92 REPORTS ON THE STATE OF SCIENCE.—1918. 
This sludge is flocculent in character, has a very high bacterial 
content and contains numerous higher forms of life especially 
ciliated protozoa. 
When sewage is aerated in intimate contact with from 20 to 25 
per cent. (by volume) of activated sludge there is a rapid initial 
removal of oxidisable matter from pseudo-solution, with the produc- 
tion cf a well clarified effluent, which is nitrified on further aeration. 
Judging by the volume of sludge produced it is presumed that 
the colloidal matters are removed from solution either by the 
absorptive action of the flocculent sludge or coagulated by intimate 
contact with the sludge particles, or as the result of enzymic action. 
While Bartow has shown that large quantities of carbonic acid 
are produced during the purification process, which is indicative of 
vigorous biolytic action, the high proportion of organic matter in the 
resultant sludge appears to preclude any material destruction or 
resolution of the organic matter present in the original sewage either 
in actual suspension or in colloidal solution although iv is evident 
from its appearance there has been a considerable alteration in its 
physical character. 
The high nitrogen content of activated sludge, which varies from 
4 to 7 per cent. (dry matter) dependent on the character of the 
sewage treated, leads to the same conclusion, as Clark has shown 
that the sewage colloids contain much more nitrogen than do the 
grosser suspended solids. 
There is however an undetermined factor, viz. :—to what extent, 
if any, is there any fixation of atmospheric nitrogen ? 
Ardern and Lockett have studied this question but further infor- 
mation is required before any definite pronouncement can be made. 
As previously stated, the sludge is quite flocculent and readily 
separates out from the purified sewage. 
Presumably on account of its gelatinous condition, the water 
content of the sludge is very bigh and cannot be reduced below 
95 per cent. by simple sedimentation. 
When this percentage has been reduced to from 88 to 90 per cent., 
either by centrifuge, or by treatment on drainage filters, the sludge 
is of the consistency of a fairly stiff jelly, whereas ordinary sludge 
obtained from the sedimentation of sewage, containing a similar 
amount of water, is quite fluid and can be readily carried and 
discharged through pipes. 
The solution of the problem of the best means of pre‘iminary 
de-watering of this sludge, prior to drying, involves a study of the 
conditions favourable to the removal of water retained by colloids in 
the “ gel”’ state. 
Chalkley Hatton has shown that such de-watering can be effected 
by treatment in a modified type of filter press, with the production 
of a cake containing 75 to 80 per cent. moisture. 
There is, however, room for research, with special reference to 
electrical effects, in regard to the most economical method of 
de-waterising and drying the sludge. 
The problem is one of considerable importance to agriculture, 
involving as it does the utilisation of the full value of the sludge as 
a fertiliser, 
