Chemical and bacteriological examination of muds from sources differing widely in the degree of pollution to which they were subject showed great differences in the contents of carbon, nitrogen and sulphide. These differences were not correlated with differences in the severity of faecal pollution. The amount of organic matter available for growth of micro-organisms in the mud of different depths was not reflected in the figures for organic carbon. A convenient index of this factor was obtained by measuring the volume of gas evolved during anaerobic digestion over a prolonged period of incubation. The rate of evolution was increased by the addition of an inoculum of digested sludge from a sewage works.Sulphate-reducing bacteria appeared to be of two different types. In samples of mud from fresh-water lakes much higher counts were usually obtained in a medium containing comparatively low concentrations of inorganic salts and of lactate than in a medium containing much higher concentrations of these constituents. In samples from locations where conditions were more saline the reverse was usually true.Counts of Bact. coli and of Strep, faecalis together probably constitute the best index of faecal pollution in the examination of samples of mud. These organisms are, however, largely confined to the surface layers.

It is well known that successive stages of treatment at a sewage works reduce the quantity of suspended solids, the content of organic matter, and the biochemical oxygen demand of sewage, so that the polluting strength of the final effluent is a small fraction of that of the sewage received at the works.Much less is known, however, about the effect of treatment on the bacterial flora of sewage. An investigation has therefore been made at two sewage works at each of which treatment in percolating filters and humus tanks is followed by treatment in a sand filter or an anthracite filter. Samples taken at various stages of treatment from both works over a period of several months were submitted to bacteriological examination to find the plate count on nutrient agar and the counts of coli-aerogenes bacteria, of faecal Bacterium coli and of Streptococcus faecali8. A large number of pure cultiires of coliform bacteria and of streptococci were isolated and their characters were determined. Chemical analyses were made on the same samples as were used for bacteriological examination.The data as a whole gave a general picture of the extent to which treatment at the works reduced the numbers of bacteria in different groups, of the relative parts played by percolating filters and by sand filters in this process, and of the numbers of bacteria discharged to a river with a final treated effluent. PREVIOUS WORKOne of the earliest processes of sewage treatment consisted in applying settled sewage intermittently to slow sand filters. Early work at the Lawrence Experiment Station (Massachusetts State Board of Health, 1887Health, -1915 established the fact that if the interval between doses was sufficiently long to allow the bed to drain and dry on each occasion, intermittent filtration through fine sand yielded an effluent in which the bacterial count was a small fraction of that of the settled sewage applied to the bed. Thus a filter containing coarse clean mortar sand (effective size 0-48 mm.), 5 ft. deep, which consistently received sewage at a rate of 74,000 gallons*/acre/day removed on an average 94 % of the organic matter and 98 % of the bacteria.A similar filter with finer sand (effective size 0'06 mm.) treated 43,000 gallons/acre/day and removed 97.5 % of the organic matter and 99-99 % of the bacteria from sewage.As with slow sand filters used in the treatment of water, the efficacy of these filters depends largely on the presence of a biological film which reduces the volume of the interstices between the grains of sand and at the same time removes organic matter in the water by using it as a source of food for growth and energy for the micro-organisms in the film.

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...

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