Water purification

“ In the purification of water containing only insignificant quantities of suspended matter, but a relatively large amount of unstable organic matter, it will be conceded at once that the chief factor in the purification is the nitrification produced by the bacteria in the upper layers of the sand. On the other hand, the purification by sand filters of a hypothetical water containing no organic matter, but only finely‐divided mineral matter in suspension, could take place only by the physical deposition of the particles upon the sand grains. ”

“ Chlorination is an invaluable adjunct to other forms of water purification and it is not improbable that, in the future, filter plants will be designed to remove æsthetic objections at the lowest possible cost and that chlorination will be relied upon for bacterial reduction. Chlorination is the simplest, most economical, and efficient process by which the removal of bacteria can be accomplished and there is no valid reason why it should not be used for that purpose. ”

“ The effluent which is turbid, putrescible and rich in organic matter cannot be discharged into streams with safety without further treatment, unless the volume of water is sufficient to complete the purification by dilution. ”

“ In order to ensure greater purification the sewage in the subsiding tanks is now usually mixed with certain chemical reagents, which, it is believed, have the effect not only of speedily precipitating the solid materials, but also carrying down injurious matters suspended in the sewage water, thus rendering it sufficiently pure to be discharged without risk to health into any watercourse. ”

“ Disposal by dilution in large streams of water is regarded as satisfactory in many places—where the water of the stream is not to be used for drinking or cooking—provided the flow of the stream is always copious enough to dilute and disperse the sewage so widely as to prevent putrefaction and substitute oxidation. For purification by bacterial action no single method is found adapted to all conditions. ”

“ However, absolute reliance cannot be placed upon natural purification of a bad water, as the objectionable organisms often have great resistive power. There is no perfectly pure water except that prepared in the chemical laboratory by distillation. All natural waters come in contact with the soil and air, and necessarily contain impurities proportional to the extent of their contamination. ”

“ If the water tables are low the beds are not underdrained. In artificial beds the size of the sand is important. While fine sand will give a more brilliant effluent than a coarser material, the sewage has to be applied in small doses with long periods of rest. The rate of purification is higher in coarse sand filters and the effluent while containing more bacteria is non-putrescible. ”

“ It must also be remembered that soils may exist which have no porosity whatever, and through which it is impossible for sewage to make its way. Such soils are not available for sewage purification, and, no matter how small the rate or how careful the method of application, such areas will fail to produce any practical purification. Soils like clay, peat, and fine water-deposited silt are of this sort. Clay soils may sometimes become pulverized by cultivation so that they will ultimately be able to take care of a moderate amount of sewage. ”

“ Long experience and accurate meteorological records may in some cases enable those in charge of chlorination plants to anticipate changes in the conditions of the water supply, but it is always preferable to provide a positive method of preventing sudden changes by using chlorination merely as an adjunct to other processes of purification. Unpurified waters that are objectionable on account of their bacterial content only are very rare, as the cause that produces the bacterial pollution usually produces other conditions that are equally objectionable though not so dangerous to health. ”

“ John Duncan Watson, of Birmingham, England, contends that the complete elimination 76of bacteria is prohibitive inasmuch as it is beyond the limits of the reasonable demands on the purse. Robert Spurr Weston, member of the American Society of Civil Engineers, at one time reminded an audience that the proper place to protect the water consumers against disease is at the water works and not at the sewage disposal plant. Authorities are in general agreed that sewage should be disposed of as the stream demands, and that local conditions should determine degree of purification required. ”

“ The next step is the partial purification of the sewage by means of a chemical process. When a town lies some distance up an estuary, as for example London, Glasgow, Rochester and many others, the dilution may be insufficient to prevent a nuisance, or the suspended matters may be deposited upon the foreshore to be uncovered at low water. The first stage of purification is then employed, namely, clarification in tanks. Practice varies with regard to tank capacity, but as a general rule it should be at least equal to, half a day's dry weather flow. ”

“ The reduction of organic matter in solution is considerable, averaging 17 percent.
This sludge at the bottom is a black muddy looking silt-like deposit containing about 60 percent of water and 5 percent of organic matter. Its value as a fertilizer is small.
27Although the amount of sludge is relatively small the accumulation requires removal occasionally, three or four times a year.
It is estimated that for every 1,000,000 gallons sewage there is about 3 cubic feet of dry matter. By chemical precipitation this would amount to from 20 to 25 cubic feet per 1,000,000 gallons. ”

“ And water in which there are no ashes, is saved by the covering bound upon it.
2. Every doubt implies cleanness in the heave-offering and cleanness in purification. Every reason for suspense in the heave-offering causes pouring away of the water in purification. If acts requiring legal cleanness be afterward performed, they are in suspense. Shallow water755 is clean for holy things, and the heave-offering and purification. R. Eleazar said, “trickling water756 is unclean for purification.” ”

“ Mere exposure to the air, even if accompanied by violent agitation, is comparatively powerless for the removal of polluting organic matter from water. Although, however, the flow of a river has thus but little effect in purifying the water by the oxidation of the dissolved organic matters, it has a most material influence in the removal by subsidence of a large proportion of the suspended impurities both organic and mineral, especially if the flow be sluggish in places. ”

“ All experiments with bacteria beds show that the objects for which they were intended, to abolish sludge, has been realized and that sewage can be purified without chemicals at a small cost, being but little more than that incurred by the labor in attending to, or supervising the filling and discharging, the filters, and that sewage purification can be carried on with little or no nuisance. ”

“ For who would deny that masonry or concrete tanks and the materials contained in the same are artificial products—i.e. products formed by man—and that land is a natural product—i.e. formed by nature—and that further the soil is the natural home of bacteria. Hence it must be perfectly clear, even to a casual observer, that the line of demarcation drawn by the Commissioners between all known systems of sewage purification is a correct and legitimate one, and that all objections to such a division are based on misconceptions. ”

“ The sludge, however, which we return and keep intimately mixed with the sewage at all times is perhaps the chief medium of purification, because that sludge is filled with microorganisms and nitrifying organisms, which really do the purification work, and that is the reason that it is called activated sludge, because it is so filled with the nitrifying organisms, and the more active the sludge is the more rapid and higher degrees of purification you secure. ”

“ Intermittent downward filtration may be used where there is a considerable area of sandy soil, and also where a high degree of purification is necessary.
Chemical precipitation does not require a large area for its operation and used alone does not give a high degree of purification. Where land and material for beds is expensive, and partial purification is sufficient, this system may be used.
The septic tank requires a small area. The odors are not offensive and cannot be noticed 100 feet away. The effluent may be discharged into small streams and soon loses its identity. ”

“ When considering natural and artificial sewage treatment, it ought to be borne in mind that in the natural treatment we have to deal with one treatment only, and that, in order to bring the results obtained from artificial processes up to the same standard, the artificial treatment ought to be supplemented by a treatment for the removal of nitrates from the effluent, and another for the removal of pathogenic micro-organisms, which means one treatment in natural, as against three separate treatments in artificial purification. ”

“ When sewage is treated by filtering through land or by broad irrigation the process is bacterial, bacteria or microbes in the soil converting injurious organic matter into innocuous mineral products. The typical process is one of nitrification. The novelty of recent methods is in utilising bacteria for the whole process of purification, and not only for its final stages. The object is, in fact, not as in chemical processes to arrest, but by confining the sewage in tanks to aid and hasten decomposition or putrefaction. ”

“ Passing now to the aerobic stage of sewage purification we find it universally admitted, that a good loam resting on very porous sand or gravel, affords the best medium for work by the oxygen-loving nitrifying organisms [Pg 6] when they are supplied with constantly moving liquid sewage, and given intermittent periods for the aeration of the pores of the soil. ”

“ To the surface of the earth we owe thus a purifying influence whose activity furnishes us vegetation and food on the one hand and preservation from disease on the other. This purifying power is not possessed by the deeper layers, and therefore the percolation of organic refuse into them from deep cesspools is wasteful to agriculture and dangerous to our storehouse of drinking water.
Even the surface of the soil when overloaded with sewage loses partially its power of purifying the organic matter. ”

“ It is convenient to divide sewage purification into two processes, the preliminary process and the final, or finishing process, and, while the preliminary process, in itself, never accomplishes purification, yet it is of considerable value in facilitating and increasing the rate and efficiency of that purification. The most common preliminary treatment is sedimentation, by which the larger solids in suspension are allowed to settle in a tank or tanks so that the filter beds later used are relieved from the accumulation of those deposits. ”

“ To obtain the highest efficiency the sewage must be applied intermittently and there must be a large percentage of voids in the sand.
Experiments made on especially prepared beds at Lawrence, Mass. show that by treating 85000 gallons per acre per day, 94 percent of the organic matter and 98 percent of the bacteria are removed, and by treating 60,000 gallons per acre per day, 97 to 99 percent of the organic matter may be removed. The above data shows that 100,000 may be successfully treated with a five foot bed of sand. ”

“ Where such a filtration area is made to form part of a sewage farm it acts as a safety valve, and enables the land and crops to have a rest when they do not require further irrigation; at the same time the process of purification is not interrupted. If open, porous land is available for sewage purification, and if it can be drained 6 ft. deep to a good free subsoil, so that the effluent can get readily away, we find that the sewage of from 600 to 700 people can be dealt with on each acre per annum with both good agricultural and sanitary results. ”

“ It may be done by discharging the tank effluent into a system of agricultural drains laid just below the surface of the ground, called sub-surface irrigation. It may be done by removing the top soil from a bed of sand placed by nature, and needing little except suitable surface distribution to insure the most efficient purification.
For a small plant, instead of a sand filter, for which the sand is found naturally in a suitable location, an artificial filter may be built by preparing an enclosure and carting in sand for filling. ”

“ In addition to this it must be understood that, owing to the great losses by evaporation and by growing plants, which are continually at work on sewage farms, especially during the summer months, when, as a rule, the flow of water in the brook that takes the effluent is smallest, the quantity of the effluent from the natural treatment is probably only from one-half to one-third that resulting from the artificial treatment, which is a point of very great importance. ”

“ Where the pollution is less, so that the dissolved oxygen contained in the raw water is sufficient for the oxidation of the organic matters, continuous filtration will give substantially as good results as intermittent filtration, and in other respects it has important advantages. ”

“ Intermittent filtration may be considered in those cases where it is proposed to use a water polluted entirely beyond the ordinary limits, and for waters containing large quantities of decomposable organic matters and microscopical organisms ”