Scientific American Supplement, No. 799, April 25, 1891 by Various, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE ELECTRICAL PURIFICATION OF SEWAGE AND CONTAMINATED WATER.
THE ELECTRICAL PURIFICATION OF SEWAGE AND CONTAMINATED WATER.
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.
Recently read before the Chemical Society, London. From the Journal of the Society.
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