EXPERIMENTS UPON THE PURIFICATION OF SEWAGE AT

THE LAWRENCE EXPERIMENT STATION.*

BY ALLEN HAZEN, CHEMIST IN CHARGE.

The year 1892 is the fifth that the experimental work of the Lawrence Experiment Station has been continued. The work is carried on under the general supervision of Hiram F. Mills, C.E., a member of the State Board of Health, with the writer in direct charge. Mr. George W. Fuller is in charge of the biological department, Mr. Harry W. Clark is assistant chemist, and Mr. F. L. Fales has compiled the records and prepared the tables and diagrams showing the results of the work. Professors T. M. Drown and W. T. Sedgwick of the Massachusetts Institute of Technology are respectively consulting chemist and biologist, with a general oversight of the chemical and biological investigations.

The following subjects are treated in this paper:

Character of the sewage.

Fats in sewage.

Fat as a clogging material.

Experiments with sand clogged by sewage.

Influence of the amount of sludge in sewage upon the rapidity of

clogging.

Scraping sewage filters.

Systematic scraping.

Stratification and the effect of horizontal layers.

Filtration of sewage containing dye-stuffs.

Construction of sewage carriers.

Purification of sewage in winter.

On the area of filters to be provided.

Removal of clogged sand.

Work of filters for 1892.

Filling of experimental sewage filters.

Measurements of sewage applied to filters.

Operation of experimental sewage filters during 1892.

A full account of the work done at the Lawrence Experiment Station for the years 1988 and 1889 is contained in a special report of the State Board of Health upon the Purification of Sewage and Water, 1890, and a similar account for the years 1890 and 1891 is contained in the Twenty-Third Annual Report of the Board for the year 1891.

CHARACTER OF THE SEWage.

The sewer from which the sewage for the experiment station is obtained drains the most densely populated section of the city of Lawrence, and the sewage is probably considerably more concentrated than would be the average sewage of the entire city. In addition, all of the sewage used at the station is pumped between the hours of 7 A.M. and 5 P.M., and is probably stronger than the sewage for the remaining hours of the day. On the other hand, some of the insoluble matters of the sewage are retained in the pipe leading to the experiment station, as was shown on page 456 of the Twentythird Annual Report for the year 1891, and this to some extent balances the increased concentration, due to time and place of collection.

An experiment showing the amount of matter retained in the pipes was given in the report mentioned above (page 456). The experiment was repeated June 28, 1892, with results which differ but little from those previously given. The two experiments are as follows:

It has been the uniform practice from the beginning of the experiments to collect for analysis a gallon of sewage from one of the measuring basins on four or six days of the week. While the sample was intended to be as far as possible a representative one, it necessarily represented accurately only a small fraction of the total amount of sewage used; and the variations otherwise unaccounted for in the nitrogen stored in different filters, and in the chlorine of their

effluents, have at times indicated that portions of the sewage differ considerably in composition from the regular samples. The routine of the station has been conducted with such great regularity that it was by no means impossible that some filters flooded, say, uniformly in the afternoon should receive sewage of quite a different composition from those flooded in the morning. There is also considerable difficulty in mixing several hundred gallons of sewage so as to get a sample with its proper share of suspended matter.

To determine more accurately the average composition of all sewage pumped, a sample has been taken every Tuesday by collecting directly from the pump (without allowing any opportunity for sedimentation) a quantity of sewage for each lot of sewage pumped, the quantity collected being in each case proportional to the quantity pumped. Collecting the sewage directly from the pump gives of course a chance of getting a sample from a small lot of sewage of exceptional composition; but when a considerable number of such samples are mixed, error from this source is mainly eliminated, and when the analyses are averaged for a month or for a year, it should entirely disappear; and it is certainly a very great gain, to remove the possibility of sedimentation and incomplete mixing. The analyses of the sewage so collected are given as "average sewage," and should represent the average composition of all sewage applied to the filters. The results do not differ widely from those obtained from the regular sewage samples.

To determine whether the sewage applied to the different filters varied from the average, samples of the sewage applied to three of the large filters were taken during the latter part of the year. These samples were also taken from the sewage as pumped; and to avoid daily variations, which might be quite serious when only one lot of sewage was taken for the sample, a mixed sample has been prepared each week, representing each dose of sewage applied to each filter. In order to keep these sewage samples for one week without decomposition, which might seriously affect the analysis, a small amount of a mercury salt was placed in the bottle. At first mercuric chloride was used, but this of course prevented the determination of chlorine. Mercuric nitrate was excluded for a similar reason. The sulphate was tried, but owing to its slight solubility, unless special care was taken, enough would not go into solution, and the little taken up would be precipitated as sulphide, and the sewage would then putrefy badly. Later, mercuric acetate, prepared by dissolving freshly precipitated mercuric

MONTH 1892.

hydrate in acetic acid, was employed with excellent results, five cubic centimeters of a solution (containing as much mercury as five cubic centimeters of saturated corrosive sublimate solution) being an ample quantity to preserve a gallon of sewage. The mercury does not affect the ammonia determination in any way. The chlorine titration in its presence is not satisfactory, and we, therefore, evaporate 50 c. c. of these samples to dryness, with a little sodium carbonate, and take up the chlorides in warm water after ignition in a radiator. The soluble and insoluble albuminoid ammonias were not determined in these samples, as early experiments indicated that some matters became insoluble in presence of the mercury. In the tables of averages the soluble albuminoid ammonia has been taken as that found in the regular samples for corresponding dates. This is an approximation to the truth, for the soluble matters are much less liable to sudden variations than are the suspended matters.

The following tables show the monthly averages of sewage samples collected in the various ways above described. The daily variations in the sewage are so thoroughly shown in the tables of analyses in the Special Report upon Purification of Sewage and Water, 1890, and in the annual report for 1891, that it is not deemed necessary to publish again the daily results in full.

Monthly Averages of Analyses of Regular Sewage Samples.

[Parts per 100,000.]

ALBUMINOID AMMONIA.