The following article will guide you about how to remove refractory organic and dissolved inorganic substances from sewage.
Removal of Refractory Organic Materials from Sewage:
In the conventional methods of sewage treatment such as activated sludge process and trickling filters various organic materials (measured by BOD) are removed, but these methods are not effective for the removal of refractory organic materials (measured by COD). The refractory organic materials include surfactants, phenols and agricultural pesticides.
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The removal of refractory organic materials can be accomplished by using the following methods:
1. Adsorption by activated carbon; and
2. Chemical oxidation by chlorine and ozone.
1. Adsorption by Activated Carbon:
Adsorption, in general, is the process of collecting soluble substances that are in solution on a suitable interface. The adsorption process can be pictured as one in which molecules leave solution and are held on the solid surface by chemical and physical bonding. The molecules are called the adsorbate and the solid is called the adsorbent.
If the bonds that form between the adsorbate and adsorbent are very strong, the process is almost always irreversible, and chemical adsorption or chemisorption is said to have occurred. On the other hand, if the bonds that are formed are very weak, physical adsorption is said to have occurred.
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In the physical adsorption process the molecules adsorbed are easily removed, or desorbed, by a change in the solution concentration of the adsorbate, and for this reason, the process is said to be reversible. In the removal of refractory organic materials from sewage by activated carbon adsorption process mostly physical adsorption occurs.
The adsorption process can be divided into three steps:
(1) Transfer of the adsorbate molecules through the film that surrounds the adsorbent,
(2) Diffusion through the pores if the adsorbent is porous, and
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(3) Uptake of the adsorbed molecules by the active surface, including formation of the bonds between the adsorbate and the adsorbent i.e., activated carbon.
Activated-Carbon Production:
Activated carbon is prepared by first making a char from material such as wood or coal. This is accomplished by heating the material to a red heat in a retort to drive off the hydrocarbons but with an insufficient supply of air to sustain combustion. The char particle is then activated by exposure to an oxidizing gas at high temperature.
This gas develops a porous structure in the char and thus creates a large internal surface area. After activation, the carbon can be separated into, or prepared in, different sizes with different adsorption capacity. The two size classifications generally are powdered, which has a diameter of less than 200 mesh, and granular, which has a diameter greater than 0.1 mm.
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Treatment with Granular Activated Carbon:
A fixed-bed column is often used as a means of contacting sewage with granular activated carbon as shown in Fig. 18.11. The sewage is applied to the top of the column and withdrawn at the bottom. The carbon is held in place with an underdrain system at the bottom of the column. Provision for backwash and surface wash is usually necessary. Backwashing is necessary to avoid building up of excessive head loss. Fixed-bed columns can be operated singly, in series or in parallel.
One of the most serious problems encountered with the use of fixed carbon beds is the surface clogging caused by the suspended solids in the sewage effluent to be treated. The problem of clogging in fixed beds is partially overcome by using a surface wash or air scour, or both. Expanded-bed and moving-bed carbon contactors have also been developed to overcome the problem of surface clogging.
In the expanded-bed system, the influent is introduced at the bottom of the column and is allowed to expand, much as a filter bed expands during backwash. In the moving-bed system, spent carbon is displaced continuously with fresh carbon. In such a system, head loss does not build up with time after the operating point has been reached.
Treatment with Powdered Activated Carbon:
An alternative means of application is that of adding powdered activated carbon. It may be added to the effluent from biological treatment processes or directly to the various biological treatment processes. In the case of biological-treatment-plant effluent, the carbon is added to the effluent in a contacting tank.
After a certain amount of time for contact, the carbon is allowed to settle to the bottom of the tank, and the treated sewage is then removed from the tank. Since carbon powder is very fine, a coagulant, such as a polyelectrolyte, may be needed to aid the removal of the carbon particles. The addition of powdered activated carbon directly in the aeration tank of an activated sludge treatment process has proved to be effective in the removal of some soluble refractory organic materials.
Carbon Regeneration:
Economical application of activated carbon depends on an efficient means of regenerating the carbon after its adsorptive capacity has been reached. Granular carbon can be regenerated easily in a furnace by oxidizing the organic matter and thus removing it from the carbon surface.
Some of the carbon (about 5 to 10%) is destroyed in this process during transport and must be replaced with new or virgin carbon. A major problem with the use of powdered activated carbon is that there is no well-defined methodology for its regeneration. As such the use of powdered activated carbon will increase only when regeneration problems are solved. However, the use of powdered activated carbon produced from solid wastes may obviate the need to regenerate the spent carbon.
Activated-carbon treatment of sewage is usually thought of as a polishing process for the sewage that has already received normal biological treatment. Under normal conditions, after treatment with activated carbon the effluent BOD ranges from 2 to 7 mg/l, and the effluent COD ranges from 10 to 20 mg/l.
Under optimum conditions, it appears that the effluent COD can be reduced to about 10 mg/l. However, both granular and powdered activated carbon appear to have a low adsorption affinity for low-molecular-weight polar organic species. If biological activity is low in the carbon contactor or in other biological unit processes, these species are difficult to remove with activated carbon.
2. Chemical Oxidation by Chlorine and Ozone:
Chemical oxidation can be used to reduce the concentration of refractory organic materials in sewage. Both chlorine and ozone can be used to reduce the concentration of residual organic materials in sewage. When these chemicals are used for this purpose, disinfection of sewage is usually an added benefit. A further benefit of using ozone is the removal of colour.
Removal of Dissolved Inorganic Substances from Sewage:
The various advanced sewage treatment operations and processes which may be used for the removal of dissolved inorganic substances are:
1. Chemical precipitation,
2. Ion exchange,
3. Reverse osmosis, and
4. Electrodialysis
1. Chemical Precipitation:
Chemical precipitation in sewage treatment involves the addition of chemicals to alter the physical state of dissolved and suspended solids and facilitate their removal by sedimentation. Removal of phosphorus in sewage is accomplished by the addition of chemicals such as alum, lime or iron salts and polymers or polyelectrolytes.
Coincidently with the addition of these chemicals for the removal of phosphorus, various inorganic ions, principally some of the heavy metals are also removed. One of the disadvantages of chemical precipitation is that it usually results in a net increase in the total dissolved solids of the sewage that is being treated.
2. Ion Exchange:
Ion exchange is a unit process by which ions of a given species are displaced from an insoluble exchange material by ions of a different species in solution. For the reduction of dissolved solids in sewage, both anionic and cationic exchange resins have to be used.
The sewage is first passed through a cation exchanger where the positively charged ions are replaced by hydrogen ions. The cation exchanger effluent is then passed over an anionic exchange resin where the anions are replaced by hydroxide ions. Thus the dissolved solids are replaced by hydrogen and hydroxide ions that react to form water molecules.
Ion exchangers are usually of the down-flow, packed-bed column type. Sewage enters the top of the column under pressure, passes downward through the resin bed, and is removed at the bottom. When the resin capacity is exhausted, the column is backwashed to remove trapped solids and then regenerated. The cationic exchange resin is regenerated with a strong acid, such as sulphuric or hydrochloric. Sodium hydroxide is the commonly used regenerant for the anion exchange resin.
This demineralization process can take place in separate exchange columns arranged in series, or both resins can be mixed in a single reactor. Sewage application rates range from 0.20 to 0.40 m3 per minute per m2. Typical bed depths are 0.75 to 2.0 m. Total exchange capacities of commercially available resins are about 50 000 to 80 000 g/m3 (as CaCO3).
High concentrations of suspended solids in influent can plug the ion-exchange beds, causing high head losses and inefficient operation. Resin binding can be caused by residual organic materials found in biological treatment effluents. All the dissolved ions are not removed equally; each resin is characterized by a selectivity series, and some dissolved ions at the end of the series are only partially removed.
3. Reverse Osmosis (Hyperfiltration):
Reverse osmosis is one of several demineralization techniques applicable to the production of a water suitable for reuse. This process has the added benefit of removing dissolved organic materials which are less selectively removed by other demineralization techniques. The primary limitations of reverse osmosis are its high cost and a general lack of operating experience in the treatment of domestic sewage.
Reverse osmosis is a process in which water is separated from dissolved salts in solution by filtering through a semipermeable membrance at a pressure greater than the osmotic pressure caused by the dissolved salts in the sewage. With existing membranes and equipment, operating pressures vary from atmospheric to 10 000 kN/m2.
The basic components of a reverse osmosis unit are the membrane, a membrane support structure, a containing vessel, and a high pressure pump. Cellulose acetate and nylon are used as membrane materials. Four types of membrane support configurations are now in use: spiral-wound, tubular, multiple-plant, and hollow-fibre configurations.
The tubular configuration is recommended for use with domestic sewage effluents. Reverse osmosis units can be arranged either in parallel to provide adequate hydraulic capacity or in series to effect the desired degree of demineralization.
A very high-quality feed is required for efficient operation of a reverse osmosis unit. Pretreatment of a secondary effluent with filtration and carbon adsorption is usually necessary. The removal of iron and manganese is also sometimes necessary to decrease scaling potential. The pH of the feed should be adjusted to a range of 4.0 to 7.5 to inhibit scale formation.
4. Electrodialysis:
In the electrodialysis process, ionic components of a solution are separated through the use of semipermeable ion-selective membranes. Application of an electrical potential between the two electrodes causes an electric current to pass through the solution, which, in turn causes a migration of cations towards the negative electrode and a migration of anions towards the positive electrode. Because of the alternate spacing of cation- and anion-permeable membranes, cells of concentrated and dilute salts are formed.
Sewage is pumped through the membranes which are separated by spacers and assembled into stacks. The sewage is usually retained for about 10 to 20 seconds in a single stack or stage.
Dissolved solids removals vary with the:
(i) Sewage temperature,
(ii) Amounts of electric current passed,
(iii) Type and amount of ions,
(iv) Permselectivity of the membrane,
(v) Fouling and scaling potential of the sewage,
(vi) Sewage flowrate, and
(vii) Number and configuration of stages.
This process may be operated in either a continuous or a batch mode. The units can be arranged either in parallel to provide the necessary hydraulic capacity or in series to effect the desired degree of demineralization. Makeup water usually about 10 percent of the feed volume, is required to wash the membranes continuously.
A portion of the concentrate stream is recycled to maintain nearly equal flowrates and pressures on both sides of each membrane. Sulphuric acid is fed to the concentrate stream to maintain a low pH and thus minimize scaling.
Problems associated with the electrodialysis process of sewage renovation include chemical precipitation of salts with low solubility on the membrane surface and clogging of the membrane by the residual colloidal organic matter in the effluent from sewage treatment plant. To reduce membrane fouling, activated-carbon pretreatment, possibly preceded by chemical precipitation and some form of multimedia filtration, may be necessary.