or occasionally, heavily charged with suspended matter, as is the case with many rivers and streams, because of the expense and inconvenience of the frequent scraping and sand- washing that would be required. In such cases there should be either a preliminary settlement tank, or a series of “roughing” filters. The latter consist of perhaps three or four filters, of gravel, ° the size of which decreases in each succeeding filter; these arrest most of the suspended matter and can be cleaned periodically by flushing them through in a backwards direction with a mixture of water and compressed air. <Callout type="tip" title="Tip">Roughing filters are effective for removing large particles before they reach more delicate slow sand filters.</Callout> " : Rapid Gravity Filters. Alternatives to the “slow sand filter” are the “rapid gravity filter”, with which chemical precipitation and sedimentation are generally necessary, and the “rapid pressure filter”, which works better without pre-sedimentation. <Callout type="important" title="Important">Rapid filters require frequent cleaning due to high flow rates, often once every 24 hours.</Callout> Hither of 186 THE BUILDING—ITS WATER SUPPLY these occupies far less space than “slow sand filters” and the pressure type can be installed on a pipe line without appreciable loss of head. Whereas the rate of flow through the older type of filter must not exceed 4 inches per hour, or 2 gallons per square foot of surface per hour, in the “rapid” type the rate may be from 10 feet up to 164 feet per hour, or 60 to 100 gallons per square foot per hour, according to the condition of the water to be treated. Prior to passing into the filter chemical treatment is necessary. The nature of this will depend entirely upon the degree of tem- porary hardness (or bicarbonates) present in the raw water. If entirely absent, chalk, hydrated lime or soda ash must be added, but prior thereto sulphate of alumina in solution is introduced and this, combining with the carbonates, forms a cloudy precipitate or ‘floc’, The success or otherwise of the process depends largely upon the nature and quantity of this “floc”. In most instances far too much “floc” results, but this is all to the good of the pro- cess and the excess must be allowed to precipitate in a suitably designed sedimentation tank, where the water will travel slowly through the tank in the space of two to four hours according to circumstances. Thence it should pass to the filter, carrying sufficient “floc” quickly to fill wp the spaces between the sand grains with this jelly-like material. It has been stated above that in the “‘slow” process all mineral and vegetable solids in suspen- sion, as well as bacteria, are caught in the surface film of the filter. This is not the case in the rapid type, where the whole thickness of the filter is necessary for the interception of bacteria, owing to the high velocity of the flow. As a consequence of the increased work put upon the comparatively small area of filter, very fre- quent cleaning becomes necessary; generally speaking this will be once in every twenty-four hours, and the necessity for it is ascer- tained from gauges showing the rate of filtration, which will drop as use of the filter proceeds, or by the difference of head on the filter at the inlet and outlet. When this amounts to 6 to 6} feet in gravity beds, or 10 feet in the enclosed pressure type, it is time to wash. The cleansing of a rapid filter is done by reversal of the direction of flow of filtered water, accompanied by an agitation of the clogged filter with suitable rakes or compressed air. Pressure Filters. It may be here noted that the “pressure” filter (often termed a “ mechanical filter”) consists of a cylindrical steel tank, at the bottom of which is a false perforated floor sup- porting the 8 or 4 feet of filtering sand. In this floor are a large number of nozzles which serve the double purpose of collecting the THE BUILDING—ITS WATER SUPPLY 187 filtered water, when the flow of water is downwards in the filtering process, and of distributing equally the wash water in the process of reversal of flow. If compressed air is used to disturb the sand the same nozzles are available. In some types of filters the com- pressed air is used at the same time as the water, in others the operations succeed each other. It is important that it should be so arranged that the sand is not washed out of the filter by an excessive flow of water or air. The flow of the wash water must also be controlled for the same reason if disturbance is done by means of rotating rakes. It is a matter for careful bacteriological investigation, but nor- mally the filter must be run to waste for fifteen minutes on restart- ing after cleansing. If the above description of the process be carefully considered it will be realised how the nature of the “floc”’ will affect the rate of efficient filtration, the frequency of washing, and the length of the period of running the early filtrate to waste. The “rapid” filter is quite as efficient as the “slow sand” pro- cess, but is far more liable to failure and it requires very constant supervision and investigation. With a raw water containing any large number of pathogenic organisms, it should not be relied upon except with the subsequent use of a sterilising agent such as chlorine, which may be made an absolute safeguard to any supply, and which, by the introduction of new methods of application, is being brought into use in nearly all large municipal works. At the Thames-side works of the Metropolitan Water Board, rapid gravity filters are being used without coagulants, merely as preparatory or roughing filters, and the result is that the water can afterwards be passed through slow sand filters at a greatly increased speed, with comparative safety; but even this double process of filtration is not permissible for Thames water without subsequent chlorination. It is particularly useful during those seasons when the raw water is heavily infested with “algae” (minute vegetable growths), under which condition the slow sand beds soon become quite unusable without the preliminary roughing filters. Domestic Water Filters. Domestic filters are of two kinds, (1) the low-pressure, the most usual form of which is the upright jar with a tap at the bottom, and (2) the high-pressure, which is attached to a tap on the pipes supplying the house. The low-pressure portable jar filters are of two types: (a) those containing a quantity or block of filtering material such as animal or vegetable charcoal, manganous carbon, spongy iron, etc.; and 188 THE BUILDING—ITS WATER SUPPLY (b) those containing hollow, candle-shaped units of baked clay. The former are to be avoided, as bacterial researches have shown that filters of this type are not really germ proof, while a good candle filter is. ) Candle Filters. The high-pressure candle filter is of various forms, but the principle in all is the same. A metal case contains a hollow candle-shaped filter of fine unglazed earthenware. The water filters through from the outside to the inside of the candle, depositing a scum on the outside of it, which is readily removed by taking the candle out of the case and cleaning it. Fig. 144 shows a section through the Pasteur-Chamberland form. A metal case contains the “candle” and is screwed on to the tap. At the bot- tom is a glazed nozzle outlet. By unscrewing the collar just above this, the candle is at once freed for cleansing purposes. Fig. 148 shows the Berkefeld filter, which is of rather different construction. It has a thicker candle formed of baked fossil earth and filters more rapidly, but its efficiency is less. The water in this case is drawn from the top of the filter through the small pipe shown. The candle is removed by unscrewing the two wing nuts at the top. The small tap at the bottom is for the purpose of scouring out. The Berkefeld filter should be cleansed daily. Another type of this filter is the Doulton. These filters all pass the water very slowly and are therefore often provided of larger size and in cases or batteries containing two or more candles. In the same way, several candles can be put into a filter of the jar form for low- pressure filtration, and this is the only type of jar filter recom- mended, It is hardly necessary to point out that domestic filters of these types, needing, as they do, frequent cleansing, are mere make- shifts and not to be compared in efficiency with the scientifically managed purification works of a water authority. They may, however, be the only appliances available to small country houses. It has already been pointed out that the number of bacteria pre- sent in a raw water is considerably reduced by sedimentation, and further reduced to quite a minute quantity by efficient filtration. With most water supplies this will be sufficient for safety. Where, however, the raw water was a highly polluted one, even the re- moval of 99 per cent. of the bacteria will not make the water quite safe, especially as the bacteria which are still present will multiply rapidly in a short period. To make such waters safe the bacteria must be exterminated and this can only be done by sterilisation by means of certain chemicals. The science of sterilisation of water has made such progress in THE BUILDING—ITS WATER SUPPLY 189 recent years that it has enabled some water authorities to take into use once more old sources of supply which had been discarded owing to their polluted state. Many authorities, too, have been enabled to bring into use local rivers and so avoid the expense of bringing water from great distances. Ozone has been tried as a sterilising agent, but the process has hitherto been too expensive for general use. It is becoming less expensive and it.is quite possible that before many years are past the method will be in general use. The use of “ultra-violet” rays has been advocated, but it seems that this method also is expensive; in any case it cannot be said to have been sufficiently tested on a large scale. The two principal methods in use to-day are “chlorination” and the ‘‘ excess lime” treatment; of these the former is in more general use. Chlorine may be obtained for adding to the water in any of the following forms: Firstly, as a solid, in the form of calcium hypochlorite (commonly known as “bleaching powder’’), this being dissolved in a small body of water before being added to the main supply; this method was used for the supply of armies during the war of 1914-18 and for some years by the Metropolitan Water Board, but the plant required for dissolving the powder is bulky and the method may be regarded as ob- solete. Secondly, as a liquid, named “Chloros”, in which form it may be convenient for small installations or in emergencies. Thirdly, as a gas delivered under pressure in steel cylinders, this being the method in almost universal use to-day. | The chief difficulties met with in any chlorination process are that chlorine in the presence of moisture, or any strong solution of chlorine, corrodes metals and that the amount to be added to the water has to be adjusted with precision; if too much is added the taste will be unpleasant; if insufficient, sterilisation will not be complete, and the margin between these two extremes 1s narrow. To mect these difficulties it is essential to install a properly de- signed apparatus, called a “chlorinator’, which is a small and compact contrivance. <Callout type="risk" title="Risk">Improper chlorination can lead to unpleasant tastes in the water and potential corrosion of plumbing.</Callout> Chlorine in a free state, if added to a water still containing organic impurities, will first of all oxidise these, and chlorine so used will not be available for destroying bacteria. It is therefore necessary to add a little more than is required for such oxidation. Chlorination consequently almost invariably is done after filtra- tion and not before it, so that the amount added shall be as small The only circumstances in which it may be desirable as possible. t wh before filtration is where it is necessary to destroy to chlorinate 190 THE BUILDING—ITS WATER SUPPLY algae (minute vegetable growths), which would block the filters in a very short time. The explanation of the effectiveness of the process is that chlorine combines with hydrogen and releases oxygen; “nascent oxygen”, i.e. oxygen newly formed by a chemical process, is a far more active agent than ordinary oxygen. The dose usually required for a filtered water is about } to $ part per million parts of water. ‘ It is found with some waters that an effective dose of chlorine must be so large that the resulting taste of the water will be most objectionable, especially when the water is boiled. Fortunately, objectionable taste can be prevented by ensuring that the water contains an excess of free ammonia, which may be done by the addition of a solution of ammonium sulphate, prior to the intro- duction of the chlorine. The substance thus formed, known as “chloramine”, is a germicide equally as effective as free chlorine, and it has the advantage of not being used up in the oxidation of organic matter. It should be noted that there is delayed action in the process and water so treated should not be allowed to reach the consumers until four hours have elapsed. Excess Lime Method. The “excess lime” method is usually rather more expensive than chlorination. It has already been explained that lime is sometimes used to soften waters containing bicarbonates, and also to neutralise the acidity of waters contain- ing acids. If more lime is added than is required for either of these chemical reactions, it will be available for the destruction of bacteria. After sufficient time has been given for sterilisation, the excess lime is precipitated by blowing carbon dioxide through the water in a “carbonating chamber”. The resulting carbonate of lime is removed, dried and heated, to drive off the carbon dioxide, so that the lime is recovered for re-use. Removal of Colour. Finely powdered “activated carbon” is now being largely used for the elimination of colour and taste from water that cannot otherwise be freed of them. This is the only method of getting rid of the very offensive taste which results from the decomposition of algae. It is not a costly process, but the carbon must afterwards be removed by filtration and this adds to the complication of the treatment works. It is now most effect- ively used at the Southend Water Works, where objectionable taste would otherwise result from the excess lime process in a river water. Impounding and Service Reservoirs. There are many ways in which the water supply works for a town can be arranged, depend- ing on the levels of the land between the source of supply and the | » RIVER ———) XY) THE BUILDING—ITS WATER SUPPLY San FEMI SEMIS St) 191 192 THE BUILDING—ITS WATER SUPPLY point of distribution, and involving either gravitation or pumping systems. A few examples will show some of the many cases that occur in practice and give an idea of the relative positions of the units forming such a scheme. Thus Figs. 151 and 152 show diagrammatic sections through gravitation schemes, and Figs. 158 to 156 similar sections through pumping schemes. Thus in Fig. 151 the waters passing through a valley are held up by a dam, D., in an impounding reservoir, I.R., the outlet of which is controlled by a valve at the foot of a valve tower, W.T. It passes on to filter beds, F.B., and then into a service reservoir, 5.R., from which the distributing mains, D.M., pass to supply the town. A somewhat similar arrangement is shown in Fig. 152, but the levels permit of an elevated position for the service reservoir nearer to the town, which is a better arrangement. In Fig. 153 the supply is drawn from a river, and passes first into a settling or sedimentation reservoir, then on to filter beds, after which it goes to a pumping station, P.S. From there it is pumped up to an elevated service reservoir, the height of which gives the necessary pressure in the distributing mains; a pressure great enough to ensure the water being thrown over the tops of the houses in case of fire. In Fig. 154 the levels are such that the water has to be pumped up into the settling reservoir, from which it flows by gravitation to the filter beds and on to the service reservoir, the pressure in the distributing mains being provided by a second pumping station. Fig. 155 is a similar case, but the levels permit of the water reaching the settling reservoir by gravitation, Fig. 156 shows a somewhat similar case to Fig. 155, but the filters discharge into a main, leading to pumps which raise the water to an elevated service reservoir, from which the distributing main descends. If the supply is from a well or borehole the general principles of the arrangements of Figs. 154 or 155 could be used, but no sedimentation reservoir would be needed. If a high site is available near the town the water could be pumped direct from the well to the service reservoir, passing through pressure filters en route; thence to town by gravity. If no such site is available the water will still be pumped to the service reservoir through pressure filters and will have to be pumped again from there into the town mains. Different Types of Reservoir. We may next briefly consider these various forms of reservoir, their construction and capacity. The reservoirs necessary for the supply of towns with water include: (1) impounding or storage reservoirs; (2) compensation THE BUILDING—ITS WATER SUPPLY 1938 reservoirs, which are often rendered unnecessary by the special form of the impounding reservoir; (8) settling tanks, or sedi- mentation or depositing reservoirs; (4) service reservoirs. Impounding Reservoirs. Impounding reservoirs are
Key Takeaways
- Use roughing filters to remove large particles before slow sand or rapid gravity filters.
- Frequent cleaning is necessary for rapid filters due to high flow rates.
- Chlorine can be used as a sterilizing agent, but requires precise dosing and proper equipment.
- Excess lime method involves adding more lime than needed to destroy bacteria.
- Activated carbon can remove color and taste from water.
Practical Tips
- Always test the water for bacteria levels before relying solely on filtration methods.
- Install a chlorine dosing system if you have access to a reliable power source, as it is an effective sterilization method.
- Regularly clean your filters to maintain their efficiency and effectiveness.
- Use activated carbon in conjunction with other filtration methods to remove unwanted tastes and odors from water.
- Consider building or using a settling tank before filtering to reduce the workload on your primary filter.
Warnings & Risks
- Improper chlorination can lead to unpleasant tastes and potential plumbing corrosion.
- Excess lime method is more expensive than chlorination but effective for certain types of water.
- Rapid filters require constant monitoring due to their high flow rates and frequent cleaning needs.
- Domestic filters are not as efficient as large municipal works, so they should be used as a last resort.
Modern Application
While the specific techniques described in this chapter may differ from modern practices, the principles of water filtration remain crucial for survival. Modern systems have improved in efficiency and safety but still rely on similar methods like sedimentation, filtration, and sterilization. Understanding these historical processes can provide valuable context and a foundation for building or maintaining your own water purification system.
Frequently Asked Questions
Q: What is the difference between rapid gravity filters and slow sand filters?
Rapid gravity filters are designed to handle higher flow rates, ofte