Treatment of sewage or sewage effluent with chlorine may be advocated for several reasons. These include disinfection of sewage in cases of emergency, destruction of pathogens or avoidance of unsightly growths where an effluent is discharged to a river or near bathing beaches, prevention of septicity in sewage before it reaches a treatment works, and the relief of ponding in trickling filters. Chlorination is much less generally advocated in this country than in America, where sewage is usually weaker and the dilution which it receives on discharge to rivers much greater than in Great Britain.Emergency chlorination was sometimes rendered necessary during the war and the chlorination of effluents discharged to British rivers is sometimes advocated in the interests of public health. It was considered advisable for these reasons to obtain data for the effect ofchlorine on samples ofEnglish sewage and to study the factors affecting destruction and aftergrowth of bacteria. Since it had been found (Allen, Blezard & Wheatland, 1946) that chlorinated effluents from sewage to which gas liquor had been admitted were particularly toxic to fish, special attention was devoted to a study of the bactericidal properties of such effluents. Houston (1910), as a result of experiments carried out for the Royal Commission on Sewage Disposal, concluded that for an effluent of good chemical quality the dose of chlorine required to ensure that no coliform organisms were detectable in 1 ml., after a period of contact of 10 hr., was between 1 and 10 p.p.m., and that the average doses required for periods of contact of 1 hr. and of 6 min. would be about 10 and 40 p.p.m. respectively.
PREVIOUS WORKThe conception of chlorine demand and the use of the o-tolidine test developed by Ellms & Hauser (1913) placed chlorination on a much sounder basis. Disinfection was found by various workers to be more effective when sufficient chlorine had been added to react with all substances capable of absorbing chlorine, and to leave residual chlorine detectable by its reaction with o-tolidine, Lea (1934) pointed out that chlorination of sewage may result in the formation of a wide range of derivatives, including chloramines and chlorinated proteins, different concentrations of which are required to give a colour with o-tolidine. For results to be significant the tests applied to sewage should, therefore, be rigidly standardized. Lea recommended the use of a more acidic reagent to ensure that the pH value of the alkaline sewage was reduced to the zone required for correct yellow colour, and a larger proportion of reagent to sewage in order to avoid the diffuse and transient colour which sometimes appeared with the weaker reagent.Tiedemann (1927) found, in studies on sewage chlorination at Huntington (U.S.A.), that doses of chlorine sufficient to give a colour with starch iodide, but not sufficient to give a colour with o-tolidine, reduced both the total count and the count of coliform bacteria by 98-99 % in 10 min., and by 99-9 % in 1 hr. When there was 0-2...