338 MODERN METHODS OF WATER PURIFICATION. 



Here, too, the washing of the hlter takes place by a reversal 

 of flow, but there is a striking- feature in this filter, that the 

 sand stirrers, revolving at the rate of 80 feet per minute, not 

 only stir and agitate the sand, but squirt out water under pres- 

 sure from the revolving arms, which are hollow, so that at all 

 parts of the filter there is a jet of water playing as the stirrers 

 go round, loosening the material and washing away dirt. Some 

 worked the stirrers by hand, others had motor power. Like 

 the American filters, sludge is formed and needs removal. The 

 amount of wash water needed is said to be less than i per cent., 

 or little over ^ per cent. 



I got excellent reports from the officials of places using Bell 

 filters that I visited, and they were satisfied that they did good 

 work. At one installation the water went through a gravel 

 bed first. At some of the places visited there was no accurate 

 measurement of the coagulant required, whether a teaspoonful 

 or a shovelful appeared immaterial. That is not the fault of 

 the filter nor its makers, as there is an automatic feed appara- 

 tus, but tlie " rather more or less " method of doing things 

 which characterises some people and their work. 



The Mather & Piatt Filters are made in two forms: (i) The 

 Pressure Filter and (2) The Gravity Filter. The former is a 

 closed steel cylinder tank, domed at the ends, and standing on 

 four feet. These filters do not use sand, gravel, etc., layers 

 as in other filters, but the filtering material consists exclusively 

 of crushed quartz crystals, graded into various layers, finest 

 above. The floor is a dished iron plate which, like a midriff, 

 separates the chamber that is the filter proper from the collect- 

 ing one below. These filters are used in quite a number of 

 Scotch waterworks, e.g.. Kirkcaldy, Dumbarton. Rothesay, 

 etc. ; also the King and Andrew Carnegie have installations, 

 and in South Africa at Maritzburg and Inchanga. 



One feature of this filter is that the brass nozzles are screwed 

 into the false bottom f)'0)n hcloiv. The advantage of this is 

 that they can be unscrewed from the collecting chamber below 

 without digging out the quartz, so that if a nozzle is broken it 

 can be replaced easily. If you have to replace a strainer in the 

 American filter you must empty the sand of the filter; net 

 so here. These nozzles are used for channels downwards for 

 the filtered water, and upwards for the wash water. The water 

 is admitted for filtering by an annular channel, over the edge 

 of which the water flows on to the filtering quartz. The wash- 

 ing is done by reversal of current, and a rake is also used, 

 rotated by hand or by power. The filtering takes place under 

 pressure. 



The Gravity filter of this firm is similar in other important 

 points, but the cylindrical tanks are open at the top. At Bolton 

 ferric-alumina is used in the filters of the Mather & Piatt pres- 

 sure type, and a very favourable opinion is held of their work. 

 Careful attention is there paid to the regulation of the quantity 

 of alum used. Bolton has an interest for us, because the water 

 there is very turbid, and it is one of the first places in England 

 where alum was used by a Public Authority. As far back as 

 1881 use of alumina-ferric was made for purposes of removing- 



