine types include Aerators, Inc.; Aqua-Aerobic Systems, Inc.; Aqua Tech Industries, Inc.; Ashbrook Corp.; Chemineer Inc.; Dorr-Oliver; Eimco Process Equip. Co.; Envirex Inc.; Enviroquip, Inc.; FMC Corp., Material Handling Systems Div.; Infilco Degremont; Koch Engineering Co., Inc.; Lakeside Equipment Corp.; Philadelphia Mixers Corp.; Smith & Loveless, Inc.; U.S. Filter; United Industries, Inc.

Manufacturers of the draft tube types include Barebo, Inc./Otterbine; Bird Machine Co.; Environmental Dynamics Inc.; Infilco Degremont.

A surface mounted aspirator aerator is manufactured by Aeration Industries Int'l. Aspirator aerators draw air down a hollow shaft and injects it beneath the water surface. The aerator utilizes horizontal mixing to reduce energy consumption and can operate in cold climates. Units can be leased or rented for a variety of applications. Horizontal mixing aspirating aerators are available from AEROMIX Systems, Inc.; Framco Environmental Technologies; I. Kruger Inc.; and Vladix Corp. Other aerators are available from SEMBLEX, Inc.

Combinations of mechanical and diffused air systems can be obtained, as well as adjustable depth turbine models. In application, the draft tube type is usually designed as a modular unit - a single aerator in a basin. The surface and other vertical turbine types can be distributed over a basin area, making them highly adaptable to lagoon or oxidation pond application. The horizontal rotor aerators are designed to operate in a comparatively narrow channel, and perform the combined functions of aiding flow movement and oxygenation. Because of the basin configuration, the plants incorporating them were termed "oxidation ditches." Lakeside Equipment Corp. refers to them as such. Envirex Inc. uses the term "Orbal;" Eimco Process Equip. Co., "Carrousel."

A static aerator with no moving parts is made by Parkson Corp. It is a low-head, gravity flow-device utilizing aerating weirs, downcomers and riser tubes for increasing dissolved oxygen levels. Another sub-surface static-type mixing-aeration device called the "Helixor" is produced by Polcon Sales Ltd. A wind-powered recirculator allows water to oxygenate naturally.

Among the devices used for aerating water are a cascade of steps or baffles, a series of trays mounted one above the other, and nozzles. The last is considered the most efficient, because the droplets formed result in exposure, of the largest interfacial area.

Variations involve the substitution of coke trays for cascades, the enclosing of the system to provide forced draft induced aeration, diffused air aeration, and mechanical surface aerators. The latter are not often used in water treatment, finding wider application in wastewater treatment practice. Greensand and other materials can be used for specialized applications such as an aid to demineralization.

Commercial forms of the spray aerators are furnished by General Filter Co.; and U.S. Filter in the shape of a distributing pan over three or more superimposed coke trays. Aluminum tray or baffled aerators are made by Washington Aluminum.

General Filter Co. provides a forced draft tray aerator, natural draft tray aerator, and a cascade type. A forced draft tray aerator is also made by Envirex Inc. Infilco Degremont also makes a cascade aerator, consisting of three concentric superimposed circular trays.

In coke tray type aerators, the water is distributed over the top tray by spray nozzles on a manifold. Pressure aerators, using steel shells containing coke are also made. Cascade aerators which can be mounted in elevated tanks and modules equipped with forced draft circulation are made by General Filter Co.

See Section A for manufacturers of electric motors for powering the aerators. There is some specialization in drive gearing, but those providing pump motors can often meet the specifications for aerator motors.

Combined Aerators & Clarifiers & Steel Basin Fabricators

Several firms have combined an aeration basin and a clarifier in one unit for installation in small plants. In the so-called "factory-built plants," manufacturers of which are listed in Section D-5, combinations of aerators and clarifiers as well as other ancillary equipment are provided.

In the "Complete Plants" section of D-5 there are listed producers of self-contained treatment plants. Brown-Minneapolis Tank; Canbar Inc.; Columbian Steel Tank Co.; Environetics, Inc.; L. O. Koven & Brother, Inc.; Pittsburg Tank & Tower Co.; Raven Industries, Inc.; and A.O. Smith Harvestore; will fabricate steel tanks from design specifications.

Modified Activated Sludge Treatment

Basic departures from the conventional activated sludge process are considered to be:

1) Providing primary settling followed by a shorter than conventional aeration period (2 to 3 1/2 hours) with reduced mixed liquor solids level, in the interest of economy of air employed and tank area, to produce an effluent of intermediate quality. This has been termed "high rate" activated sludge process and "activated aeration."

2) With primary settling optional, providing a "mixing aeration" period of 20 to 40 minutes, followed by secondary settling and re-aeration of the sludge for five to seven hours prior to return to the inflow. Here, the significant departure is expending the air supply primarily on the sludge instead of the influent plus sludge. This modification has been termed generally "contact stabilization" and sometimes "biological coagulation." Infilco Degremont, Inc.'s process is of this type. U.S. Filter can provide modular plants that employ contact stabilization.

3) Providing 24 hours' aeration of raw sewage followed by final settling and up to 100 percent return of Sludge. This is generally classified as "extended aeration."

4) Providing rapid and thorough mixing of all of the sludge with all of the raw sewage plus a shorter-than-24 hour aeration period. This has been termed the "complete mixing" or "completely mixed" activated sludge process. The Infilco Degremont Aero-Accelator has been used in this procedure. The Infilco Degremont Co. also offers a complete mixing activated sludge system.

The extended aeration and complete mixing concepts have been used in the design of factory built plants available from several manufacturers. These are essentially small units requiring little or no sludge disposal facilities and providing a high degree of treatment. As such, they have become popular for use by institutions, restaurants, motels, and residential subdivisions.

The units manufactured are described under "Complete Plants," Section D-5.

Other Uses of Aeration

The activated sludge returned for mixing with the incoming sewage is sometimes re-aerated to maintain aerobic conditions and to prevent deposits in the channel, for which purpose diffusers are placed in the channel. Diffusers and other devices suitable for oxygen systems are produced by Ferro Corp., Filtros Plant, Specialty Ceramics Div.

In the case of treatment of digester supernatant, facultative anaerobes are converted to aerobic metabolism, permitting aerobic oxidation systems to function more efficiently. The use of the system in wet wells, force mains, and recirculation lines is advocated especially in new systems that are under-loaded or wherever septic conditions are likely to develop. Air is extensively used in small plants for pumping of sludge from settling tank hoppers for recirculation.

The sludge removal system of FMC Corp., Material Handling Systems Div. employs squeegees and an air lift pumping arrangement.

EnviroQuip Int'l. Inc. has a "quick flush spray nozzle" used for foam control in aerated tanks. Spray nozzles are also available from F.B. Leopold Co., Inc.; Orthos, Inc.; and Pollution Control, Inc.

Oxygen Application. Instead of applying air in activated sludge treatment, increased efficiency of the process can be attained by applying oxygen directly. Since the microorganisms utilize only the oxygen in the air, the principles are essentially the same and energy is saved, assuming that more energy is required to dissolve the oxygen from in water than to separate oxygen in the conventional manner. Superior oxygen transfer occurs when it is applied in the relatively pure form.

Air Products and Chemicals, Inc. provides a direct oxygen application process called the OASES[R] system. It involves baffling oxygenation stages with sequential flow through the stages. Conventional aeration tanks can be converted to using this system.

Pure oxygen injection systems are provided by Liquid Air Corp.

The "Unox" process employs covered aeration tanks with oxygen introduced in the first stage and flowing concurrently with the wastewater. The tanks are kept at atmospheric pressure or about two to four in. water column (w-c), sufficient to maintain control and prevent backmixing. Effluent mixed liquor is separated by conventional means and the thickened sludge is returned to the first stage. Either surface aerators or submerged turbines with diffusers may be employed for mixing. The oxygen flow can be controlled to meet effluent requirements. The process was originally marketed in this country by the Linde Division of Union Carbide.

Zimpro Environmental, Inc. furnishes the Marox pure oxygen line of technology for wastewater treatment, including a rotating diffuser head. Oxygen application systems are provided by Taiho Machinery Industry Co., Ltd.

BOC Gases has the Vitox system employing a land-based or submersible pump drawing a mixed liquor sidestream from an open, activated sludge tank. Pure oxygen from a cryogenic storage container is vaporized and combined into the sidestream and then returned to the bottom of the tank through high velocity mixing nozzles. Post aeration systems using oxygen are available from Environmental Dynamics Inc.
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Title Annotation:Water Pollution Control
Publication:Public Works
Date:Apr 15, 1995
Previous Article:Settling tanks.
Next Article:Oxidation ponds.

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