Our experience with New Haven sewage lends no color to the hope that a net financial profit can be obtained by the use of the Miles Acid Process, except with sewage of exceptionally high grease content and low alkalinity. They do, however, suggest that for communities where clarification and disinfection are desirable—where screening would be insufficient and nitrification unnecessary—the process of acid treatment comes fairly into competition with the other processes of tank treatment, and that it is particularly suited to dealing with sewages that contain industrial wastes, and to use in localities where local nuisances must be avoided at all costs and where sludge disposal could be provided for only with difficulty.
The conclusions reached as a result of the Chicago experiments are:[[194]]
The results on hand indicate that treatment of this sewage with acid results in a somewhat greater retention of fat. An apparent reduction in the oxygen demand over that resulting from plain sedimentation, while remarkable, is probably not real, being simply due to a retardation of decomposition by the sterilization of the bacteria present, the organic matter being left in solution.... However, there appears the added cost of acid treatment and the cost of recovery of the grease, as well as the uncertainty of the price to be received for the grease recovered.
The cost of the treatment is estimated by Dorr to be $18 per million gallons, and the value of the sludge obtained from the Boston sewage as $24 per million gallons, giving a net margin of profit of $6 per million gallons. At New Haven, the total return is estimated at $7.09 per million gallons. Based on the production of sulphur dioxide by burning sulphur (assumed to cost $36 per long ton) and on drying from 85 per cent to 10 per cent moisture with coal assumed to cost $7.50 per ton, it appears that the acid treatment of sewage should be materially cheaper than either the Imhoff treatment or fine screening under the local conditions. A comparison of the cost of the treatment of the East Street and the Boulevard sewage at New Haven and the Calf Pasture sewage in Boston is given in Table 100. The cost of construction was estimated by Dorr and Weston in 1919 as greater than $15,000 per million gallons of sewage per day capacity.
| TABLE 100 | ||||||||
|---|---|---|---|---|---|---|---|---|
| Estimated Cost of Sewage Treatment at New Haven and Boston by Three Different Processes | ||||||||
| Cost in Dollars per Million Gallons Treated | ||||||||
| (Engineering and Contracting, Vol. 51, p. 510) | ||||||||
| Miles Acid Process | Imhoff Tank and Chlorination | Fine Screens and Chlorination | ||||||
| East Street | Boulevard | Calf Pasture | East Street | Boulevard | Calf Pasture | East Street | Boulevard | |
| Tanks and Buildings Int. and Dep. | 2.47 | 2.47 | 2.47 | 5.28 | 4.44 | 4.60 | 4.60 | |
| Acid treatment | 6.93 | 10.74 | 18.65 | |||||
| Drying sludge | 2.09 | 2.04 | 10.34 | |||||
| Degreasing sludge | 1.78 | 1.91 | 9.12 | |||||
| Superintendence | 1.06 | 2.65 | 1.06 | 0.46 | 1.15 | 0.47 | 1.15 | |
| Labor on tanks and screens | 1.00 | 1.00 | 1.00 | 1.20 | 1.50 | 1.42 | 2.05 | |
| Disposal of sludge or screenings | 1.00 | 1.00 | 0.50 | 0.50 | ||||
| Chlorination | 4.05 | 4.05 | 4.05 | 4.05 | ||||
| Gross cost | 15.50 | 20.98 | 42.75 | 11.99 | 12.14 | 11.03 | 12.35 | |
| Revenue | 6.57 | 10.66 | 47.59 | |||||
| Net cost | 8.93 | 10.32 | 4.84 | 11.99 | 12.14 | 11.03 | 12.35 | |
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.