The analysis is given in Table I. and the results show that the beds are not working as efficiently as might be expected from a first class plant. This may be accounted for from the fact that the filtration is too rapid and that the material which is less than two feet in depth is not suitable for such a purpose.
| TABLE I. | |||||||
|---|---|---|---|---|---|---|---|
| Analyses of Sewage and of Effluent from Intermittent Downward Filtration Plant at Mendota, Ills. | |||||||
| Parts per 1,000,000 | |||||||
| April 20, 1900 | May 4, 1900 | ||||||
| Sewage | Effluent | Efficiency | Sewage | Effluent | Efficiency | ||
| Oxygen consumed: Total | 48.5 | 6.50 | 86% | 27.7 | 12.0 | 55% | |
| In suspension | 33.4 | 15.5 | 4.6 | 70% | |||
| In solution | 15.1 | 12.2 | 7.4 | 40% | |||
| Nitrogen as albuminoid ammonia | |||||||
| Total | 10.4 | .32 | 97% | 3.68 | .768 | 79% | |
| In suspension | 9.28 | 2.816 | .416 | 85% | |||
| In solution | 1.12 | .864 | .352 | 59% | |||
| Total organic nitrogen | 17.960 | .920 | 95% | 9.60 | 1.860 | 81% | |
| In suspension | 15.04 | 7.500 | 1.152 | 85% | |||
| In solution | 2.92 | 2.100 | .708 | 66% | |||
| Nitrogen as free ammonia | 7.20 | 2.40 | 67% | 3.84 | 1.28 | 67% | |
| Nitrogen as nitrites | .675 | .375 | 000 | 000 | |||
| Nitrogen as nitrates | 4.00 | 18.00 | .120 | .120 | |||
| Chlorine as chlorides | 53.0 | 49.0 | 47.0 | 44.0 | |||
Chemical Precipitation
When certain chemicals are added to sewage a precipitate is formed which under favorable conditions may carry down with it all the suspended matter as well as a portion of the dissolved organic matter. The addition of the chemicals together with the working of the various appliances for grinding and mixing the same, the decanting of the effluent, and the caring for the sludge all constitute what is known as the chemical treatment of sewage, the complete process being in reality partly chemical and partly mechanical.
The following matter concerning the theory of precipitation is taken from Baker and Rafter’s Sewage Disposal.
The reagents chiefly used at the present time are lime, sulphate of alumina, and ferrous sulphate. In the case of lime, there is a combination of some of the lime and with free and partially combined carbon dioxide to form an insoluble carbonate of lime. There is probably a further combination of an additional part of the lime with a portion of the organic matter in solution. The insoluble substances so formed sink to the bottom, carrying with them the major portion of the suspended matter in the sewage in the form of sludge.
Sulphate of alumina exercises a precipitating effort by a combination of the sulphuric acid with lime while alumina hydrate forms a flocculent precipitate which entangles and carries down the suspended organic matters.
On November 11, 1899 the writers visited the chemical precipitation plant at Madison Wisconsin and through the kindness of Mr. McClellen Dodge, City Engineer, were shown the plant at that time in operation.
The sewage was screened as it came from the city and emptied into a well. Here it was dosed with lime and then pumped to one of the four settling tanks. These are 15 feet deep and 25 feet in diameter. The sewage enters near the bottom of the tanks. The sludge settles to the bottom and the effluent rises to the top, where it is carried away to three of the filter beds. These three beds are in continuous use. A fourth bed is out of use while it is washed and allowed to rest for one day.
The total area of the beds is 5550 square feet. The flow through the beds is about 8,000,000 gallons per acre per day.