Processes of Treatment
The processes for treating sewage may be divided into three main groups—the preliminary or preparatory, the main or final, and disinfection.
The processes in the preliminary or preparatory group remove more or less of the solids, especially the suspended matter, but the effluent, or liquid that is discharged into the stream, is chemically unstable and will decompose and putrefy. These are the simplest methods of treatment, and, except when sewage is discharged into very large bodies of water where it is desired only to improve the esthetic condition or where the water is capable of rapid self-purification, at least one of these processes is used in combination with some other form of treatment in the next group. The preliminary processes are dilution, screening (coarse or fine), plain sedimentation, straining or roughing filters, chemical precipitation, slate beds, colloidal tanks, septic tank treatment, and single contact beds.
The main or final processes are more complex. These remove a substantial proportion of the dissolved and suspended matter. The effluent is generally stable. When any one of these processes is used it is customary to provide some preliminary treatment. The processes in this group are double contact beds, trickling (also called percolating), sprinkling filters, intermittent sand filtration and broad irrigation or sewage farming.
In the third group is the process of disinfection, either by hypo-chlorite of lime or liquid chlorine. Some authorities call this third group the finishing process and preface two others, secondary settling tanks and secondary filters. The chemical elements of this group destroy the bacteria, especially the disease producing kind, and are used in combination with one or more of the processes in the other two groups to produce a highly purified effluent.
Several other processes have been developed within the last few years. The electrolytic process is now being used in a few American cities, and has been included in almost all of the experiments now being made by municipalities. The activated sludge process has been adopted by two large cities, Milwaukee, Wis., and Houston, Texas, and two small cities, San Marcos, Texas, and Escanaba, Mich., and is being tested in at least eighteen others, among them Baltimore, Cleveland and Brooklyn. Jersey City, N. J., has tentatively adopted the activated sludge process. Another process, known as the Miles Acid Sludge Process, is being experimented with by the city of Boston.
These processes or variations of them may be used singly or in combinations of two or more to yield different degrees of purification that will meet varying local requirements. Which of these or what combination of processes to use according to local requirements is the all important question for a city to answer. Several cities either have adopted or are planning to adopt the plan advocated by John A. Giles, Commissioner of Public Works of Binghamton, New York, to include a number of the different stages of treatment in the original design so that when future installation is necessary on account of increased population, with its increased pollution, or the need for a greater degree of purification becomes imperative, the addition can be made on the site already provided for and each unit will fit into the complete structure at a minimum cost. The consensus of opinion is that a disposal works can be designed and constructed which will produce an effluent that will not deteriorate the water into which it is discharged, that will create no nuisance from odor or from flies and that the cost will be strictly proportionate to the sanitary and esthetic results achieved.
An approximate idea of the efficiency of the various well known processes in the removal of bacteria was given by Professor George G. Whipple, Professor of Sanitary Engineering, Harvard University, before the New York State Conference of Mayors and Other City Officials:
| Process | Percentage of Bacteria Removed |
|---|---|
| Fine screens | 10 to 15 |
| Settling tanks | 60 to 70 |
| Septic tanks | 60 to 70 |
| Chemical precipitation | 80 to 90 |
| Contact filters | 75 to 85 |
| Percolating filters | 85 to 95 |
| Intermittent sand filters | 95 to 99 |
| Broad irrigation | 95 to 99 |