SAVING THE SEWAGE SLUDGE
In matters pertaining to sanitation and the movement of sewage Great
Britain undoubtedly leads the world. There our conquest ends. From
that point onwards we can only point to lamentable inefficiency.
For instance, the lay-out of the main drainage system of London,
undoubtedly the finest illustration of such engineering in the world,
has involved a capital expenditure of £12,514,606--$62,573,030. By the
provision of enormous conduits and feeders the excrementitious matter
from residences, offices, workshops, and factories of the metropolis
is borne for miles to central stations. In this manner those natural
and trade wastes, construed as being inimical to health, are removed
swiftly and hygienically, and we compliment ourselves upon our prowess,
which certainly is justifiable so far as it goes.
But when we come to the treatment of this material we fail miserably.
At the central station the solid matter, in reality a mud or sludge,
is separated from the free liquid. The disposal of the latter offers
little or no difficulty. It can be rendered innocuous, and is therefore
permitted to resume its part in the scheme of Nature. But the sludge:
that is a different proposition. A few figures concerning the situation
in regard to London may prove illuminating. Certainly they will serve
to demonstrate the magnitude of the volume of this waste. During the
year over 100,000,000,000 million gallons of sewage are received
from approximately 5,350,000 people occupying 95,000 acres. Each
million gallons of sewage yields about 25 tons of sludge. The total
quantity of solid matter is approximately 200,000 tons. It costs about
30s.--$7.50--to treat and dispose of each million gallons of raw
sewage.
The total yield of sludge exceeds 2,600,000 tons a year. It is an
incubus having no ostensible commercial value, so is transferred
to vessels to be carried out to sea where it is thrown overboard.
Seeing that it costs about £17 13s.--$88--to run each vessel out
and back again, and that some 111,000 journeys are made during the
year, dumping the sludge costs the ratepayers of London nearly
£2,000,000--$10,000,000--a year. The crime incidental to London is
repeated throughout the country, and in this way, as Sir William
Crookes pointed out, the nation is deliberately discarding 16,000,000
tons of valuable nitrogenous material which, were it subject to proper
treatment, might be reclaimed to participate in the nourishment of our
broad acres. Estimating the value of this potential fertilizing agent
at the modest figure of one ¹⁄₂d.--1 cent--per pound we are, of malice
aforethought, throwing away a round £35,000,000--$175,000,000--per
annum. But this is not the most disturbing feature. For the most part
the sludge, and in the case of seaside towns the crude sewage, is
discharged upon potential valuable fishing grounds, to the destruction
or infection of the fish, especially shell-fish. Furthermore, one must
not imagine because the objectionable and dangerous refuse is abandoned
well out to sea its serious dangers are removed. Tides and currents
play strange tricks, the result being that much of this filth is thrown
back upon the coasts, perhaps at a distant point, to wreak possible
havoc.
Civilization breeds a strange fastidiousness. The idea of reclaiming
sewage for exploitation is repulsive to the average individual,
although he does not turn a hair at the use of the comparative material
derived from the animal kingdom for the nourishment of the soil, and
the feeding of produce cultivated essentially for the table. The
argument often raised against any exploitation of excrement is that it
has become associated with many other deleterious substances, which
have been thrown or allowed to run down the drain, as the readiest
avenue for their disposal. But the very circumstance that such waste
has become compounded with other residues, many of which are worth
reclamation, should be sufficient to induce us to regard sewage not
as an incubus or danger, but as a mine worthy of development to its
fullest extent.
Fortunately, the objection to the exploitation of sewage for its
commercial contents is in process of being over-ruled by the growth of
a more enlightened attitude towards the whole issue, although it is
to be feared, in accordance with the precept that what the eye does
not see the heart does not grieve, the more progressive policy is
being sanctioned unconsciously. It is safe to assert that, but for the
war, which retarded the hands of progress very pronouncedly, the new
movement in regard to the handling of this material would have made a
material advance. Even to-day the outlook is not hopeless, inasmuch as
the accentuated need to make every use possible of waste products may
result in the sewage exploitation problem being attacked with enhanced
energy.
What can be done with sewage is revealed by the action of one or two
towns which have taken their courage into their own hands, notably
Bradford and Oldham. In these two instances the modern handling of
sewage was assumed before the war, so that the experience gathered
during the past six years may prove sufficiently convincing to permit
the whole subject to be attacked more in consonance with contemporary
thought, which views all wastes in one light--potential raw materials
for other industries.
Changing conditions and the need to cope with this residue along more
comprehensive lines, in accordance with the growth of the population
and the quantity of material to be handled, were responsible for the
change from the old method to the new in both instances. In the case
of Bradford the Corporation found it necessary to establish new works
about six miles distant from the centre of the city, and was faced with
the necessity to expend £1,250,000--$6,250,000--in connection with the
undertaking. In view of such a heavy capital committal perhaps it was
only logical to consider the possibility of rendering the sewage more
remunerative in the future than it had been in the past. Any revenue
to be derived from exploitation in such a field must react to the
advantage of the community affected, more especially when such action
does not jeopardize the health of the citizens to the slightest degree.
Of course, the situation in so far as it concerns Bradford was somewhat
unusual. The city is the hub of the wool-scouring trade of the country,
and in treating the sewage much of the wealth allowed to slip down the
drains from cleaning the wool is open to reclamation. The one great
mistake, if such it may be called, of which Bradford has been guilty,
in view of the volume of grease contained in the effluents, is ever to
have permitted these wastes to pass into the drains and sewers. They
should have been collected and treated as a separate entity. But, as
this would have entailed combination of the interests concerned, an
admittedly difficult undertaking under voluntary conditions, the city
authorities decided to repair the sins of omission upon the part of
its industrial citizens and to assume the recovery of the valuable
materials which were being allowed to escape.
This manifestation of commendable enterprise and initiative owes its
origin mainly to the activity of Mr. Joseph Garfield, A.M.I.C.E., the
sewage engineer. Many years ago the idea of turning the sewage of the
city to industrial account occurred to him, and he embarked upon a
prolonged series of exhaustive experiments. These were sufficiently
conclusive and sufficiently promising of profit as to persuade the
adoption of the methods he advocated at the critical moment, which
arrived when the provision of a new sewage station became imperative.
The plant for dealing with the sludge was moved from the old situation
to new buildings specially erected for the purpose at Esholt, and the
raw material is fed to the latter station through a special main. The
sludge contains only 80 per cent. of water, the free water having been
previously removed by settling. It is fed into the main by compressed
air. Upon its arrival at the station the sludge is lifted, also by
compressed air, into large vats, where it is heated by the waste
steam from the engines of the power plant. In this heated condition
the sludge passes into close-sealed vessels from which, still at a
temperature approaching boiling point, it is forced by compressed air
through the filter presses. Each of these presses, of which there are
about 100 disposed in rows, contains 47 chambers, each 3 feet square.
As already stated, the sewage of Bradford is heavily charged with
grease resulting from wool-washing and other industries, and it is this
heavy proportion of grease which renders the process so attractive.
Moreover, by keeping the sludge in a heated condition during the
pressing process the expression of the fatty content is more readily
effected. From 40 to 48 hours are required to fill a press with
residuum, that is to say this period of time must elapse before the
whole of the available space within the press is occupied by the dry
cake from which the grease has been expressed, by which time from
four to five tons of sludge have been passed through. Each cake is 3
feet square by 1¹⁄₂ inches thick and weighs about 30 cwt. The grease
and water which is driven out of the sludge is carried away from the
presses into tanks. Here the water and grease are separated, the water
to be re-discharged into the sewage, while the grease is led to the
purification tanks. Subsequently the fat is either drawn off into
barrels or is pumped into tank wagons for dispatch to the works where
it is worked up into articles of commerce, including soap. The oil is
found to yield three valuable products--olein, stearine, and pitch. The
two last named enter extensively into the dressing of leather, as well
as the manufacture of candles and as an insulator for electric cables,
respectively.
The installation yields from 12 to 15 tons of grease throughout the
twenty-four hours, working, of course, being continuous. This product
in the days before the war commanded from £8 to £10--$40 to $50--per
ton, but the price is now higher. The sludge-cakes find favour as a
fertilizer, mainly from the fact that they are free from lime and
carry only from 28 per cent. to 30 per cent. of moisture. This residue
fetched from 3s.--75 cents--upwards per ton at the works in pre-war
days, when a healthy export was recorded, the product being shipped
in appreciable quantities to France and even to the Southern States
of America. The output of cake averages from 50 to 60 tons per day.
In addition to proving useful as a fertilizer it has been found to
furnish, when blended with coal-dust, a serviceable fuel.
The revenue derived from this example of sewage industry is certainly
such as to attract widespread attention. In the early days of the
process, when only two presses were maintained to establish its
possibilities, the grease sales reached £222 10s. 6d.--$1,112.62--per
annum. In 1911 the annual revenue had risen to a figure ranging between
£20,000 and £30,000--$100,000 and $150,000--from the enlarged battery
of presses. When the new works were opened it was anticipated that the
Corporation would be deriving £50,000--$250,000--a year from the sale
of the products derived from its sewage upon the attainment of the
designed maximum output. Up to the year 1911 the total sales amounted
to no less than £100,000--$500,000. From the recital of these figures
it must be conceded that Bradford has a very profitable commercial
enterprise in its sewage works.
Yet even the foregoing figures are undoubtedly capable of improvement
owing to the advances made in the whole issue of the recovery of fats
from wastes. The pressing system, even when conducted along the most
modern lines with up-to-date plant, leaves much to be desired in point
of yield. Under the most favourable pressing conditions at least 10
per cent. of the original volume of grease is left in the residue. The
presence of this grease reacts against the value of the residue as a
fertilizer, grease being the bugbear of the farmer. With the latest
process for grease extraction this content can be reduced down to 1 per
cent. Not only does this represent an increased yield of 9 per cent.
of fat with its attendant enhanced financial return, but it gives a
fertilizer which, being exceedingly low in fat, appeals more strongly
to the farmer, and accordingly is able to command a higher price. This
fact appears to have become appreciated by the Bradford authorities
according to recent developments.
Because such a striking success has been recorded at Bradford, it is
not to say that the self-same method would be equally profitable at
other places, especially those handling what might be termed purely
domestic sewage. The conditions existing at the Yorkshire city are
peculiar, owing to the wool-washing trade. The process which is more
likely to make the widest appeal, being the one adapted to meet the
average conditions, is that which has been installed in the borough
of Oldham. This is the invention of Mr. J. Grossmann, M.A., Ph.D.,
F.I.C., the well-known chemical engineer, who has made the exploitation
of sewage his life-long study. The plant in question was laid down
in 1912, being set in operation in October of that year, since which
date it has been working without a break, giving the most satisfactory
results. At the time the installation was carried out the population of
the borough was 148,840, and both the water-carriage and sanitary-pan
system were in vogue, although the latter was giving way to the former
method at the rate of about one thousand per year. As the conversion
system was carried into effect the quantity of sludge which the sewage
works were called upon to handle increased, the quantity pressed in
1911 being nearly 8,000 tons a year as compared with 4,000 tons in
1899. This did not include the several hundred tons which were dealt
with in lagoons without pressing. As the quantities of pressed sludge
increased so did the difficulty of disposing thereof.
The outlook was somewhat disconcerting. The agricultural land in the
vicinity could only absorb a portion of the available volume. The
necessity to incur the expense of carrying the residue a considerable
distance to dispose of it, which solution would have proved somewhat
costly, appeared to be inevitable. Experiments innumerable were carried
out, but to no purpose. Agriculture, which is regarded as the obvious
outlet for such material, was adverse to the proposal to absorb the
accumulation for the land, because it carried approximately 15 per
cent. of grease. The only escape from the dilemma appeared to be
the installation of further presses with the attendant expense for
auxiliaries to yield a dry material, and then to pay for the cartage
of this residue to some convenient tipping ground or carriage of the
settled sludge to sea to be dumped. As a round 30,000 tons of sludge
would have been involved, the sea-dumping expedient would have been
extremely costly. Further consideration of the question established the
possibility of converting the material into a marketable manure, but
this would have required the utilization of a trade process and also
would have incurred expense.
At this juncture the attention of the Corporation was attracted to Dr.
Grossmann’s process. It was investigated and submitted to searching
experiments spread over a period of three years at the sewage works.
From the results obtained and the experience gathered, it gave promise
of being completely successful when conducted upon a large scale. So it
was adopted.
The Grossmann process may be said to represent the most logical
exploitation of sewage yet attempted in accordance with the severe
hygienic conditions imposed to-day. Curiously enough, when the disposal
of sewage by water-carriage was first introduced, the critics of the
principle did not hesitate to point out that it represented the most
wasteful solution of the problem which had ever been accepted for
practice. But against these contentions the advocates of the idea urged
that the hygienic advantages to be gained were so overwhelming that the
question should not be considered from the commercial view-point at all.
Other days, other manners. In this instance, however, not many years
passed before the issue attracted such widespread attention as to
demand searching investigation, the difficulty and cost attending
the disposal of the sludge being responsible for a pronounced outcry
against the method. The sludge problem was thoroughly probed by a Royal
Commission, by which the opinion was expressed that the value of this
waste, calculated upon the volume of dry substance contained therein,
was no more than 10s.--$2.50--per ton at the very outside. But as the
sludge is produced in a form showing a high percentage of water it
was hopeless to expect farmers to absorb it, owing to the transport
charges involved for such a comparatively low manurial return, unless
their land happened to be situate close to the centres of production.
To overcome the water difficulty attempts were made to dry the sludge,
in the effort to reduce its bulk, but it was discovered that drying did
not constitute a complete sterilization process, with the result that
the material was liable to carry infection. But the greatest objection
to drying is that this very process, while it achieves one end--the
transport difficulty--provokes another disability. The sewage is worth
less after drying than in the saturated form.
The presence of fat in material quantities has always been responsible
for agricultural hostility towards this waste as a fertilizer. The fat
is due to soap used in the household, and which is thrown down the
drains, as well as the grease resulting from other domestic operations.
The great objection to grease is that it has the tendency to clog the
soil.
In turn efforts were made to dispose of the nuisance as a fuel, the
heavy proportion of oil present in the dried cake being the attractive
feature prompting this application. This recommendation found scanty
favour. Another brilliant mind conceived the idea of consuming the
refuse in gas-producers, thus obtaining a low-grade gas for power
purposes. This attempt failed to meet approbation. A third expedient
was its conversion into an illuminating gas, but this likewise failed
to overcome the obstacle. In so far as lighting is concerned, in many
places the practice is followed of allowing the gas thrown off by
the decomposing fæcal matter during its passage through the sewers,
to be led to the burners of adjacent street lamps to mix with the
ordinary town gas and thus be consumed. But this is merely a safety
precaution; it is not followed from economical motives. Now that
electricity is widely displacing gas for street illumination, even this
quasi-utilitarian system is meeting with defeat.
Under the Grossmann system, as practised at Oldham, the sludge is
subjected to a complete scientific treatment. The process is continuous
and automatic throughout. Moreover, the plant is designed and built
upon the unit principle, which allows the standardization of parts and
ability to meet any desired demand by merely acquiring a sufficient
number of units to comply with the sewage resulting from a given
population. Each unit is capable of dealing with sludge arising from
the purely domestic sewage of 20,000 inhabitants. Thus a town of
100,000 inhabitants would require 5 units, a city of one million souls
50 units, and so on in arithmetical progression. Furthermore, any
number of units can be worked together, so that in those centres where
the population fluctuates according to season or other conditions, a
certain number of units can be shut down during the off period.
The sludge passes to a special tank and is permitted to settle down
to approximately 20 per cent. solid matter. It is then scooped up by
bucket elevators to be lifted and discharged into another tank at the
top of the building. This acts as the storage tank or hopper, whence
it is moved automatically by means of screw conveyors and distributed
among six hoppers. Each of these hoppers feeds a drying machine. The
driers, set out in pairs with their brickwork casings and flues, occupy
the upper room. The machines themselves comprise iron cylinders set in
the brickwork and coal-fired furnaces. They are fitted with a specially
designed gearing and pulley mechanism which gradually moves the crude
wet sludge from the inlet towards the opposite end or outlet. Being
exposed to heat during this passage the sludge is naturally deprived of
the water it contains, this being evaporated to be led to the furnace
where any offensive gases and other matter associated therewith in
suspension are consumed before passing to the chimney to escape into
the outer air. By the time the sludge reaches the outlet it has been
completely dried.
The arrangement of the feed from the hopper to the drier is such that
only a measured quantity of sludge can be passed through in a given
time, which ensures the condition of the sludge at the outlet being
uniform. The provision of a similar measuring system at the outlet of
the drier ensures only a measured quantity of sludge being discharged
at that point. It will be observed that these protective devices guard
against forcing the apparatus to the detriment of the delivered sludge
which emerges from the drier in the form of a dry powder.
If desired this residue may be burned. Mixed with coke it forms an
excellent fuel, and can be employed towards raising the requisite steam
to conduct the treatment of further sewage. But, in view of the fact
that this powder contains about 15 per cent. of fat, its disposal as a
fuel would constitute about the most wasteful conceivable. Accordingly,
the next stage is the extraction of the fatty content. As it comes
from the drying apparatus the sludge is passed automatically into a
distilling retort which is bricked-in and heated. Above this retort
is a tank containing acid, a certain quantity of which is passed
into the retort to be automatically mixed with the powdered sludge.
Simultaneously superheated steam is driven through the mass in such a
manner as to permeate the whole. The interior of the retort is fitted
with gearing and pulleys similar to those provided to the drier and
for a similar purpose--the steady gradual movement of the sludge from
one end to the other. By the time it has reached the outlet from the
machine the sludge, completely deprived of fat, is automatically
discharged as a valuable manure and is ready for distribution upon the
land.
The superheated steam charged with the grease is passed into a
condenser, where water from a feed tank condenses the water and throws
down the grease. The mixture of condensed steam and grease is passed
into a recovery tank. The grease settling out on the top is removed
for boiling up in a separate vessel, upon the completion of which
treatment it is ready for packing and sale. The fatty matter consists
largely of stearine and palmitine, which to-day meet with a prompt sale
at lucrative prices.
But it is the solid residue in the dry powdered form which attracts the
greatest measure of attention. Disposal of the grease from sewage has
never occasioned so much difficulty as the utilization of the ultimate
residue from reasons already explained. In this particular instance
the great problem has been solved. The manure is in the form of a fine
powder, containing nitrogen, phosphoric acid, and potash, as well as
about 40 per cent. of organic material. It is very fine, brownish in
colour, odourless, and what is more to the point, absolutely innocuous,
having been completely sterilized. Consequently there is no risk of
infection being disseminated by its use.
The circumstance that the process is absolutely automatic from the
time the sludge is charged into the hopper to the finished article
issuing from the distilling retort, is a distinct recommendation. Not
only does it conduce to extremely economical operation, but it reduces
the necessity to bring human labour into one of the most offensive of
industries, inasmuch as the atmosphere of such an establishment is
scarcely fragrant, as may well be imagined, although familiarity breeds
strange contempts. The only labour essential is that required for
heating up the drying machines and retorts.
There is one overwhelming advantage incidental to this process which
cannot fail to arouse attention. Pressing in any form is eliminated.
This not only signifies a very pronounced saving in capital expenditure
in the first instance, but contributes to lower working charges, while
there is an enhanced recovery of grease and an absolutely grease-free
residue.
Before the Corporation of Oldham decided to install this system upon a
practical scale searching experiments were conducted with the resultant
manure, to determine its plant-feeding value. It was the promise of
being able to find such a ready market for the ultimate residue which
constituted one of the attractions of the process. Experiments were
conducted at several farms with various produce, and these proved that
the manure gives remarkably good results and is more effective than any
other plant-feeder containing the same proportion of nitrogen, potash,
and phosphates. Finally it contains an ingredient which is absolutely
missing from every chemical fertilizer. The latter is certainly a
plant food, but it is imperative that the ground should be treated
with a certain quantity of organic matter to assure the physical and
mechanical working of the soil. Decaying organic matter fulfils this
end admirably, as one would suppose, being a natural process, but
during the past five years the bestowal of sufficient quantities of
necessary humus has been impossible, owing to the shortage in supplies
of farmyard manure.
For this reason every farmer regards a grease-free manure carrying
substance of a humus-like nature for the improvement of his soil with
a particularly friendly eye, and he is prepared to pay a good price
for such an article. The sewage sludge fertilizer prepared under the
Grossmann process offers the agriculturist just what he desires in this
connection, inasmuch as it carries about 30 per cent. of the humus-like
substance. Then, again, the active manurial ingredients are distributed
over the mass in such a fine state of division as cannot possibly be
attained by resort to mechanical grinding. Finally, it is excellently
balanced, and the farmer keenly appreciates a well-balanced fertilizer.
Here he gets it because the essential operation has been conducted
by Nature, whose process cannot be rivalled. Applied to gardens this
manure is found to prevent the growth of yellow leaves, while the green
of the foliage is particularly rich and dark. In some quarters there
has been a certain degree of hesitation to utilize the fertilizer
merely because it is derived from sewage, owing to the prevalence of
many fallacious notions. Its origin is regarded with revulsion, and
its utilization with a certain degree of dread, but there need be no
apprehensions whatever concerning its use. The fact that in the course
of the treatment the material is raised to a temperature approaching
600 degrees Fahrenheit--where the superheated steam comes into contact
with the waste to expel the fat--effectively disposes of all germ life
inimical to the health of both animals and human beings, while it is
also clean to handle and odourless, it being impossible, from mere
cursory examination of the fertilizer, for the lay mind to determine
its origin. Finally, it may be stored for any length of time without
creating a nuisance, or deteriorating.
The whole of the output from the Oldham sewage works, which, owing
to the process of concentration, is really limited, notwithstanding
the volume of crude sewage handled, is readily absorbed by farmers.
Disposal was entrusted to a firm to act as the selling agents for the
Corporation. Owing to the number of repeat orders received, year after
year, this house declares that it could easily place 20,000 tons of the
fertilizer, were it forthcoming, without increasing its present staff
of travellers.
While the outbreak of hostilities militated against the expansion of
the process, although many other Corporations have expressed their
readiness to introduce the process into their respective sewage works,
the past five years have not been allowed to represent dead time.
Improvement upon improvement has been incorporated with the object
of securing still higher efficiency. In this direction the inventor
has made many distinct progressive strides. The one objection levied
against the process was the heavy expense incurred in regard to fuel
charges for drying the sludge, and these costs naturally have become
accentuated by the 200 to 300 per cent. rise in the price of coal. But
in this direction it is now possible to record noticeable reductions.
As a result of experiment the inventor has evolved a new method for
settling the sludge. He found that, by adding a very slight amount of
sulphuric acid--about 1 part to 1,000--to the sludge coming from the
settling tanks, the usual settling process is completely reversed.
Instead of the sludge settling to the bottom, the addition of the acid
causes it to rise to the surface, and in a much more concentrated form.
The water settles to the bottom in a clearer condition and can be drawn
off. By further settling and draining this top layer--virtually a
thick scum--a sludge can be obtained carrying about 30 per cent. solid
matter, and therefore as a less volume of water needs to be evaporated
a considerable saving in the consumption of fuel and cost of drying is
achieved.
Moreover, it is suggested that in laying down new installations, it
will be possible and profitable to install a destructor upon the sewage
works. In such cases it would be feasible to draw upon the waste
heat from the destructor to conduct the drying and other operations
demanding the application of heat. In combining the destructor
with the sewage plant the question of transport of the refuse from
the collecting ground to the destructor would demand very careful
consideration when horse haulage is employed, but with mechanical
traction the question of an extra mile or two in distance hauled is
not of such moment, especially as it would be off-set by the saving
of fuel which would attend the diversion of the waste heat to this
useful application. In fact, in cases where new lay-outs are being
contemplated it is a matter for serious reflection as to whether it
would not be found profitable to centralize destructor, electric
generating station and Grossmann sewage treatment plant in one
centralized spot, interlocking them together, and taking full advantage
of such inter-connection. The destructor would furnish the necessary
steam from the combustion of cinders and other refuse which it does not
pay at present to exploit, or preferably other low-grade fuel to drive
the electric plant, the waste steam being carried to the sewage works
for the drying and other machines together with the desired proportion
of live steam, while the electric station would furnish the requisite
power for operating the automatic mechanical appliances.
The grease recovered from the sewage, which is of a domestic character,
is essentially that from soap, cooking and washing operations. It
is totally free from all objectionable smell. It can be purified
very easily and is of distinct value. In its crude condition the fat
contains about 70 per cent. of stearic acid.
During the past few years the dry powdered residue has not only
been utilized in a direct form, but has also been exploited in the
production of compounded fertilizers. Mixed with phosphates, sulphate
of ammonia, and other nitrogenous products it has yielded a manure
which has given most excellent results in farming. Considerable
improvements with regard to greater efficiency and cheaper production
have been made in the manufacture of compounded fertilizers from this
residue, and there is every indication that still greater developments
are possible in this direction.
Were all the sewage of this country treated along these lines British
agriculture would derive distinct benefit, while industry would
also be presented with a new source of supply of essential raw
material. It would go a long way to enable us to use our greases over
and over again, because the drain is the most popular avenue for
the escape of this material. Sewage represents the greatest waste
incidental to this country. Dr. Grossmann estimates its value at
approximately £22,000,000--$110,000,000--per annum, of which but only
an infinitesimal fraction is at present recovered. The value of the
fat alone thrown down our drains, and reclaimed in a marketable form
would realize from £500,000 to £1,000,000--$2,500,000 to $5,000,000--a
year. The value of the manurial product, of which at least 1,000,000
tons are recoverable during the twelve months, may be set down at least
at £2,000,000--$10,000,000--the contents thereof being equivalent to
50,000 tons of phosphates, 50,000 tons of potash salts, with nitrogen
equal to that forthcoming from 100,000 tons of sulphate of ammonia.
This manure would suffice for the fertilization of at least 3,000,000
acres of land from which we might safely anticipate gathering, at a
modest estimate, additional crops worth £5,000,000--$25,000,000.
There is one other fact which deserves mention. Sewage is eminently
adapted to the feeding of sandy soils and other land which, at the
moment, is considered too poor for agricultural purposes. If this
manure were reserved for such land many thousand additional acres
might be brought under cultivation in these islands. At the present
moment these acres are being allowed to run to seed, constituting what
we erroneously term waste land, but only waste because we are not
sufficiently enterprising and energetic to reclaim it.
From the point of view of the towns and cities called upon to handle
the sewage, the Grossmann process holds out many inducements. It
complies with the demands of sanitation because it precipitates
no nuisance. It is the most hygienic process yet evolved for the
disposal of sewage sludge. The revenue derived from the sale of the
by-products--manure and grease--is such as to render the operation
of the plant not only self-supporting but money-making. As a rule
the sewage works of the average town represent a sink in more senses
than one, more especially when it becomes incumbent to resort to the
tipping, dumping or other disposal of the enormous accumulations of
the sludge. But signs of awakening are apparent. The Oldham plant has
been investigated by Corporations and other authorities, not only of
this country but from other parts of the world, who have been satisfied
as to its commercial practicability. With the restoration of normal
trading conditions it is anticipated that the process will become more
extensively adopted, especially as during the past five years ceaseless
effort has been devoted to the perfection of details to assure the
establishment of the process upon a firm commercial basis.