LONDON WATER SUPPLY-FILTRATION.
[Specially written for FIRE AND WATER).
THE whole of London is built upon sands and gravels belonging to the drift period, which mark the ancient bed of a river or estuary of much greater magnitude than the Thames of to-day. The average breadth of this sand deposit may be taken as two miles, and it is superimposed directly upon the bed of London clay, which extends in a cup-like form beneath the whole London area from Willesden and Islington on the north, to Wandsworth, Camberwell, and Clapham on the south, and eastward and westward to Woolwich and Hammersmith respectively;or roughly, at an underestimate, 12×8 (96) square miles. This I .ondon clay covers varying deposits of sands and pebbles, which rest upon the chalk. For our purposes this latter may be considered the basic rock of the London district, as being the bed whence are derived all those supplies of water to which filtration is not an absolute necessity. The depth of the different strata varies, of course,at many points in the area to which we have referred. Thus at Mile End the made ground or alluvium is given at 20 1-2 feet and the London clay at 84 1-2 feet, while at Kentish Town the alluvium is “ nil” and the depth of London clay 536 feet. It would serve little purpose to give more detail of the geological structure of the basin,and one specimen, containing the above figures for Mile End, may suffice. Woolwich or Thanet sand (fine) follows the clay to a depth of 41 feet; loams and pebbles, 55 feetjafter which comes the chalk. 654 feet in thickness. The so-called alluvium or gravel deposits above the clay may be thus subdivided. Bagshot sand (partial), 210 feet; pebble gravel (partial), 20 feet; glacial gravel, too feet; gravel sand, 20 feet; loam or brick earth, 30 feet; river deposits, e., silt, peat, and marsh clay, 40 feet. These figures, however, are the “probable maxima” as estimated at the Royal Museum of Geology, and may be gauged by the item fot London clay,. e., 450 feet; whereas, as shown above, borings have found that stratum to be as low as 236 feet and 84 1-2 feet respectively. The London clay is acceptedly the bed of a tropic or subtropic ocean, it being evident from the fossils it contains that, after the deposits of the sands above noted as Woolwich (Thanet), loams and pebbles, which point to an estuarine condition, a vast, gradual, and enduring depression must have occurred to secure the regular and even deposit of clay bearing subtropical fossils which exists. The “ Handbook” of the above museum says that these fossils comprehend great numbers of kinds that must have existed in a fairly warm and deep ocean.
No uniformity would appear to have been attempted, or even to have been arrived at by “ the evolution of the fittest ” in the constitution of the filter-beds actually in use by the London companies. This would point to some special variations in the requirements of the water drawn from the Thames at different spots. It may be convenient to tabulate these data.
They are arranged in regular order under the following heads : Name of company; number of beds ; area of beds; ands—Thames, Harwich, Barnes, unclassified; hoggin; shells ; gravel—fine, coarse ; boulders:
Chelsea, 7, 6 3-4, 4 feet 6 inches, —, —, —, —, 3 inches, 3 feet, 3 inches, —. — ; East London, 34, 3t, —, —, —, 2 feet, 6 inches,—, —, 1 foot. —; Grand Junction,17, 21 3-4,—, 2 feet, 6 inches, —, —, 6 inches, —, 9 inches,9 inches, : foot; Grand Junction (New), —, —, —, 2 feet, —,—, —, 6 inches, 3 inches; Lambeth, 12, 12 1-4, 3 feet, —, —, —, —, 1 foot. -, 3 feet. – ; New, 20, 16 1-2. -, -, 2 feet. 3 Inches, —, —, 3 feet, 4 Inches, A* — ; West Middlesex, la, 15, —, 1 foot, q inches, 1 foot, —, —, —, 1 foot, B*—; Southwarkand Vauxhall, 18,20 1-2, —, 3 feet, —, —, 1 foot, —, 9 inches, 9 inches. —.
The variation in the composition of the seven sets of filterbeds dealing with Thames water only seems unaccountable; but, as it appears by the analyst’s reports to be justified by the results, there is doubtless some special reason in each case, which is not obvious.
1 he question of the mechanical and chemical effect of filtration upon suspended impurities would appear to be pretty well decided; but the isolation and numeration of colonies of bacteria, enabling the degree of purity of any water before and after filtration to be compared with any approach to accuracy, is only, says Dr. Frankland, the outcome of the recent discoveries of Dr. Koch. The method of this savant accomplished the isolation of each individual microbe, and further placed it in conditions favorable for development, which it was found took place with such amazing rapidity that around one solitary microbe, a visible colony of innumerable members, comparable to London itself for population, sprang up in from two to three days. By operating upon a known volume of water, say, a cubic centimetre, the number of separate organisms before and after filtration can now be determined with quantitative exactness. The attention of experts has been of late specially directed to this point, and the result of simple sand-filtration upon bacteria is found to be simply astounding. Thus a single drop of raw Thames water on its way to the filters was found to contain nearly 3,000 separate living organisms; but it issued from the beds so cleansed that only two or three such organisms were detected in a single drop—and sometimes it would be absolutely sterile. In fact, the purification of Thames water, bacteriologically, has been so perfected, that, although the water from the Kent chalk wells (being before pumping bacteriologically pure) absorbs a very small percentage of organisms before being delivered, the Thames water under the present stringent system of filtration is very ltttle inferior in point of purity. It is, of course, a matter of very great congratulation to this vast community that the sources of so vital a matter as their water supplye,, the various rivers and streams emptying into the Thames bed, have their origin in the best water-bearing strata in the kingdom, namely,the chalk, oolite, and lower green sand,whence, again quoting Dr. Frankland, they issue “ in copious springs of unsurpassed purity. For dietetic purposes there is no better water than the underground water of the Thames valley.”
Thus since, according to experts, the supply appears fully eqllal t0 meetinS a11 possible requirements of this metropolis for fift years to come, after making allowance for an improb able increase of population in the same ratio as during the half century expired, the magnificent schemes of the present county council to fetch water for London from the middle of Wales, a distance of 150 miles, appear to some to be wholly unnecessary for the efficiency of the supply.
A Incresing in coarsnes towards the bottom. 8 Screened to different sizes and arranged in layers.
It may be noted in this connection, that, although, as has been already reported, the government has introduced a comrehensive measure, dealing with the whole of the metropolitan companies, the London County Council, at a recent meeting, reaffirmed its intention of pursuing the initial experiments for its enterprise.
Accompanying this article are traces of a plan of the filtering beds of the Southwark and Vauxhall Company; a tracing of sectional drawing of one such bed; a view of one filtering bed showing the Harwich sand, gravel, and hoggin resting upon the drain pipes; and a general view of the same company’s works at Hampton, showing the operation of skimming the surface of the filtering medium. These illustrations, which must prove of considerable interest to our readers, have been supplied by the courtesy of Mr. I. W. Retler, the chief engineer to the company, who has kindly lent them for reproduction in our pages.
The basis of the beds is 9 inches of concrete, on which follow the materials as above. The water is admitted from the river to a depth of 4 feet and filters to the conduit, 3 feet by 2 feet 9 inches, whence it is pumped by the engines. .The cleansing of the beds takes place on an average once a month. The process of cleansing is as follows: The top surface, to a depth of 3-8 of an inch, which includes deposited mud, and other impurities, is totally removed and rewashed. After an interval, a second 3-8 of an inch is removed, and this is continued periodically until 5 or 6 inches of sand are taken off. When the bed has been skimmed thus to the extent considered advisable, about 5 inches of the remaining sand-bed is removed, in order to preclude the possibility of any organisms or impurities remaining in the sand, and fresh material is laid down to the maximum original thickness. The impure sand is not thrown away, but is thoroughly cleansed, and rendered perfectly available for future use.