from those manufacturing industries in the State which pollute or threaten to pollute our rivers and ponds.

In connection with the study of permanency, investigations of different methods of separating the sludge of the sewage from the main body of the liquid, and thus disposing of the clogging matters which tend to shorten the life of sewage filters, have been continued.

Methods for hastening and increasing the purifying action of filters by different means of increasing the air supply are of considerable scientific and practical interest, and experiments on these subjects have been continued throughout the year.

In addition to the chemical and mechanical examinations of filtering materials in use at the station, a large number of soils and sands from the following places have been examined during the year, either in connection with applications made to the Board for advice with reference to sewerage systems and water supplies or in connection with the Neponset River investigation: Norwood, East Walpole, Canton, Taunton, Provincetown, Danvers Lunatic Hospital, Rutland, Stockbridge and Leicester.

PURIFICATION OF MANUFACTURING SEWAGE.

Various industries produce a large volume of waste liquor or sewage. It can be stated that the volume of sewage turned out daily by one manufacturing plant is often as great as the sewage of a village of considerable population, and the organic matters contained in manufacturing sewage are often much greater than in the same volume of town sewage.

These excessive amounts of organic matters, together with various chemicals used in the manufacturing processes and also present in the sewage, often render the problem of successful and economical disposal of this sewage exceedingly difficult.

During the past year the composition and disposal of the sewage of two tanneries, engaged in different lines of work, three paper mills and two wool-scouring establishments have been studied.

TANNERY No. 1.

This tannery is engaged in preparing and tanning calf skins. Of these skins large numbers are imported, and, to prevent decomposition en route, come packed in sulpho naphthol, a germicide, and

this has been present in the sewage throughout the period of examination.

To aid in the process of freeing the skins of hair, a ton or more of sulphide of arsenic is used each month, mixed with lime to form a soluble salt of arsenic, and as a result of this the sewage always contains arsenic in suspension and solution. Various other chemicals and dyes are used in the tannery and are present in the sewage.

The sewage as it flows from the tannery is a heavy, thick and offensive liquid, containing a very large amount of organic matter and generally colored by the dyestuff's present in it. A large amount of the organic matter present is in suspension, and experiments have shown that it will settle out from the main body of the sewage very completely in one hour, this rapid sedimentation being aided by the lime and other chemicals in the sewage.

There is generally enough arsenic in this sewage to prevent or almost prevent the growth of bacteria, and hence if it were applied directly to filtration areas the bacterial actions, which cause the purification of sewage and upon which the life and value of a sewage filter depend, could not take place. A considerable proportion of the arsenic is held by the sludge, or organic matter in suspension, and is carried downward with it when the sewage is allowed to stand and settle. The supernatant sewage, after this sedimentation, contains, however, generally enough arsenic to check bacterial action, and if applied to filtration areas there would probably be an accumulation of arsenic in the filter in the course of time in sufficient quantity to permanently check nitrification.

The volume of sewage flowing from the tannery generally exceeds 200,000 gallons daily, and its character is shown by the following monthly averages of the chemical analyses of a large number of samples.

In regard to the bacteria in this sewage the numbers given in this table are of those present in the supernatant sewage, samples for planting being taken without shaking the bottle. The reason of this way of recording the numbers present is that if the bottle were shaken, and the sample planted contained its proportionate amount of the sludge, this sludge generally contained enough arsenic to sterilize the plate and prevent bacterial growth. This was particularly true of the samples collected during the last six months of the year.

Removal of Arsenic by Coke or Iron.

Methods for removing this arsenic have been studied, and we have found that by passing the sewage through a filter or strainer of coke breeze it is quite completely freed from the arsenic. This removal is probably due to a combination of the arsenic with the iron in the coke and the formation of an insoluble double salt of iron and arsenic which is retained in the coke. The same result is accomplished by passing the sewage through iron filings or turnings.

These methods of removing arsenic are important and can undoubtedly be applied in the case of other arsenic-bearing manufacturing

sewages.

Average Amount of Arsenic (AS2 03) in Entire and Supernatant Sewage and Effluent of the Coke Strainer.

Much of the effluent of the coke strainer contained no arsenic, but occasionally, when an excessive amount was applied and the strainer was overworked, some would pass through.

In Solution.

Chlorine.

Oxygen Consumed.

Bacteria per Cubic

Centimeter.

Examination of the coke proved that a large percentage of the arsenic was retained in the upper few inches of the strainer. Thus, for example, a small filter containing 2 feet in depth of coke breeze was put in operation October 1 and flooded each day for two weeks with this sewage. The effluent was examined each day and contained no arsenic, and on October 15 the coke itself was examined with the following result:

Two sand filters and the coke strainer before mentioned have been operated, receiving the supernatant sewage, and are designated as filters Nos. 71, 72 and 73.

Filter No. 71.

This filter contains 4.5 feet of sand of an effective size of 0.23 millimeter and has received the supernatant sewage at the rate of 50,000 gallons per acre daily. It has produced a satisfactory effluent generally, but nitrification has been entirely destroyed in it at those periods when the applied sewage has contained rather more arsenic than usual, as during September and again during December, as shown by the table beyond. In warm summer weather nitrification is quickly re-established in a filter, but if it ceases in winter it does not start again until warmer weather returns, thus causing poor purification and excessive storage of organic matter in a filter during the cold weather.

Filter No. 72.

This filter contains 2 feet in depth of coke breeze and has received the supernatant sewage at the rate of 100,000 gallons per acre daily. This rate kept the surface of the coke flooded for about two hours. daily and could have been increased and still resulted in the removal of the arsenic. It has removed considerable organic matter and generally all the arsenic from the sewage. The object of this filter has been to free the sewage of germicides and thus get it into such a condition that the organic matter can be attacked by the bacteria of

decomposition, putrefaction and nitrification. That this result is accomplished is shown by the tables beyond, for when the sewage applied to this coke strainer or filter has contained so much arsenic that only a few hundred bacteria per cubic centimeter were found growing in it, its effluent has contained several million per cubic centimeter.

Filter No. 73.

This filter, containing 4.5 feet of sand of an effective size of 0.23 millimeter, has received the effluent of Filter No. 72 at the rate of 100,000 gallons per acre daily, and has maintained uniformly good nitrification and purification.

The following tables give the monthly averages of the analyses of the effluents of these three filters: