Part 2

Although the amount of sludge is relatively small the accumulation requires removal occasionally, three or four times a year.

It is estimated that for every 1,000,000 gallons sewage there is about 3 cubic feet of dry matter. By chemical precipitation this would amount to from 20 to 25 cubic feet per 1,000,000 gallons.

Another good example of the septic tank method is the one in use at Exeter, England.

This plant has been in operation since 1896. It was designed for 98000 gallons capacity. The sewage is allowed to remain in the tank nearly 24 hours. The tank is used in conjunction with five beds in which further purification is used. The beds are filled with coke breeze and one is always resting for a week at a time. The tank is made nearly air tight by an arched roof and any gasses collecting may be burned at a vent.

The rate of working this tank is much slower than the process at Champaign Illinois.

At Southwold England, there is another good illustration of this method. This has been called an open septic tank and is used in conjunction with two beds, an anaerobic, and an aerobic bed, the latter containing polarite.

This plant is interesting from the fact that the effluent from the anaerobic bed is distributed over the aerobic bed by means of a revolving sprinkler which prevents the liquid from passing unequally through the large grain, porous material. The action upon these beds is continuous.

From a report of this system the analysis of gas found in three samples are as follows:—

+---------------------------------------------------------------------+ ¦ ¦ ¦ ¦ Carbon ¦Sulphuretted ¦ ¦ ¦ Oxygen ¦ Nitrogen ¦ dioxide ¦ hydrogen ¦ +-------------+-------------+-------------+-------------+-------------¦ ¦Septic tank ¦ 0.25%¦ 28.46%¦ 70.03%¦ 1.26%¦ ¦Anaerobic bed¦ 7.26%¦ 39.10%¦ 52.37%¦ 0.91%¦ ¦Aerobic bed ¦ 20.62%¦ 78.63%¦ 0.75%¦ 0.00%¦ +---------------------------------------------------------------------+

_Contact Bed_

The contact bed as distinguished from the bacteria bed may be said to be made up of fine material, while the bacteria beds are built of coarse material. The latter are used for taking out the rougher solids, and the former for taking out the more finely divided material, and the organic matter in solution. While this distinction is not commonly made it seems to be growing into usage.

In the process of purification by means of contact beds the sewage is applied intermittently by distributing pipes or troughs so as to slowly fill the beds which are filled from a depth of from three to six feet with any kind of hard, porous, jagged material. The change which takes place is due to the action of certain bacteria in the presence of air. Although this process is not a new one, the method by which the results were obtained was not fully understood until the more recent discoveries in the science of bacteriology were made.

The beds are made water tight to a depth of about four feet, the bottom being channeled or tiled to drain the effluent either to a secondary bed or into the effluent channel.

Many eminent men have advocated special material as coal, coke, clinker slag, sand, and gravel, and even glass for filling material. The results do not differ materially and Prof. L. P. Kennicutt in an article in the Journal of the Association of Engineering Societies, February 1900 makes the following statement, “The material should be more or less porous so as to have a larger water absorbing area and have a jagged surface on which the gelatinized micro-organisms can be easily retained.”

“The quantity of sewage that can be successfully treated by intermittent filtration has been shown not to be over 100,000 gallons per acre per day, a quantity so small as to be quite useless for towns and cities which would be obliged to construct beds with sand not “in situ”. This point was quickly perceived in England where sand “in situ” is not of common occurrence. These bacteria beds were not used long before the problem arose: Can the amount of land required by the intermittent filtration method be so reduced that the construction of artificial bacteria beds will be a practical possibility?

“The results of the investigation started by this problem have given us what is known as the contact system of treatment and the septic tank treatment and have apparently shown not only that by combining these two methods the amount of area required for 100,000 gallons can be reduced from one acre to about one seventh of an acre but also that the bacterial treatment is possible with sewage containing manufacturing refuse, and have outlined how sewage containing storm water may be treated.”

It has been found by experiment that after two or three weeks there is a marked reduction in the initial capacity of the tank due to breaking down of the filling material and also to the filling material becoming charged and coated over with a gelatinous slime consisting of living organisms and organic matter in the process of transformation.

If the bed is not overworked the capacity of the tank will remain constant after the first two or three weeks, showing that the durability is unlimited.

The beds are usually filled in half an hour, and the sewage allowed to remain on them about two hours when the effluent is run off and the beds allowed to rest several hours before again being filled.

By contact beds in series, the number depending upon the kind of sewage, almost any degree of purification can be obtained.

The Engineering Record of Jan. 27, 1900 gives a very interesting account of the sewage disposal works at Sutton, England. The works were constructed in 1891–93, and comprise an area of 28 acres, 18 acres only being capable of irrigation. They were originally designed for chemical precipitation and broad irrigation, but after giving these methods a two years’ trial the local board found itself unable to satisfy the requirements of the conservators of the River Thames, into which the effluent flowed.

Mr. W. J. Dibdin advised the construction of filters or fine grained bacteria beds. Two beds having a combined area of a quarter of an acre and a capacity of 200,000 gallons, were built in 1895–96 for the purification of the chemically treated sewage. In 1896 the first bacteria bed was constructed for the treatment of crude sewage.

As previously stated, many tests were made to determine the best kind of material for filling the beds.

The crude sewage, after being screened to intercept floating matter, is run directly onto the filters without the addition of any chemicals. After remaining in the beds for a period of two hours the effluent is in a fit condition to be discharged into the brook leading to the Thames and is uniformly superior to the effluent obtainable by local land treatment.

All experiments with bacteria beds show that the objects for which they were intended, to abolish sludge, has been realized and that sewage can be purified without chemicals at a small cost, being but little more than that incurred by the labor in attending to, or supervising the filling and discharging, the filters, and that sewage purification can be carried on with little or no nuisance.

_Discussion_

The amount of purification to be obtained by dilution depends upon the size of the stream into which the sewage is discharged and also upon the amount of oxygen contained in the stream. The latter condition is controlled very largely by the rate of flow of the stream and its previous condition of pollution. Mr. E. P. Stearns in his report to the Massachusetts State Board of Health, 1890, on Pollution and Self-Purification of Streams, gave a table showing the calculated amount of free ammonia, dissolved solids and chlorine which sewage adds to running streams.

Much interest is being taken in the effect of the discharge of the sewage of Chicago, and the waters of Lake Michigan, into the Illinois River, and the outcomes of the analyses of samples taken along the river is awaited with interest. Published reports have not been given out, but information from the most reliable sources seem to show that a considerable purification takes place in the passage down the river.

No reliable data could be obtained giving the percent of purification by irrigation. At Berlin the sewage is purified by this method, and the effluent comes well within the requirements of the German law.

With chemical treatment about 90 percent of the matter in suspension and a small percent of that in solution is removed, and the purification is about 53 percent of the total organic matter.

The best examples of intermittent downward filtration show an efficiency of 95 percent on the total organic matter. However, if this process is used in connection with other methods the organic matter may be reduced 99 percent and the chemical analyses of the effluent may fill the drinking water requirements.

Results of analyses of sewage and effluent from septic tanks show an efficiency of from 85 to 90 percent in the organic matter in suspension. Very little change takes place in the matter in solution.

The result of experiments with a single contact bed at Sutton, England, from November 1896 to March 1898 shows a purification of 64 percent; if, however, two beds are used in series, a further purification of 50 percent is obtained or a total purification of 82 percent of the crude sewage.

The capacities of irrigation, and intermittent downward filtration plants, and contact beds are usually stated in terms of the number of gallons per acre per day.

TABLE III.

_RATE OF SEWAGE TREATMENT._

System. Location. Sewage treated in gallons per acre per day. ----------------------------------------------------------------------- IRRIGATION. Berlin, Germany. 18,000. Manchester, Eng.—Estimated but not constructed. 18,500.

INTERMITTENT DOWNWARD FILTRATION. Framingham, Mass. 19,000. Leicester, Mass. 38,750. Brockton, Mass. 44,000. Marlborough, Mass. 54,000. Gardner, Mass. 140,000. Mendota, Ills. 184,000. Worcester,[1] Mass. 322,000. Worcester,[1] Mass. (Fletcher’s estimate). 500,000. Madison,[1] Wis. 8,000,000.

CONTACT BEDS. Manchester, Eng. (crude sewage) 500,000. Manchester,[1] Eng. 700,000. Manchester, Eng. (storm water sewage) 2,500,000.

Footnote 1:

Secondary filtration.

Table III shows the capacity of representative plants. Best authorities consider 100,000 gallons per acre per day to be the maximum rate permissible under the best conditions for the treatment of crude sewage by the intermittent downward filtration system. With unfavorable conditions the quantity of sewage should be limited to as low as 20,000 gallons. Wherever too large a dose is applied to the bed the sewage is not properly purified and the beds soon become clogged up and unfit for further use.

When the beds are used for secondary purification, 750,000 gallons may be treated per acre per day.

The results of extensive experiments made at Manchester, England in 1898 and 1899 show that by means of contact beds crude sewage may be treated at the rate of 500,000 gallons per acre per day. When the beds are used for secondary purification, 750,000 gallons may be successfully treated, and storm water sewage treated at the rate of 2,500,000 gallons per acre per day.

The capacity of the chemical precipitation plant at Madison, Wisconsin is 68 percent of the daily flow, which in 1899 was estimated at 300,000.

The company that built the plant agreed that the plant should have a daily capacity of 1,200,000 for each and every day in the year. This shows the estimated capacity of the tank to be 28 percent.

At Worcester, one of the best examples of this process is in operation. The size of the tanks is equal to 28 percent of the total flow of 17.1 million gallons per day of which 10.1 million gallons is the out flow from a comparatively clear pond. If the actual amount of sewage is considered the tanks have a capacity of 65 percent of the total flow.

Rafter and Baker in their discussion of the size of tanks recommends that the total capacity should be nearly 50 percent of the average daily flow.

The two typical types of septic tanks are those at Exeter, England and at Champaign Illinois. The first has a cubic capacity of 93 percent of the total daily flow, while the second has a capacity of 7.5 percent of the daily flow.

When the capacity of the tank is as small as at Champaign the bacteria do not have time to act upon the sewage as they would if the flow was not so rapid.

It is conceded now by the highest authorities on this subject that the tank should have a capacity equal to from one fourth to one half of the average daily flow.

Concerning the conditions for which the various processes are applicable, it may be said that the dilution process is used when advantage may be taken of natural resources. This method can be utilized by most cities situated along rivers or streams large enough to sufficiently dilute and carry away the sewage, and fulfill sanitary requirements.

As an example of the use of dilution the Chicago River may be cited. There the uncertain flow of the river was made to pass inward toward the Illinois River with a speed of 2½ miles per hour and a discharge of 20,000 gallons per minute per 100,000 inhabitants. The Blackstone River was used by the cities of Massachusetts until the pollution of the river became unbearable and the state was compelled to pass a law forcing the cities to purify their sewage.

Broad irrigation is a method which cannot generally be used on account of the large amount of land and labor required. This method is especially applicable to asylums, alms houses, and reformatories where cost of labor is small. It is used in the West where the value of all available water leads to the application of sewage to crops, and also on account of the low stage of western streams during the summer which renders sewage discharged into them an unbearable nuisance.

Intermittent downward filtration may be used where there is a considerable area of sandy soil, and also where a high degree of purification is necessary.

Chemical precipitation does not require a large area for its operation and used alone does not give a high degree of purification. Where land and material for beds is expensive, and partial purification is sufficient, this system may be used.

The septic tank requires a small area. The odors are not offensive and cannot be noticed 100 feet away. The effluent may be discharged into small streams and soon loses its identity.

Contact beds are used where a high degree of purity is required; if the effluent is emptied into rivers or sources of water supply. The beds do not require a large area and the process may be recommended if suitable material is at hand.

The cost of construction and maintenance of the different systems vary so largely according to local conditions that a fair estimate is not possible. As the most striking example of maximum and minimum cost the disposal system at Chicago and at Lowell, Massachusetts may be cited. The purification in both cases is by dilution, the cost being practically nothing at Lowell while the Chicago Drainage Canal cost $33,000,000. It may not be fair to charge this entire cost to the sewerage system; nevertheless up to date it has been used for no other purpose.

At Berlin, Germany where broad irrigation is used, the sewage has to be pumped to the farms and involves a considerable cost. Aside from this the receipts derived from the sale of farm products are enough to pay the running expenses of the farm.

The first cost of the intermittent downward filtration plant is estimated by the writers at $90,000 per million gallons per day of sewage treated, where the beds are artificial. Where natural beds can be used the first cost may be reduced to $7,500 per million gallons per day. The cost of maintenance is about $2.00 per 1,000,000 gallons.

The average cost for operating a chemical disposal plant is about 58 cents per inhabitant per year, or $16.00 per 1,000,000 gallons treated. At Manchester England where broad irrigation and chemical precipitation are used in combination, the average cost was $63.00 per 1,000,000 gallons treated.

A septic tank for treating 1,000,000 gallons of sewage per day will cost less than $10,000 and the cost of maintenance will be about $1.00 per 1,000,000 gallons treated.

_Conclusion_

In selecting a method of sewage disposal, the conditions, surroundings, and requirements of the city should be carefully studied and analyzed, and judgment and discretion must be used. A matter of so much importance to the community should be placed in the hands of men qualified to make a proper solution of the problem. While in general several methods of purification may be applied to the requirements of the city, usually local conditions and considerations will narrow the choice to two plans or possibly to a single method.

The governing features of the dilution process are so distinct that it is not usually difficult to determine where this method is applicable, and likewise, the broad irrigation process has peculiar conditions. In both, the plans involve only matters of construction.

The other methods have more in common and the determination of their relative value is not so easy. As before stated, the recently developed biolytic processes promise to displace chemical precipitation. Where a suitable deposit of sand or gravel is conveniently located on cheap land which may be made without great expense into filtration areas and the sewage discharged upon them by gravity, intermittent downward filtration may be the most satisfactory, especially if a highly purified effluent is desired. The item of expense of attendance, and labor of maintenance must be considered in connection with the cost of this method. In the absence of such favorable conditions and especially where complete purification is not required the septic tank may be the most suitable. For higher purification the combination of the septic tank with filter beds, or contact beds run at comparatively high rates, makes a satisfactory purification and is applicable to a wide range of conditions. A bacteria bed may be substituted for the septic tank for the roughing process but its applicability in not so general.

In conclusion it may be said that if the next ten years gives as much development in sewage purification as has the last decade, some of the processes herein outlined will have been discarded and sanitary engineering will have achieved still greater triumphs.

TRANSCRIBER’S NOTES

1. Table of Contents added by transcriber. 2. All text after the Contents was handwritten. 3. Silently corrected typographical errors and variations in spelling. 4. Retained anachronistic, non-standard, and uncertain spellings as printed. 5. Footnotes have been re-indexed using numbers. 6. Enclosed underlined text in _underscores_.