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.