before and after recewing the Cumbridge sewage effluent. 361 
the effluent to 12 and 20 of the river water. It is obviously 
impossible to increase and maintain a larger volume of the river, 
even if it were desirable from other considerations; and, it 
cannot yet be decided whether it will be necessary to spend 
more money on a larger area of filter beds, until it is clearer 
that the additional settling tanks in course of construction and 
the beds now in use are giving their best results. For the 
present, it is more important to aim at the production of an 
effluent, which shall contain fewer suspended solids and consume 
less oxygen in five days. This can be accomplished by a more 
complete separation of the solids in the settling tanks, and the 
filtration of all the liquid through the beds. The various sections 
of the filter beds should always be kept in good condition by 
periodic scarifying, ploughing and resting, in order to break up 
the surface of the soil, and thoroughly rate the subsoil and the 
gravelly sand below. 
In connection with this investigation it is desirable to re- 
member that the disposal and purification of the sewage effluent 
is concerned with “the harm caused by allowing unpurified, or 
“imperfectly purified, sewage to flow into streams, thereby causing 
“the de-zration of the water of the river, and consequent injury 
“to fish; the putrefaction of organic matter in the river to such 
“an extent as to cause nuisance; the production of sewage fungus 
“and other objectionable growths; the deposition of suspended 
“matter, and its accumulation in the river bed or behind weirs ; 
“the discharge into the river of substances, in solution or 
“suspension, which are poisonous to fish or to live stock drinking 
“from the stream; the discoloration of the river; and the dis- 
“charge into the river of micro-organisms of intestinal derivation, 
“some of which are of a kind lable, under certain circumstances, 
“to give rise to disease” (5th Report of the Royal Commission 
on Sewage Disposal, p. 217). It has not yet been very closely 
concerned with the bacterial purification as distinct from the 
chemical purification. Like all effluents, the effluent from the 
Cambridge sewage farm is polluted with all kinds of bacteria, 
and is therefore potentially dangerous. It has been shown, for 
example, by Purvis and Rayner (loc. cit.) that the Bacillus coli, 
an intestinal bacillus found in the sewage effluent, can be traced 
down the Cam for at least four miles below the effluent outfall. 
Whether it will be necessary to sterilise sewaye effluents before 
they are discharged into streams is a question which has not yet 
received adequate attention ; but if a river, which is the receiver 
of sewage pollutions, is the source of supply of water for drinking 
purposes, it should undergo an elaborate system of bacterial puri- 
fication. The researches of Dr Houston, the Director of the 
Laboratories of the London Metropolitan Water Board, are of the 
greatest value in this direction. 
