MODERN' METHODS OF WATER PLRIFICATIOX. 327 



American writer, referring- to the popular prejudice against the 

 use of akmi to purify the water, states that complaints are com- 

 mon that the consumers could taste the alum, so))icti)}ics even 

 before it zvas used. We had been advised just a few months 

 before by a special report, making reference to our first mov- 

 ings towards chemical precipitation assistance in purifying" our 

 water supply, ag'ainst the addition of any chemical precipitant 

 to quicken or otherwise facilitate the precipitation of suspended 

 matter. 



From the time that we adopted, or rather got into regular 

 working" of, chemical precipitation, we have had clean water, 

 and a much diminished Typhoid incidence. 



Besides the questions of how temperature affects the work- 

 ing of chemical sedimentation processes, there is an impor- 

 tant consideration raised, namely, the questions of layering or 

 stratification of the water, also the effect of " baffling " the 

 sedimentation basin or settling tanks, as we call them here. 

 Bafides are used to prevent direct passage from inlet to outlet, 

 and assists sedimentation. Some baffles skim the water from 

 the first baffled compartment and deliver it to near the bottom 

 of the next compartment. 



There is one further consideration I heard discussed in con- 

 nection with the deterioration in quality of certain turbid 

 waters in the coldest and the hottest weather, that there is a 

 seasonal " turn-over " of the whole body of the water in the 

 river-bed or reservoir due to the effect of the temperature, 

 and the depths to which the heating or the cooling" of the 

 body of water extends. This, perhaps, throws a light on the 

 hitherto puzzling phenomenon that in the non-rainy months of 

 June, July and August when, from absence of flood, the water 

 should be clear in the Alodder River, and is so in shallow parts 

 and the little streams of tributaries, it is in the deeper water 

 of the river discoloured to greater degree than the still flow 

 or almost stagnant condition of the river in the dry months 

 would lead one to expect. 



Another important consideration is that the amount of pre- 

 cipitant added should vary with the requirements, and this 

 is where the chemical sedimentation system makes demands on 

 the intelligence of the oflicial in charge of the plant. If with 

 the precipitation the mechanical filter is used with water re- 

 quiring" careful attention to precipitation to secure good work, 

 it cannot be left to itself, and to get the best results the same 

 care is required as in running" an engine or a steam pump. 

 Here there is an advantage with the old, slow sand filters that 

 they are more " fool-proof." 



So far these remarks have dealt with precipitation and its 

 necessity in certain waters. From here the next step in purifi- 

 cation may be regarded as a matter of careful choice, slow 

 sand filtration or rapid mechanical filtration. The two systems 

 are known in America as the (i) English, i.e., slow sand 

 filtration after preliminary settling" without chemical addition 

 just by time and subsidence (modified English, where chemical 

 precipitation precedes slow sand filtration j ; and (2) the Ameri- 

 can filtration, i.e., where, after adding according" to require- 



