Volume 2

Survey of the river Tees.

  • Great Britain. Water Pollution Research Board.
Date:
1931-1937
Catalogue details

Licence: Public Domain Mark

Credit: Survey of the river Tees. Source: Wellcome Collection.

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    in a sealed bottle was also greater than when the diluent was sewage diluted with tap water (Chapter XVII). The decomposition of f-cresol and of spent still liquors in which the most toxic constituents are tar acids can be effected by the action of bacteria, and it is known that bacteria capable of causing this decompo- sition are present in sewage. It appears, however, that there is in the Estuary and not in diluted sewage some factor capable of accelerating the breakdown of phenolic substances. This acceleration does not seem to be due to inorganic salts nor to an optimum condition of hydrogen ion concentration. It is possible that the high rate of decomposition in the Estuary is due to the presence of a specially active bacterial flora. The amount of decomposition which occurs when tar acids are diluted with tap water containing sewage, whilst not so great as that obtained with Estuary water as diluent, is nevertheless considerable. It can hardly be doubted that the discharge of sewage into the Estuary accelerates the decomposition of tar acids. It is evident that, during the period of about 1 to 5 days for which tar acids may remain in the Estuary, a considerable reduction, if not a complete loss, of toxicity occurs. The presence in the Estuary of a large volume of water which remains there for several days tends therefore to reduce the general concentration of tar acids below the value estimated from the amount of dilution afforded by the new fresh and salt water entering the Estuary every day. DECOMPOSITION OF EFFLUENTS FROM GAS SCRUBBERS AND OF PURE CYANIDES In comparing the rate of decomposition of a substance under different conditions by observations of the change in its toxicity to fish, considerable varia- tions in the results may be expected, since the difference between the resistance of individual test fish to poisons may be large. Moreover, in solutions containing approximately equal concentrations of sewage the dissolved oxygen content may fall at a different rate, either because of small differences in the composition of the sewage used or because of inequalities in temperature; thus, since the toxicity of a direct poison, as determined by its effect on fish, is affected by the degree of oxygenation of the solution, a further source of variation in the results is intro- duced. The time taken by solutions of potassium cyanide in tap water with added sewage, stored under similar conditions, to become non-toxic is given in Table 87. The temperature during Series 1 was higher than during Series 2. From these figures it appears that the effect of a given factor can only be regarded as significant if it causes a larger variation than 2 days in 7 in the time taken by a solution of cyanide to become non-toxic. TABLE 87—WNo. of Days Required for Solutions of Potassium Cyanide in Tap Water containing Sewage to become Non-toxic Concentration of cyanide: 0:04 gm. (CN) per 100 litres Diluent : tap water containing 0-5 per cent. sewage Series 1. Mean temperature 6-3° C. Series 2. Mean temperature 5:4° C. 5 8 6 9 5 9 7 S 6 9 fi 8 Effect of Sewage The rate of decomposition of potassium cyanide in solutions containing varying concentrations of sewage is shown in Table 88. No significant change in the rate of decomposition of cyanide is brought about by the addition of sewage. The results of two similar series of experiments in which an effluent from coke oven gas scrubbers was used as the source of cyanide are summarised in Table 89. The initial concentration of effluent was 0-5 per cent. in each case; the only toxic constituent of the effluent in this concentration was the cyanide, which was present in a concentration of approximately 0-04 gm. per 100 litres. As in the case of pure cyanide solutions sewage does not accelerate the rate of decomposition of the effluent.