There are a good many patents in connection with the various methods of sewage purification, and some caution is necessary to avoid their infringement, though of course the general principles of settling and filtration, and the destruction of organic matter by bacterial action, are open to all.
As a general rule the waste-liquors from a tan-yard or leather dye-works are exceedingly impure. They contain the organic matter (in a state of great putrefaction) from the soaks, bates and puers; other organic matter, also more or less putrefied, from the tan-pits; the lime liquors, with their large proportion of lime and of dissolved hide-substance, and in addition the various dyes and other chemicals which may have been used in the conversion of the raw hide into the finished leather; and hence their efficient purification has presented difficulties which do not occur in most other trades.
The different waste liquids are best run into a capacious tank, and, after being thoroughly mixed up together, are allowed to settle for some hours. By this means the greater part of the tanning matter will combine with the lime also present to form a heavy, brown insoluble substance; some of the dye and other organic matter will become entangled in this, and thus be removed from the liquid. The clear liquid is next run off into a bacterial filter (preferably a septic tank, followed by an open coke filter), and then into the nearest stream. If the tannery is near to a town, and the corporation sewers can be utilised, it is probable that a filter made of spent tan may be substituted, as this material will not only remove all excess of lime from the liquid but will also fix much of the colouring matter as well (Koenig). The tan, after being used for this purpose, contains so much lime in its pores that it is said to be useful as manure.
In tanneries where large quantities of disinfectants such as mercuric chloride, carbolic acid, etc., are used, it is necessary that the mixed liquids shall contain so much lime as to make them distinctly alkaline. In this way most of the disinfectants will be either precipitated or rendered inactive. Where arsenic is used in the limes it may be advisable to add a little ferrous sulphate (green vitriol or copperas), in order that the arsenic may form an insoluble compound with the iron, and so be removed along with the sludge. The ink produced by the action of the iron salt on the tan liquors will be completely removed by the bacterial filter.
APPENDICES.
APPENDIX A.
METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER TRADES CHEMISTS FOR THE ANALYSIS OF TANNING MATERIALS.
Including Alterations adopted at the Leeds Conference in 1902.
Section I.—Sampling from Bulk.[189]
[189] See London Report, pp. 22-29 and 124.