Management of Wastewater Solids
Reprinted from Pollution Engineering
Selection of a treatment process depends on the nature of the wastewater and the quality of the effluent desired. Hazardous components of the wastewater may be either separated or converted to non-hazardous forms in order to allow disposal of the wastewater effluent by conventional methods. Conversion processes can be done simply or in multiple steps. Hazardous components which are separated from the wastewater must be disposed of and this may take additional steps, such as sludge dewatering, incineration, et cetera.
Liquid-Solids Separation
Separation of suspended matter from wastewater can be accomplished by a number of different processes. Large heavy solids are easier to remove than finely divided light solids.
Screening Devices:
Screening devices are used to remove large pieces of solid matter that would interfere with subsequent processing operations or would cause damage to equipment such as pumps. Coarse screening devices may consist of parallel bars, rods or wires, perforated plates, gratings or wire mesh.
Gravity Sedimentation:
Gravity sedimentation involves the containment of wastewater for a sufficient period of time to allow some or all of the suspended materials to either settle out or float to the surface of the wastewater. In its simplest form as a batch process, a given volume of wastewater is transferred to a vessel and held there until nearly all the settleable and floatable matter separates. The floating matter can be skimmed off and the wastewater decanted for discharge or further treatment. Sludge may be allowed to collect until several batches of wastewater have been processed, then, it is removed. The vessel may have a conical bottom so that the sludge can be removed via a valve. Large settling ponds may be constructed which are drained periodically to permit sludge removal. Solids-contacts or sludge-blanket clarifiers are useful for sludges that are flocculents and of low density. Such clarifiers are designed with large mixing and reaction zones that, coupled with the sludge blanket, account for greater efficiency in solids removal.
Dissolved-Air Flotation:
Dissolved-Air Flotation is useful for suspended matter that does not sink or float in a reasonable period of time. Separation is brought about by the introduction of finely-divided gas bubbles which become attached to the particulate matter, causing it to float to the surface where it is removed by skimming. Introduction of the gas bubbles is usually accomplished by reducing the pressure of the wastewater causing dissolved gases to be released. This method is commonly used to separate greasy or oily matter from industrial wastes.
Granular-Media Filters:
Granular-Media Filters or deep bed filtration is a polishing step that removes small amounts of suspended solids and produces a highly-clarified water. Chemical coagulation and sedimentation typically precedes this stage. Graded sand and pulverized coal are commonly used in the filter beds. Conventional operation is usually by down-flow. The ability of the granular media filter beds to produce a clear effluent results from the straining action and adhesion, which removes particles finer than the pore space.
Surface Filters:
Surface Filters make use of a fine medium such as a cloth or close mesh screen. In the rotary vacuum filter, the medium is in the form of a continuous belt and it rotates over a perforated drum that is partially submerged in the slurry to be filtered. Water is pulled through the filter cake that forms on the belt to the inside of the drum, where it is transferred to the vacuum system.
Centrifugation:
Centrifugation is a useful alternative to filtration for sticky sludges that do not dewater rapidly on a filter. They operate by a rapid rotation of a liquid suspension, which induces a much greater force than gravity to hasten the separation of the suspended matter
Wastewater Screeners
Screening is an important pretreatment operation and is the separation of solid-solid or solid-Liquid mixture by mechanical means, purely based on the size difference, between the particles. It is one of the primitive material processing arts, still very much neglected. In screening, a mixture of solid-solid or solid-Iiquid is fed onto a screen surface and, depending on the motion of the screen surface or the screen opening, a certain amount of particle, smaller than the screen opening, will go through the screen. Thus, the feed material is separated into different fractions using various screens and collected separately. The screen surface can be made of woven wire, perforated plates, polymer cloth, grizzly bars, wedge wire, etc.
In wastewater treatment, screeners can separate a large amount of suspended solid and insoluble BOD5 prior to the primary and secondary treatments. The capital and running costs for the screener is small compared to the whole wastewater treatment facility. A proper screener can reduce the solids loading in the primary settling tank and the secondary treatment, thereby reducing large capital costs.
There are various kinds of screeners used in wastewater treatment. The following are descriptions of the most commonly used screeners for this application: vibratory screener, inclined static screener, and rotary screener.
Circular vibratory screener:
Vibration is accomplished by the unbalanced mass mounted on the top and bottom of the motor. This type of unit has the advantage of having a combination of horizontal and vertical motion which translates into three-dimensional motions. By setting the proper mass on the top and bottom and proper angle between them, a maximum screening efficiency and throughput can be achieved for a particular application.
The following is a table for the effluent BOD5 and TSS data from a laundry wastewater analysis done with and without a vibratory screener:
* analysis was done by the Sanitary District of Rockford, IL. Average 200 GPM of water through a 60 in. Kason VIBROSCREEN (vibratory screen).
The most important thing shown from the above data is that there is a noted reduction in costs for a processing plant effluent by using a finer mesh screen, An 80-mesh screen (178 micron opening) was able to remove an average of 45% TSS and 37% BOD5. The same screener removal efficiency was increased to 67% for TSS and 68% for BOD5 only by changing the screen to a fine 325-mesh (45 micron opening).
The advantage of the vibratory screener is that it can handle a wide variety of slurries. As an example, a higher motor RPM used for thixotropic fluids enhances the separation capacity. Water or steam spraying systems and anti-blinding devices can be used with the screen to prevent screen clogging. There is a limitation in that heavy solids loading (large metal pieces, etc.) may damage the screen surface.
Static inclined screener:
The static inclined screener is simple in construction with no moving parts. The slurry is fed into the bottom of the headbox. This eliminates the feed turbulence and the flow fluctuation similar to an equalization basin. The process stream overflows the headbox onto the acceleration deck where optimum velocity is attained. If there are fibers in the slurry, they orient themselves in the direction of the flow, which increases the dewatering capacity. This is true because the fiber length. instead of the fiber diameter, becomes a critical factor in choosing the screen opening due to their orientation in the direction of the flow and this allows use of a larger screen opening. The accelerated liquid/solid stream then passes over the dewatering screen made up of profile wire. This effluent design causes a thin layer of the flowing liquid to be sliced and, in accordance with the "Coanda" effect, to cling to the wall of the preceding bar. As a result, the liquid passes through the screen and oversize solid particles are discharged from the lower edge of the screening deck.
It has been experienced, depending on the slurry characteristics (i.e. viscosity. etc.) at a particular inclination or angle the static screen gives the optimum separation. Therefore, an adjustable angle feature is helpful for this purpose. The rule of thumb in choice of wire screen profile is that the screen opening should be twice the minimum particle size which is to be separated. The following are typical flow loading capacities for various plant wastewater feed materials for a 27 sq ft wedge wire screen area.
In this case, limitation exists in that, for fine particles (less than 100 microns), the screening area requirement becomes too large and the screening surface will need regular cleaning.
Rotary screener:
This is the most commonly used screener for wastewater pretreatment. It is a rotating shaft and wrapped-around drum shaped screen made of wire mesh, plastic material, or perforated plate. The drum is partially submerged in the effluent chamber. The wastewater goes inside the drum through the screen opening, and discharges from the side of the cylinder. The solids collected on the screen surface are periodically or continuously cleaned with spray water or by a knife blade placed on the opposite side of the slurry inlet. Removed solids are collected in a trough.
The usual screen opening is 0.4 inches to 150 microns. The typical solids removal is 40% -60% of insoluble BOD5 and 50% -70% for TSS. The drum RPM can be varied.
Listed below are the screen openings and percent solids removal for various industries.
Cannery wastewater:
Screeners are used to separate solids from washing, peeling, slicing, rinsing and packing.
Meat packing:
Solids separation from wastewater of manure, slaughtering, "plucking," eviscerating, cutting and trimming.
Dairy:
Chemicals/Metals:
Pulp & Paper:
Brewery:
Centrifuges:
Centrifuges are used for sludge dewatering. The centrifugal force is used to increase the sludge solids sedimentation rate. The choice of centrifuge depends on:
1. Properties of the slurry
2. properties of the solids
3. properties of the liquid and
4. process requirements
Centrifuges are very sensitive to the changes in solids concentration or composition of the sludge. A cake dryness of 15-17% from a feed solids concentration of 0.4 to 6% is considered a good performance for a centrifuge.
Advantages:
1. Process automation
2. Small space requirements
3. Flexibility in handling thickened or dilute sludge
Limitations:
1. Sensitive to the change of slurry properties
2. High energy and maintenance cost
The three most common types of centrifuges are commonly used for sludge dewatering:
1. Solid bowl
2. Disc
3. Basket
Solid bowl:
The centrifuge assembly consists of a bowl and a screw conveyor joined through a gear system. The bowl and conveyor rotate at different RPM. The solid bowl or shell is supported between two sets of bearings and it is conical at one end. This conical section forms the dewatering sector over which the screw pushes the sludge solids to outlet ports. The opposite end of the bowl is equipped with an adjustable outlet weir plate to regulate the sludge level in the bowl. The sludge pool is the concentric annular liquid ring on the inside wall of the bowl. The sludge slurry is fed into the rotating bowl through a fixed feed pipe into an abrasion-resistant chamber through several baffles.
Then, it is discharged through the feed ports of the rotating screw conveyor and into the sludge pool. The centrate is discharged through the outlet ports either by gravity or by a pump attached to the shaft at one end of the bowl. The material of the construction of the screw and the bowl is usually mild or stainless steel. However, depending on the nature of the sludge slurry, special metals or coatings can be used. The above described centrifuge is a countercurrent type. If the feed slurry is fed into the bowl through a feed pipe opposite to the effluent outlet, then it is called a concurrent centrifuge. Concurrent flow helps to produce dryer cake and to reduce flocculent demands.
Design criterion:
Bowl length to diameter ratio: 2.5 4.0. Bowl angle: 5 to 10 degrees. Acceleration: 1000 -2700 G. Psylmeric coagulants, aluminum, lime, or other coagulants are added internally to the bond at a concentration of 0.1 to 0.2% to increase cake dryness. Solid bowl centrifuge solids recovery is between 50 to 75% without chemical addition and 80 to 95% with chemical addition.
Disc centrifuge:
It is well suited for low solids concentration, high volume sludge slurries. It is mostly used for chemical process industries. Here conical discs are stacked to create narrow channels inside the bowl. The feed slurry is distributed among those channels. The suspended solids travel a small distance to settle and discharge continuously through the small orifices in the bowl wall.
Limitation:
Feed must be pre-screened to prevent orifice clogging. Sludge concentration is limited by the discharging of the sludge through the small orifices of 1.2 mm to 2.4 mm sizes. It works more like a thickener than a dewatering device. It increases the solids concentration of the sludge from one percent to six percent.
Basket centrifuge:
It is suitable for small plant wastewater slurry where solids are difficult to filter and there is a variation in solids concentration and characteristics. It is also a batch device, with alternate feed charging and dewatered cake discharging. The slurry is fed at the bottom of the centrifuge and moved toward the perforated basket. Cake continuously builds up within the basket until feed is shut off. A knife cuts the cake and the cake discharges through the bottom of the centrifuge. It has lower power consumption and maintenance costs than other types of centrifuges. Approximately 90 to 97% solids recovery is achieved here, without chemical addition. About 90% solids recovery, with minimum chemical addition, is the goal for centrifugation.
Chemical Treatment
Chemical treatment is a widely used process for the destruction or separation of hazardous constituents in wastewater. This can be done by neutralization of acidic or alkaline wastewater until a suitable pH is obtained. Precipitation/Coagulation/Flocculation is useful for the removal of heavy metals. Precipitation refers to the formation of a solid phase; coagulation is where the containment is trapped by the formation of a precipitate; and flocculation is the agglomeration of coagulating chemical.
Oxidation-reduction:
Oxidation-Reduction or the redox processes are used for converting toxic pollutants to harmless forms or less toxic materials into other forms that are more easily removed. These processes involve the addition of chemical reagents to wastewaters, causing changes in the oxidation states of substances, both in the reagents and in the wastewaters. In order for one substance to be oxidized, another must be reduced. Ozone is a powerful oxidizing agent that is usually used to eliminate traces of organics. Wet oxidation utilizes oxygen at temperatures and pressures up to 350 C and 180 atmospheres to treat organic wastes.
Ion exchange:
Ion exchange involves a change in the chemical form of a compound -the exchange of ions in solution with other ions held by mixed anionic or cationic groups or charges. Typically, a waste solution is percolated through a granular bed of the ion exchanger, where certain ions in solution are replaced by ions contained in the ion exchanger. If the exchange involves cations, the exchanger is called a cation exchanger and correspondingly an anion exchanger is one that involves an ion exchange.
Other Physical Treatment Methods
There are several methods used for separating pollutants from wastewater: Activated carbon, steam stripping, evaporation, reverse osmosis and solvent extraction. The chemical and physical characteristics of the pollutant are important in the selection of the physical removal method. Steam stripping is effective for substances that have an appreciable vapor pressure at the boiling point of water, whereas evaporation is effective for those chemicals that will not volatilize. Soluble, small organic molecules are absorbed by activated carbon, large ions are separated by reverse osmosis.
Activated carbon adsorption:
Inorganic and organic chemicals are adsorbed onto activated carbon. Usually, hydrophobic chemicals are more likely to be removed from the waste-stream. The degree of adsorption is linked to the molecular weight, methanol-water coefficient or solubility; these are also linked to the recalcitrance and/or toxicity. The smaller the size of the grain, the more surface area is available and so equilibrium is reached quicker with powdered activated carbon, compared to the granular form. But then, the powdered form needs more pumping to get the wastewater through and hence the costs are increased. There are two principle adsorption systems: One is downward flow through the bed (pressure or gravity flow) and the other is up-flow through a packed or expanded bed.
Activated carbon adsorption is applicable to the treatment of dilute aqueous wastes, but they should be treated first to remove suspended solids, oil and grease. Temperature and pH are also important for the different compounds to be treated. The carbon is either disposed of, or regenerated. Carbon has also been added directly to biological treatment of effluent in a contacting basin. The advantage of this method is that the sludge toxicity is reduced by selectively removing the toxic organics from solution and that the carbon adsorption capacity is extended by bio-regeneration of the "biocompatible" species adsorbed on the surface. For aqueous solvent waste containing contaminants in concentrations up to 10,000 mg/L, the activated sludge process has been proposed as a potential applicable treatment. However these concentrations may be toxic to the sludge or they may be easily stripped to the atmosphere, thereby creating another hazard. The sludge may also contain recalcitrant waste, due to sorption of the contaminants and be difficult to dispose of.
Evaporation:
Evaporation is the process that heats the liquid, venting the vapors to the atmosphere and concentrating the pollutants into a slurry.
Reverse osmosis:
Osmosis is the process where a solvent (e.g. water) moves from an area of low-concentration-to-high, across a semi-permeable membrane which does not allow the dissolved solids to pass. In reverse osmosis, a pressure greater than the osmotic pressure is applied so the flow is reversed. Pure water will then flow through the membrane from the concentrated solution.
Solvent extraction:
Solvent Extraction is a process whereby a dissolved or adsorbed substance is transferred from a liquid or solid phase to a solvent that preferentially dissolves that substance. For the process to be effective, the extracting solvent must be immiscible in the liquid and differ in density so that gravity separation is possible and there is minimal contamination of the raffinate with the solvent. The hydrophobic solutes are more likely to be extracted. Solvent extraction can be performed as a batch process, or by the contact of the solvent with the feed in staged or continuous equipment.
Steam stripping:
Steam stripping is a process in which water vapor at elevated temperatures is used to remove volatile components of a liquid. Countercurrent flow is generally used to promote gas-Liquid contact, thus allowing soluble gaseous organics from the liquid waste to be continuously exchanged with molecules within the stripping gas. Again, this is useful only for waste with low water solubilities.
Incineration
Incineration is a high temperature oxidation process that converts the principal elements (carbon, hydrogen and oxygen) in most organic components to carbon dioxide and water. Because of the problems of land disposal, incineration may take on a lead role in waste treatment. However, it is not without its problems -air pollution. The destruction of the molecular structure usually eliminates the toxicity of the chemical. But the existence of other elements in a waste may result in the production of pollutants, that require removal in off-gas treatment systems. There are several types of incinerators available.
Liquid injection:
The liquid injection incinerators operate by spraying combustible waste mix with air into a chamber where flame oxidation takes place. The purpose of spraying is to atomize the waste into small droplets which present a large surface area for rapid heat transfer, thereby increasing the rate of vaporization and mixing with air to promote combustion. Air is supplied to provide the necessary mixing and turbulence. These incinerators are widely used for destruction of liquid organic wastes.
Rotary-kiln:
Rotary-Kiln incinerators are designed to process solids and tars that cannot be processed in the liquid incinerator. The rotary kiln is a cylindrical shell lined with refractory material that is horizontally-mounted at a slight incline. It is rotated from 5 to 25 times an hour at high temperatures, 1500 to 3000 F with excess air, and the residence time varies depending on the nature of the waste. The rotation causes a tumbling action that mixes the waste with air. The primary function is to convert, through partial burning and volatilization, solid wastes to gases and ash/residue. If the ash is free of dangerous levels of hazardous wastes, it is put in a landfill.
Fume incineration:
Large quantities of organic vapor fumes are produced by many industries, including fat rendering, metal painting and varnishing and various types of printing. These vapors are generally mixes of hydrocarbons alcohols and acetates. The mixes may not be acutely toxic but they do cause odor problems.
Multiple hearth incinerator:
The multiple hearth incinerator is used for wastes that are difficult to burn or that contain valuable metals that can be recovered. It consists of a refractory-lined circular steel shell, with refractory hearths located one above the other. Solid waste or partially dewatered sludge is fed to the top of the unit, where a rotating plow rake pulls it across the hearth to drop holes. The uncombusted material falls to the next hearth and the process is repeated until the combustion is complete.
Fluidized-bed incinerators:
Fluidized-bed incinerators (FBI) are applicable to the destruction of halogenated organic waste streams. This type of incinerator consists of a vessel in which inert granular particles are fluidized by a low velocity air stream which is passed through a distributor plate below the bed. A FBI consists of a windbox, through which combustion air is introduced to the reactor, a reactor zone, containing a bed of sand and waste injection and removal ports. Temperatures are in the range of 1,300 to 2,100 F, gas residence times are usually a few seconds. They have been used to treat municipal sewage sludge, low quality fuels, pulp and paper effluents, food processing waste, refinery waste, and miscellaneous chemical waste.
Molten salt incinerator:
A molten salt incinerator uses a molten salt such as sodium carbonate as a heat transfer and reaction medium. In the process, waste material along with air is added below the surface of the bed so that any gases formed during combustion are forced to pass through the melt. Reaction temperatures in the bed range from I ,500 to 2,000 F and residence times are less than a second. Any acidic gases formed are neutralized by the alkalinity of the bed. This can change the fluidity of the bed and so it needs replacement frequently.
Plasma arc incineration:
Plasma arc incineration is based on the concept of reducing or pyrolyzing waste molecules to the atomic state using a thermal plasma field. The system uses very high energy at temperatures near to 10,000 C to break bonds of hazardous waste chemical molecules down to the atomic state. An electrode assembly ionizes air molecules which create a plasma field. Hazardous waste mixtures interact with the field, forming simple molecules such as carbon dioxide, hydrogen, hydrogen chloride, and other minor matrix compounds such as acetylene and ethene.
Lime or cement kiln incineration:
A cement kiln is basically a large rotary kiln into which raw materials are fed countercurrent to combustion gas flow. The wet process kilns using a 30% water slurry feed are the most suitable for hazardous waste destruction. The products formed are alkaline and so act as a scrubber, removing acid gases formed during combustion. Such a system operates at 2,800 degrees resulting in very efficient removal of wastes.
Wet air oxidation:
This is based on the principle that with the proper temperature, pressure reaction time, and in presence of sufficient oxygen, any solids capable of burning can be oxidized to any degree. This process is good for difficult-to-dewater waste liquors or where solids concentration is very low in the waste sludge stream. It does not require preliminary drying or dewatering. It can operate with one percent solids concentration. Oxidation is obtained at temperatures as low as 300 -400 F.
Solidification Techniques
There are several innovative non-thermal processes that have been developed under EPA's SITE program that immobilize wastes by vitrification or other types of solidification. The SITE program is the Superfund Innovative Technology Evaluation a $20 million/yr program which has been developed to encourage the private development and demonstration of new technologies for cleaning up hazardous wastes. For example, researchers at Battelle Pacific Northwest Laboratories have developed an in situ vitrification process (which was originally designed for the containment of nuclear wastes) in which electrode are sunk into a contaminated area and attached to a diesel-powered generator. The current produced temperatures of about 3,600 F which is much higher than the fusion temperature of soil. An exhaust hood is placed over the site to collect and treat any combustion products. The result is a massive glass-Iike product consisting of completely immobilized organics, inorganics, steel drums and other components that are essentially locked up and inert. The time taken to complete the process depends on the electrode depth and frequency.
Another solidification process uses a reagent called Urrichem that immobilizes slurried hazardous components. The contaminated soil is excavated and intimately mixed with the Urrichem off-site. After blending, the slurry is pumped out of the mixer and hardens into a concrete-like mass within 24 hours. Chemfix is a process developed by Chemfix Technologies (Metaire, LA). In this technique, a proprietary blend of soluble silicates and additives is used to convert high molecular weight organic and inorganic slurries into a cross-linked, clay-like matrix.
Alternate methods have also included landfills, deep well injection disposal and ocean dumping. Landfills were developed because it was believed that by placing waste solids in designated ground areas, there would be a natural decomposition over time. Unfortunately, the water table rises in a landfill and this effect means that it is possible to get a leaching of the water-containing toxics since the water flow is always from areas of high-to-low head. This is a special problem for areas that rely on groundwater and not surface water for domestic/industrial supplies. Deep injection wells have similar problems. When aquifers in which the deep well is located are pressurized by pumping, the contaminants are drawn into the wall. Ocean dumping of wastewater solids has upset the delicate ecosystem balance in some areas. Thus, there remains a need for effective economical techniques that can be safely used to combat the nation's solids management problems. PE
Paul N. Cheremisinoff is a Professor of Civil & Environmental Engineering at New Jersey Institute of Technology in Newark, NJ. Paul is a Registered Environmental Manager. Kasinath Banerjee is a Research Engineer with International Technology Corporation in Pittsburgh, PA. Goutam (Gary) Datta is a Senior Application Specialist with Kason Corporation in Linden, NJ.
Selection of a treatment process depends on the nature of the wastewater and the quality of the effluent desired. Hazardous components of the wastewater may be either separated or converted to non-hazardous forms in order to allow disposal of the wastewater effluent by conventional methods. Conversion processes can be done simply or in multiple steps. Hazardous components which are separated from the wastewater must be disposed of and this may take additional steps, such as sludge dewatering, incineration, et cetera.
Liquid-Solids Separation
Separation of suspended matter from wastewater can be accomplished by a number of different processes. Large heavy solids are easier to remove than finely divided light solids.
Screening Devices:
Screening devices are used to remove large pieces of solid matter that would interfere with subsequent processing operations or would cause damage to equipment such as pumps. Coarse screening devices may consist of parallel bars, rods or wires, perforated plates, gratings or wire mesh.
Gravity Sedimentation:
Gravity sedimentation involves the containment of wastewater for a sufficient period of time to allow some or all of the suspended materials to either settle out or float to the surface of the wastewater. In its simplest form as a batch process, a given volume of wastewater is transferred to a vessel and held there until nearly all the settleable and floatable matter separates. The floating matter can be skimmed off and the wastewater decanted for discharge or further treatment. Sludge may be allowed to collect until several batches of wastewater have been processed, then, it is removed. The vessel may have a conical bottom so that the sludge can be removed via a valve. Large settling ponds may be constructed which are drained periodically to permit sludge removal. Solids-contacts or sludge-blanket clarifiers are useful for sludges that are flocculents and of low density. Such clarifiers are designed with large mixing and reaction zones that, coupled with the sludge blanket, account for greater efficiency in solids removal.
Dissolved-Air Flotation:
Dissolved-Air Flotation is useful for suspended matter that does not sink or float in a reasonable period of time. Separation is brought about by the introduction of finely-divided gas bubbles which become attached to the particulate matter, causing it to float to the surface where it is removed by skimming. Introduction of the gas bubbles is usually accomplished by reducing the pressure of the wastewater causing dissolved gases to be released. This method is commonly used to separate greasy or oily matter from industrial wastes.
Granular-Media Filters:
Granular-Media Filters or deep bed filtration is a polishing step that removes small amounts of suspended solids and produces a highly-clarified water. Chemical coagulation and sedimentation typically precedes this stage. Graded sand and pulverized coal are commonly used in the filter beds. Conventional operation is usually by down-flow. The ability of the granular media filter beds to produce a clear effluent results from the straining action and adhesion, which removes particles finer than the pore space.
Surface Filters:
Surface Filters make use of a fine medium such as a cloth or close mesh screen. In the rotary vacuum filter, the medium is in the form of a continuous belt and it rotates over a perforated drum that is partially submerged in the slurry to be filtered. Water is pulled through the filter cake that forms on the belt to the inside of the drum, where it is transferred to the vacuum system.
Centrifugation:
Centrifugation is a useful alternative to filtration for sticky sludges that do not dewater rapidly on a filter. They operate by a rapid rotation of a liquid suspension, which induces a much greater force than gravity to hasten the separation of the suspended matter
Wastewater Screeners
Screening is an important pretreatment operation and is the separation of solid-solid or solid-Liquid mixture by mechanical means, purely based on the size difference, between the particles. It is one of the primitive material processing arts, still very much neglected. In screening, a mixture of solid-solid or solid-Iiquid is fed onto a screen surface and, depending on the motion of the screen surface or the screen opening, a certain amount of particle, smaller than the screen opening, will go through the screen. Thus, the feed material is separated into different fractions using various screens and collected separately. The screen surface can be made of woven wire, perforated plates, polymer cloth, grizzly bars, wedge wire, etc.
In wastewater treatment, screeners can separate a large amount of suspended solid and insoluble BOD5 prior to the primary and secondary treatments. The capital and running costs for the screener is small compared to the whole wastewater treatment facility. A proper screener can reduce the solids loading in the primary settling tank and the secondary treatment, thereby reducing large capital costs.
There are various kinds of screeners used in wastewater treatment. The following are descriptions of the most commonly used screeners for this application: vibratory screener, inclined static screener, and rotary screener.
Circular vibratory screener:
Vibration is accomplished by the unbalanced mass mounted on the top and bottom of the motor. This type of unit has the advantage of having a combination of horizontal and vertical motion which translates into three-dimensional motions. By setting the proper mass on the top and bottom and proper angle between them, a maximum screening efficiency and throughput can be achieved for a particular application.
The following is a table for the effluent BOD5 and TSS data from a laundry wastewater analysis done with and without a vibratory screener:
| Date | Screen | BOD5 | TSS* | Approx Cost |
|---|---|---|---|---|
|
Used
|
(mg/L)
|
(mg/L)
|
3-Month Period
|
|
|
3/83
|
None
|
1669
|
1003
|
$10,000
|
|
12/83
|
80 mesh
|
1063
|
460
|
$5,000
|
|
10/84
|
80 mesh
|
1039
|
672
|
$6,000
|
|
6/85
|
325 mesh
|
504
|
320
|
$3,000
|
|
9/85
|
325 mesh
|
573
|
338
|
$3,000
|
* analysis was done by the Sanitary District of Rockford, IL. Average 200 GPM of water through a 60 in. Kason VIBROSCREEN (vibratory screen).
The most important thing shown from the above data is that there is a noted reduction in costs for a processing plant effluent by using a finer mesh screen, An 80-mesh screen (178 micron opening) was able to remove an average of 45% TSS and 37% BOD5. The same screener removal efficiency was increased to 67% for TSS and 68% for BOD5 only by changing the screen to a fine 325-mesh (45 micron opening).
The advantage of the vibratory screener is that it can handle a wide variety of slurries. As an example, a higher motor RPM used for thixotropic fluids enhances the separation capacity. Water or steam spraying systems and anti-blinding devices can be used with the screen to prevent screen clogging. There is a limitation in that heavy solids loading (large metal pieces, etc.) may damage the screen surface.
Static inclined screener:
The static inclined screener is simple in construction with no moving parts. The slurry is fed into the bottom of the headbox. This eliminates the feed turbulence and the flow fluctuation similar to an equalization basin. The process stream overflows the headbox onto the acceleration deck where optimum velocity is attained. If there are fibers in the slurry, they orient themselves in the direction of the flow, which increases the dewatering capacity. This is true because the fiber length. instead of the fiber diameter, becomes a critical factor in choosing the screen opening due to their orientation in the direction of the flow and this allows use of a larger screen opening. The accelerated liquid/solid stream then passes over the dewatering screen made up of profile wire. This effluent design causes a thin layer of the flowing liquid to be sliced and, in accordance with the "Coanda" effect, to cling to the wall of the preceding bar. As a result, the liquid passes through the screen and oversize solid particles are discharged from the lower edge of the screening deck.
It has been experienced, depending on the slurry characteristics (i.e. viscosity. etc.) at a particular inclination or angle the static screen gives the optimum separation. Therefore, an adjustable angle feature is helpful for this purpose. The rule of thumb in choice of wire screen profile is that the screen opening should be twice the minimum particle size which is to be separated. The following are typical flow loading capacities for various plant wastewater feed materials for a 27 sq ft wedge wire screen area.
| Feed Material | Screening Opening | U.S. GPM |
|---|---|---|
| Storm Water |
0.060
|
1,000
|
| Pulp Mill Effluent |
0.020
|
600
|
| Packing Waste |
0.040
|
500
|
| Sanitary Sewage |
0.060
|
800
|
| White Water |
0.020
|
775
|
| Press Liquor |
0.010
|
425
|
| Brewery Caustic Wastewater |
0.040
|
300
|
| Typical Plan Wastewater |
0.040
|
500-700
|
| Typical TSS Removal |
|
25% - 75%
|
| Typical BOD5 |
|
25% - 40%
|
In this case, limitation exists in that, for fine particles (less than 100 microns), the screening area requirement becomes too large and the screening surface will need regular cleaning.
Rotary screener:
This is the most commonly used screener for wastewater pretreatment. It is a rotating shaft and wrapped-around drum shaped screen made of wire mesh, plastic material, or perforated plate. The drum is partially submerged in the effluent chamber. The wastewater goes inside the drum through the screen opening, and discharges from the side of the cylinder. The solids collected on the screen surface are periodically or continuously cleaned with spray water or by a knife blade placed on the opposite side of the slurry inlet. Removed solids are collected in a trough.
The usual screen opening is 0.4 inches to 150 microns. The typical solids removal is 40% -60% of insoluble BOD5 and 50% -70% for TSS. The drum RPM can be varied.
Listed below are the screen openings and percent solids removal for various industries.
Cannery wastewater:
Screeners are used to separate solids from washing, peeling, slicing, rinsing and packing.
| Profile wedge wire: | 0.020 |
| Wire mesh: | 20 - 50 mesh |
| BOD5 removal: | 10% - 65% |
| TSS removal: | 30% - 85% |
Meat packing:
Solids separation from wastewater of manure, slaughtering, "plucking," eviscerating, cutting and trimming.
| Profile wedge wire: | 0.020_ - 0.04_ |
| Wire mesh: | 20 mesh (very common) - 80 mesh |
| BOD5 removal: | 40% - 50% |
| TSS removal: | 40% - 60% |
Dairy:
| Profile wedge wire: | 0.02 |
| Wire mesh: | 20 U.S. mesh - 200 mesh |
| BOD5 removal: | 30% - 65% |
| TSS removal: | 40% - 90% |
Chemicals/Metals:
| Profile wedge wire: | 0.01 |
| Wire mesh: | 50 - 325 mesh |
| TSS removal: | 50% - 99% |
Pulp & Paper:
| Profile wedge wire: | 0.01 |
| Wire mesh: | (varies widely depending on the specific application) 60325 U.S. mesh |
Brewery:
| Profile wedge wire: | 0.02 |
| Wire mesh: | 30 - 60 U.S. mesh |
| BOD5 removal: | 10% - 65% |
| TSS removal: | 30% - 85% |
Centrifuges:
Centrifuges are used for sludge dewatering. The centrifugal force is used to increase the sludge solids sedimentation rate. The choice of centrifuge depends on:
1. Properties of the slurry
2. properties of the solids
3. properties of the liquid and
4. process requirements
Centrifuges are very sensitive to the changes in solids concentration or composition of the sludge. A cake dryness of 15-17% from a feed solids concentration of 0.4 to 6% is considered a good performance for a centrifuge.
Advantages:
1. Process automation
2. Small space requirements
3. Flexibility in handling thickened or dilute sludge
Limitations:
1. Sensitive to the change of slurry properties
2. High energy and maintenance cost
The three most common types of centrifuges are commonly used for sludge dewatering:
1. Solid bowl
2. Disc
3. Basket
| Centrifuge | Feed Particle | Feed Solids | Feed Type |
|---|---|---|---|
|
Type
|
Size (Micron)
|
Percent
|
|
| Basket |
3 - 10,000
|
10-60
|
Low Feed rate - high recoveries |
| Solid Bowl |
1 - 50,000
|
5-40
|
Coarse Feed and high solids |
| Disc. |
0.1 - 500
|
5-25
|
Fine Feed particles, low solids loading and high volume |
Solid bowl:
The centrifuge assembly consists of a bowl and a screw conveyor joined through a gear system. The bowl and conveyor rotate at different RPM. The solid bowl or shell is supported between two sets of bearings and it is conical at one end. This conical section forms the dewatering sector over which the screw pushes the sludge solids to outlet ports. The opposite end of the bowl is equipped with an adjustable outlet weir plate to regulate the sludge level in the bowl. The sludge pool is the concentric annular liquid ring on the inside wall of the bowl. The sludge slurry is fed into the rotating bowl through a fixed feed pipe into an abrasion-resistant chamber through several baffles.
Then, it is discharged through the feed ports of the rotating screw conveyor and into the sludge pool. The centrate is discharged through the outlet ports either by gravity or by a pump attached to the shaft at one end of the bowl. The material of the construction of the screw and the bowl is usually mild or stainless steel. However, depending on the nature of the sludge slurry, special metals or coatings can be used. The above described centrifuge is a countercurrent type. If the feed slurry is fed into the bowl through a feed pipe opposite to the effluent outlet, then it is called a concurrent centrifuge. Concurrent flow helps to produce dryer cake and to reduce flocculent demands.
Design criterion:
Bowl length to diameter ratio: 2.5 4.0. Bowl angle: 5 to 10 degrees. Acceleration: 1000 -2700 G. Psylmeric coagulants, aluminum, lime, or other coagulants are added internally to the bond at a concentration of 0.1 to 0.2% to increase cake dryness. Solid bowl centrifuge solids recovery is between 50 to 75% without chemical addition and 80 to 95% with chemical addition.
Disc centrifuge:
It is well suited for low solids concentration, high volume sludge slurries. It is mostly used for chemical process industries. Here conical discs are stacked to create narrow channels inside the bowl. The feed slurry is distributed among those channels. The suspended solids travel a small distance to settle and discharge continuously through the small orifices in the bowl wall.
Limitation:
Feed must be pre-screened to prevent orifice clogging. Sludge concentration is limited by the discharging of the sludge through the small orifices of 1.2 mm to 2.4 mm sizes. It works more like a thickener than a dewatering device. It increases the solids concentration of the sludge from one percent to six percent.
Basket centrifuge:
It is suitable for small plant wastewater slurry where solids are difficult to filter and there is a variation in solids concentration and characteristics. It is also a batch device, with alternate feed charging and dewatered cake discharging. The slurry is fed at the bottom of the centrifuge and moved toward the perforated basket. Cake continuously builds up within the basket until feed is shut off. A knife cuts the cake and the cake discharges through the bottom of the centrifuge. It has lower power consumption and maintenance costs than other types of centrifuges. Approximately 90 to 97% solids recovery is achieved here, without chemical addition. About 90% solids recovery, with minimum chemical addition, is the goal for centrifugation.
Chemical Treatment
Chemical treatment is a widely used process for the destruction or separation of hazardous constituents in wastewater. This can be done by neutralization of acidic or alkaline wastewater until a suitable pH is obtained. Precipitation/Coagulation/Flocculation is useful for the removal of heavy metals. Precipitation refers to the formation of a solid phase; coagulation is where the containment is trapped by the formation of a precipitate; and flocculation is the agglomeration of coagulating chemical.
Oxidation-reduction:
Oxidation-Reduction or the redox processes are used for converting toxic pollutants to harmless forms or less toxic materials into other forms that are more easily removed. These processes involve the addition of chemical reagents to wastewaters, causing changes in the oxidation states of substances, both in the reagents and in the wastewaters. In order for one substance to be oxidized, another must be reduced. Ozone is a powerful oxidizing agent that is usually used to eliminate traces of organics. Wet oxidation utilizes oxygen at temperatures and pressures up to 350 C and 180 atmospheres to treat organic wastes.
Ion exchange:
Ion exchange involves a change in the chemical form of a compound -the exchange of ions in solution with other ions held by mixed anionic or cationic groups or charges. Typically, a waste solution is percolated through a granular bed of the ion exchanger, where certain ions in solution are replaced by ions contained in the ion exchanger. If the exchange involves cations, the exchanger is called a cation exchanger and correspondingly an anion exchanger is one that involves an ion exchange.
Other Physical Treatment Methods
There are several methods used for separating pollutants from wastewater: Activated carbon, steam stripping, evaporation, reverse osmosis and solvent extraction. The chemical and physical characteristics of the pollutant are important in the selection of the physical removal method. Steam stripping is effective for substances that have an appreciable vapor pressure at the boiling point of water, whereas evaporation is effective for those chemicals that will not volatilize. Soluble, small organic molecules are absorbed by activated carbon, large ions are separated by reverse osmosis.
Activated carbon adsorption:
Inorganic and organic chemicals are adsorbed onto activated carbon. Usually, hydrophobic chemicals are more likely to be removed from the waste-stream. The degree of adsorption is linked to the molecular weight, methanol-water coefficient or solubility; these are also linked to the recalcitrance and/or toxicity. The smaller the size of the grain, the more surface area is available and so equilibrium is reached quicker with powdered activated carbon, compared to the granular form. But then, the powdered form needs more pumping to get the wastewater through and hence the costs are increased. There are two principle adsorption systems: One is downward flow through the bed (pressure or gravity flow) and the other is up-flow through a packed or expanded bed.
Activated carbon adsorption is applicable to the treatment of dilute aqueous wastes, but they should be treated first to remove suspended solids, oil and grease. Temperature and pH are also important for the different compounds to be treated. The carbon is either disposed of, or regenerated. Carbon has also been added directly to biological treatment of effluent in a contacting basin. The advantage of this method is that the sludge toxicity is reduced by selectively removing the toxic organics from solution and that the carbon adsorption capacity is extended by bio-regeneration of the "biocompatible" species adsorbed on the surface. For aqueous solvent waste containing contaminants in concentrations up to 10,000 mg/L, the activated sludge process has been proposed as a potential applicable treatment. However these concentrations may be toxic to the sludge or they may be easily stripped to the atmosphere, thereby creating another hazard. The sludge may also contain recalcitrant waste, due to sorption of the contaminants and be difficult to dispose of.
Evaporation:
Evaporation is the process that heats the liquid, venting the vapors to the atmosphere and concentrating the pollutants into a slurry.
Reverse osmosis:
Osmosis is the process where a solvent (e.g. water) moves from an area of low-concentration-to-high, across a semi-permeable membrane which does not allow the dissolved solids to pass. In reverse osmosis, a pressure greater than the osmotic pressure is applied so the flow is reversed. Pure water will then flow through the membrane from the concentrated solution.
Solvent extraction:
Solvent Extraction is a process whereby a dissolved or adsorbed substance is transferred from a liquid or solid phase to a solvent that preferentially dissolves that substance. For the process to be effective, the extracting solvent must be immiscible in the liquid and differ in density so that gravity separation is possible and there is minimal contamination of the raffinate with the solvent. The hydrophobic solutes are more likely to be extracted. Solvent extraction can be performed as a batch process, or by the contact of the solvent with the feed in staged or continuous equipment.
Steam stripping:
Steam stripping is a process in which water vapor at elevated temperatures is used to remove volatile components of a liquid. Countercurrent flow is generally used to promote gas-Liquid contact, thus allowing soluble gaseous organics from the liquid waste to be continuously exchanged with molecules within the stripping gas. Again, this is useful only for waste with low water solubilities.
Incineration
Incineration is a high temperature oxidation process that converts the principal elements (carbon, hydrogen and oxygen) in most organic components to carbon dioxide and water. Because of the problems of land disposal, incineration may take on a lead role in waste treatment. However, it is not without its problems -air pollution. The destruction of the molecular structure usually eliminates the toxicity of the chemical. But the existence of other elements in a waste may result in the production of pollutants, that require removal in off-gas treatment systems. There are several types of incinerators available.
Liquid injection:
The liquid injection incinerators operate by spraying combustible waste mix with air into a chamber where flame oxidation takes place. The purpose of spraying is to atomize the waste into small droplets which present a large surface area for rapid heat transfer, thereby increasing the rate of vaporization and mixing with air to promote combustion. Air is supplied to provide the necessary mixing and turbulence. These incinerators are widely used for destruction of liquid organic wastes.
Rotary-kiln:
Rotary-Kiln incinerators are designed to process solids and tars that cannot be processed in the liquid incinerator. The rotary kiln is a cylindrical shell lined with refractory material that is horizontally-mounted at a slight incline. It is rotated from 5 to 25 times an hour at high temperatures, 1500 to 3000 F with excess air, and the residence time varies depending on the nature of the waste. The rotation causes a tumbling action that mixes the waste with air. The primary function is to convert, through partial burning and volatilization, solid wastes to gases and ash/residue. If the ash is free of dangerous levels of hazardous wastes, it is put in a landfill.
Fume incineration:
Large quantities of organic vapor fumes are produced by many industries, including fat rendering, metal painting and varnishing and various types of printing. These vapors are generally mixes of hydrocarbons alcohols and acetates. The mixes may not be acutely toxic but they do cause odor problems.
Multiple hearth incinerator:
The multiple hearth incinerator is used for wastes that are difficult to burn or that contain valuable metals that can be recovered. It consists of a refractory-lined circular steel shell, with refractory hearths located one above the other. Solid waste or partially dewatered sludge is fed to the top of the unit, where a rotating plow rake pulls it across the hearth to drop holes. The uncombusted material falls to the next hearth and the process is repeated until the combustion is complete.
Fluidized-bed incinerators:
Fluidized-bed incinerators (FBI) are applicable to the destruction of halogenated organic waste streams. This type of incinerator consists of a vessel in which inert granular particles are fluidized by a low velocity air stream which is passed through a distributor plate below the bed. A FBI consists of a windbox, through which combustion air is introduced to the reactor, a reactor zone, containing a bed of sand and waste injection and removal ports. Temperatures are in the range of 1,300 to 2,100 F, gas residence times are usually a few seconds. They have been used to treat municipal sewage sludge, low quality fuels, pulp and paper effluents, food processing waste, refinery waste, and miscellaneous chemical waste.
Molten salt incinerator:
A molten salt incinerator uses a molten salt such as sodium carbonate as a heat transfer and reaction medium. In the process, waste material along with air is added below the surface of the bed so that any gases formed during combustion are forced to pass through the melt. Reaction temperatures in the bed range from I ,500 to 2,000 F and residence times are less than a second. Any acidic gases formed are neutralized by the alkalinity of the bed. This can change the fluidity of the bed and so it needs replacement frequently.
Plasma arc incineration:
Plasma arc incineration is based on the concept of reducing or pyrolyzing waste molecules to the atomic state using a thermal plasma field. The system uses very high energy at temperatures near to 10,000 C to break bonds of hazardous waste chemical molecules down to the atomic state. An electrode assembly ionizes air molecules which create a plasma field. Hazardous waste mixtures interact with the field, forming simple molecules such as carbon dioxide, hydrogen, hydrogen chloride, and other minor matrix compounds such as acetylene and ethene.
Lime or cement kiln incineration:
A cement kiln is basically a large rotary kiln into which raw materials are fed countercurrent to combustion gas flow. The wet process kilns using a 30% water slurry feed are the most suitable for hazardous waste destruction. The products formed are alkaline and so act as a scrubber, removing acid gases formed during combustion. Such a system operates at 2,800 degrees resulting in very efficient removal of wastes.
Wet air oxidation:
This is based on the principle that with the proper temperature, pressure reaction time, and in presence of sufficient oxygen, any solids capable of burning can be oxidized to any degree. This process is good for difficult-to-dewater waste liquors or where solids concentration is very low in the waste sludge stream. It does not require preliminary drying or dewatering. It can operate with one percent solids concentration. Oxidation is obtained at temperatures as low as 300 -400 F.
Solidification Techniques
There are several innovative non-thermal processes that have been developed under EPA's SITE program that immobilize wastes by vitrification or other types of solidification. The SITE program is the Superfund Innovative Technology Evaluation a $20 million/yr program which has been developed to encourage the private development and demonstration of new technologies for cleaning up hazardous wastes. For example, researchers at Battelle Pacific Northwest Laboratories have developed an in situ vitrification process (which was originally designed for the containment of nuclear wastes) in which electrode are sunk into a contaminated area and attached to a diesel-powered generator. The current produced temperatures of about 3,600 F which is much higher than the fusion temperature of soil. An exhaust hood is placed over the site to collect and treat any combustion products. The result is a massive glass-Iike product consisting of completely immobilized organics, inorganics, steel drums and other components that are essentially locked up and inert. The time taken to complete the process depends on the electrode depth and frequency.
Another solidification process uses a reagent called Urrichem that immobilizes slurried hazardous components. The contaminated soil is excavated and intimately mixed with the Urrichem off-site. After blending, the slurry is pumped out of the mixer and hardens into a concrete-like mass within 24 hours. Chemfix is a process developed by Chemfix Technologies (Metaire, LA). In this technique, a proprietary blend of soluble silicates and additives is used to convert high molecular weight organic and inorganic slurries into a cross-linked, clay-like matrix.
Alternate methods have also included landfills, deep well injection disposal and ocean dumping. Landfills were developed because it was believed that by placing waste solids in designated ground areas, there would be a natural decomposition over time. Unfortunately, the water table rises in a landfill and this effect means that it is possible to get a leaching of the water-containing toxics since the water flow is always from areas of high-to-low head. This is a special problem for areas that rely on groundwater and not surface water for domestic/industrial supplies. Deep injection wells have similar problems. When aquifers in which the deep well is located are pressurized by pumping, the contaminants are drawn into the wall. Ocean dumping of wastewater solids has upset the delicate ecosystem balance in some areas. Thus, there remains a need for effective economical techniques that can be safely used to combat the nation's solids management problems. PE
Paul N. Cheremisinoff is a Professor of Civil & Environmental Engineering at New Jersey Institute of Technology in Newark, NJ. Paul is a Registered Environmental Manager. Kasinath Banerjee is a Research Engineer with International Technology Corporation in Pittsburgh, PA. Goutam (Gary) Datta is a Senior Application Specialist with Kason Corporation in Linden, NJ.
Easy adjustment of screen angle assures efficient separation of varying streams of prewash.
Dewatering screens installation for solids removal process wastewaters.