Chemicals Used.

For the purpose of precipitating the organic matter in the sewage several chemical substances are employed, depending chiefly on the nature of the sewage to be dealt with. These include ferrozone, copperas, lime, sulphuric acid, terricsulphate, alumina-ferric (a trade name for iron and alumina), either alone, or in combination with one or more. For domestic sewage of average strength, alumina-ferric, in quantities varying from three to ten grains per gallon of sewage, and upwards, is used in by far the majority of works where chemical precipitants are used. It is very simple in its method of employment, because a given quantity can be added to the incoming sewage, and allowed to dissolve in it.

This compound is also used in combination with other chemical substances, especially where a good deal of liquid wastes from different trade and manufacturing process has to be dealt with. Of some nineteen installations under observation on behalf of the Fifth Royal Commission, alumina-ferric was used alone as the precipitating agent at nine of them. At five others, in combination with lime; at another, in company with sulphuric acid; at another (Kingston-on-Thames) it was used as part of Sillar's A.B.C. process, with alum, blood, charcoal, and clay. The clay used in this method is of a special white kind. Alumina-ferric, used alone for precipitating purposes, acts in a variety of ways with different

sewages.

The percentage reduction of suspended matter in the crude sewage is affected by the addition of different chemical precipitants.

At Horfield, near Bristol, where the sewage is domestic in character, and the water supply a rather hard one, aluminaferric produces a percentage of ninety-three. At Normanton, on the other hand, it only amounted to sixty-seven per cent., with a slop-water sewage, the water supply being a soft moorland one.

As regards alumina-ferric, mixed with lime, the opinion of the same Commission, formed from experiments conducted at Dorking with a sewage of average strength, is that better results are obtained from the combination than when alumina is used alone.

Lime, although not in itself very valuable as a precipitant, is useful in neutralising acids present in the sewage; otherwise it has but little to recommend it, the results being inferior to other chemicals, and owing to the skill required

in mixing accurately to fix the quantity of water, the cost of superintending its use is high.

An interesting feature as regards chemical precipitants, and the use of local means as far as possible, is afforded at Buxton, where the local natural iron water is employed with milk of lime.

The A.B.C. process, carried on at Kingston-on-Thames, in which a combination of alumina-ferric, alum, blood, charcoal, and clay is used, and which was spoken of by the Fifth Royal Commission as "undoubtedly a very efficient form of chemical precipitation," produced a higher percentage in the reduction of the suspended solids, as the result of precipitation in the tanks, than from the employment of other chemicals under observation, viz., as much as ninety-five per cent.

With different classes of sewage various kinds of chemical precipitants have to be used; and this is so even with sewages which are similar in character, where the circumstances of each case are different. The addition of large quantities of liquid trade and manufacturing waste calls for special kinds of precipitating agents, or for some modification in the use of others. At Bradford, for instance, sulphuric acid is added to the sewage, which has large quantities of woolscouring refuse turned into it, for the purpose of separating the fat; this is afterwards extracted from the sludge. another case, at Burton-on-Trent, large quantities of lime are added to deal with the very large quantities of brewery

wastes.

Cost of Chemicals.

In

Chemical substances, used as precipitants, vary very much in price in different parts of the country, a wide difference being found in the cost of the same precipitants. Aluminaferric, for instance, has cost on an average £5 6s. per million gallons of sewage at Withnell; £2 2s. at Guildford, and 1 5s. at Normanton.

Alumina, used in combination with lime, costs only about 12s. 8d. at Friern Barnet.

In each of the above cases the sewage dealt with is very strong in character, and the calculation is based on the dryweather flow.

At Chorley, where the alumina-ferric is made in the works, the cost is only £1 for the quantity added to each million gallons.

At Kingston-on-Thames the A.B.C. process costs £3 45., and the natural iron water, combined with milk of lime, used at Buxton, costs only 4s. 6d. for the same quantity.

CHAPTER XVIII.

Settlement of the Suspended Solids by Sedimentation.

Tanks of the same kind, and constructed in a similar manner to those used for chemical precipitation, are used for the purpose of attaining the settlement of the suspended organic solids by sedimentation. All tanks so used are, in reality, sedimentation ones, because, although the methods employed are different, the object is the same the elimination of as large a proportion as possible of the suspended solids in the crude water-borne sewage, and at the same time the production of as good a top water, or tank effluent, as the particular process will produce, according to the nature of the sewage. Whatever method is employed, solid matter, or sludge, as it is called, settles to the bottom of the tanks. This has to be removed at varying intervals, depending on the means employed to ensure the settlement of the solids, the composition of the particular sewage dealt with, the length of time intervening between the cleansing of the tanks, the nature of the matters in suspension, and the shape of the tanks.

The principal methods employed to produce this sedimentation process are:-Continuous flow settlement with chemicals; the same without the chemicals; quiescent settlement, with and without chemicals; and septic tank treatment.

As regards quiescent settlement, no definite rule can be laid down as to the length of time that should be occupied in dealing with the suspended solids; but with some mixed sewages, such as those containing wool-scouring liquors, two or three hours have proved sufficient to remove a large proportion of the solids entering from the tanks with the crude sewage.

Table V. gives some approximate details as regards the time needed in the tanks to produce sedimentation, the approximate cost of the tanks, and of "settling" 1,000,000 gallons of sewage.*

# See p 70.

CHAPTER XIX.

The Septic and other Bacterial Systems of Sewage

Treatment.

These methods are the antithesis of the chemical processes, and are the most recently discovered. As we have noticed in the employment of chemicals for the preliminary process of facilitating the precipitation of the suspended solids, in the crude water-borne sewage, the underlying ideas are to effect such settlement and arrest decomposition in the sewage matter, whilst, at the same time, preserving its manurial value as far as possible; destroying a good many of the pathogenic (disease-producing) organisms; preventing secondary decomposition in the tank effluents; and, as far as can be ensured, the prevention of nuisances on discharge from the tank, and subsequently in the final effluent on its discharge from the works. By the bacterial treatment, however, the decomposition and putrefaction of the sewage is encouraged.

The Use of Natural Agents.

For the purpose of ensuring liquefaction in the crude sewage when it enters the septic tanks, natural agents, in the shape of microscopic organisms, bacteria or microbes, as they are more commonly called, are employed.

These minute forms of life were discovered by the eminent French scientist, Professor Pasteur, and it is from him they obtained the name of microbes, from two Greek words, micros, small, and bios, life. A good deal of controversy has been waged around the question as to whether these organisms are descended from parents of exactly the same species as themselves, or are the result of spontaneous generation. The writer favours the former theory. The work of these microbes is exceedingly interesting, and they play an all-important part in Nature's marvellous economy. Some exercise exceedingly harmful functions. Of these, the most dangerous are the specific organisms of different infectious diseases, each of which has its particular germ. Thus the tubercle bacillus is the cause of tuberculosis, and the bacillus typhosis of typhoid, or enteric fever. Decay, as a process in Nature, takes place in almost everything around

us.

As

To take vegetable life as an illustration. The process of decay in this case is caused by micro-organisms, by means of which decomposition goes on, and vegetable matter is prepared and adapted as the plant food for another year; thus Nature makes use of the same food again and again. Her agents appointed to prepare this refuse matter for plant food are particular species of organisms, so minute that the most powerful microscope is needed to detect them. The chemical composition is altered and simplified by the microbes which carry out this work, by their feeding on the vegetable matter which in due course they excrete. Although the process is by no means thoroughly understood, sufficient is known to determine that several species are employed to carry out this work, and that the life products of one class usually serve as the food of another, each species being killed or poisoned by its own products. Thus to carry on this work several varieties of bacteria are employed, each performing a definite part, only to succumb to its microbic successor. regards sewage purification, the latest discovery has been. that, given suitable conditions, by no means at present thoroughly understood, aided also by suitable apparatus, and possibly by other low forms of life, such as worms and larvæ, micro-organisms can be relied upon to carry out some portion, at all events, of the work. That bacteria could be utilised in any way for the purification of sewage was discovered as the result of investigations by certain French scientists some twenty-five years ago; but it was not until some dozen years since that it first occurred to Mr. Donald Cameron, then City Surveyor of Exeter, to utilise certain definite species for the purpose. When it was suggested the idea was received with considerable scepticism in sewage disposal circles, because micro-organisms had, up to that time, been looked upon almost solely as the cause of various infectious diseases, and also to the fact that their employment was a complete reversal of the methods which had been hitherto followed under chemical treatment.

The idea underlying the employment of the bacteria. present in the sewage and their life products was that the organic solids in suspension in the sewage could be

digested" by passing them through a sealed tank, in which putrefactive organisms were encouraged to multiply, and as the system followed the lines employed to bring about decomposition in vegetable matters, as we have noticed above, it seemed a very suitable thing to utilise the natural agencies as far as possible.