SEWAGE PURIFICATION 301 



surface area. On the other hand, the material may be so far 

 subdivided that the interstices rapidly fill up with gelatinous 

 matter, which in its turn holds up water, so that the interstices 

 become reduced and the circulation of air is interfered 

 with. 



In general, therefore, it will be found economical to use the 

 smallest material which allows of free circulation of air. 

 The main direction of air circulation in a trickHng filter is 

 probably from above downwards, the air being drawn through 

 the filter by the percolation of the hquid. An exception to 

 this rule may occur in cold weather, when the higher tempera- 

 ture of the filter, as compared with the outside air, may induce 

 an upward current. 



It is obvious that filtering material must be avoided which 

 tends to weather and break down, as the interstices will then 

 tend to be filled with small pieces of broken-down material, 

 and air circulation wiU be impeded. In order to obtain the 

 greatest possible surface area, material of an irregular character, 

 such as hard furnace cUnker, gives the best results, but other 

 available material can be used, so long as it is not so smooth as 

 to exert httle or no retaining power, or retentivity, oh the 

 gelatinous matter deposited upon it. 



Further, thorough drainage is essential ; otherwise water 

 will tend to be held up by capillary attraction in the bottom 

 layers of the filter, and wiU interfere with air circulation. For 

 this reason a concrete bottom for the filtering medium is advis- 

 able, the thickness of which will depend on engineering con- 

 siderations. These conditions of efficiency apply equally 

 both to contact beds and to trickhng filters. 



It is obvious that if the filtering material is to be fully 

 made use of, efficient and equable distribution of the efiluent 

 over every part of the filter is essential ; it may, however, be 

 pointed out that in certain circumstances, especially in small 

 works, it is well to have an ample surface area of material, so 

 that the efficiency of the process shall not be too dependent on 



