A typical drinking water treatment plant is able to clarify the water and makes it hygienically safe. It may also add lime and sodium carbonate to make the water softer, and it may mix the water with activated carbon to improve the odor and the taste. The water is generally made to chlorinate at the end of treatment (sometimes at the beginning, too) with enough chlorine to kill essentially all bacteria and viruses and leave a residue of chlorine sufficient to keep the water safe until it reaches the water tap.
As water being treated for drinking purposes often is contaminated by organic substances, chlorination can produce some organic compounds of chlorine some that have been detected have been chloroform and carbon tetrachloride. These compounds are suspected of being carcinogens.
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No alternative technology to chlorination is immediately available. Ozone is used by special permission in one very small system. Ozone has been highly toxic to bacteria, and it destroys viruses more efficiently than chlorine. It leaves no taste. The technology for the use of ozone has been somewhat more expensive than for chlorination, and we have slight long-term experience in the use and effectiveness of ozone. Thus the government must move very carefully before requiring a switch from chlorination. Eventually the change could be made.
The methods of purifying waste domestic and industrial waters are largely traditional and have proved extremely effective in the past. However, in recent years further methods of purification have been introduced, particularly in the field of industrial water treatment.
1. Use of Coagulants:
For effective removal of impurities from water it becomes essential that these have first flocculated, that, have been in a form that could be removed readily by filtration method. The aim has been to obtain particles in excess of 120 microns in diameter; these tend to settle readily in typical aqueous medium. Particles between about 3 and 100 microns tend to settle too slowly for practical purposes.
Flocculation seeks to achieve agglomeration of small particles into larger units and thereby speed up the settling process. The most widely used flocculating agent has been alum, a complex salt of aluminium. When this has been added to an alkaline solution a voluminous aluminium hydroxide flocculate has been produced, which readily sinks to the bottom because it has been heavier than water. In its passage downwards it traps tiny particles suspended in the water and thus removes them.
A recent development has been the use of a number synthetic and natural and polymers. These form ‘bridges’ between adjacent solid particles and cause them, to sediment. In many instances polymer sedimentation has been not as effective as alum, because small particles, particularly those with a diameter below 2 microns, tend to possess a (usually negative) electric charge.
Alum is able to dissipate this charge through the presence of positively charged aluminium ions in solution. However, metal salts have been not without their disadvantages. They are expensive, sometimes give inadequate flocculation and may also leave a ‘carry-over’ of fine turbidity.
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Special synthetic polymers are being manufactured which are having the properties of bridging and discharge of electrical potential. Only water soluble monomer polymer systems can be used for waste water clarification.
The following poly-electrolytes have been used:
(a) Non-ionic types – Polyols, polyethers, polyamides, polyvinyl heterocyclics;
(b) Anionic groups – Carboxylates, sulphonates, phosphonates;
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(c) Cationic groups – Amines, quaternary ammonium compounds, sulphonium and phosphonium compounds.
The most common of all flocculants have been the polyacrylamides which are having molecular weights in the range 4-10 million.
2. Flash Distillation:
Pure evaporation of seawater and other brackish water has been still the most widely used of all methods of recover the pure water. The heat needed to boil a kilogram (litre) of pure water at 100°C has been nearly 2.27 MJ; but the heat needed to boil concentrated solutions of salts is greater as the presence of the salts elevates the boiling point. The main economy of the process has been dependent upon obtaining adequate heat economy.
This could be mainly achieved in two ways. First, solutions can be evaporated at lower pressure when the latent heat needed has been markedly reduced; and second, attempts can be made to use low grade waste heat for the process, especially using waste fuels like refuse, petroleum tail gases, etc., for operating seawater distillation plants. There has been a considerable future in the operation of seawater desalination plants which use distillation on the total energy principle. The most commonly used process employing distillation as a basis has been flash distillation.
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Flash distillation is involving a technique of allowing the seawater to boil successively in a large number of chambers, each of which has been maintained at a lower pressure than the one before, to match the lower temperature of the water. The method of flash distillation of seawater has been especially promising when related to the use of waste heat produced in nuclear power stations. It has been feasible to design a nuclear power stations. It has been feasible to design a nuclear power station, operating on the total energy principle, to produce 400 MW of electricity and 283,000 m3 of freshwater per day.
Operating cycles of a flash evaporator. The flash evaporator is having twenty to fifty chambers is sequence, which are operating at successive lower pressure. When heated brine flows from one chamber to the next, some of it ‘flashes’ off into water vapour, which then condenses on colder condenser tubes and drops as distillate into trays to be led away into storage.
The brine, when passing from chamber to chamber, gets progressively cooler and it has been this same brine which is eventually pumped back through the condenser tubes to act as the coolant in the condenser section of each chamber. It becomes progressively hotter as it does this; consequently, when it reaches the heat input section before re-entering the first flash chamber, its temperature need only be raised a few degrees. The heat would be normally supplied by low-pressure steam which, in its turn, has been readily obtained by utilization of waste heat from primary power generating sources.
As can be seen from Fig. 5.1, the heated brine has been passing from the heat input section to the first flash chamber (1) and from there successively through all the chambers down to the coolest one (6), flashing off a certain amount of water vapour at each stage. It could be then extracted by the brine circulator pump and returned to the tubes of the heat recovery section at (4). In the last few stages —the heat rejection section (5) and (6) – cold crude sea water has been pumped through the tubes.
This allows condensation to occur in these stages and also extracts an amount of heat equal to that put into the evaporator at the heat input section, thereby allowing permitting a continuous cycle of operation. Some of the seawater, after chemical treatment to disallow scale formation, is added to the circulating brine to make up for the distillate extracted and for brine which must be discarded in order to keep the solute concentration in the evaporator within specific limits.
When seawater has been heated gases such as oxygen and carbon dioxide are given off. These could be extracted by a vacuum pump or ejector system.
The heat needed in a flash evaporator to produce unit weight of distillate has been given by the following equation:
(te – t1) / (te – tf) = L
Where, te represents the temperature of the water entering the flash stage in °C;
t1 represents the temperature of the water leaving the tube system in °C;
tf represents the temperature of the water leaving the first flash stage in °C;
L represents the latent heat of flashed vapour in kJ/kg.
Capital costs for typical evaporator plants have been found to depend upon the operating efficiency. Low efficiency plants are having a (te – t1) / (te – tf) ratio of 0.25, while the ratio for high-efficiency plants has been of the order of 0.1 or less. Thus low efficiency plants are having a fuel consumption which has been 21/2 times as high as that of high efficiency plants.
3. Solar Stills:
These methods have been likely to gain importance in areas of the world where fuel is scarce but there is adequate heat from the sun. Solar radiation energy available in many parts of India and Pakistan has been of the order of 20 MJ per m2 land surface area per day for some five months in the year, and even in mid-winter the solar heat has been still more than 10 MJ/m per pay.
A system developed in India uses a large area of glass positioned between and 60 cm above the surface painted black, along which seawater is allowed to flow. Solar heat makes water to evaporate, leaving the salt behind, and condensation takes place on the glass cover. The water drips off to an aluminium channel and runs to a storage vessel. The unit has been also equipped to collect rainwater.
The cost of getting water from the experimental 2 m3 per day stills, which have been able to extract around 3-4 litres of distilled water from each m3 of still surface erected, is about 18p per m3. It is now thought that with larger installations it becomes possible to use this system anywhere the present cost of water exceeds 70p per 1000 US gallons or 18.5p per m3. Such conditions have been found in many parts of India and other arid regions of the earth.
A large solar distillation plant was built near Daytona Beach, Florida. The feed water is having 32 per cent dissolved salts and the area of land covered is 220 m2. Each unit is consisting of grooved concrete curbs laid 1 m apart to form a bay 20 m in length.
There have been plastic basin liners and clear plastic cover domes. Underneath the brine has been a thick layer of insulation, so that the heat generated inside the still has been not conducted away to the soil. Water evaporates from the feed water and condenses against the plastic dome cover, running down towards galvanized steel distillate troughs at the sides of the stills. Solar stills, based on similar designs to the above, are also erected in Greece and Spain.
For a basin type solar distillation plant, with a capacity of 180 m3 of water per day, covering a ground area of 50,000 m3 plant investment cost has been about £ 2,00,000; as far as running costs are concerned, amortization accounts for 70 per cent of the daily costs.
The next major items of expenditure have been taxes, insurance and interest on working capital, which add a further 18 per cent. The final cost of water supplied works out at 35p per m4 very much higher than with distillation systems which use purchased fuels.
However anomalous it may be that where the fuel has been free the costs have been higher, it has been certain that solar stills will not be economic in comparison with other methods of water desalination for many years to come.
4. Ion Exchange:
The cations commonly present in natural waters including seawater, have been calcium, sodium, potassium, magnesium, iron, and traces of other metals like manganese, aluminium, copper and zinc. The usual anions have been bicarbonate, chloride, sulphate and traces of nitrate and fluoride.
Ion exchange purification takes place by passing water first through an exchanger where cations get exchanged for hydrogen ions, followed by passage through an anion exchanger bed where acid radicles have been replaced by hydroxyl ions so that-
H+ (hydrogen ions) + OH– (hydroxyl ions) → H2O (water).
Naturally, it becomes necessary to reclaim both the cation and the anion exchange resins as they become spent. This is normally carried out by reacting the former with strong acid and the latter with caustic soda. There have been a number of more sophisticated plants including so-called ‘mixed bed’ deionizers where special methods of fluidization are able to separate the different ion exchange resins to enable them to be reclaimed separately. In the past it was regarded uneconomic to use ion-exchange plants for the desalination of water with a solid content in excess of 1.5 per litre.
By employing a cascade system of desalination it now becomes possible to treat water with a salt content well in excess of 10 g per litre. Some Russian plants have been using ion- exchange membranes for the desalination of waters having up to 15 g of salt per litre. The technique used has been a combination of electro-dialysis, and ion exchange using membranes which are referred to simply as MK – 40 and MA – 40.
These are having as surface electrical resistance of about 12 ohm/cm2, measure 150cm x 50 cm have been able to exchange between 2.3 and 4.4 mg equivalent of ions per hour. The total electric power consumption needed has been 9 kWh per m3 of drinking water produced.
A new American technique employs weak electrolyte ion exchangers to change sodium chloride into sodium bicarbonate first followed by removal of the cation (sodium) and then recovery of the carbon dioxide which is still present in solution. It has been reported that with this technique really concentrated solutions of salt could be treated efficiently and economically. The ‘Sirotherm’ process has been an ion-exchange process in which regeneration of the resin has been by heat and not chemicals.
At present this process has been still at the laboratory stage, but there have been indications that it has been technically, feasible to treat water with a salinity of up to 3 g per litre. It is thought that some other process may be required to reduce the salt content of seawater from its usual 35 g per litre to that at which an ion exchange process is feasible. This implies a combination of electro-dialysis or reverse osmosis with an inexpensive ion-exchange technique like ‘Sirotherm’.
5. Reverse Osmosis:
When we keep a solution on one side of a semi-permeable membrane and pure solvent on the other, the solvent will pass through this membrane to dilute the solution. The pressure which gets exerted by the solvent as it passes through is called the osmotic pressure.
Indeed, it is to be argued that in solution a dissolved salt behaves like a gas and obeys the gas laws. This implies that one gramme mole of cation or anion dissolved in 22.414 litres of water gives an osmotic pressure of exactly 1.013 bars (1 atmosphere) at a temperature of 0°C. Reverse osmosis has been found to depend upon the use of a semi-permeable membrane, which is capable of being traversed only by water and not by salt, combined with pressurizing of the solution so that water passes through the membrane, but the salt stays behind.
Due to the high pressures involved, the main difficulty has been in the making of a semi-permeable membrane of adequate strength. In general, membranes could be supported by a special carrier.
The following semi-permeable membrane materials are tried, with favourable results:
(a) Cellulose acetate dissolved in glacial acetic acid together having small quantities of additives like magnesium per–chlorate before being allowed to solidify on the carrier;
(b) A cellulose acetate/formamide solution in acetone;
(c) Ethyl cellulose mixes.
The working pressure on one side of the reverse osmosis cell has been normally of the order of 100 bars, while the other has been at atmospheric pressure. Supporting the semi-permeable membrane to avoid breakdown has been therefore a problem of considerable magnitude.
The first reverse osmosis plants was using perforated stainless steel plates and tubes as supports, but these have been reported to be too expensive for commercial plants. The best material found which was having the requisite strength and at the same time was not too expensive was fibre-glass cloth laminate about 3 mm thick; this is having a compressive strength of 15 MN per m2 and a tensile strength in excess of 150 MN/m2. As far as tubular support materials are concerned, phenolic resins having a wall thickness of 2.5 mm seemed to provide some of the best results at the lowest cost.
The desalination cell has been so designed as to bring the saline solution at high pressure in contact with the semi-permeable membrane, with either plates or tubes to support the membrane itself. As osmotic pressure tends to increase with increasing salinity, the through put rate for a given membrane is less with seawater (3.5 per cent dissolved solids) than typical brackish water (less than 0.5 per cent dissolved solids). The final product usually is having about 2000 ppm of dissolved solids, and may have to be purified further by other means.
Reverse osmosis has been found to be more economical for low salt content water than for seawater. As the osmotic pressure of seawater has been approximately 25 bars, it has been obviously necessary to overcome this pressure before reverse osmosis can begin.
Osmotic pressure builds up as the feed stream is concentrated and this very much is able to limit the percentage of water in the feed which can be usefully recovered as freshwater; only about a third of the water can, in fact, be recovered. Recovery of some of the power from the flow of the waste stream of concentrate seawater has been usually attempted.
It is found that a reverse osmosis plant producing 40 m3 of water from seawater would cost about £3.5 million, and its operating cost would be 4.5 p per m3 of water obtained. A plant to recover pure water from brackish sources would be costing about £ 1.1 million with an operating cost of only 2.3p per m3.
In spite of the disadvantages of reverse osmosis, this system has been likely to become one of the most important in the field of water desalination.
Practical Reverse Osmosis Plants:
Unlike many of the other desalination processes, reverse osmosis has emerged from the pilot plant stage and has been now used industrially. Paterson Candy International make reverse osmosis plants able to supply drinking water from brackish sources to private houses, hotels, farms, small industrial undertakings, etc.
The units are using a feed of brackish water with a maximum dissolved solids content of about 1 per cent and pH between 3 and 7.5. The salt reduction achieved has been between 85 and 95 per cent, depending upon the nature of the ions. The semi-permeable membrane in Paterson Candy units has been modified cellulose acetate cast on the inside of disposable paper/polyster tubes.
Normally membranes can find use continuously for between 12 and 24 months before replacement becomes necessary. An electric motor has been used to produce a pressure of 40 – 80 bars, depending upon the nature of the feed. Power consumed is, on average, about 2.5 kWh/m3 of desalinated water—which compares favourably with 12 kWh/m3 for distillation and 7 kWh/m3 for freezing processes.
A plant measuring 2.6 + 1.5 x 1.3 m and weighing under 2 tonnes has been able to produce 45 m3 of desalinated water per day. This has been incidentally, the largest plant currently made by this firm, which produces equipment having capacities from 2.3 to 45 m3 day.
6. Electrodialysis:
In electrodialysis, as in reverse osmosis, a semi-permeable membrane permits pure water to pass, but stops salts. Whereas in reverse osmosis the passage of water through the membrane has been induced by physical pressure acting on the saline solution, in electro-dialysis this work has been carried out by an electric current.
The basic idea has been that the saline solution has been enclosed by membranes and an electric current induces cations (metals) to migrate outward towards a cathode, and anions (the non-metallic ions) to migrate towards an anode, thereby progressively desalinating the solution.
Unfortunately, the procedure in practice has been not quite as simple as this. For progressive desalination to continue, the energy exerted during the electrodialytic process must also overcome osmotic pressure which is able to force clean water back into the solution.
As in reverse osmosis, membranes have to be very tough indeed for withstanding the high pressures applied. At the present time electro-dialysis has been most widely used in the Soviet Union, and the most common membrane has been polyethylene backed by nylon fibre.
In general the greater the current density used in any given dialysis cell, the smaller the area of membrane required. But increased current density also necessitates increased flow rate to prevent the membrane from being polarized. Optimum current density has been found to vary with the concentration of salts in water to be treated, the cost of the membrane and the cost of the power expended.
For seawater having a salt content of 3.5 per cent and a power cost per kWh amounting to 0.14 per cent of the cost of membrane per m2, the optimum current density has been 1.06 A/dm2. For other conditions, the optimum current density may decrease between 0.7 and 3 A/dm2.
Electrodialysis desalination finds use in the USSR mainly for water supply in sparsely populated areas, farms and watering places at distant pastures as well as for expeditions venturing to areas where there has been no fresh water but there has been access to underground or surface salt water. Although the process has been basically an expensive one both from the point of view of initial cost and of power needed it is having the considerable advantage that it can be completely automated, needing no resident labour.
In Western countries electro-dialysis for seawater desalination has been not generally regarded economically viable compared with other methods, mainly due to the large quantities of power needed. However, if at some future date the cost of electric power must fall appreciably, electro-dialysis may become competitive.
Separators of expanded rigid PVC have to be used to support the membranes and to create turbulence in the stream. Two plants have been built, one with a daily output of 77 m3 and the other with a daily output of 151 m3, using membrane areas of 12 and 40 m2 respectively.
The biggest American electro-dialysis plant is the 2460 m3 per day plant at Buckeye, Arizona. This brings down the salinity of the water from approximately 2.1 per cent to 0.5 per cent at a cost of about 5p per m3 of water treated. Electro-dialysis provides the most economic results when the installed plant capacity is large and when the feed water salinity is comparatively low.
7. Ion Exchange Techniques:
There are four basic ion exchange resins which have been able to absorb even small quantities of dissolved cations from solutions. These have been sulphonic phenolic resins, sulphonated coal, carboxylic resins and sulphonated polystyrene. For all of them the reaction which occurs can be expressed as –
Where, M+ represents the metallic ion (its valency does not have to be one) and r– is the net like resin structure. Once the ion exchange resin gets absorbed its capacity of metallic ions, it could be reclaimed by means of a strong acid –
The metal can then be able to recover from the concentrated chloride or other salt. Ion exchange of this type has been found to be very effective for many metals especially, lead, copper, zinc, mercury, chromium and nickel. For arsenic an anion exchange resin is able to remove the metal in the form of arsenates and arsenites; reduction of arsenic contents of waste liquors to below 5 ppm can be attained without difficulty.
Copper and zinc can be removed from waste liquors so effectively that a final effluent is having less than 0.001 ppm of either metal. It has been vitally necessary, as it has been shown that even concentrations of 0.02 ppm of these metals are having very harmful effects on fish.
Ion exchange techniques must be used, where possible, in conjunction with precipitation methods, because they have been most successful when the initial concentration of dissolved metal ions has been already fairly low.
8. Self-Purification of Waters:
In the various zones of receiving streams (surface waters such as streams, rivers and lakes into which unpurified or purified waste waters are channeled and by which they are carried away), living creatures are to be found which are able to utilize any nutrients that are available.
Natural self-purification of the waters by the decomposition of putrefactive substances, after a given period and distance of flow of the receiving streams, is due to these creatures. Under favorable conditions the pollution of the waters is substantially corrected in this way, so that the original pure state is at least approximately restored.
The self-purification processes depend mainly on biological processes (self-purification by organisms) or chemical processes (primarily oxidation and reduction processes), the effectiveness of which is determined by physical factors such as velocity of flow, state of the water bed, ratio of foul to clean water, temperature and depth of water, intensity and duration of solar radiation and fineness of waste substances. The main burden of self-purification falls on the biological processes.
The organisms contributing to the process of self-purification include fungi, algae, bacteria, protozoa, crustaceans, shellfish, worms, insect larvae, snails, fish, big predatory animals and waterfowl.
In the metabolism of the organisms these substances are incorporated into the body matter or ultimately broken down into water and carbon dioxide for the purpose of providing energy. In order to sustain these processes, dissolved oxygen is absorbed from the water. Owing to their autotrophic manner of feeding, the algae containing chlorophyll are in a position by day to return the oxygen so absorbed.
If the water is over-rich in nutrients, i.e., overloaded with plant nutrients such as nitrogen or phosphorus, there can well be an explosive proliferation of algae. This will result in a shortage of oxygen as, owing to the limited absorptive capacity
depends largely on the water temperature. At higher temperature the saturation point is reached more rapidly. If algae are present in the water in overabundance, the oxygen requirements at night will lead to a severe oxygen deficiency.
The protozoa, which absorb bacteria and algae as nutrients, are also able to utilise dissolved organic substances to some extent. Crustaceans, snails and worms feed on undissolved, deposited or suspended matter. They also ingest protozoa. Bigger crustaceans and insect larvae, for their part, feed on small crustaceans, worms and also on protozoa. Fish and waterfowl, which are also liable to fall victims to predatory animals in their turn, often prey on crustaceans and worms.
The intensity of self-purification is determined not only by optimum functioning of the living communities but also by the conformation of the riverbed. In naturally formed river-beds that is to say, in waters with large surfaces and strong turbulence (irregular strong water currents of high turbulence), the higher input of oxygen makes for more favorable living conditions for the organisms than are found in corrected or dammed river courses.
Even minor changes in one or another of the determining biological, chemical or physical factors can completely upset the self-purification capacity of the waters.
9. Sewage Purification:
Until relatively recent times, the self-purification capacity of static and running waters was still sufficient to reduce the contaminants contained in them. Loading the waters with waste and sewage from different sources has proliferated, however; and owing to the accretion of these contaminants, the reduction processes in the waterways have either come to a standstill by the extermination of micro-organisms or been intensified by the increase in nutrients.
With the multiplication of organisms, more oxygen has been extracted from the waters. The self-purification capacity of the waters has been overloaded, and the regenerative powers of the waters have been disrupted. The immediate consequences of such pollution have been the extermination of fish life, deposition of mud, and putrefaction processes.
The most serious loading of the waters stems from domestic and industrial waste disposal. The sewage is conducted through the drainage system to a purification plant, which as a rule operates in two stages – mechanical and biological treatment stages. The mechanical system comprises a grid, a sand trap and the preliminary settler. The coarser elements in the sewage are separated and retained by the gird.
As the flow speed of the water slows, smaller mineral elements settle in the sand trap; the other removable matter is deposited in the preliminary settler, and the floating elements are removed by means of a scoop. The purification performance at the mechanical stage stands at about 20% — 30%.
The water thus purified passes finally to the biological stage, which comprises a restoration basin (percolating filter) and a second filter, or final sedimentation tank, in the restoration basin, bacteria and other micro-organisms convert the suspended and dissolved matter into removable sludge by absorbing the particles and dissolved matter as a nutritive substratum and by forming cell lumps as they proliferate, which, owing to their increase in weight, sink to the bottom. The whole process is facilitated and intensified by artificial induction of air by means of bellows, rotors and cylinder pumps.
In order to ensure that the micro-organisms required for decomposition are present in sufficient quantities, parts of the micro-organism sludge deposited on the floor of the secondary filter tank are returned to the restoration basin. The purified water on the surface of the secondary filter tank is fed into the receiving stream.
The sludge from the primary and secondary settlers is digested in septic towers with the aid of anaerobic bacteria. By this process, methane gas is formed which can be used to provide energy for the plant. Previously the digested sludge was usually dried out in beds to compact it. In new plants the preference is for mechanical dehydration plants, such as centrifuges and presses, because of the smaller space they take up. The dehydrated sludge can then be further processed together with household refuse.
Under optimum conditions the mechanical-biological sludge drying plant can achieve a purification performance of over 90%. For further purposes, however, this performance level is still too low, so that a further stage, the chemical stage, needs to be added. The chemical precipitation process employed serves to eliminate phosphates and other (especially industrial) pollution.
With the aid to micro-sieves and similar technical appliances, organic residues can be further reduced. In addition, a hygienic improvement in the sewage may be achieved by means of chlorination, radiation, heating or ozonizing.
10. The Corrugated Plant Filter:
For the purification of contaminated waste waters, a plant with corrugated-plate-filter muck piles can be installed wherever the water is charged with floating or deposited matter. The oil filters can be installed at any point in the petrochemical or mineral oil processing industry, in tanker depots, at airports, in big garages, etc. Plant for the removal of solids is installed for the purification of filter return water and for sludge filtration (e.g., in the foodstuff and textile industries).
The corrugated plate filter depends for its operation on the force of gravity. In the filter the particles of contaminants do not need to rise the whole way to the surface of the basin, nor to sink to its floor, since the filter contains a number of corrugated plates, one above the other, representing in effect a series of surfaces and of floors. Any one particle need to cover only the distance between two plates and is then effectively deposited.
The corrugations on the plates make for closer pooling of the contaminants from the water for onward transmission to the filter channels. Additional collecting channels on the inlet and outlet sides of the corrugated plate will ensure that the filtered matter is collected and carried away without being diverted (by the inlet or outlet currents) into the sewage water again or contaminating the clean water.
According to the specific gravity of the contaminants to be eliminated, the filter will be fed from above downward (for matter that is lighter than water, such as oil) or from below upward (in the case of matter than is heavier than water).
The way in which the filter plant and the filter itself function is illustrated in the following example, relating to oil filtering. The inlet tank produces an even distribution of the incoming contaminated water between the individual parallel cells of the filter tank, so that all the plate packs are equally loaded. In front of each pack a flow filter spreads the water flow evenly over the whole field.
The oil and water mixture makes its way from above into the pack, which is set at an angle of 45° to the vertical. It flows down diagonally between the corrugated plates set one above the other. The flow between the plates is laminar; the oil droplets rise unhindered to the underside of the plate above, collect in the crests of the plate, and flow back upward to the entrance of the plate pack. There the oil is caught up in the mainstream and carried away from the flow of water coming in to the surface of the basin.
The sludge present in the water collects in the troughs of the undulations, sinks down with the current flow of the water to the outlet of the pack, where it is similarly taken up by the collecting channel, and is carried in the mainstream to the floor of the basin or into an elutriating funnel.
The cleansed water passes into the outlet tank, and from there over an overflow dam into the outlet channel. The separated oil collects as a slick on the surface of the water in the filter tank and flows over an oil scoop or skin pipe into the sump. The filter tank is covered with floating plates of plastic form in order to reduce evaporation and offensive fumes.
For the separation of solid matter the direction of flow is reversed, but the way in which the entire plant works is, in other respects, the same.
The average performance of a set of plates, with a distance between the plates of 19 mm, is 30 cubic meters per hour. In special cases this average performance is reduced, however, to enable particles with lower rates of rise or fall to be thoroughly separated.
11. Composting of Sewage Sludge- The Counterflow Process:
The counterflow process, which was developed for the composting of raw sludge from local sewage plants, is suitable for use in all sewage plant systems. The reactors are available in sizes of 25 cu m, 50 cu m, 100 cu m, and up to 500 cu m. Anaerobic or aerobic sludge stabilization (digestion tower) is superfluous. What is necessary, however, is drainage of the sewage sludge by mechanical means. For composting the sludge, owing to its high nitrogen and water content, an organic carbon carrier, such as peat, sawdust, chopped straw or lignite, is also required.
The sludge, fed into the sewage plant with a water content of about 98%, is dehydrated mechanically by means of presses and centrifugal driers down to a water content of 75%-80%. The dehydrated sludge can be mixed with the required organic carbon carrier by means of a mixing screw in the ratio of 1:1. In the case of a moving operation, instead of the carbon carrier, backflow (reflux) matter can be used in the proportions of 50% sludge, 40% reflux and 10% carbon carrier.
The mixed material is raised by a sloping conveyor into the bioreactor, which consists of combined metal elements. In order to prevent heat dissipation, the reactor is insulated over its entire outer wall. The material provided passes slowly and continuously through the reactor from top to bottom (period of passage about 10 days). Ventilation is effected at the base of the reactor. The air injected through nozzles passes upward through the material. Control of the airflow is possible.
Air is constantly removed at three points in the material store (reactor filling; and its carbon dioxide content examined with analysis equipment. Resistance pyrometers constantly check the temperature at six different points, which is then recorded on a six-point ink-writer.
The rapid group system is adopted for utilizing clean sewage sludge. By a regular supply of oxygen, optimum living conditions are provided for the reducing and converting microorganisms already present in the sludge. In this way they are stimulated to exercise higher metabolic activity and faster germination.
The air injected has an initial oxygen content of about 21%. It penetrates the material slowly by a counterflow process in an upward direction. This promotes the respiration of oxygen by the organisms. Oxidation processes thus get under way and the oxygen content of the air is reduced from below upward. In the upper field of the reactor the content of oxygen in the air falls to 7% – 10%. In accordance with the airflow the temperature rises from below upward, in the upper region immediately below the condenser water zone the temperature builds up to levels of 75° – 82°C (167° – 180°F).
The material, which is slowly and continually traversed by the reactor from top to bottom, must necessarily therefore pass through this heat accumulation zone so that hygienic treatment of the sewage sludge is assured. The end product considerably reduced in volume and weight compared with the original material, is a hygienically flawless, friable material.
12. Water Supply – Preparation of Surface Water:
Water consumption in big cities amounts to some 200-300 liters (210-315 quarts) per person per day, rising in peak periods to as high as 450 liters (472 quarts). On top of this, comes industrial water requirements. To produce 1kg (2.2 lb) of plastics, up to 500 liters (525 quarts) of water may be needed, and to produce 1 kg of paper up to 3000 liters (3150 quarts). Of the total supply of drinking water today about half comes from groundwater, about one-third from spring-water, and about 15% or 16% from surface water in lakes and rivers.
In thickly populated industrial centers, groundwater supplies are now utilized practically to the limits of their capacity. Increasing recourse must consequently be had in the future to the surface water in lakes and rivers. But because of the high cost of treating surface water for drinking, the supply of drinking water from this source to wider sectors of the population will raise the price of water considerably.
An example of a river water treatment plant will be described – The untreated water is drawn from the river by means of a suitable suction plant and conveyed to the waterworks by a remote-control pumping plant. There the river water is sterilized with chlorine in premixes and treated with a flocculation agent. This products heavy sludge flakes which sink to the bottom. The clear water remaining above flows on to the ozonizing plant.
There ozone gas is emitted into the water in fine bubbles, by which means (much more intensively than by chlorine) additional sterilization and oxidation of dissolved organic matter are achieved. The ozone gas needed for this purpose is generated from oxygen at the waterworks by means of electric energy, and it reverts to oxygen again afterward, leaving no trace.
In the adjoining rapid filter plant all the other still-dissolved foreign bodies are removed from the water, first of all the remaining suspended particles with the aid of a 2-3 meter thick quartz sand layer and then all the other still- dissolved foreign bodies by means of the active carbon filter inserted below it. The treated water is retained until its introduction into the filtrate reservoir.
A further possibility is to allow the treated water to seep away into suitable gravel deposits in the river valleys, to lower its temperature and assimilate it in other respects to natural groundwater, and after a specified period to remove it in the same way as groundwater together with natural groundwater. A pure water reservoir is needed to retain the water removed, from which the drinking water is conveyed by pumps to the final consumer.
Consumption of drinking water has risen in the past 50 years about 40-fold. This is explained, for the most part, by the enormous expansion in industrial production, by increased awareness of hygienic requirements, and by changes in man’s consumption habits. Furthermore the population has practically doubled in the period.
The proportion of surface water in total supplies of drinking water is likely to expand in line with the constant development of industry.
13. Fluorine and the Fluoridation of Drinking Water:
Fluorine, a member of the halogen family of elements, is the most corrosive chemical element that is known. It is a greenish yellow gas, which, like the fluorine compounds, is highly toxic. The addition of fluorides to drinking water to prevent dental caries (tooth decay) had been opposed by some doctors and toxicologists but now has been endorsed by all major U S. and international health organizations. The application of fluorides in tooth-pastes or for internal consumption hardens the enamel on teeth and so provides protection against destruction of the teeth by caries.
Against the fluoridation of drinking water, various toxicological and legal arguments have been adduced. Water is necessary for life, whereas fluorine and its compounds can be toxic substances. It is for the waterworks to provide drinking water that satisfies the hygienic requirements of comestibles (food). Fluoride at high concentration produces a corrosive effect on glass, steel and a number of other metals.
Drinking water is treated with 1 mg of fluoride per liter of water. At concentrations over 2 mg per liter, people begin to react gradually with symptoms of mild dental fluorosis, as part of the daily intake of fluorine accumulates in the body. The fluorosis manifests itself as white flecks in the dental enamel. Pitting of the enamel can result from fluorine concentrations of 3-4 mg per liter. Only in a few places in the United States is the concentration above 4 mg per liter. In those areas the Environmental Protection Agency is working to reduce the fluorine level.
People do not absorb fluorine into their system only as an additive in drinking water, but fluorine is present in their diet in natural and artificial forms. This supply of fluorine in the daily diet varies in different countries and for different types of food, so that the total intake of fluorine in drinking water and foodstuffs cannot be assessed precisely. A further factor is the loading of the atmosphere with the fluorine waste gases from aluminum works and similar industrial plants, which can also result in an uncontrollable intake of fluorine.
Although fluorine has been shown active in a dental context only for children under 15 years of age the entire water supply system is treated with fluorides. It is estimated that about 1000 liters of water would have to be treated with fluoride to enable a child whose teeth are still developing to drink 1 liter of treated water.
A difficult legal problem now arises from the fact that the consumer can in no way avoid being supplied with fluoridated drinking water. The situation amounts to enforced medication. However, the legal validity of fluoridation has been upheld in courts in the United States and in several other countries.
In many cities in the world, however, the fluoridation of drinking water introduced in the past on a trial basis has been abandoned. In 1971 the model project introduced in 1952 in Kassel-Wahlershausen, Germany, was suspended. In 1972 the fluoridation of drinking water in Sweden was discontinued. In the United States more than half the population drinks fluoridated water and that proportion is increasingly slowly.
Caries is not a disease caused by fluorine deficiency but is the result of bacterial action exacerbated by general nutrient deficiencies and lack of dental care. Fluorine medication can, at best, influence the symptom of caries, but not the disease itself.