THE BACTERIOLOGICAL TREATMENT OF SEWAGE.

Taranaki Daily News, Volume XXXXII, Issue 170, 26 June 1900, Page 4

THE BACTERIOLOGICAL TREATMENT OF SEWAGE.

The bacteriological methods of sewage disposal have been gradually evolved, in response to the necessity for finding simple and hygienic methods of destroying town refuse, and so preventing . the wholesale pollution of rivers which was formerly common in this and other countries. An ideal method of destroying sewage would involve the conversion of the putrescrible matters contained in it into simple inorganic substances, which could be either allowed to flow into the sea, or made use of as a manure for the sake of the nitrogenous substances present. In the case of towns situated on the banks of a river, the latter forms the natural carrier of the town's refuse, and this should be first brought into such a condition that it will neither cause destruction of the animal life of the river nor render the river water unfit for domestic use. These desiderata have never been fully met by any of the chemical methods of sewage purification so far introduced, and the latter may be regarded only as temporary methods of preventing serious pollution, to be abandoned in favor of a suitable process when one could be found. Bacteriological methods of decomposing refuse have now been under investigation for some considerable time, and much valuable information respecting the utility of such processes has been gained, although, . even now, it is necessary to carry out initial experiments in each locality in order to make such alterations in the general method as local conditions may require. Such initial experiments are now being carried out by the London County Council in their experimental ground at Barking. These methods of sewage disposal are carried out essentially in two stages, which may be conviently called the " liquefying " and the " nitrifying " stages. The liquefaction of the refuse involves ,the decomposition, into soluble bodies, of such substances as albumen, cellulose, starch, etc. These changes are brought about by a great variety of bacteria, which are, however, alike in the fact that they are only active, or at all events most active, in the absence of oxygen—i.e., they are anaerobic. As examples of these may be mentioned the following types:— Bacillus amylo bacter dissolves cellulose, forming marsh gas and carbonic acid gas. Bacillus putrificus coli liquifies albumen, forming ammonia gas. Bacterium sulphureum liquifies gelatine and casein, forming sulphuretted hydrogen, There are also present in this pre- , liminary treatment bacteria capable of converting ammonia into nitrous acid, so that some of the nitrogen of the albuminous material originally present passes out in the effluent in the form of nitrites.

This first stage of the process is carried out in various ways; thus, in I the Barking experiments, it takes place when the coke beds are filled with sewage. (Imp. Inst. Journ., vol. iv., p. 314.) In the Scott-Moncrieff process it occurs in the cultivation tanks, into which the sewage flows through a false bottom and remains for some time, being eventually drawn off by an outflow pipe at the top of the tank. The liquid produced in this preliminary treatment contains ammonia nitrites and simple albumenoid substances, which are easily oxidised, and are not putreseible, so that it might safely be run into a river without further treatment, where by the action of the oxygen dissolved in the river water, these albumenoid substances would be still further decomposed. It is, however, more satisfactory to expose it to the action of a new set of bacteria having the power of converting the ammonia into nitrates, and of completing the oxidation of the organic matter. This part of the process was at first a matter of some difficulty, the means usually adopted being to allow the effluent 'to pass slow through long J nitrifying channels, consisting of pipes containing coke impregnated with nitrifying organisms. This apparatus acted very well for some time, but eventually became useless. This was found to be due to lack of oxygen, and the " nitrifying " power was rapidly restored after a rest and free exposure to air. The most complete conversion into nitrates is probably obtained in the Scott-Moncrieff process, where the effluent is allowed to fall upon trays containing coke bearing nitrifying bacteria. These trays are arranged in series, and each is filled with effluent for six minutes and " rests " for a similar period. The following table of analysis made by Dr. Eideal shows the extent to which this change can exactly be

In other processes, as in the Barking experiments, an attempt is made to carry out both stages at one step. Tor this purpose the sewage is run into coke beds, which are allowed to remain full for some hours and then emptied and allowed to rest for a similar period. In such a method of working the final effluent is less satisfactory, as the following table shows:—

Objections have been raised to bacteriological methods of sewage purification on account of the great in-: crease in the number of bacteria which would occurr in river water into which such effluents might be emptied; but it has been pointed out by Dr. Bidtal (Soc. of Arts Journal, July 21, 1899) that the ordinary filtering process adopted by water companies would remove most of these, just as they do at present in the case of bacteria occurring naturally in river water, and further that experiments carried out by Dr. Cartwright Wood at Exeter on the patho-genicity of bacteriological effluents showed that the bacteria present were quite innocuous.—lmpend Institute Journal,.

carried:— Chlorine. si s j Albuminoid Ammonia. Nitrogen as Nitrite. Nitrogen as Nitrate. s § s?s> |l Effluent fromj tank Effluent from 1st tray ... Effluent from 3rd tray... Effluent from 6th tray ... Effluent from 7th tray ... Effluent from 9th tray... 9.0 9.0 8.5 7.75 7.5 7.5 12.5 10.5 5.0 1.6 .35 .25 1.6 1.25 .60 .15 .30 .60 .99 .78 .48 nil nil .12 .096 1.87 4.68 6.6 9.0 3.05 2.85 2.48 .51 .56 .394

Coke Bed Method. I. II. in. IV. Chlorine Nitrogene as nitrite Nitrogene as nitrate Total Oxidised nitrogen... 22.4 1.67 11.0 .95 12.8 .067 1.53 2.2 7. trace ,1.06 i.OG Tbay Method. I. II. III. Chlorine Nitrogene as nitrite ... Nitrogene as nitrate ... Total oxidised nitrogen 7.5 trace 9.0 9.0 6.4 .06 5.98 6.01 6.5 .034 4.34 4.37