IMPROVEMENTS IN FILTERING WATER ON A LARGE SCALE
Year: 1856

page_text: LETTERS PATENT to Charles Frederic Vasserot, of the firm Wells and
Vasserot, of 45, Essex Street, Strand, London, Patent Agents, for the
Invention of "IMPROVEMENTS IN FILTERING WATER ON A LARGE SCALE."-
A communication from Henri Darcy, of Paris, General Inspector of
High Roads, Officer of the Legion of Honour, and Knight of the Order
of the King Leopold.
Sealed the 17th March 1857, and dated the 19th September 1856.
PROVISIONAL SPECIFICATION left by the said Charles Frederic Vasserot
at the Office of the Commissioners of Patents, with his Petition, on the
19th September 1856.
I, CHARLES FREDERIC VASSEROT, of the firm Wells and Vasserot, of 45, Essex
Street, Strand, London, Patent Agents, do hereby declare the nature of the
said Invention for "IMPROVEMENTS IN FILTERING WATER ON A LARGE SCALE," &
communication from Henri Darcy, of Paris, General Inspector of High Roads,
Officer of the Legion of Honour, and Knight of the Order of the King
Leopold, to be as follows, and consists of an apparatus, made either of sheet
iron or of brick, having at its bottom a filter of two beds, the lower bed com-
posed of gravel, and the upper bed of fine sand; the gravel bed shall have
for its base a kind of skimmer, made of thick boards or of iron, perforated
with a great number of small holes. Those thick boards or iron are supported
by iron circular bands placed around the bottom of the apparatus. The inside

page_text: of the bottom of the apparatus is made slightly concave, and in the centre is
placed the pipe which receives the filtered water to convey it to the reservoir,
or otherwise. The feeding of the apparatus may be accomplished by a pipe,
which pours the water into it about a yard above the filter. This feeding pipe
is in connexion with the discharge pipe, for the purpose hereafter mentioned,
but the communication is intercepted by a cock.
The above disposition presents a double advantage; 1stly, in allowing the
filter to be cleansed by an upward current, & for performing this it is only
necessary to close the feed pipe, and open the communication between it and
the discharge pipe, when the water enters the apparatus through the said
discharge pipe; and, 2ndly, in allowing the filter to be put to work without
guttering it by the fall of the water on its surface, and also to send away the
air that the filter may contain. To obtain this result it is only necessary to
dispose everything as if the filter were to be cleansed by the said upward
current, and when the water has reached the feeding pipe the communication
between it and the delivery pipe is to be closed, and the cock of the feeding
pipe opened. A vertical pipe is placed down the inside of the apparatus, and
bent horizontally above the level of the filter, passing through the side of the
apparatus for the purpose of giving a free passage to the wash that the feeding
pipe may introduce.
For emptying the apparatus to the level of the filter, four or more hori-
zontal orifices shall be disposed at the distance of half a radius from the
centre, being the mouths of four pipes, which passing through the apparatus
shall be adjusted to a circular pipe closed with a cock. The small superficial
extent of the filtering apparatus allow it to be cleansed without much difficulty.
The one or the other of the following processes may be employed, or the two
simultaneously.
First process. If instead of feeding the water in the usual manner it is
directed tangentially against the inside of the apparatus, it will drag along all
the liquid matter, which shall by that means receive a perpetual rotary motion,
keeping always the mud in constant suspension on the surface of the filter
without adhering to it, and without taking the slightest consistence. A current
of water, as the one just described, would suffice to give to the whole mass
of water a rotary motion superior in speed to that required for keeping the
mud in suspension; it will consequently be necessary to regulate and not
surpass the limit required, since the fine sand has a speed of six inches per
second, and that it is essential not to overthrow the surface of the filter. It is
very easy to moderate the action of the speed of the water when leaving the
pipe, if it were greater than required, or this rotary motion could be given by

page_text: mechanical means, since there is always a machine at command. If by pro-
longing the feeding pipe towards the centre of the apparatus the rotary motion
of the water should surpass the suitable speed for the stability of the sand, the
result would be, that the surface of the sand would take little by little the
form of a cone, very slightly inclined, the top of which will be at the centre
of the apparatus, which would not be detrimental to filtering. It is certain
that if (without stopping the working of the apparatus) there is taken away
a portion of the water from the surface of the filter, it will remove the
greater part of the muddy deposit which, by reason of its density and the
feeble rotary motion of the water, shall occupy principally the lower part
of the apparatus. To accomplish this, it will be only necessary to open the
cock of the before-mentioned circular pipe which is in connexion with the
four pipes, and the said water containing the muddy deposit will be discharged.
(Experience will indicate the time to keep open the cock.) If it were judged
necessary, it would be easy during the discharge of the muddy deposit to
turn round on the surface of the filter a flexible broom, of a length equal to
the radius of the apparatus. This broom should be fixed to a horizontal bar,
adjusted to a vertical rod reposing on a socket, & which rod should be moved
from above the apparatus.
Second process. It is again possible to employ the mechanical power for
giving a constant rotary motion to the said broom. This broom should be
made either similar to the street-sweeping machines, or composed of a series
of moveable and independent rakes, adjusted to a horizontal bar. These rakes
should be kept in position by means of springs, sufficiently strong to contend
against the influence of the water, but to allow of their moving when they
meet with an obstacle on the surface of the bed of sand. This broom shall
always keep the mud in suspension, which can be taken away from the
apparatus in the manner above described.
Independently of these processes, to prevent the encumbrance and solidi-
fication of the mud, it will be, however, necessary to cleanse the filter (at
intervals experience will show) by means of the upward current before
mentioned, and also to take away a small portion of the bed of sand which,
notwithstanding the care & processes above mentioned, will always end by
being spoiled by the mud.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Charles Frederic Vasserot in the Great Seal Patent Office on
the 19th March 1857.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, CHARLES
FREDERIC VASSEROT, of the firm Wells and Vasserot, of 45, Essex Street,
Strand, London, Patent Agents, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Nineteenth day of September, in the year of our Lord
One thousand eight hundred and fifty-six, in the twentieth year of Her reign,
did, for Herself, Her heirs and successors, give and grant unto me, the said
Charles Frederic Vasserot, Her special license that I, the said Charles Frederic
Vasserot, my executors, administrators, and assigns, or such others as I, the
said Charles Frederic Vasserot, my executors, administrators, and assigns,
should at any time agree with, and no others, from time to time and at all
times thereafter during the term therein expressed, should and lawfully
might make, use, exercise, and vend, within the United Kingdom of Great
Britain and Ireland, the Channel Islands, and Isle of Man, an Invention for
"IMPROVEMENTS IN FILTERING WATER ON A LARGE SCALE," a communication
from Henri Darcy, of Paris, General Inspector of High Roads, Officer of the
Legion of Honour, and Knight of the Order of the King Leopold, upon
the condition (amongst others) that I, the said Charles Frederic Vasserot,
by an instrument in writing under my hand and seal, should particularly
describe and ascertain the nature of the said Invention, and in what manner
the same was to be performed, and cause the same to be filed in the Great
Seal Patent Office within six calendar months next and immediately after
the date of the said Letters Patent.
NOW KNOW YE, that I, the said Charles Frederic Vasserot, do hereby
declare the nature of the said Invention, and in what manner the same is
to be performed, to be particularly described and ascertained in and by the
following statement :-
The large filters employed in this country for supplying water to towns, &c.
produce on an average in 24 hours only 4 cubic yards of water to every
square yard of the filter, it is therefore necessary to have very extensive
establishments to filter sufficient water. From the results of experiments
made it is found there are two ways or means to prevent this inconvenience,
viz., by augmenting the head of water upon the filter, or by diminishing the
pressure under the filter. The application of these results allow the surface
of filters to be considerably reduced, in fact, the height of the water on the

page_text: filter can be increased or the pressure under the filter diminished, and the
thickness of the supporting and filtering beds made less. It is useless to give
to the supporting bed a thickness of more than 10 to 20 centimetres, and for
the filtering bed a thickness of 30 centimetres is more than sufficient for
obtaining the desired result. I shall first mention and describe the mode of
filtering, and an apparatus for the same, by augmenting the head of water
upon the filter, as well as the means, equal new, of cleansing the said
apparatus.
Calling K a co-efficient, depending on the permeability of the sand employed,
S, the surface of the filter, e, its thickness, H, the height of water on the filter,
the discharge would be (H+e). If the pressure under the filter were less
than the atmospheric pressure (P-H.) for instance, the discharge would be
(H+Ho+e).
The co-efficient K will be determined by means of experiments made in
Paris with the Fouvielle's filters.
Water-works of the Arcade.-A filter of a surface of one square metre
produces under the charge of 15 metres 984 cubic metres in 24 hours. The
filter is composed of pounded gritstone, 25 centimetres thick, and an equal
supporting bed; it has consequently,
981= K
15.50
0.50
Here the thickness of fine sand or pounded gritstone is a little more.
Filter of Marseilles, produces per square metre of filter 13 cubic metres in
24 hours, with a charge of 40 centimetres. The total thickness of the beds
is 80 centimetres, of which the filtering bed has 38 centimetres; it has
+1..

page_text: The accompanying Drawing fully illustrates the process of filtering by
augmenting the head of water upon the filter; in which Fig. 1 is a vertical
section; Fig. 2 is a part section at X, X, and part plan with filter removed;
Figs. 3 & 4 are enlarged parts of the broom or cleansing machine.
Supposing it were necessary to filter the quantity of water required for a
town of 100,000 inhabitants, at an average of 127 quarts per person, the
quantity required to be filtered would be upwards of 15,000 cubic metres in
24 hours, and this would be obtained by making, either of sheet iron or
bricklayers' work, a filtering apparatus or tub A, 730 metres in height, and
7 metres inside diameter; and at the bottom of the tub a filter should be
made composed of two beds; the first, a filtering bed of fine sand, 30 cen-
timetres in thickness; and the second, a supporting bed of gravel, 10
centimetres in thickness. The gravel should be smaller as it meets the bed
of sand. The supporting bed shall have for its base a kind of skimmer a,
made of thick boards, or of iron, perforated with a great number of small
holes. The said skimmer shall be supported by iron circular bands, (see
Fig. 2,) placed circularly at the bottom of the apparatus, at a distance of
about one metre from each other; the bands of iron shall be supported and
fastened to cast-iron brackets or bearers of about 10 centimetres in height.
The inside of the bottom of the apparatus is made slightly concave, and in
the centre is placed the pipe e, which receives the filtered water, to convey it
to the reservoir or well D. The said skimmer a may (if desired) be composed
of perforated tiles, reposing on small circular brick walls, and made in such
a manner as to let the filtered waters have free access to the pipe e. The
feeding of the apparatus may be accomplished by means of the pipe chaving
a valve or sluice R, which pours the water into it, about a metre above the
filter. This feeding pipe c is in connexion with the discharge pipe e, for
the purpose hereafter mentioned, but the communication is intercepted by a
valve or cock R'. The above disposition presents a double advantage; 1stly,
in allowing the filter to be cleansed by an upward current, and for performing
this it is only necessary to close the cock or valve R of the feed pipe c and
the sluice i, and open the cock or valve R¹, when the water enters the
apparatus through the said discharge pipe e; and, 2ndly, in allowing the filter
to be put to work without guttering it by the fall of the water on its surface;
and also to send away the air that the filter may contain. To obtain this
result it is only necessary to dispose everything as if the filter were to be
cleansed by, the said upward current, and when the water has reached the
feed pipe c the cock or valve R¹ is to be closed, and the cock or valve R and

page_text: the sluice i to be opened. A vertical pipe g is placed down the inside of the
apparatus, and is within 10 centimetres of the top; it is bent horizontally
just above the filter, passing through the side of the apparatus, for the purpose
of giving a free passage to the wash that the feeding pipe may introduce.
For emptying the apparatus to the level of the filter, four or more hori-
zontal orifices shall be placed at the distance of half the radius of the centre,
being the mouths of 4 pipes f, which passing through the apparatus shall be
adjusted to a circular pipe E closed with a cock, valve, or sluice j.
Before to treat of cleansing the filter it will be shewn what it will produce.
The height of water on the filter in a tub of 7'30 metres deep may be
brought to 6'60 metres, and under those circumstances the Marseilles filter
will produce per square metre, in 24 hours,-
13 775 cubic metres,
1.20
and 100 cubic metres at least if the thickness of the filtering and supporting
beds had been reduced to 40 centimetres, according to the principle above
described. This result is much inferior to those obtained in the Arcade and
Pantheon water-works, where the composition of the filtering beds is analogoas
to that of the filtering apparatus. It is found in those filters that the co-
efficient K is equal to 30 and 28. By taking the third of one of those
figures, say 10, the produce per square metre of the filter will be,
10 180 cubic metres (about).
0.40
In the filters which have served for some time, the permeability of the
sand may be weakened (notwithstanding the cleansings), by reason of the
introduction of a certain quantity of mud in the interstices, and it is therefore
preferable to take always the less favourable circumstances, say, consequently
100 cubic metres only, for the product of the filtering tub per square metre,
its superfice being 150 metres, the product in 24 hours will be 15,000 cubic
metres of water, being the quantity required for a town of 100,000 inhabitants.
The small superficial extent of the filtering apparatus allow it to be cleansed
without much difficulty.
The reposing of the mud on the surface of the filter gives to it a prejudicial
consistence, and it is therefore of the greatest consequence to find a means
to keep the mud in a moving state. It is well known that water when in
motion has the power to keep in suspension matters of a density considerably
superior to its own. From experiments made, brown argile, although very
heavy, does not begin to resist the action of a current of water until the
speed descends to 3 inches per second. Supposing a filtering tub discharging
15,000 cubic metres of filtered water in 24 hours, it receives per second

page_text: about 12 cubic centimetres, which, delivered through a pipe of about 30
centimetres in diameter, will take a speed equal to 2'45 metres. If instead
of feeding the apparatus in the usual manner it is directed tangentially
against the inside of the apparatus (see feeding pipe c, Fig. 2), the water
will drag along all the liquid matter, which shall by that means receive a
perpetual rotary motion, keeping the mud in constant suspension, which will
cause it to travel on the surface of the filter without adhering to it, and
without taking the slightest consistence. A current of water, as the one just
described, would suffice to give to the whole mass of water a rotary motion,
superior in speed to that required to keep the mud in suspension. It will
consequently be necessary to regulate and not to surpass the limit required,
since the fine sand has a speed of about 6 inches per second, and that it is
essential not to overthrow the surface of the filter. It is very easy to moderate
the speed of the water when leaving the pipe, if it were greater than required,
or the rotary motion could be given by mechanical means, since there is
always a machine at command. By prolonging the feeding pipe c towards
the centre of the apparatus the speed of the water would be lessened, and
should the rotary motion then surpass a little the suitable speed for the
stability of the sand the result would only be that the surface of the sand
would little by little take the form of a cone, very slightly inclined, the top of
which would be at the centre of the apparatus; however, it would not be in
the slightest detrimental to filtering. Admitting now, on the one part, that
the filter produces 15,000 cubic metres in 24 hours, and, on the other part,
that each cubic metre contains 1 lb. of mud in suspension, this last hypothesis
is the most unfavourable, since it is supposed that there is placed in the filter
the muddy waters of the Seine, for instance, without extracting the grossest
impurities by means of deposit or otherwise. Of this double hypothesis it
results that at the end of nearly 2 hours a quantity of mud, weighing about
1,500 lbs. will float at a certain height above the surface of the filter. It is
certain that if without stopping the working of the filter there is taken away
a portion of the water, say, about 50 centimetres, from the surface of the
filter, it will remove the greater part of the muddy deposit, which, by reason
of its density, and the feeble rotary motion of the water, shall occupy principally
the lower part of the apparatus. To accomplish this the cock, valve, or sluice
j of the circular pipe E must be opened, which said pipe is in connexion with
the four pipes f, and the said water containing the mud will be discharged;
(experience will indicate the time to keep open the cock). If it were judged
necessary it would be easy during the discharge of the said muddy water to

page_text: turn round on the surface of the filter a flexible broom F, of a length equal
to the diameter of the apparatus. This broom should be fixed on a horizontal
bar 1, suitably adjusted to a vertical rod m, reposing in a socket b, and which
rod should be operated by the gearing h from above the apparatus. Mecha-
nical power may be employed for giving a constant rotary motion to the said
broom. This broom should be made either similar to the street-sweeping
machine or composed of a series of moveable and independent rakes adjusted
to the said horizontal bar l. These rakes should be kept in position by
springs, sufficiently strong to contend against the influence of the water, but
to allow of their moving when they meet with an obstacle on the surface of
the bed of sand. This broom (the weight of which may be diminished as
much as may be required by enveloping the vertical rod m with a hollow
cylinder a¹, made of sheet iron, and acting as a counterpoise) shall always
cause the water to have a rotary motion, thereby keeping the mud in sus-
pension, which can be taken away from the apparatus in the manner above
described and as often as required. The operation of discharging the muddy
water, if even renewed 10 times in 24 hours, would present only a loss of a
20th of the filtered water; probably, only 25 centimetres in depth of muddy
water may only be required to be removed, which would reduce the loss to a
40th, and even experience will show that this operation may be repeated less
frequently than before mentioned. Independently of the above process, for
the purpose of preventing the encumbrance and solidification of the mud on
the filter, (which processes have the advantage of being employed without
stopping the work of the filter,) it will be, however, necessary to cleanse the
filter (at intervals that experience will show) by means of the said upward
current, and also to take away 2 to 3 centimetres of the bed of sand, which,
notwithstanding the care and processes above mentioned, will always end by
being spoiled by the mud.
In some instances it will be found inconvenient to use a filter of the
height as above described, as it would involve the expense of raising or forcing
the water therein; and to obviate this, instead of obtaining a great quantity
of filtered water by having a great head of water upon the filter, I obtain
the same results with a head of water, in accordance with the present filters
of water-works, by diminishing the pressure under the filter, the thickness
To better explain my
of the supporting and filtering beds being the same.
meaning I use a few letters; say, P, the atmospheric
head of water on the filter; e, the thickness of the sand.
virtue of which the water flows cut, supposing that the
pressure; H, the
The pressure by
part underneath

page_text: the filter is equally submitted to the pressure of the atmosphere P, will be
(P+H+c) P or H+e, it is proportionally to that quantity that the flowing
out is operated. But if instead of having underneath the filter a pressure
P equal to the pressure of the atmosphere, it was only P-H', the total pressure
would become (P+H+e)-P-H', or H+H+ e. It will be seen by that
result, even supposing the direct pressure H to be very feeble, a very consider-
able pressure may be obtained by giving to H¹ a sufficient value, that is to say,
by diminishing properly the pressure under the filter, since the total pressure
by virtue of which the water flows out, is equal to the amount of direct
pressure H, and of the decrease of pressure under the filter H¹. To realize
this result practically I adjust to the bottom part of the small reservoir of
the filter (the space under the skimmer upon which the sand is placed) a pipe
of a diameter sufficient for discharging the whole produce of the filter. This
pipe descends 5 to 6 metres in the well or reservoir D, which is made of
sufficient depth for such purpose. But before working the filter the precaution
must be taken of expelling all the air contained in the small reservoir or
space above mentioned, which is accomplished by filling it with water as
well as the pipe which descends in the well or reservoir D, the air which
was there being expelled through a pipe with a cock or valve fastened to the
side of the apparatus, and when water flows out of it without air, the cock
or valve shall be closed, and then the apparatus is ready for working. The
water in the well will be low, but that in the discharge pipe from the filter
will be kept there by atmospheric pressure, and if the difference in level
between the water in the well and the under part of the filter, or the upper
part of the filter's reservoir is H', the upper pressure being P+H+e, the
water will flow by virtue of the charge or head of water P+H+e-(P-H¹)
=H+H+e. Supposing, therefore, H to be very feeble, we can have, how-
ever, a charge of, say, 5 metres, by arranging in such a manner that the
difference in level between the water in the well and the under part of the
filter's reservoir be equal to about 5 metres. This is not only very clear
theoretically, but numerous experiments have proved its practicability. It is
well to say that the pressure under the filter being less than the atmospheric
pressure, a part of the air in the water may disengage itself and get through
the sand of the filter, but the pressure of this air, when existing, may be very
easily indicated by means of a small floating sphere placed in a glass tube,
the extremities of which are engaged in metallic pipes in communication
with the upper and lower parts of the filter's reservoir. This small sphere,
following in its oscillations the height of water, will indicate by its successive

page_text: lowerings the quantity of air imprisoned, and in such case the charge of the
filter will be evidently diminished as the air accumulates, which air is easily
removed by means of the pipe and cock or valve before mentioned.
The mode of cleansing the filter, keeping the and in suspension, and the
withdrawal of the same without retarding the filtering, as described in the
first-mentioned process of filtering with a head of water upon the filter, is
equally applicable to the last-mentioned process of filtering water by diminish-
ing the pressure under the filter.
In witness whereof, I, the said Charles Frederic Vasserot, have here-
unto set my hand and seal, this Nineteenth day of March, in the
year of our Lord One thousand eight hundred and fifty-seven.
CH FRED VASSEROT. (L.S.)