Bacterial Water Purification

Abstract

THE twenty-seventh annual report of the State Board of Health of Massachusetts for 1895 has just been issued. This is the eighth year that the valuable experimental work of the now famous Lawrence Experiment Station, has been continued. Although no very remarkable novel features have been recorded in the practice of bacterial water purification, it is highly satisfactory to find that the previous important work of the station is fully confirmed by the investigations conducted during the past year. An interesting point to which attention is called is the tendency exhibited by sand-filters to increase in bacterial efficiency in proportion to their period of service. In support of this the working of the oldest experimental filter at the station, and one of those with the greatest effective size of sand grain, is cited. This filter in 1893, filtering at an average rate of 2,000,000 gallons per acre daily, had a bacterial efficiency equal to 96˙75 percent.; during 1894 its rate of filtration was 4,500,000 gallons per acre daily, and its bacterial efficiency reached 98˙97 per cent., whilst, in 1895, although working at approximately the same rate as in the previous year, its bacterial efficiency rose to 99˙57 per cent. This increased bacterial efficiency, caused by greater length of service period, was considerably more marked in the case of filters constructed of medium coarse or coarse sands than with those in which medium fine sand was employed. It would be out of place in these columns to discuss the various technical questions dealt with in the report, but there is one point which is very clearly brought out, and which is of particular interest in connection with the controversy which has recently arisen over the bacterial examinations of the London water-supply. An attempt has been made more than once to discount the value attaching to early bacterial examinations of the London waters, which first exhibited the efficiency of the purification processes employed, on the ground that the samples for investigation were not collected direct from the filters but from the delivery pipes. It has been contended that the numerical results obtained from samples drawn from the mains do not represent the bacterial efficiency of the purification processes in operation at the works, and this contention is based upon the hypothesis that the bacteria present in the effluent multiply in the pipes before delivery. The examinations made at the Lawrence Experiment Station show that there is no foundation whatever for this supposition. Thus in, the monthly averages of daily bacterial examinations made of the Lawrence water conducted over six months, we find the raw river water contained 7533 bacteria per cubic centimetre, the effluent taken direct from the filter 134, the reservoir outlet 119, and the samples taken from the City Hall tap 86 bacteria per cubic centimetre. These results are sufficiently striking and instructive, and require no further comment. Another point to which considerable interest attaches is the effect upon the total number of bacteria which appear upon a gelatine plate, produced by the time during which the latter is kept and the colonies counted. Thus a water-plate poured from raw river-water exhibited 913 colonies per cubic centimetre on the first day, after two days the number rose to 8613, after three days to 12,317, whilst after four days they numbered 15,017 per cubic centimetre. Similarly in a sample of the same water filtered, whilst only three colonies could be counted on the first day, 48 made their appearance after two days, 72 after three days, and 87 per cubic centimetre after four days. The bacteriological examination of water, it cannot be too frequently insisted upon, is surrounded with subtle pitfalls into which the unwary may very easily be decoyed, and if the method is to take its position as a scientific process, too much attention to the details upon which its accuracy depends cannot be expended. In conclusion, in the section devoted to bacteriological technique, we note the introduction of a new word. Hitherto we have spoken of plate-cultivating a given water, but this expression we find cut down to “plating” a water; as, however, the general practice is now to substitute dishes for plates, we shall probably be reduced to the ugly phraseology of “dishing” a water.