It is almost needless to say that but few of these patents were of any practical value. Those processes which have some merit and are now best known are, I believe, included in the following list:—
List of Chemical Processes for
the Treatment of Town Sewage.
- The A. B. C. (or Sillar’s) process
- Anderson’s process
- Bird’s „
- Blyth’s „
- Campbell’s „
- Collin’s „
- Forbes and Price’s process
- Fulda’s process
- Goodall’s „
- Hanson’s „
- Higg’s „
- Hille’s „
- Holden’s „
- Lenk’s „
- Lundy’s „
- Manning’s „
- Scott’s „
- Smith’s „
- Spence’s „
- Stothert’s „
- Suvern’s „
- Whitthread’s process
- Wickstead’s „
Space will not permit me to describe these processes, and probably most of them, if not all, are familiar to my readers. Suffice it to say that in nearly all cases the modus operandi is that of mixing certain chemicals with the sewage by mechanical agitation, or by passing the sewage over “salmon ladders,” &c., then allowing the sewage to remain perfectly still whilst the solids are gravitating, and then dealing with the effluent in different manners.
Amongst the numerous chemical ingredients which are used for this purpose may be mentioned the following:—
Alum, animal charcoal, ashes, blood, bone ash, carbolic acid, chalk, chloride of lime, chloride of zinc, chloride of iron, clay, creosote, hæmatite, hydrate of lime, lead nitrate, magnesian salts, oxide of manganese, perchloride of iron, salt, soda, sulphate of zinc, sulphate of iron, sulphuric acid, and tar.
One of the great difficulties in connection with the precipitation of sewage is the disposal of the sludge which is left behind in the tanks.
This sludge contains about 90 per cent. of moisture, and if left to dry atmospherically, a thin crust forms over it, thus protecting that which is underneath, and it will not dry for many months. In some cases it is sought to dispose of the accumulations of sludge by digging it into the land; in others it is mixed with house ashes, &c., and sold as manure. Its bulk, however, in proportion to its manurial value is so excessive as to render it almost valueless, and it is difficult to get rid of it for this purpose even when fortified with ammonia or other chemical. A frequent practice now adopted is to reduce its bulk by exposure or by presses to a semi-dried condition; in other cases it is dried to a powder by heat, and General Scott has patented a method where, in connection with the lime process, it is burned and manufactured into cement. Sometimes it is squeezed in presses, such as Needham and Kite’s or Johnson’s, or it is filtered by Milburn’s or Weare’s apparatus. For drying the sludge by heat, Borwick’s or Forrest’s machines have been used with some success, but there is no doubt that the slimy, glutinous, albuminous, offensive mixture technically known as sludge is a difficult matter to dispose of in all sewage works.
In connection with the chemical treatment of sewage, it is of advantage to pass the effluent over land filters on the intermittent downward filtration principle. Another very effective plan is to pass the effluent (or even crude sewage) through land which is thickly planted with the Anacharis or American weed, duckweed, sedges, rushes, reeds, &c., or through beds of osiers or alder trees. An acre of land thus planted is said to purify more than three million gallons of sewage per diem.[192]
There is no doubt that plants of this description have a powerful action in purifying sewage or foul water of any kind, and where land is scarce this method has many advantages.