ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 91 
Comparison of the conditions obtaining in these filters with those 
of the modern ‘artificial’ filters in which sewage is treated at very 
much higher rates, leads to the conclusion that the results obtained 
may certainly be considered a maximum effect with respect to the 
resolution of the organic matter deposited in the filter, at any rate so 
far as any practicable scheme of sewage filtration is concerned. 
The fact that the sewage treated contained its original solids 
in suspension unfortunately precludes any definite conclusion with 
. regard to the actual resolution of the solids derived from matters in 
pseudo solution. 
In the Second Annual Report of the State Department of Health 
of Massachusetts (1916), Clark reports the results obtained with the 
operation of a series of sand filters receiving respectively, (@) un- 
treated sewage, (0) settled sewage, (c) sewage after treatment in 
straining filter, and (d) sewage clarified by precipitation with 
sulphate of alumina The volume of sewage applied to each filter 
- was varied so that each received as nearly as possible equal amounts 
of organic matter (measured by the organic nitrogen). [t is interest- 
ing to observe that working in this manner there was no material 
difference in the amount of nitrogenous matter stored in the respective 
filters, measured by determining the amount of albuminoid ammonia 
in the sand. 
It would thus appear that the nitrogenous organic constituents of 
sewage whether in actual suspension or in colloidal solution are more 
or less equally amenable to biolytic action, and consequently the 
percentage resolution of total organic matter observed in connection 
with the sand filters treating crude sewage should apply fairly well 
to the matters coagulated from pseudo solution. 
In conclusion it is evident that the total unresolved organic matter 
either retained in the filter or discharged with the effluent, will 
depend on the character (suspended solids and colloid contents) of 
the sewage treated and the rate of application with respect to the 
capacity of the filter, which will vary according to the type em- 
ployed. 
A survey of the subject would not be complete without reference 
to the large scale operations developed by O’Shaughnessy with 
reference to the subsequent treatment of the sludge removed from 
the Birmingham sewage by treatment in sedimentation on septic 
tanks. 
This sludge is submitted to prolonged anaerobic fermentation, 
whereby appreciable auto-digestion of the sludge occurs with loss of 
colloidal character. By this treatment a dense and granular material 
is produced without nuisance. (Vide Reference, 1914, J.S.C.1., 
No. 1, vol. xxxiii.) 
The following reference to the “Activated Sludge” process of 
sewage purification is made on account of the rapid removal of 
oxidisable matter in pseudo-solution, which is effected without the 
aid of actual surfaces, other than is provided by the particles of 
sludge. 
An “active” sludge is built up as the result of the oxidation by 
aeration of successive quantities of screened and detritus free 
sewage. 
