Cantor lectures on bacterial purification of sewage / by Samuel Rideal.

  • Rideal, Samuel.
Date:
1899
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Licence: Public Domain Mark

Credit: Cantor lectures on bacterial purification of sewage / by Samuel Rideal. Source: Wellcome Collection.

Provider: This material has been provided by London School of Hygiene & Tropical Medicine Library & Archives Service. The original may be consulted at London School of Hygiene & Tropical Medicine Library & Archives Service.

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    14-3 times its volume of fully aerated water. This explains the comparative failure and frequent collapse of filter beds in large masses, especially if the fluid is a raw sewage or a merely screened or precipitated effluent with- out preliminary hydrolytic change, as with every 100,000 gallons of sewage, at least 50,000 gallons of air must be continuously supplied. Contrivances like fountains, cascades, and weirs can only raise the dissolved oxygen to the saturation point of about 7 cc. per litre, or NH3 + HNO2 = (NH-.) CHj COOH + HNOo = Amido-acetic acid. NH2 C2H8 + HNO2 = Ethylamine. The change is therefore accompanied by a great loss of nitrogen, and a disappearance of odour. It takes place in the resting-full period of filters, and causes disappearance of more nitrogen than carbon. Nitrification proper, or the production of nitrates, is due to one or more organisms capable of growing in culture solutions which are practically free from organic carbon. But, under natural circumstances, they act in succession to nitrous organisms, and in the presence of organic material, which they do not, however, by themselves decompose.* Some of the difficulties of the subject have been cleared up by Adeney's researches, who, by cultivation in known solutions, has eliminated disturbing factors. His conclusions are :— 1. In inorganic solutions, containing ammonia, nitrous organisms thrive, but nitric organisms gradually lose their vitality. 2. Nitrous organisms cannot oxidise nitrites to nitrates in inorganic solutions. 3. Nitric organisms thrive in inorganic solutions containing nitrites. 4. The presence of peaty or humous matter appears to preserve the vitality of nitric organisms during the fermenta- tion of ammonia, and establishes con- ditions whereby it is possible for the nitric organisms to thrive simultane- ously in the same solution as the nitrous organisms. In an effluent which is properly prepared and well-aerated, nitrification can often be encou- raged by seeding with a small quantity of a fertile garden soil. The conditions of nitrification have been often stated but may be recapitulated. • Winogradsky, Ccntr. Bakt., 1896, 2, ii. ,|is and 440. 700 gallons per 100,000; although useful, if simple, like the aerator at Exeter, they are quite inadequate. The nitrosification change is, however, very valuable in the second stage, as getting rid of the transition products, ammonia, amido- acids, and the amines by double decomposi- tion into water, or hydroxy-compounds (which are afterwards broken up by fermentation) and nitrogen gas. As simple instances we have :— 2H2O + N2 (OH) CH2 COOH + HiO 4- Na Glycolic acid. CsHsOH -f H2O + Nj Alconol. {a) In every case the formation of ammonia by some other organism precedes the appear- ance of nitrous or nitric acid. {b) Some fixed base must be present to com- bine with the acid formed. Therefore in a sewage farm, if the soil is devoid of lime it must be added. Ordinary sewage contains fixed alkali derived from washing soda, and any acid discharges are generally neutralised by this and by the free ammonia. E. Chuard found that nitrification may occur in an acid medium, but that it was very slow.* Hence in strong manufacturing effluents a treatment with lime may be necessary before nitrification will take place. (c) The solution must not be too strong, nor too alkaline. Warington found that a 12 per cent, solution of urine was the highest strength nitrifiable, and that the maximum alkalinity corresponded to 36-8 parts per 100,000 of N as ammonia carbonate, equal to 44*6 parts of ammonia. These are strengths which only under special circumstances would be ap- proached in sewage. In the runnings firom urinals, stables, &c., dilution would be necessary. {d) Darkness and free admission of air.f • Compies Rendus, cxiv., i8i. + At this point I may incidentally dr.-iw attention to a curious fact. In nearly ,a]l published analyses, the chloride in the effluent is slisflitly lower than that in the corresponding sewage. Muntz pointed out th.'it in iiitrification, bromides and iodides were oxidised to bromates and iodates. Chili saltpetre (nitrate of soda), which has been produced by n.atural nitrification, almost invariably contains chlorates and perchlorates, sometimes amounting to 7 per cent, of the former and 5 per cent, of the latter; they arc supposed to have been formed by bacteria. Dr. Tidy, some 20 years .ago, found a loss of chloride in waters running over aerating wooden shelves, and suggested that it might be due to the formation of chlorates. CJourn. Soc. Chem. Iiiii., Dec. 1898, p. 1160).