Separation Techniques

Up to now, only the water treatment aspects relating to the efficient run­ning of a plant have been covered. It is necessary to consider the discharge of the water from any cleaning process into the waterways or drainage systems, in order to ensure that statutory regulations are not violated. The following techniques are briefly considered:

• Separation of a liquid from a solid.

• Separation of a solid from a liquid.

The selection of the method used depends on many complex factors, which are not covered in this paper. The basics of each of the two methods are briefly covered. Only a few of these techniques would be used in the water treatment procedure encountered in the industrial ventilation field; however, for the sake of completeness others are also covered.

Separation of a Liquid from a Solid

The removal of the various solid particulates from a gas stream may be achieved by washing either with chemicals or with water. Once this process has been carried out, the problem is to remove the solids from the liquid in order to

• Reuse the liquid

• Collect any valuable particulate matter for recycling

• Ensure that the drainage system is not contaminated

Numerous techniques may be used, each with disadvantages and advan­tages. The methods used are

• Gravitational force

• Centrifugal force

• Application of a vacuum

• Application of mechanical force

• Action of a solvent

• Displacement

• Vaporization

In considering each of these techniques, the properties and principles on which they depend are related to the liquid only.

The solid in the liquid mixture must be considered as

• Nonvolatile

• Insoluble

• Unaffected by the treatment to which it is subjected

• Capable of being reused with a minimum of treatment

• Not causing any problems when discharged into the drains

• Discharging into drains less than 40 °C with pH in the 6-11 range

Removal by Gravitational Force. This method simply involves a settling chamber in which the fact that the liquid-solid mixture-solid pore interface has little holding power. Thus, given sufficient time, the solids will settle into the base of the chamber due to gravitational forces.

The efficiency of this operation can be improved by the use of finely per­forated vee troughs, which will contain the particulate matter and allow the liquid to descend to the base of the container, where it then drains away either for further treatment or for reuse.

Removal by Centrifugal Force. This method is more efficient in extract­ing the particulate matter from a liquid due to the fact that the forces devel­oped are many times greater than the force of gravity.

The machine used is normally the basket-type centrifuge, which is a rotating perforated drum with a vertical axis. The solids remain in the drum and the liquid passes out through perforated holes. The smaller the holes, the greater the collection efficiency. However, there is the risk of hole clogging, causing a rapid fall in operating efficiency. The fluid viscos­ity and the particulate size are of prime importance. Plant arrangement in series using different-size perforations tends to overcome the clogging problems.

Vacuum Removal. This approach is used in the paper industry for de­naturing the paper, in which a vacuum is applied under the paper stock.

Mechanical Force. Liquid can be readily expelled from a spongelike particulate mass of solid by using various pressing techniques. With this method, mechanical energy is used to force the liquid containing the partic­ulate matter through a porous bed. The particulate matter is held in the pores in the bed. When the pressure drop reaches a certain level, replace­ment or backwashing takes place. This process may be either intermittent or continuous.

Solvent Action. Materials that tend to respond well to extraction by pressing will be more effective in solids removal when solvents are used. The complication is that it becomes necessary to separate not only the solids and the containing liquid from the finished process, but the solvent as well.

Displacement. This approach, which displaces one liquid from a solid mass by the introduction of another, is seldom used.

Vaporization Methods. The above methods may be unacceptable on certain counts, and if complete removal of the liquid from the solid is re­quired, vaporization methods are used. A nonvolatile solid can be removed from a volatile liquid by the application of heat, a vacuum, or both.

The various techniques can be classified as

• Heat applied at atmospheric pressure

• Heat applied at reduced pressure

• Vapor distillation

The dryers make use of warm air, flue gases, and direct radiant heat to the liquid-particle mixture. This method allows complete extraction of the solid through removal of the liquid by vaporization. Due to the energy input re­quired with this method, it is the most costly.

Separation of a Solid from a Liquid

A solid in a liquid medium may be in either of the conditions:

1. The solid may be dissolved into the liquid medium.

2. It may be insoluble and remain suspended in the liquid.

The liquid is assumed to be saturated if undissolved material is present, with solids both in suspension and in solution in the liquid phase.

The efficiency of separation will obviously affect the purity of the liquid, and it may be necessary to provide a series of separate stages to meet the stan­dard required by the specification.

The most important consideration is the actual condition of the solid in the liquid. Is it in solution, or is it in suspension? Other consider­ations are

1. The relationship of the solid to the liquid

(a) Solids in solution in the liquid

•The solid concentration in the solution •Degree of solubility

•The relationship between temperature and solubility •The viscosity of the liquid

(b) Solid in suspension in the liquid

•The relative amounts of solids in suspension

•The size of solid particulate matter

•The density of the particulate matter

•The compacting properties of the solid

•The viscosity of the liquid medium

•The nature of the solid— spongelike, gritty, etc.

2. The degree of separation required

3. The heat-sensitive properties of the solid or liquid

4. Corrosive nature of solids and liquids

5. The degree of separation required for both the liquid and the solid

6.The value of the recovered solids or liquids

7.The volume of materials to be treated

The following list provides an indication of the various techniques on which the separating methods are based.

• Relative vapor pressure of the solid and liquid

• The phase relationship between the solid and liquid

• The relative solubility of immiscible solvents

• Reduction of solubility

• Chemical precipitation

• Ion exchange

• Electrolytic deposition

• Adsorptive properties

The separation of solids in suspension in liquids can be achieved by either of the following techniques

• Density difference

• The cross-section of the solid particulate matter

• The electrostatic properties of the solid

Next we briefly consider each of the above in turn.

Relative Vapor Pressure of Solid and Liquid. If the dissolved solids in a liquid have a low vapor pressure relative to the liquid in which they are dis­solved, provided the solid is not affected by the liquid boiling point, it is an easy matter to vaporize the liquid, leaving a dry residue.

Phase Relationship between the Solid and Liquid. A phase relationship may involve a number of crystalline forms from which materials can be sep­arated. When a solid material is precipitated as a result of the solution be­coming supersaturated, crystallization occurs. Crystallization may be achieved by

• Cooling alone

• Concentration

• Concentration followed by cooling

• Simultaneous concentration and cooling

Relative Solubility of Immiscible Solvents. Many solid materials in so­lution can be removed by transferring them to a second solvent; it is essentia! that the solvents be mutually insoluble.

This approach will not produce a solid; it can, however, be used to remove a solid from one solution or solvent and transfer it to another, from which it can be readily removed.

Reduction of Solubility. It is possible to remove a solid from a solution by changing the condition of a solvent. One method is the addition of a sec­ond solvent miscible with the first, in which the solid in solution is relatively

Insoluble. Another method depends on the fact that if the substance can be ionized, its solubility can be suppressed. This is achieved by adding a highly ionized second substance having one ion in common with the original dis­solved solid.

Chemical Precipitation. If physical separation techniques do not work, separation may be achieved by chemical conversion to a soluble precipitate.

Ion Exchange. Certain solid substances have the property of exchang­ing one ion for another if placed in a solution containing the ions. Typical sub­stances with this property are the zeolites and certain synthetic resins.

Electrolytic Deposition. The separation of a metal from a solution can be achieved by electrolysis.

Adsorptive Properties. Substances such as silica gel and activated char­coal can be used to collect (adsorb) certain solids from solution. The adsorber bed may be discarded when depleted or recycled by washing and heating.

Separation of Solids in Suspension in Liquids. This can be achieved using

1. Density difference

2. The difference in cross-section between the solid particulate matter and the liquid molecule

3. The electrostatic properties of the solid

Density Techniques. Many methods are in use, with the selection de­pending on

• Particulate size

• Settling velocity

• Quantity of solids in suspension

In the case of gravitational settling, the unit design depends on the method of solid removal after settling. The methods in use are

Vacuum Filters. If, due to the nature of the liquid, the gravity filter be­comes unsuitable, a vacuum filter is used to create a substantial pressure dif­ference. Vacuum filters can be divided into the following types:

• Intermittent filters

• Leaf filters

• Continuous (e. g., rotary vacuum) filters

• Vacuum pressure filters, as used in desulfurization plant

• Pressure filters

In the true gravity case, pumps are not used. If, however, the liquid is highly viscous, to achieve efficient operation, pumps are required to force the fluid through the pressure filters. The pump can be considered essentially as a press with a plate-and-frame filter. The plate-and-frame filter consists of a se­ries of frames over which the filter medium is stretched. A centrifugal basket of fine mesh is another method of particulate removal.

Dialysis. If a solution containing colloidal particle is placed on one side of a dialysis membrane, the water on the other side will allow the solution to be reduced in concentration as it passes through the membrane.

Electrostatic Properties of Solids in Suspension. Some solids in suspen­sion will migrate from one pole to another when placed between direct current electrodes. The phenomenon of solids moving toward an electrode is known as cataphoresis.

To close this section on treatment, two more methods that depend on bac­teriological action are considered:

• Aerobic treatment

• Anaerobic treatment

Aerobic Treatment. The activated sludge process depends on aerobic biological action. In this case the microorganisms, in searching for food, break down the complex organic substances into simple stable substances. This process results in the removal of soluble and suspended organic matter from wastewater.

The growth of microorganisms in the presence of dissolved oxygen re­moves the majority of pollutant matter; in turn, protozoa grow and feed on these organisms. The resulting balance is of a living culture in suspended form in the activated sludge floe. This process is ideally suited for the removal of carbonaceous matter and nitrification from wastewater.

The principal elements of the system include an aeration tank in which the wastewater is thoroughly mixed with continuously activated sludge and oxy­gen. From this part of the process, it passes into a clarifier tank, where the set­tled sludge is removed from the purified water to be recycled by the return activated sludge pumps.

For this system to work, two exacting requirements must be met. The aer­ation device must be capable of both transferring oxygen from the atmosphere to the liquid, and distributing this oxygen throughout the wastewater to the suspended living microorganism. This type of system is suited for low-strength waste, typically on the order of 50-200 mg L-1 BOD.

To enhance the purification process and increase the degree of purifica­tion, powdered activated carbon (PAG) may be added directly to the aeration tank, or the biologically treated wastewater may be filtered through granu­lated activated carbon (GAG) for posttreatment.

Pre — or post-treatment with ozone of wastewater may also be applied. Pre­treatment with ozone takes place in the presence of biorefractory compounds, as ozone increases the BOD/COD ratio.

Anaerobic Treatment. Typical of this method is the upflow anaerobic sludge blanket. This consists of a corrosion-resistant tank complete with sepa­rators. The flow network enters the reactor base without short-circuiting, en­suring the proper formation of the granular sludge. New bacterial cells are formed in the reactor and aggregate into tiny granules, which have good set­tling characteristics.

Biogas is produced by the bacteria in the form of small bubbles; these float upward through the sludge bed/blanket, providing a good mixing ac­tion. When the biogas reaches the top of the reactor, it is collected and used as a fuel.

Design Considerations

In determining the best method of treatment, the following factors have to be considered:

Owning and Operating Costs

• Initial cost

• Maintenance

• Energy costs

• Water treatment

• Corrosion costs

• Odor treatment

• Abrasion problems

• Slurry pumping problems

• The maximum temperature the drains and the water sinks can accept (bearing in mind thermal stresses and corrosion in discharge pipelines, and algae growth and oxygen depletion in the watercourses)

Properties of Liquid Used

• Vapor pressure

• Temperature of decomposition

• Viscosity

• Density

Properties of Solid Being Extracted

• Temperature of decomposition

• Solubility

• State of subdivision

• Surface absorptive properties

• Elasticity

In some cases, fragility related to the dryness of the resultant solid, which will influence the removal technique

Relationship between Liquid and Solid

• Mechanically held liquid

• Liquid absorbed on solid surfaces

Removal Efficiency Required. This depends on design requirements and current legislation.

Requirements for Regeneration

• Of liquid

• Of solid

• Of both liquid and solid

Equipment Availability. In many cases the equipment is not available “off the shelf,” and the delivery time may be lengthy; hence, adequate planning is necessary to ensure that the commissioning date can be met with the plant se­lected.