[Footnote: Recently read before the Chemical Society, London. From the
_Journal_ of the Society.]
By WM. WEBSTER.
The term sewage many years ago was rightly applied to the excremental
refuse of towns, but it is a most difficult matter to define the liquid
that teems into our rivers under the name of sewage to-day; in most
towns "chemical refuse" is the best name for the complex fluid running
from the sewers.
It is now more than ten years since I first commenced a series of
experiments with a view of thoroughly testing various methods of
purifying sewage and water contaminated with putrefying organic matter.
It was while investigating the action of iron salts upon organic matter
in solution and splitting up the chlorides present by means of
electrolysis, that I first became aware of the importance of
precipitating the soluble organic matter in such manner that no chemical
solution should take the place of the precipitated organic matter. If
chemical matter is substituted for the organic compounds, the cure is
worse than the disease, as the resulting solution in most cases sets up
after precipitation in the river into which it flows.
My first electrolytical experiments were conducted with non-oxidizable
plates of platinum and carbon, but the cost of the first and the
impossibility of obtaining carbon plates that would stand long-continued
action of nascent chlorine and oxygen made it desirable that some
modification should be tried. I next tried the effect of electrolytic
action when iron salts were present, but did not think of using iron
electrodes until after trying aluminum. I found that the action of
non-oxidizable electrodes was most efficacious after the temperature of
the fluid acted upon rose 4° or 5°; but the cost of working made it
impossible on a large scale.
After a long series of experiments, iron plates were used as electrodes,
with remarkable results, for the compounds of iron formed not only
deodorized the samples of sewage acted on, but produced complete
precipitation of the matters in suspension, and also of the soluble
organic matter; the resulting effluents remaining perfectly free from
putrefaction. The first part of the process is well illustrated by the
small experiments now shown; the organic matter in suspension and in
solution separates into flocculent particles, which rise to the top of
the liquid and remain until the bubbles of hydrogen which have carried
them up escape, when the solid matter will precipitate. In the
arrangement adopted on a working scale, the separated particles
precipitate readily. As an illustration of the action upon organic
matter in solution I take a small quantity of dye, mix it with water,
and placing the connected iron electrodes in the mixture, the dye in
solution separates into flocculent particles. The electrolytical action
is of course easily understood, but the chemical changes that take place
need an explanation. At the positive pole, hypochlorite of iron seems to
be formed at first, but this is quickly changed into a protochloride,
and as at the negative pole an alkaline reaction takes place, the iron
salt is precipitated in the form of the ferrous hydrated oxide, together
with the organic matters in suspension and solution. Owing to the
carbonates that are always present in sewage, ferrous carbonate is also
formed.
The success of these laboratory experiments led me to a trial of the
process on a larger scale, for hitherto only a gallon at any one time
had been treated.
Small brick tanks were erected at my wharf at Peckham and iron
electrodes fitted to them.
Wrought iron plates were fixed about an inch apart, and connected in
parallel in the tanks, forming one big cell. Sewage to the amount of
about 200 gallons was run into the electrode tank and then treated, the
results being so satisfactory that larger works were erected, when a
supply of sewage equal to 20,000 gallons an hour could be obtained.
After a number of experiments had been carried out it was decided to run
the sewage as rapidly as possible through electrodes, six cells or two
rows in series fixed in a long channel or shoot, for experience showed
that the motion of the liquid acted on reduced the back E.M.F. and
hastened the formation of the precipitate.
A channel is kept at the bottom of the electrodes for the silt to
collect, with a culvert at side to flush it into, so as to prevent any
block occurring; the advantage of this is obvious. The plates in each
section may be from half an inch to an inch thick, and can be of any
length up to 6 ft. It may possibly be objected that a large number of
plates is required. This may be so, but the larger the number of plates,
the less the engine power required, and the longer they last. In each
section the electrodes are in parallel, and any one section is in series
with the other, the arrangement being exactly like that of a series of
primary battery cells.
By actual experience I have been able to prove that at least 25 sections
of electrodes should be in series and across any one of these sections
the potential difference need not be greater than 1.8 volts, the current
being of any desired amount, according to the surface of plates used.
The electrical measurements taken by Dr. John Hopkinson during these
experiments for the Electrical Purification Association, to whom I had
sold my patents, entirely corroborated my contentions as to E.H.P. used,
and agreed with the measurements of the managing electrician, Mr.
Octavius March.
The process was then thoroughly investigated by Sir Henry Roscoe, who
had control of the works for one month. He reports as follows:
"The reduction of organic matter in solution is the crucial test of the
value of a purifying agent, for unless the organic matter is reduced,
the effluent will putrefy and rapidly become offensive.
"I have not observed in any of the unfiltered effluents from this
process which I have examined any signs of putrefaction, but, on the
contrary, a tendency to oxidize. The absence of sulphureted hydrogen in
samples of unfiltered effluent, which have been kept for about six weeks
in stoppered bottles, is also a fact of importance. The settled sewage
was not in this condition, as it rapidly underwent putrefaction, even in
contact with air, in two or three days.
"The results of this chemical investigation show that the chief
advantages of this system of putrefaction are:
"First.--The active agent, hydrated ferrous oxide, is prepared within
the sewage itself as a flocculent precipitate. (It is scarcely necessary
to add that the inorganic salts in solution are not increased, as in the
case where chemicals in solution are added to the sewage.) Not only does
it act as a mechanical precipitant, but it possesses the property of
combining chemically with some of the soluble organic matter and
carrying it down in an insoluble form.
"Second.--Hydrated ferrous oxide is a deodorizer.
"Third.--By this process the soluble organic matter is reduced to a
condition favorable to the further and complete purification by natural
agencies.
"Fourth.--The effluent is not liable to secondary putrefaction."
Mr. Alfred E. Fletcher also investigated the process subsequently, and
reports as follows:
"The treatment causes a reduction in the oxidizable matter in the
sewage, varying from 60 to 80 per cent. The practical result of the
process is a very rapid and complete clarification of the sewage, which
enables the sludge to separate freely.
"It was noticed that while the raw sewage filters very slowly, so that
500 c.c. required 96 hours to pass through a paper filter, the
electrically treated sewage settled well and filtered rapidly.
"Samples of the raw sewage, having but little smell when fresh, stank
strongly on the third day. The treated samples, however, had no smell
originally, and remain sweet, without putrefactive change.
"In producing this result two agencies are at work, there is the action
of electrolysis and the formation of a hydrated oxide of iron. It is not
possible, perhaps, to define the exact action, but as the formation of
an iron oxide is part of it, it seemed desirable to ascertain whether
the simple addition of a salt of iron with lime sufficient to neutralize
the acid of the salt would produce results similar to those attained by
Webster's process.
"In order to make these experiments, samples of fresh raw sewage were
taken at Crossness at intervals of one hour during the day. As much as
10 grains of different salts of iron were added per gallon, plus 15.7
grains of lime in some cases and 125 grains of lime in another, and the
treated sewage was allowed to settle twenty-four hours; the results
obtained were not nearly as good as the electrical method."
During the present year a very searching investigation of the merits of
various processes of sewage treatment has been made by the corporation
of Salford; among others of my electrical process. As the matter is at
present under discussion by the council, I am not in a position to give
extracts from the reports of the engineers and chemists under whose
supervision and control the work was done, but I may go so far as to say
that the results of my system of electrical treatment have proved its
efficiency and applicability to sewages of even such a foul nature as
that of Salford and Pendleton. The system was controlled continuously
for the corporation by Mr. A. Jacob, B.A., C.E., the borough engineer;
Mr. J. Carter Bell, F.I.C., etc., county analyst; Messrs John Newton &
Sons, engineers, Manchester; Mr. Giles, of Messrs. Mather & Pratt,
electrical engineers, Manchester; Dr. Charles A. Burghardt, lecturer in
mineralogy at Owens College.
I would also refer you to a paper recently read before the Manchester
Section of this Society by Mr Carter Bell, the borough analyst for
Salford, in whose remarks Dr. Burghardt, an independent authority,
permits me to add that he concurs. He cannot give details until his
report has gone in, which will be very shortly.
Mr. Carter Bell's report _has_ gone in, and although he is precluded
also from giving full details, he has kindly put at my disposal samples
sealed by him of the effluents produced by the electrical treatment,
which I now submit, together with the analyses in the table.
The samples are taken at random.
Whether the process will or will not be adopted by the Salford
authorities I am of course unable to say, but I think I may safely say
that the electrical process has now absolutely proved its case in regard
to the solution of the sewage problem. It is simple, efficient and, I am
sure, more economical than any other known process where duration is
taken into account.
In regard to the Salford trials it may be interesting to give the
following particulars:
______________________________________________________________________
|
| Parts in 100,000.
|________________________________________________
| | | |
| May 15. | June 7. | June 30. | July 25.
|_____________|___________|___________|__________
|Not filtered.| | |
Total solids. | 109 | 125 | 141 | 132
Loss on ignition. | 33 | 21 | 29 | 23
Chlorine. | 32 | 44 | 42 | 43
Oxygen required | | | |
for 15 minutes. | 2.56 | 0.76 | 0.27 | 0.79
Oxygen required | | | |
for three hours. | 4.27 | 0.79 | 0.50 | 1.00
Free ammonia. | 2.20 | 0.88 | 0.50 | 0.92
Albuminoid am- | | | |
monia. | 0.32 | 0.17 | 0.092 | 0.19
_____________________|_____________|___________|___________|__________
The electrical shoot was built in brick and contained 28 cells arranged
in series.
Each cell contained 13 cast iron plates 4 in. × 2 ft. 8 in. × ½ in.
thick connected in parallel.
The available electrode surface in each cell was 256 sq. ft.
The ampere hour treatment required for Salford was found to be about
0.37 ampere hours per gallon, and the I.H.P. per million gallons based
on these figures would be 37.
NOTE.--In estimating for the plant necessary for treating the whole of
the Salford sewage, a margin was allowed on above figures. The A.H.T.
was taken at 0.4 and the I.H.P. per million at 39 to 39.5.
Mr. Octavius March, electrical engineer, who has followed the process
from the commencement, and who superintended the electrical details both
at Crossness and Salford, will give you on the blackboard a rough sketch
of the above trial plant.
The Salford tanks are admirably adapted to the application of the
electrical or in fact any process of precipitation. They are 12 in
number, and it is proposed to take two end tanks for the electrical
channels, in which the iron electrodes would be placed.
The total I.H.P. required for treating the whole of the Salford and
Pendleton sewage, taken at 10,000,000 gallons per 24 hours, is
calculated at 400 I.H.P., based on the actual work done during the
trial. The electrical plant would consist of four engines and dynamos,
any three of which could do the whole work, and three boilers, each of
200 I.H.P.
The total cost of plant, including alterations, is estimated at £16,000,
to which must be added the cost of about 5,000 tons of iron
plates--ordinary cast iron--at say £4 per ton. These plates would last
for several years.
If filtration were required, there would be an extra expenditure for
this, but it will be remarked that as the treated sewage is practically
purified when it leaves the electrical channels, these filters would be
only required for complete clarification, which for most places would
not be a necessity.
The filtering material used could be gradually prepared from the sludge
obtained after electrical treatment, unless it could be more profitably
sold as a manure, and I am not a believer in the value of sewage sludge
in large quantities. This sludge, a waste product, is converted into
_magnetic oxide of iron_, of which I have here two small samples. This
magnetic oxide is a good filtering material, but, like every other
filtering material, it would of course require renewal. There would,
however, always be a supply of the waste product--sewage sludge--on the
spot, and the spent magnetic oxide recarbonized could be used
indefinitely.
The annual cost for dealing with the Salford sewage is estimated at in
round figures £2,500 for coal, labor, maintenance of engines, boilers
and dynamos. To this must be added the consumption of iron and its
replacement, which would have to be written off capital expenditure.
If a colorless effluent were required, absolutely free from suspended
matter, the additional cost is estimated at from £1,200 to £1,500.
* * * * *