Sewer Air. Though not of material importance as regards bacterial treatment of sewage, this subject calls for some remark. For long it has been known that air polluted by sewage emanations is capable of giving rise to various degrees of ill-health. These chiefly affect two parts of the body; one is the throat, and the other is the alimentary canal. Irritation and inflammation may be set up in both by sewer air. Such conditions are in all probability produced by a lowering of the resistance and vitality of the tissues, and not by either a conveyance of bacteria in sewer air or any stimulating effect upon bacteria exercised by sewer air. What evidence we have is against such factors. (See p. 105.)
Several series of investigations have been made into the bacteriology of sewer air, amongst others by Uffelmann, Haldane, Laws, and Andrewes. From their labours we may formulate four simple conclusions:
1. The air of sewers contains very few micro-organisms indeed, sometimes not more than two organisms per litre (Haldane), and generally fewer than the outside air (Laws and Andrewes).
2. There is no relationship between the microbes contained in sewer air and those contained in sewage. Indeed, there is a marked difference which forms a contrast as striking as it is at first sight unexpected. The organisms isolated from sewer air are those commonly present in the open air. Micrococci and moulds predominate, whereas in sewage bacilli are most numerous. Liquefying bacteria, too, which are common in sewage, are extremely rare in sewer air. Bacillus coli communis, which occurs in sewage from 20,000 to 200,000 per cc., is altogether absent from sewer air.
3. Pathogenic organisms and those nearly allied to them are found in sewage, but absent in sewer air. Uffelmann isolated the Staphylococcus pyogenes aureus (one of the organisms of suppuration), but such a species is exceptional in sewer air. Hence, though sewer air is popularly held responsible for conveying diphtheria and all sorts of other virulent bacteria, there is up to the present no evidence of a substantial nature in support of such views. Sewer air neither conducts pathogenic organisms nor stimulates the virulence of such.
4. Lastly, only when there is splashing in the sewage, or when bubbles are bursting (Frankland), is it possible for sewage to part with its contained bacteria to the air of the sewer.
Whilst we cannot here enter more fully into an account of the bacteria found in sewage or of their functions, it is necessary to remark upon one distinguishing feature. A very large number of sewage bacteria are decomposing and denitrifying, that is to say, breakers down, by means of putrefaction, of organic compounds. The knowledge of this fact has recently been applied, in conjunction with oxidation, to the biological treatment of sewage. As this illustrates in a marked degree some of the facts we have dwelt upon in considering the bacteriology of soil, and as it is likely that the future will witness a still wider application of these same facts, it will be necessary to refer in some detail to the matter.
Hitherto there has been adopted one of four methods of treatment of sewage. In the first place, in towns situated on the coast the sewage has, by means of a conduit, been carried out to sea. It is clear that such a course, which is in itself open to criticism, is applicable to but few towns. In the second place, methods of chemical treatment have been practised. This has generally been of the nature of a "precipitation" process. Six to twelve grains of quicklime have been added to each gallon of sewage. The process is simple and cheap, but it does not remove the organic matter in solution. On the one hand, it does not produce a valuable manure; on the other, it fails to purify the effluent. A dozen other methods have been tried, but all based on the addition of chemical substances to precipitate or change the organic matter of the sewage. Electrolysis, too, has been proposed. The third mode adopted in the past has been that known as intermittent downward filtration. This may be defined as "the concentration of sewage at short intervals on an area of specially chosen porous ground, as small as will absorb and cleanse it, not excluding vegetation, but making the product of secondary importance" (Metropolitan Sewage Commission). The action is mechanical and biological, that is to say, due in part to nitrification by bacteria in the upper layers of soil. The fourth plan is that of irrigation, or "the distribution of sewage over a large surface of ordinary agricultural ground, having in view a maximum growth of vegetation (consistently with due purification) for the amount of sewage supplied." Like the former, there is biological influence at work here, though in a less degree. About one acre is required for every hundred persons in the population. These two latter modes are much to be preferred to chemical treatment, yet on account of space and management, as well as on account of the non-removal of the "sludge," their success has not been all that could be desired. Until comparatively recent times the above methods of treating sewage were the only ones available.
In 1881 it appears that M. Louis Mouras, of Vesoul (Haute Saône), published an account of a hermetically sealed, inodorous, and automatically discharging cesspool, in which sewage was anaërobically broken down by "the mysterious agents of fermentation." This is the first record we have of the newly applied treatment of sewage by simply allowing Nature to fulfil her function by means of bacteria. We shall most easily arrive at an appreciation of the recent developments of the process in England by describing the so called septic tank and cultivation beds.
