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.