Electrolytic Treatment
276. The Process.—This process has been generally unsuccessful in the treatment of sewage and has grown into disrepute. In the words of the editor of the Engineering News-Record:[[195]]
Thirty years of experiments and demonstrations with only a few small working plants built and most of them abandoned—such in epitome is the record of the electrolytic process of sewage treatment.
It is probably true that the process has never received a thorough and exhaustive test on a large scale, but the small-scale tests have not been promising of good results. Among the most extensive tests have been those at Elmhurst, Long Island,[[196]] Decatur, Ill.,[[197]] and Easton, Pa.[[198]]
Whatever degree of popularity the method has possessed has been due possibly to the mystery and romance of “electricity” and to the personality of its promoters. The process should, nevertheless, be understood by the engineer in order that it may be explained satisfactorily to the layman interested in its adoption.
In this process, sometimes called the direct-oxidation process, all grit is removed and the sewage is passed through fine screens before entering the electrolytic tank. In the electrolytic tank the sewage passes in thin sheets between electrodes and an electric current is discharged through it. A recent development has been the addition of lime to the sewage at some point in its passage through the electrolytic tank. From the electrolytic tank the sewage flows to a sedimentation tank, where sludge is accumulated, and from which the liquid effluent is finally disposed of.
It is claimed that the action of the electricity electrolyzes the sewage, releasing chlorine, which acts as a powerful disinfectant. The constituents of the sewage are oxidized so that the dissolved oxygen, nitrates, and relative stability are increased and the sludge is rendered non-putrescible. It is said that the addition of lime increases the efficiency of sedimentation and enhances the effect of the electric current. The results obtained by tests at Easton, Pa., are shown in Table 101. It will be observed from this table that the combination of lime and electricity does not have a more beneficial effect than either one of them alone. The amount of sludge produced by the combination is about the same as by chemical precipitation alone, but the character of the sludge produced with electricity is less putrescible. The cost of the treatment as estimated at Elmhurst is shown in Table 102.
As a result of the tests at Decatur, comparing lime alone with lime and electricity together, Dr. Ed. Bartow stated:
The purification by treatment with lime alone was greater than that obtained in several of the individual samples treated with lime and electricity.
| TABLE 101 | ||||||
|---|---|---|---|---|---|---|
| Comparative Results Obtained from the Treatment of Sewage by Lime Alone, Electricity Alone, and Lime and Electricity Combined | ||||||
| (Creighton and Franklin, Journal of the Franklin Institute, August, 1919) | ||||||
| Lime and Electricity | Lime Alone | Electricity Alone | ||||
| Change, Parts per Million | Change, Per Cent | Change, Parts per Million | Change, Per Cent | Change, Parts per Million | Change, Per Cent | |
| Chlorine | +1.2 | +1.9 | +12.3 | +18.2 | +1.6 | +2.2 |
| Nitrites | +0.014 | +58.3 | -.005 | –10.0 | –0.01 | –20.0 |
| Nitrates | +0.13 | +23.6 | +.005 | +0.8 | –0.15 | –20.0 |
| Ammonia | –3.3 | –18.3 | +0.2 | +1.3 | +0.9 | +6.6 |
| Albuminoid ammonia | –3.6 | –12.1 | –0.4 | –1.7 | –0.5 | –2.3 |
| Oxygen demand | –13.0 | –20.5 | –7.7 | –8.9 | –6.5 | –10.0 |
| Dissolved oxygen | +1.78 | +40.9 | –0.93 | –19.1 | +1.61 | +40.1 |
| Total bacteria at 37° (Thousands) | –343 | –92.7 | –373 | –82.4 | –165 | –37.8 |
| Total bacteria at 20° (Thousands) | –688 | –92.7 | –1074 | –90.1 | –635 | –70.0 |
| B. Coli (Thousands) | –77.9 | –99.85 | –96.3 | –92.3 | –45 | –81.8 |
| Oxygen absorbed in 5 days | –3.40 | –81.6 | –1.03 | –21. | +1.24 | +31 |