Screening
The screening process consists of running the sewage through coarse or fine screens, either hand cleaned or mechanically operated, to remove suspended and floating matter. There is almost an unanimity of opinion now in favor of the use of mechanically operated fine screens. The efficiency depends largely although not entirely, upon the size of the mesh or openings through which the sewage passes. Coarse screens, which are cleaned by hand, will remove from two to ten per cent. of the suspended matter and fine screens which are mechanically operated will in some cases remove as much as 25 per cent. Screening will not materially change the turbidity of the liquid or the greasy appearance nor will it remove all of the suspended matter.
Experience has shown that the screening process is valuable in connection with sewage pumping works and inverted siphons, when sewage is disposed of by dilution and when raw sewage is applied without any other preliminary treatment to a final process as it prevents the clogging of machinery and filters.
When the process is used the screenings must ordinarily be disposed of within twenty-four hours on account of fermentation and decomposition which sets in quickly. In some cities the deposits are buried and in others they are burned after having been artificially dried. Robert Spurr Weston says that it seems unwise to attempt to dispose separately of two kinds of sludge, namely that removed before and that remaining after subsidence. “On the other hand,” he continues, “the screening of the effluent from a settling tank in order to reduce the operative charges for cleaning sprinklers is an economical practise. Furthermore, the actual amount of material screened from the effluent is small in comparison with that removed from unsettled sewage and its subsequent disposal is not a serious burden.”